MOSFET amp - thump at switch on.
 

Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a conventional
power supply?

Comments welcome. ;-)

--
*A computer once beat me at chess, but it was no match for me at kickboxing.

Dave Plowman London SW
To e-mail, change noise into sound.

MOSFET amp - thump at switch on.
 

In article , Dave Plowman (News)
wrote:
Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Some amp designs might 'thump' anyway as the bias points passed magic
values that caused various parts of the circuit to 'wake up'.

Comments welcome. ;-)

Personally, such thumps have never bothered me. The only change I made to
my Armstrong 700s was to eventually bypass their output relays. After a
decade or more they did as I'd predicted originally and started to give
unreliable connections. So I shorted across them.

Provided the thump doesn't bother the speakers or blow you across the room
I simply regard it as an audible 'switched on' indicator.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

"Jim Lesurf" wrote in message
...
In article , Dave Plowman (News)
wrote:
Common way to stop the thump you can tend to get with a MOSFET amp
at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it
might
be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Some amp designs might 'thump' anyway as the bias points passed
magic
values that caused various parts of the circuit to 'wake up'.

Comments welcome. ;-)

Personally, such thumps have never bothered me. The only change I
made to
my Armstrong 700s was to eventually bypass their output relays.
After a
decade or more they did as I'd predicted originally and started to
give
unreliable connections. So I shorted across them.

Provided the thump doesn't bother the speakers or blow you across
the room
I simply regard it as an audible 'switched on' indicator.


Surely the risk is d.c. offset as almost all modern amps and certainly
a MOSFET amp would have split supplies.

I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!


--
Woody

harrogate3 at ntlworld dot com

MOSFET amp - thump at switch on.
 

On 22/12/2015 6:06 AM, Woody wrote:
"Jim Lesurf" wrote in message
...
In article , Dave Plowman (News)
wrote:
Common way to stop the thump you can tend to get with a MOSFET amp
at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it
might
be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Some amp designs might 'thump' anyway as the bias points passed
magic
values that caused various parts of the circuit to 'wake up'.

Comments welcome. ;-)

Personally, such thumps have never bothered me. The only change I
made to
my Armstrong 700s was to eventually bypass their output relays.
After a
decade or more they did as I'd predicted originally and started to
give
unreliable connections. So I shorted across them.

Provided the thump doesn't bother the speakers or blow you across
the room
I simply regard it as an audible 'switched on' indicator.


Surely the risk is d.c. offset as almost all modern amps and certainly
a MOSFET amp would have split supplies.

I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.

--
Trevor Wilson
www.rageaudio.com.au

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MOSFET amp - thump at switch on.
 

Dave Plowman (News) wrote:

Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a conventional
power supply?

Comments welcome. ;-)



** Ramping up the AC with a Variac will show you if the idea works or not - usually its no help or even makes the thump worse.

What you normally see is the output swings DC from the get go and then at some point it suddenly jumps back to zero.

The problem is not restricted to MOSFET amps, most amps thump at switch on unless there is a speaker muting relay.

My own BJT amp has no thump if switched on after a long break, but a rather loud one if cycled back on after 10 or 20 seconds - cos one rail drops faster.



.... Phil

MOSFET amp - thump at switch on.
 

On 21/12/2015 11:46 PM, Dave Plowman (News) wrote:
Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a conventional
power supply?

Comments welcome. ;-)


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.

--
Trevor Wilson
www.rageaudio.com.au

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MOSFET amp - thump at switch on.
 

Trevor Wilson wrote:


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.


** The amp is fully symmetrical / complementary from input to output. I note it still has relay input muting and relies on rail fuses for self protection - but has enough outputs, 10 x 150W devices per channel, for that to work.

http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png

Such a topology allows for a very wide operating voltage range, down to a +/- a few volts and this stops the thumps.

But I am a bit worried about the bias arrangements - having six transistors in the bias loop makes for thermal instability.


..... Phil

MOSFET amp - thump at switch on.
 

On Mon, 21 Dec 2015 23:24:14 -0800 (PST), Phil Allison
wrote:

Trevor Wilson wrote:


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.


