Ah, Morgan! Good to see you're keeping him off the streets. Andy

It would be refreshing to see him on this NG.
But little chance I fear:-(


Makes me think of Groucho Marx "I refuse to belong to any such club as would
accept myself as a member"......

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Don Pearce wrote:

On Thu, 13 Jan 2005 14:35:24 +0200, "Iain M Churches"
wrote:


"Don Pearce" wrote in message
...

I've been the minimalist route for amp design, and the problem is that
it doesn't stay that way. You always end up thinking "if I just do
this, it will work SO much better" and by the time you are done, you
have put in all the buffered current mirrors etc that really make it
work. And in the end, why not? The bits cost bugger all.

The cost is not really a factor. The idea was to design and build the
SS equivalent of Andre's simple SET amplifier, so that those
interested could build both for listening comparison.

Iain

I understand that, and it could be an interesting experiment -
although the very different transfer characteristics of a BJT and a
triode would make it of limited value. And of course in SS it is so
much more trivial a task to do all the good stuff.

If only you could get valves that worked on positrons!

I wouldn't buy tubes that worked by emitting positrons....

Its hard to conjure up such a device, and just because that would enable
some of the combinations routinely used with NPN and PNP
SS devices may not bring more music to anyone's ears.

I think it will be quite some time before anyone comes up with an
8 watt two transistor single ended amp which has no global NFB
and which sounds as well as a 300B amp when correctly set up.

Stewart Pinkerton and I discussed the prospects of a kiSS amp
at RAT some weeks back, and so far, no SS prototype has arrived on the
doorstep of our
consciousness from Mr Pinky.

There are several problems with BJTs ensuring that simplicity is almost
impossible when one uses them.
Its impossible to use them without NFB, since the collector resistance is so
much higher
than a speaker's, like an SE pentode amp.
OK, so lets say we allow the BJT amp to use the same amount of NFB as the
internal NFB
which exists in a 300B and then proceed.
This rules out the use of setting up the output trannie as an emitter
follower, since that is a case
of a vast amount of series voltage NFB.
And input voltage must be below 1vrms.
The low base input resistance of the bjt can spoil the simple is best
idea.....
Even if you set up a single MJE21194 or 93 as an EF , the Iin will be
emitter output current / current gain, so for 1 amp of speaker current, and
8v into 8 ohms,
and if current gain is 30, then you need to apply about 8.1 v at the base
at 33 mA, so Rin = 242 ohms, and needing that other driver transistor
If the driver transistor was also in EF, and had a current gain of 100,
then Rin is still a lousy low 24k ohms.
Methinks any other form of applied loop FB to get Vin 1v, Ro 1 ohm,
and thd 1% will still result with Rin as too low.

Simple SE SS amps were routinely used in auto radios of the 60s and 70s,
and often had a few driver transistors or an opamp to power a single BJT
in class A with a transformer coupled speaker, all running off the
12 volt car supply which allows a swing of 7 vrms, giving 12 watts into 4
ohms
if the idle current is high enough.
All used lots of NFB, and were not as simple as an SET 300B amp
when considered in terms of electronic elements used.
I once repaired such a radio from an Aston Martin.
The output tranny was a large germanium type in SE class A.


But why muck around using BJT's?

They are OK in complex amps working in nearly class B circuits, and their
cheapness is why they are used.

Mosfets provide a far better solution because they have a high gate input
impedance,
and The Nelson Pass Zen amp using just a single mosfet as the output and gain
element
with a CCS instead of an OPT with some 12 dB of shunt voltage NFB is
comparable with
a 300B amp.

People may complain that mosfets are horribly non linear.
But when used in the middle of their transfer range and with some NFB
they are quite adequately linear for a few watts, and most of the thd is
a combination of mainly 2H and 3H, rather like a pentode.
Efficiency is about 45% with SE, so for 8 watts output
you only need about 19 watts of idle power, much less than a triode amp
with 25% efficiency..
So its comparable with the triode amp to begin with and Rin is quite high.
Cin is also high, but not high enough to stop the use of a single j-fet
driver device such as the
2SK369 from being used very effectively, and in a circuit like the cap
coupled and
transformer output coupled tube amp.

BJTs have appalling voltage linearity when set up in common
emitter with the load in the collector circuit and the emitter grounded.
They have a lot of voltage gain, but linearity is worse than a pentode with
no NFB.


One thing seems certain to me, it is easier to make a good sounding tube amp
than a good sounding solid state amp.

The knockers will maintain that the tube amp measurements are always going
to be **** compared to the 0.0001% available from an SS amp,
but so what?

