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Old 25th Jun 2018, 11:25 pm   #81
ms660
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Default Re: Valve sound?

Not forgeting ol' T Bone, with Mr Gillespie in the wings:

https://www.youtube.com/watch?v=pFqK6PBq-hA

Lawrence.

Last edited by ms660; 25th Jun 2018 at 11:35 pm.
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Old 25th Jun 2018, 11:26 pm   #82
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Re the Quad 50D/E “industrial” amplifier, well one could say that it was an honorary valve amplifier, in that it had a valve amplifier topology – an input stage with the main feedback loop returned to its emitter, a phase splitter, a two-transistor driver for each side (Williamson-like, with drivers after the phase-splitter), identical output stages, with the transformer primary doing the recombination. A separate secondary winding (a Baxandall idea from the late 1940s) was used for feedback, but that looks to have been because an isolated output was required for driving 100-volt lines, etc.

In the Quad case, the transformer was “integral”, whereas in the McIntosh case it was an impedance and power matching device outside of the amplifier proper, which [the amplifier] would have been capable of directly driving a load without the (auto)transformer.

There is some more on the McIntosh transistor amplifier output transformer here: http://www.roger-russell.com/mcintosh1.htm; scroll down to “Autoformer”.

It is curious that both Quad and McIntosh built transistor amplifiers with output transformers, and that both had embraced partial cathode loading in the valve era.

Where output transformers were used in small transistor amplifiers (non-hi fi) in radio receivers and record players, my understanding is that it was not normal to take feedback (if used at all) from the output transformer secondary, but rather from one or other of the output transistors. So transistor amplifiers with output transformers within the feedback loop might have been quite scarce overall.

The original Lin quasi-complementary circuit (Electronics, 1956 September, p.156ff.) did not use an output transformer. The earliest reference I can find to the fully complementary circuit is in Electronics, 1953 September, p.140ff, and the audio version of that did not have an output transformer.

(Those Electronics magazine articles are available here: https://www.americanradiohistory.com...aster_Page.htm.


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Old 26th Jun 2018, 9:19 am   #83
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Quote:
Originally Posted by Synchrodyne View Post
Where output transformers were used in small transistor amplifiers (non-hi fi) in radio receivers and record players, my understanding is that it was not normal to take feedback (if used at all) from the output transformer secondary, but rather from one or other of the output transistors.
Yes that is right. The only radio I have with this technique in my entire collection is the unique and interesting NZ made Pacemaker radio from 1957, check out the schematic (figure 5) on page 9 of this article on it, the feedback taken from one output transistor. I had to transcribe the schematic from the radio. The audio circuit is interesting in many other ways too, with a direct coupled pre driver and driver and the bias for the opt stage derived from the driver emitter circuit . The pre driver transistor is used an an output transistor temp compensation device!

http://worldphaco.com/uploads/THE_PA..._FROM_1957.pdf

(Also this radio has been said to be one of the very first transistor MW band radios with an RF stage).
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Old 16th Jul 2018, 5:48 pm   #84
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The whole solid state vs Valve sound is shot through with all sorts of interesting things and opinions. I recall in the early days (late 50's) thinking the valve radio we had sounded sort of bassy while the transistor radio in the car sounded trebly and tinny. Things seemed to get better with tape and various other innovations but even so there appears to be a sort of valve sound musicians at least prefer. When Fender changed the designs of the 50's era valve amplifiers they made, musicians simply did not like them and they have been forced by the market to return to the circuitry of earlier times - attempts by Fender and Vox to create and sell solid state amplifiers simply failed in the early days. It has really only been the more recent marshall valvestate and some modelling type solid state amps that had any success in this area from what I've seen. However just to muddy the waters Polytone and Roland came up with solid state guitar amps that sold reasonably well especially to Jazz players. I find it a bit like Vinyl vs cd - for me the cd clarity is wonderful, but for others the vinyl 'warmth' is more important perhaps it simply comes down to a range of opinion that different people will always have.
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Old 16th Jul 2018, 6:11 pm   #85
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Default Re: Valve sound?