** The amp is fully symmetrical / complementary from input to output. I note it still has relay input muting and relies on rail fuses for self protection - but has enough outputs, 10 x 150W devices per channel, for that to work.

http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png

Such a topology allows for a very wide operating voltage range, down to a +/- a few volts and this stops the thumps.

But I am a bit worried about the bias arrangements - having six transistors in the bias loop makes for thermal instability.


.... Phil

I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?

d

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MOSFET amp - thump at switch on.
 

On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?
Or any amp with a regulated PSU.

--
Eiron.

MOSFET amp - thump at switch on.
 

On Tue, 22 Dec 2015 08:25:21 +0000, Eiron
wrote:

On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?
Or any amp with a regulated PSU.

Finite output impedance sees to that, strictly. But on a practical
level, if your amp will produce double the power into 4 ohms, you've
over-designed it for 8 ohms.

d

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MOSFET amp - thump at switch on.
 

I have a denon here with a little relay like this, the problem seems to be
that relays get dirty, or wear aas this one has and e distortion varies and
the levels wobble when its time to take the lid off and do another contact
cleaning job...
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"Dave Plowman (News)" wrote in message
...
Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a conventional
power supply?

Comments welcome. ;-)

--
*A computer once beat me at chess, but it was no match for me at
kickboxing.

Dave Plowman London SW
To e-mail, change noise into sound.

MOSFET amp - thump at switch on.
 

In article , Don Pearce
wrote:

Finite output impedance sees to that, strictly. But on a practical
level, if your amp will produce double the power into 4 ohms, you've
over-designed it for 8 ohms.

Which may be fine given that the use will be with speakers the user chooses
without getting your approval. 8-]

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

Don Pearce wrote:

Trevor Wilson wrote:


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.


** The amp is fully symmetrical / complementary from input to output. I note it still has relay input muting and relies on rail fuses for self protection - but has enough outputs, 10 x 150W devices per channel, for that to work.

http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png

Such a topology allows for a very wide operating voltage range, down to a +/- a few volts and this stops the thumps.

But I am a bit worried about the bias arrangements - having six transistors in the bias loop makes for thermal instability.



I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?


** What about the 330pF cap - C608 ?

Along with the 33kohms, it forms a pole at 160kHz.

There is no load isolating inductor at output either, so it might not like capacitor values around 10nF to 100nF plonked right across the terminals.

..... Phil

MOSFET amp - thump at switch on.
 

In article ,
Don Pearce wrote:
http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png


I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?

It does have what I used - a snubber across the first long-tail pair(s).
Also a cap across them *and* some later caps. If anything it seems to have
roll-off applied in multiple places. Which one may dominate I haven't
checked. But it doesn't seem to lack HF roll-off.

Interesting that it apparently omits having any output inductor. I tended
to find one useful if only for helping to reduce any RF injection coming in
via the speaker leads.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

Trevor Wilson wrote:



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


** High power, class D types with a PFC switching PSU get very close.

Regulated DC rails and super low internal impedance is the key.


..... Phil

MOSFET amp - thump at switch on.
 

On Tue, 22 Dec 2015 01:25:37 -0800 (PST), Phil Allison
wrote:

Don Pearce wrote:

Trevor Wilson wrote:


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.


** The amp is fully symmetrical / complementary from input to output. I note it still has relay input muting and relies on rail fuses for self protection - but has enough outputs, 10 x 150W devices per channel, for that to work.

http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png

Such a topology allows for a very wide operating voltage range, down to a +/- a few volts and this stops the thumps.

But I am a bit worried about the bias arrangements - having six transistors in the bias loop makes for thermal instability.



I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?


** What about the 330pF cap - C608 ?

Along with the 33kohms, it forms a pole at 160kHz.

There is no load isolating inductor at output either, so it might not like capacitor values around 10nF to 100nF plonked right across the terminals.

.... Phil

I see that but in that position it is a kind of luke-warm pole. I
expect it to be placed so it reinforces the Miller capacitance between
collector and base of Q613 and Q615. That has the advantage of not
creating two poles associated with the voltage amp - itself a
potential source of instability.

d

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MOSFET amp - thump at switch on.
 

