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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