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

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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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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

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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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

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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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

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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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

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. ;-)

--
*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.