On Tue, 22 Dec 2015 09:31:10 +0000 (GMT), Jim Lesurf
wrote:

In article ,
Don Pearce wrote:
http://srv2.umlib.com/1129f1c8443296...r-sm.pdf-3.png


I've never seen that topology before. There doesn't seem to be a
dominant pole in the voltage amplifiers - how did they keep it stable?

It does have what I used - a snubber across the first long-tail pair(s).
Also a cap across them *and* some later caps. If anything it seems to have
roll-off applied in multiple places. Which one may dominate I haven't
checked. But it doesn't seem to lack HF roll-off.

Interesting that it apparently omits having any output inductor. I tended
to find one useful if only for helping to reduce any RF injection coming in
via the speaker leads.

Jim

This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling. Now I know this is
easiest when you have open loop gain by the bucketload, as you have in
a modern integrated op amp. But this is still an op amp and it seems
strange to dot bits of this and bits of that around it.

I have used the snubber circuit when adding discrete transistors to
the front end of an op amp for low noise work. You have to do this
because the extra voltage gain completely screws the phase margin of
the bigger loop. If you could get inside the IC and beef up the
dominant pole cap that wouldn't help, because the current in the tails
would be insufficient to drive it at high frequencies, and you'd get
slew rate limiting.

Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.

d

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MOSFET amp - thump at switch on.
 

On Tue, 22 Dec 2015 09:25:18 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:

Finite output impedance sees to that, strictly. But on a practical
level, if your amp will produce double the power into 4 ohms, you've
over-designed it for 8 ohms.

Which may be fine given that the use will be with speakers the user chooses
without getting your approval. 8-]

Jim

I'm just thinking of the cost of that huge mains transformer - and the
heat sinks.

d

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MOSFET amp - thump at switch on.
 

On 22/12/2015 09:54, Don Pearce wrote:

I'm just thinking of the cost of that huge mains transformer - and the
heat sinks.

Get with the 21st century, grandad!
Switched mode PSUs are the in-thing now. :-)

--
Eiron.

MOSFET amp - thump at switch on.
 

In article , Don Pearce
wrote:
It does have what I used - a snubber across the first long-tail
pair(s). Also a cap across them *and* some later caps. If anything it
seems to have roll-off applied in multiple places. Which one may
dominate I haven't checked. But it doesn't seem to lack HF roll-off.


This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling.

Yes, it does seem the 'automatic' choice of many designers. To the extent
that I've seen articles on amp design that take it for granted. However
when I experimented I found a suitable snubber across the long-tail pair at
the input of the designs I worked on worked much better. I can't say if
that was peculiar to the designs I worked on, or if I simply didn't pick up
the usual hammer other chose.


Now I know this is easiest when you have open loop gain by the
bucketload, as you have in a modern integrated op amp. But this is still
an op amp and it seems strange to dot bits of this and bits of that
around it.


Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.


Yes. The Rotel does seem rather complicated to me. Maybe the designer got
desperate and fitted caps everywhere and played "hunt the battleship" until
it worked. ...but then forgot to add an output inductor. An omission that
might have ruined a few proud buyer's day. 8-]

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

On Tue, 22 Dec 2015 10:27:38 +0000, Eiron
wrote:

On 22/12/2015 09:54, Don Pearce wrote:

I'm just thinking of the cost of that huge mains transformer - and the
heat sinks.

Get with the 21st century, grandad!
Switched mode PSUs are the in-thing now. :-)

I've designed them, so yes, I'm with that. But did you know that
"normal" power supplies are also switch mode? They just switch at
100Hz instead of a few hundred kHz.

d

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MOSFET amp - thump at switch on.
 

In article , Don Pearce
wrote:

I'm just thinking of the cost of that huge mains transformer - and the
heat sinks.

Tell me about it! :-/ I had to make a +200W pc amp to meet the old IHF707
specs. Then have reviewers whine that the result was expensive. of course
it was when you added up all the heavy metel required for 1/3rd power
running for an hour before testing. Oh, and the external heatsinks weren't
allowed to exceed 60 C.

Then factor in that I was determined to have the amp happily drive loads
down to below 2 Ohms, etc. And have a transformer that didn't 'leak' too
much field and spoil the quality.