A class A PP williamson amp designed and built to the original
spec of Mr DTN Williamson in 1947 with a pair of KT66
will make 16 watts at a max of 0.1 % thd with 20 dB of series voltage NFB.
One can remove that NFB and still only get 1% thd.
There are 4 input triodes and two outputs.
School boys found such amps easy to assemble and get working.

At a few watts, thd is 0.02% with NFB and perhaps 0.2% without.
I'd suggest that 0.02% of thd at 2 watts of signal level is inaudible, so
the reduction of thd down 200 times to 0.0001% as Halcro claim to achieve
cannot make much difference.


Then if we were to consider the use of KT88 in lieu of KT66, but with exactly

the same circuit, we'd get nearly twice the power.
The Quad 40 is an example of a simple PP tube circuit using 2 x KT88,
and two signal pentode driver tubes, and although pentodes have a reputation
for
more thd than triodes, the Quad 40 and the preceding Quad II amps
have only 0.025% thd at a couple of watts when all the tubes are matched
fairly close, and arranged more maximal 2H cancellation.

I recently built a high current BJT 300 watt/channel stereo amp using 5 x
MJE21193, and 5 x MJE21194
in the outputs, and with a shirtload of NFB so thd is negligible at all
levels and
its just does not sound as smooth and non fatiguing as many tube amps.
There are not too many school boys who could manage to build and get such a
thing working.
Its not as good as the same power mosfet amps I have built.
It ought to be great, and I have followed all the rules Douglas Self and
others recommend,
even with a choke filtered collector rails, but methinks its only good for
sub-woofer
power.
The crossover distortion with a hefty idle current is utterly negligible
at audio F and the dominant thd is the non linearity of the main voltage
swings.
Still, it measures very well since a total of over 100 dB od NFB is used.
I didn't like the way the cross conduction during clipping at just above the
audio
band caused enormous currents between rails.
I tamed all that as best I could, but mosfets don't have all these ****ty
problems,
so the drive circuit and the mosfet PP complementary pair outputs is a lot
simpler than that
needed for the BJT amp.

Give me tubes or fets, and to hell with the rest.

Patrick Turner.



Pearce Consulting
http://www.pearce.uk.com

"Patrick Turner" wrote in message



I think it will be quite some time before anyone comes up with an
8 watt two transistor single ended amp which has no global NFB
and which sounds as well as a 300B amp when correctly set up.

Given the incredibly convoluted criteria used to establish that the 300B amp
sounds good...

On Fri, 14 Jan 2005 01:03:33 +1100, Patrick Turner
wrote:



Don Pearce wrote:

On Thu, 13 Jan 2005 14:35:24 +0200, "Iain M Churches"
wrote:


"Don Pearce" wrote in message
...

I've been the minimalist route for amp design, and the problem is that
it doesn't stay that way. You always end up thinking "if I just do
this, it will work SO much better" and by the time you are done, you
have put in all the buffered current mirrors etc that really make it
work. And in the end, why not? The bits cost bugger all.

The cost is not really a factor. The idea was to design and build the
SS equivalent of Andre's simple SET amplifier, so that those
interested could build both for listening comparison.

Iain

I understand that, and it could be an interesting experiment -
although the very different transfer characteristics of a BJT and a
triode would make it of limited value. And of course in SS it is so
much more trivial a task to do all the good stuff.

If only you could get valves that worked on positrons!

I wouldn't buy tubes that worked by emitting positrons....

Its hard to conjure up such a device, and just because that would enable
some of the combinations routinely used with NPN and PNP
SS devices may not bring more music to anyone's ears.

I think it will be quite some time before anyone comes up with an
8 watt two transistor single ended amp which has no global NFB
and which sounds as well as a 300B amp when correctly set up.

Well, the question has to be - why would they, given that conventional
amps do the job so much better. I well remember the single ended
transistor amps from car radios, and I can think of no reason to
revisit that topology.

Stewart Pinkerton and I discussed the prospects of a kiSS amp
at RAT some weeks back, and so far, no SS prototype has arrived on the
doorstep of our
consciousness from Mr Pinky.

Given that BJTs operate from current, and valves from voltage it is
impossible to achieve quite the degree of simplicity one would wish.

There are several problems with BJTs ensuring that simplicity is almost
impossible when one uses them.
Its impossible to use them without NFB, since the collector resistance is so
much higher
than a speaker's, like an SE pentode amp.

Are you saying the anode impedance of a pentode is similar to a
speaker? Of course it isn't - that is why you use a transformer.

