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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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

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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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.

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

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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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

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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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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