No doubt Rotel faced similar 'expectations' from reviewers and buyers. When
in reality an end user would almost never push such an amp anywhere near
its limits. If they did, their speakers and ears would probably have been
the main limit. 8-]

I have wondered since about modifying the design to run as a lower output
Class-A. In some ways it would have been simpler.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

On Tue, 22 Dec 2015 10:34:32 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:
It does have what I used - a snubber across the first long-tail
pair(s). Also a cap across them *and* some later caps. If anything it
seems to have roll-off applied in multiple places. Which one may
dominate I haven't checked. But it doesn't seem to lack HF roll-off.


This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling.

Yes, it does seem the 'automatic' choice of many designers. To the extent
that I've seen articles on amp design that take it for granted. However
when I experimented I found a suitable snubber across the long-tail pair at
the input of the designs I worked on worked much better. I can't say if
that was peculiar to the designs I worked on, or if I simply didn't pick up
the usual hammer other chose.


Now I know this is easiest when you have open loop gain by the
bucketload, as you have in a modern integrated op amp. But this is still
an op amp and it seems strange to dot bits of this and bits of that
around it.


Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.


Yes. The Rotel does seem rather complicated to me. Maybe the designer got
desperate and fitted caps everywhere and played "hunt the battleship" until
it worked. ...but then forgot to add an output inductor. An omission that
might have ruined a few proud buyer's day. 8-]

Jim

Adding the snubber across the input long tail pair will allow you to
lighten up a bit on the dominant pole on the voltage amp and give you
a bit more open loop gain, hence less distortion. But it is a bit of a
balancing act, and you have to do a pretty careful tolerance analysis
to make sure you've got away with it in all circumstances. A normal
dominant pole is a bit more of a blunt weapon.

d

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MOSFET amp - thump at switch on.
 

In article ,
Phil Allison wrote:
Dave Plowman (News) wrote:

Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it
might be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Comments welcome. ;-)



** Ramping up the AC with a Variac will show you if the idea works or
not - usually its no help or even makes the thump worse.

Sadly, not something I possess.

What you normally see is the output swings DC from the get go and then
at some point it suddenly jumps back to zero.

If I feed it from the bench power supply and start that at zero volts no
thump at any point as you turn it up to the working voltage.

The problem is not restricted to MOSFET amps, most amps thump at switch
on unless there is a speaker muting relay.

This one is pretty loud. ;-)

My own BJT amp has no thump if switched on after a long break, but a
rather loud one if cycled back on after 10 or 20 seconds - cos one rail
drops faster.



... Phil


--
*It's o.k. to laugh during sexŒ.Œ.just don't point!

Dave Plowman London SW
To e-mail, change noise into sound.

MOSFET amp - thump at switch on.
 

In article , Dave Plowman (News)
wrote:

The problem is not restricted to MOSFET amps, most amps thump at
switch on unless there is a speaker muting relay.

This one is pretty loud. ;-)

Can you show a circuit diagram? It occurs to me that someone might be able
to spot a 'tweak' that helps reduce the thump.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

On 22/12/2015 7:25 PM, Eiron wrote:
On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?

**Not even the mighty ME1500, with it's 5kVA power transformer and
310,000uF of filter capacitance.

Or any amp with a regulated PSU.


**Nope. Output device saturation is the problem.


--
Trevor Wilson
www.rageaudio.com.au

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MOSFET amp - thump at switch on.
 

On 22/12/2015 19:23, Trevor Wilson wrote:
On 22/12/2015 7:25 PM, Eiron wrote:
On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?


I'm sure that's something Stuart Pinkerton claimed of his Krell (not
sure of any of that spelling) amps on this NG, many years back. He used
to trot it out quite regularly and I don't remember anyone challenging
it . . .


--
Cheers, Rob

MOSFET amp - thump at switch on.
 

On 23/12/2015 6:47 AM, RJH wrote:
On 22/12/2015 19:23, Trevor Wilson wrote:
On 22/12/2015 7:25 PM, Eiron wrote:
On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?


I'm sure that's something Stuart Pinkerton claimed of his Krell (not
sure of any of that spelling) amps on this NG, many years back. He used
to trot it out quite regularly and I don't remember anyone challenging
it . . .