And of course by using NFB to convert a current source into a voltage
source, you can make the output impedance of a transistor stage as low
as you want.

OK, so lets say we allow the BJT amp to use the same amount of NFB as the
internal NFB
which exists in a 300B and then proceed.
This rules out the use of setting up the output trannie as an emitter
follower, since that is a case
of a vast amount of series voltage NFB.
And input voltage must be below 1vrms.
The low base input resistance of the bjt can spoil the simple is best
idea.....

You say the low base resistance is a problem - what this really means
is that the device is current controlled. This is not a problem -
merely a difference.

Even if you set up a single MJE21194 or 93 as an EF , the Iin will be
emitter output current / current gain, so for 1 amp of speaker current, and
8v into 8 ohms,
and if current gain is 30, then you need to apply about 8.1 v at the base
at 33 mA, so Rin = 242 ohms, and needing that other driver transistor
If the driver transistor was also in EF, and had a current gain of 100,
then Rin is still a lousy low 24k ohms.
Methinks any other form of applied loop FB to get Vin 1v, Ro 1 ohm,
and thd 1% will still result with Rin as too low.

Too low for what, exactly? I don't follow.

Simple SE SS amps were routinely used in auto radios of the 60s and 70s,
and often had a few driver transistors or an opamp to power a single BJT
in class A with a transformer coupled speaker, all running off the
12 volt car supply which allows a swing of 7 vrms, giving 12 watts into 4
ohms
if the idle current is high enough.
All used lots of NFB, and were not as simple as an SET 300B amp
when considered in terms of electronic elements used.
I once repaired such a radio from an Aston Martin.
The output tranny was a large germanium type in SE class A.


But why muck around using BJT's?

Do you remember valve amps in car radios? Vibrating oscillators to
generate HT volts. That is why they "mucked around" with BJTs. That
and the fact that they worked so much better.

They are OK in complex amps working in nearly class B circuits, and their
cheapness is why they are used.

Mosfets provide a far better solution because they have a high gate input
impedance,
and The Nelson Pass Zen amp using just a single mosfet as the output and gain
element
with a CCS instead of an OPT with some 12 dB of shunt voltage NFB is
comparable with
a 300B amp.

People may complain that mosfets are horribly non linear.
But when used in the middle of their transfer range and with some NFB
they are quite adequately linear for a few watts, and most of the thd is
a combination of mainly 2H and 3H, rather like a pentode.
Efficiency is about 45% with SE, so for 8 watts output
you only need about 19 watts of idle power, much less than a triode amp
with 25% efficiency..
So its comparable with the triode amp to begin with and Rin is quite high.
Cin is also high, but not high enough to stop the use of a single j-fet
driver device such as the
2SK369 from being used very effectively, and in a circuit like the cap
coupled and
transformer output coupled tube amp.

BJTs have appalling voltage linearity when set up in common
emitter with the load in the collector circuit and the emitter grounded.
They have a lot of voltage gain, but linearity is worse than a pentode with
no NFB.

What is this hangup with no NFB?


One thing seems certain to me, it is easier to make a good sounding tube amp
than a good sounding solid state amp.

I could build a good sounding SS amp before you had even finished
hammering and punching out the chassis.

The knockers will maintain that the tube amp measurements are always going
to be **** compared to the 0.0001% available from an SS amp,
but so what?

A class A PP williamson amp designed and built to the original
spec of Mr DTN Williamson in 1947 with a pair of KT66
will make 16 watts at a max of 0.1 % thd with 20 dB of series voltage NFB.
One can remove that NFB and still only get 1% thd.
There are 4 input triodes and two outputs.
School boys found such amps easy to assemble and get working.

As did I when I was a schoolboy. I've since grown out of it. I notice
the comparison you make carefully avoids the performance of a SET amp.

At a few watts, thd is 0.02% with NFB and perhaps 0.2% without.
I'd suggest that 0.02% of thd at 2 watts of signal level is inaudible, so
the reduction of thd down 200 times to 0.0001% as Halcro claim to achieve
cannot make much difference.


Then if we were to consider the use of KT88 in lieu of KT66, but with exactly

the same circuit, we'd get nearly twice the power.
The Quad 40 is an example of a simple PP tube circuit using 2 x KT88,
and two signal pentode driver tubes, and although pentodes have a reputation
for
more thd than triodes, the Quad 40 and the preceding Quad II amps
have only 0.025% thd at a couple of watts when all the tubes are matched
fairly close, and arranged more maximal 2H cancellation.

