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

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"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. ;-)

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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

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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

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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

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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
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