**I did and I explained EXACTLY how Krell did it. Stuart agreed that my
analysis was bang-on.

Here's how Krell managed the seemingly impossible:

The Krell KSA50 was rated at 50 Watts @ 8 Ohms, 100 Watts @ 4 Ohms, et al.

Krell actually built a 75 Watts @ 8 Ohm amplifier and called it a 50
Watt amp. That's it. That's the big secret.


--
Trevor Wilson
www.rageaudio.com.au

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On 22/12/2015 8:25 PM, Brian-Gaff wrote:
I have a denon here with a little relay like this, the problem seems to be
that relays get dirty, or wear aas this one has and e distortion varies and
the levels wobble when its time to take the lid off and do another contact
cleaning job...
Brian


**When I encounter such relays, I (wherever possible) replace the
original relay with a 4 pole C/O type, that uses gold over silver
contacts. They tend to last a great deal longer and with 4 sets of
contacts, some redundancy as well.

--
Trevor Wilson
www.rageaudio.com.au

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MOSFET amp - thump at switch on.
 

On 22/12/2015 9:34 PM, Jim Lesurf wrote:
In article , Don Pearce
wrote:
It does have what I used - a snubber across the first long-tail
pair(s). Also a cap across them *and* some later caps. If anything it
seems to have roll-off applied in multiple places. Which one may
dominate I haven't checked. But it doesn't seem to lack HF roll-off.


This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling.

Yes, it does seem the 'automatic' choice of many designers. To the extent
that I've seen articles on amp design that take it for granted. However
when I experimented I found a suitable snubber across the long-tail pair at
the input of the designs I worked on worked much better. I can't say if
that was peculiar to the designs I worked on, or if I simply didn't pick up
the usual hammer other chose.


Now I know this is easiest when you have open loop gain by the
bucketload, as you have in a modern integrated op amp. But this is still
an op amp and it seems strange to dot bits of this and bits of that
around it.


Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.


Yes. The Rotel does seem rather complicated to me. Maybe the designer got
desperate and fitted caps everywhere and played "hunt the battleship" until
it worked. ...but then forgot to add an output inductor. An omission that
might have ruined a few proud buyer's day. 8-]


**As a Rotel service agent, I don't recall seeing any amps that have
landed on my bench after being fried by ESLs. That said, I happen to
have an RB991 on the bench right now. When it has been completed, I'll
run it into a couple uF and see what transpires.


--
Trevor Wilson
www.rageaudio.com.au

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Phil Allison wrote:


**Have a look at the Rotel RB991. No output relay, no switch-on thump.
Nicely done, Rotel.


** The amp is fully symmetrical / complementary from input to output. I note it still has relay input muting and relies on rail fuses for self protection - but has enough outputs, 10 x 150W devices per channel, for that to work.

http://srv2.umlib.com/1129f1c8443296...-sm..pdf-3.png

Such a topology allows for a very wide operating voltage range, down to a +/- a few volts and this stops the thumps.

But I am a bit worried about the bias arrangements - having six transistors in the bias loop makes for thermal instability.



** The current source transistors for the differential pairs are biased from a string of 4 diodes and a 47kohm resistor that connect from one rail to the other. This arrangement is very deliberate and not done as an economy.

If one or other rail fuse opens, all input stage bias is lost and the amp becomes dead - so no output current can be delivered. Done the conventional way, if one fuse blew it would send send the amp hard DC to the opposite rail.

A common way around the problem is to fuse only the output stage and leave the rest of the amplifier permanently powered. Phase Linear amps and many others are wired this way. The Quad 405 has a permanent connection for the negative supply for the input op-amp.

I know of one amp where the designer got it wrong, removing the plus side fuse was harmless enough but removing the minus side one was fatal. The output would swing to full opposite rail and deliver 90VDC to the loudspeaker. I wound up modifying about 30 examples for a hire business so this did not happen again.

Most amps have no rail fuses as an even simpler answer to the problem.



.... Phil

MOSFET amp - thump at switch on.
 