I recently built a high current BJT 300 watt/channel stereo amp using 5 x
MJE21193, and 5 x MJE21194
in the outputs, and with a shirtload of NFB so thd is negligible at all
levels and
its just does not sound as smooth and non fatiguing as many tube amps.
There are not too many school boys who could manage to build and get such a
thing working.
Its not as good as the same power mosfet amps I have built.
It ought to be great, and I have followed all the rules Douglas Self and
others recommend,
even with a choke filtered collector rails, but methinks its only good for
sub-woofer
power.
The crossover distortion with a hefty idle current is utterly negligible
at audio F and the dominant thd is the non linearity of the main voltage
swings.
Still, it measures very well since a total of over 100 dB od NFB is used.
I didn't like the way the cross conduction during clipping at just above the
audio
band caused enormous currents between rails.
I tamed all that as best I could, but mosfets don't have all these ****ty
problems,
so the drive circuit and the mosfet PP complementary pair outputs is a lot
simpler than that
needed for the BJT amp.

Give me tubes or fets, and to hell with the rest.

Patrick Turner.

Your arguments are far from compelling - they seem to be of the "I
know it is worse, but I like it, so there" variety.

d
Pearce Consulting
http://www.pearce.uk.com

"Jim Lesurf" wrote in message
...
In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...

As the result of a phone call, I now know that the amp uses a 211
preceded by a 6SN7 octal.

OK. Can you also confirm that the output impedance *is* as high as around
8
Ohms - as your quoted DF of 1 implies?

No. Happily I cannot:-) This was only a figure quoted to me my Andre, when
he said that some SET amps can have a DF=1. I think we would be most unwise
to assume that this is always the case.

OK. These have a much less 'difficult' impedance characteristic than the
57s. However an amp with an o/p impedance of 8 ohms would still cause a
quite marked change in response as a result of impedance interaction.

Please don't assume that Zo=8 ohms. It may keep you awake at night:-)

They have handles on the side, and were a version for the USA only he
thinks.

I am not sure as I am not a quad 'expert'. but I think that, yes, handles
were something that only appeared on models not intended for domestic UK
useage.

It was supposed top be a "prof" model for studio use in the US.
They were some other improvements, I noticed a bottom edge trim also which I
do not recall ever seeing before. But I am even less of a Quad expert than
yourself.
It is a great many years since I have seen a pair.



Iain

In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...


I don't think we should take this DF=1 as a norm. It is just something
that Andre has mentioned to me, in connected with amplifiers without
NFB. There are several on this, and other groups who use SET amps, so
maybe they can provide us with a more reliable figure.

We would need a figure for the amp in question. If specific is unavailable
we could only work on the basis that - from what I have seen in magazines -
values of the order of 1 Ohm seem common. A DF of 1 strikes me as a
remarkably low value even for SET.

Yes indeed. I am still intrigued to know how an amplifier with such
a
poor test bench specification can sound so good. I have no axe to
grind here. I do not own an SET amp.

From what you say, maybe you preferred the resulting frequency
response which would have been *very* different than when used with an
amp with a nominally flat response and a low o/p impedance.

There are still factors in this equation which we have to mark as
unknown. I can hardly go to the gentleman's house and ask to take his
amp to bits to look for the presence of a feedback loop. It would be a
bit like asking the owner of a vintage Bentley for permission to strip
the engine to measure the crankshaft:-))

That is fair comment w.r.t to being polite. :-) However it means that
without the required info then it may be the case that a large portion of
the 'sound' you heard was due to marked changes in the frequency response.

A complication is that the 57 can provoke oscillations in amps that
are not unconditionally stable. The low and reactive speaker
impedance can also aggravate 'd.c. ducking' effects that can generate
off LF effects as well with musical dynamics. No idea if these have
any relevance in this case, though.

We have now established that the speakers are not type 57 but a later
improved export model known as type 63 Mk II.

So far as I know, there was no "Mk 2" as such. There were various issues of
circuit boards, which have some changes of components, etc. So the
electrical properties did alter a bit over the period of the production
runs. But these were mostly slight IIUC. I am not sure if the ones I have
were issue 1 or issue 2, but again IIRC I think these mainly differred in
terms of the LF behaviour at low powers.

I would need to check to be sure, but I think the early versions all show
dips down to around 3 Ohms or less at LF and around 10kHz. With an amp o/p
of 1 Ohm this implies response changes of the order of 2.5 dB. With an o/p
impedance of 8 Ohms (i.e. DF=1) this becomes 11 dB. In both cases I'd
expect this change to be noticable. Indeed, I'd expect that with a lot of
types of music 10dB changes would be quite marked! The results would be
further complicated as the speaker is also fairly reactive. The upshot is
that I would expect the results to sound distinctly different to using an
amp with a low o/p impedance.


