MOSFET amp - thump at switch on.
 

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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MOSFET amp - thump at switch on.
 

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

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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

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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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