Trevor Wilson wrote:



**As a Rotel service agent, I don't recall seeing any amps that have
landed on my bench after being fried by ESLs.


** No ESL is the same as a capacitor where the impedance drops near zero ohms at some frequency. The ESL57 dropped to about 1.7 ohms at 18kHz and then rose sharply, IOW it was resistive at 18kHz.

A better HF load simulation for typical ESLs is a 6.8uF cap in series with a 1 ohm resistor. The low end is a bigger problem cos input transformers suffer core saturation at low audio and sub sonic frequencies and this is a real amp destroyer.

IME, testing audio amps with capacitors connected right across the speaker terminals is an unnecessary torture with one exception - if the owner uses high capacitance speaker cables. Amps from Naim and Phase Linear ( having no output inductors) were notoriously unstable into capacitances around 20 to 50nF.


That said, I happen to
have an RB991 on the bench right now. When it has been completed, I'll
run it into a couple uF and see what transpires.



** Try feeding the amp with a 5kHz square wave at the 1watt level and apply the cap briefly while watching for oscillations on the CRO. Expect to see some ringing.



.... Phil

MOSFET amp - thump at switch on.
 

On 23/12/2015 2:43 PM, Phil Allison wrote:
Trevor Wilson wrote:



**As a Rotel service agent, I don't recall seeing any amps that
have landed on my bench after being fried by ESLs.


** No ESL is the same as a capacitor where the impedance drops near
zero ohms at some frequency. The ESL57 dropped to about 1.7 ohms at
18kHz and then rose sharply, IOW it was resistive at 18kHz.

A better HF load simulation for typical ESLs is a 6.8uF cap in series
with a 1 ohm resistor. The low end is a bigger problem cos input
transformers suffer core saturation at low audio and sub sonic
frequencies and this is a real amp destroyer.

IME, testing audio amps with capacitors connected right across the
speaker terminals is an unnecessary torture with one exception - if
the owner uses high capacitance speaker cables. Amps from Naim and
Phase Linear ( having no output inductors) were notoriously unstable
into capacitances around 20 to 50nF.


That said, I happen to have an RB991 on the bench right now. When
it has been completed, I'll run it into a couple uF and see what
transpires.



** Try feeding the amp with a 5kHz square wave at the 1watt level and
apply the cap briefly while watching for oscillations on the CRO.
Expect to see some ringing.


**Oh I do. I've tested A LOT of Rotel amps over the years. With a
simulated speaker load (with some parallel capacitance, they all exhibit
some ringing. The topology has remained pretty much the same for a
couple of decades.


--
Trevor Wilson
www.rageaudio.com.au

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On 23/12/2015 02:41, Trevor Wilson wrote:
On 22/12/2015 9:34 PM, Jim Lesurf wrote:
In article , Don Pearce
wrote:
It does have what I used - a snubber across the first long-tail
pair(s). Also a cap across them *and* some later caps. If anything it
seems to have roll-off applied in multiple places. Which one may
dominate I haven't checked. But it doesn't seem to lack HF roll-off.


This is my problem. A single dominant pole placed so that the phase
doesn't wander too far from 90 degrees until you are comfortably clear
of unity gain gives you a warm comfortable feeling.

Yes, it does seem the 'automatic' choice of many designers. To the extent
that I've seen articles on amp design that take it for granted. However
when I experimented I found a suitable snubber across the long-tail
pair at
the input of the designs I worked on worked much better. I can't say if
that was peculiar to the designs I worked on, or if I simply didn't
pick up
the usual hammer other chose.


Now I know this is easiest when you have open loop gain by the
bucketload, as you have in a modern integrated op amp. But this is still
an op amp and it seems strange to dot bits of this and bits of that
around it.


Anyway, I'm sure this works, but it has a look of an amplifier that
oscillated on the bench, so they just added bits until it stopped.


Yes. The Rotel does seem rather complicated to me. Maybe the designer got
desperate and fitted caps everywhere and played "hunt the battleship"
until
it worked. ...but then forgot to add an output inductor. An omission that
might have ruined a few proud buyer's day. 8-]


**As a Rotel service agent, I don't recall seeing any amps that have
landed on my bench after being fried by ESLs. That said, I happen to
have an RB991 on the bench right now. When it has been completed, I'll
run it into a couple uF and see what transpires.