At lowish listening levels, the SET distortion was not even faintly
audible. I daresay that at higher levels it might have been so. I
was not invited to touch the level controls, and did not do so.

OK. At low powers I would expect the distortions to fall and hence
what you say seems entirely plausible to me.

Andy tells me that the 211 can deliver 20W with lowish THD. So, if our
listen level was 3-5W, this would explain why there was no audible
distortion.

That seems quite plausible to me in the absence of more specific
information.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
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Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...
Well, one of the aims of the kind of points I make (e.g. about o/p
impedance) is to see if it may be possible to establish that the
'cause' of the preference might be due to an identifiable factor
which could then be replicated - if people so choose - with amp
designs that do not share the power limitations or practical
difficulties of a SET.


People who choose and listen to SET amps probably do not realise that
power limitations or practical difficulties which you mention, even
exist, given the type of music to which they listen on these amplifiers.

I am not sure. Valve amps and SET seem to be well-recommended at times in
magazines, and my impression is that this is often for all kinds of music.
The reviews do sometimes seem to me to use rock/pop of kinds I suspect
might be 'unsuitable', but I'm not in a position to judge as I don't listen
to these types of music a great deal.

Hence the recommendations do not seem to be based on, say, just
modest-volume small acoustic classic/jazz.

Although there may well be a distinction here between what magazine
reviewers recommend, and what people end up actually choosing to buy/use.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

In article , Iain M Churches
wrote:

"Don Pearce" wrote in message
...

I've been the minimalist route for amp design, and the problem is that
it doesn't stay that way. You always end up thinking "if I just do
this, it will work SO much better" and by the time you are done, you
have put in all the buffered current mirrors etc that really make it
work. And in the end, why not? The bits cost bugger all.

The cost is not really a factor. The idea was to design and build the SS
equivalent of Andre's simple SET amplifier, so that those interested
could build both for listening comparison.

Well, I don't know the design of SET amp you refer to, so can't comment on
it specifically. However when I had a student build a transistor SET amp
some years ago the expensive and difficult part was the o/p transformer.
This is a bit easier in some ways than for valve as it didn't have to
isolate very large voltages, and didn't have a large turns ratio. But you
still end up requiring a significant (expensive) transformer if you want it
to work OK even down to LF.

I don't know if I still have a copy of his project report. When I get a
chance I'll have a look for it. My (perhaps unreliable) recollection is
that the performance was limited by the transformer not by the choice of
gain devices. Hence such theoretical 'simplicity' can become quite complex
and costly to attain. Looks simple on a circuit schematic, though. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
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Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

"Jim Lesurf" wrote in message
...
In article , Iain M Churches
wrote:
I listened carefully to the 1st violin. It was perfectly clean. The
limited lower range of the cello did not give me a chance to evaluate
the LF performace of the amplifier. The transformer was, I assume,
Russian made.

Listening to single instruments, or to ones where the players may adapt
their pitch whilst playing may not show up low-order distortion as clearly
as other combinations of instruments.

Perhaps my meaning was unclear. I followed the first violin within the
quartet. I was unable to listen to single instruments. All players, with
the
exception of pianists, and players of some percussion instruments
have the ability adapt their pitch, so I am not clear what you mean by
your statement. Are you referring to intonation?

Hence it is possible that what you
describe was not a very 'hard' test of the distortion performance of the
amp or speakers.

It is the only evaluation available to me, as I heard only the one CD and
even that was a pre-production disc.

Also, if the power level was the order of a Watt, the
distortion level even with a SET might have been reasonably low. Hence it
may be simple factors like these that allow the SET to sound OK with such
music.

OK is not an adequate expression. I was greatly impressed. That doesn't
happen very often:-)


Can only speculate in the absence of reliable data on the actual
conditions
of use, etc. But the above seems to me to be at least plausible.


I am sorry that I cannot offer enough data for a complete technical
analysis.
But I can tell you that, for the music in question, the listening experience
was
quite remarkable, and I would recommend it to anyone.

Iain

"Arny Krueger" wrote in message
...
"Patrick Turner" wrote in message



I think it will be quite some time before anyone comes up with an
8 watt two transistor single ended amp which has no global NFB
and which sounds as well as a 300B amp when correctly set up.

Given the incredibly convoluted criteria used to establish that the 300B
amp sounds good...

I find it surprising that many of those who speak out so vehemently
against the SET have never actually heard one:-)

Iain