I used a Rotel RB980BX power amp successfully with ESL57s,
obviously keeping the level down.
Until I forgot to turn off the power before swapping input leads,
which blew the internal fuses but caused no other problems.
(I blame the design of phono plugs which connects signal before ground.)

--
Eiron.

MOSFET amp - thump at switch on.
 

In article , Trevor Wilson
wrote:

**As a Rotel service agent, I don't recall seeing any amps that have
landed on my bench after being fried by ESLs. That said, I happen to
have an RB991 on the bench right now. When it has been completed, I'll
run it into a couple uF and see what transpires.

Afraid that an 'ESL' isn't the only reason for using an output series
inductor. Omitting one can give rise to two other problems.

1) Instability with *small* amounts of capacitative load. e.g. few tens of
thousand pF rather than the microF levels of a QUAD. Its quite easy to have
a design that is stable and safe into, say, 2.2uF but which hoots wildly
into a much smaller capacitance.

2) Injection of RF back into the amp via the speaker or speaker leads.

These problems can take more time to deal with than ensuring an ESL won't
damage the amp.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

MOSFET amp - thump at switch on.
 

Yes but that would be major surgery of an old unit, and I cannot do this
myself now so I'll just patch it up. Incidentally I've just had similar
issues on an aerial attenuator switching relay on an ICOM communications
receiver. It was peculiar as when poor contact was being made the cross
modulation generated at first made me think an fet had died in the early
stages and gone non linear on me.
However a quick fiddle proved that it was just the effect of partial
contacting.
On the Denon, its really weird as when its in dodgy mode turning the
volume up produces some terribly broken up sounds as it presumably, once
again acts a little like diode and a resistor.
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"Trevor Wilson" wrote in message
...
On 22/12/2015 8:25 PM, Brian-Gaff wrote:
I have a denon here with a little relay like this, the problem seems to
be
that relays get dirty, or wear aas this one has and e distortion varies
and
the levels wobble when its time to take the lid off and do another
contact
cleaning job...
Brian


**When I encounter such relays, I (wherever possible) replace the original
relay with a 4 pole C/O type, that uses gold over silver contacts. They
tend to last a great deal longer and with 4 sets of contacts, some
redundancy as well.

--
Trevor Wilson
www.rageaudio.com.au

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MOSFET amp - thump at switch on.
 

On 23/12/2015 10:18 PM, Brian-Gaff wrote:
Yes but that would be major surgery of an old unit, and I cannot do this
myself now so I'll just patch it up. Incidentally I've just had similar
issues on an aerial attenuator switching relay on an ICOM communications
receiver. It was peculiar as when poor contact was being made the cross
modulation generated at first made me think an fet had died in the early
stages and gone non linear on me.
However a quick fiddle proved that it was just the effect of partial
contacting.
On the Denon, its really weird as when its in dodgy mode turning the
volume up produces some terribly broken up sounds as it presumably, once
again acts a little like diode and a resistor.
Brian


**What you have found is something I've seen hundreds of times. Which is
why I employ the fix I described, if possible.

--
Trevor Wilson
www.rageaudio.com.au

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MOSFET amp - thump at switch on.
 

I'm sure, but the fact is in the end I still cannot do it myself any more,
so it will probably remain like this until IIpop my cloggs or a more
terminal fault occurswhen it will be decision time as to what to do.
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"Trevor Wilson" wrote in message
...
On 23/12/2015 10:18 PM, Brian-Gaff wrote:
Yes but that would be major surgery of an old unit, and I cannot do this
myself now so I'll just patch it up. Incidentally I've just had similar
issues on an aerial attenuator switching relay on an ICOM communications
receiver. It was peculiar as when poor contact was being made the cross
modulation generated at first made me think an fet had died in the early
stages and gone non linear on me.
However a quick fiddle proved that it was just the effect of partial
contacting.
On the Denon, its really weird as when its in dodgy mode turning the
volume up produces some terribly broken up sounds as it presumably, once
again acts a little like diode and a resistor.
Brian


**What you have found is something I've seen hundreds of times. Which is
why I employ the fix I described, if possible.

--
Trevor Wilson
www.rageaudio.com.au

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