Quote:
Originally Posted by Synchrodyne View Post
Where output transformers were used in small transistor amplifiers (non-hi fi) in radio receivers and record players, my understanding is that it was not normal to take feedback (if used at all) from the output transformer secondary, but rather from one or other of the output transistors.
The Mullard Bible from 1960 shows plenty of 'recommended' circuits which take feedback from the speaker-side of the output transistors (whether single-transformer or double-transformer).
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Old 16th Jul 2018, 7:23 pm   #86
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Originally Posted by Radio Wrangler View Post
There's a good minute of a Hammond C being distorted to hell and back at the beginning of the Machine Head version of 'Lazy' It's quite obvious that the fuzz is well pedalled back when he wants to play a chord. There's a whole lotta expression going on!

David
Not sure of the context when you say "fuzz" here David. As far as I am aware dear JL just used to overdrive his Marshall amp (that may have been modded to distort more) directly from the output of his Hammond. I think he must be 'riding' the output from the Hammond with a volume or 'swell' pedal to get the effect you describe. Great playing and great track. Great album as well!
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Old 16th Jul 2018, 8:01 pm   #87
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Feedback isn't a yes/no sort of thing.

Even in something with no overall loop which you can point at, there may be several places where local degeneration is at work.

For things with an obvious feedback loop encompassing all or most of the stages, there is the matter of how much feedback is in operation. And this is where people often run into confusion.

Let's say we have an amplifier be it valve, transistor or witchcraft. Let's say it has a feedback network of 50k Ohms series working into 1k shunt which somehow opposes the input. This means that the feedback system is trying to limit the amplifier gain to 50 if it's in inverting mode, or 51 if it's the more common non-inverting sort Close enough, so let's say the feedback is set for 34dB in logarithmic-speak.

But we need to look at the gain that the amplifier proper can do. If the main bit can't make 34dB, no amount of negative feedback will get it to 34dB. (positive feedback could, but that brings tons of trouble)

If the amplifier proper does do 34dB, then the overall thing will fall a bit short as the negative feedback and the basic amplifier are both limiting the gain. Only as the forwards amplifier gain tends towards infinity does the thing with feedback get to the 34dB programmed by the feedback resistors. Infinite gain sounds infinitely scary, but in reality once the forwards amplifier gain gets well over 34dB, it's probably doing something useful.

If the forwards gain happened to be 44dB then the gain with feedback is going to be a couple of dB short of 24. Another term is the loop gain. if we break the loop anywhere and analyse it we have +44dB gain in the amp, and -34dB gain (=24dB loss) in the feedback network... totalling 10dB of loop gain. That's a better defined way of saying what sometimes is phrased as "10dB of negative feedback".

10dB is about 3.16 in voltage ratio. And that's the ratio by which feedback reduces distortion, reduces output impedance and maybe increases input impedance.

Mr Leak engineered his basic open-loop amplifier to be a bit under 1% distortion, but he wanted point 1 % for his adverts. So he needed a loop gain of 10 at the frequencies his distortion was going to be measured... 20dB in logarithmic-speak Those amps are rather sensitive, around 100mV in for 12W into then 15 Ohms. That's 13.4v RMS out, so a gain of 134. To get that tenfold reduction in distortion, he needs a gain of 1340 in the valves and that gives him a loop gain (That is the gain round the loop - not to be confused with the gain the amp would give with the feedback network snipped) of 20dB. So his closed-loop gain is 42.5dB his loop gain is 20dB and his forwards gain is 62.5dB.

Now, he needs that 20dB of loop gain (or more) at all the frequencies he wants to claim "point one percent" at AND his amplifier then has to roll off to bring that 20dB of gain down below zero dB comfortably before its phase lag builds up above 180 degrees. So his phase needs to stay good up to about ten times the top frequency he claims "point one" at. And as the gain rolls off, so does the distortion (etc.) reducing effect.

These are just fag-packet numbers, not an accurate analysis of a Leak amp, but they're in the right cricket-pitch to show the issues.

The confusion is that to me as a regular designer of feedback systems, "Open loop gain" means the gain right round the loop, including the feedback network. So far I've tried to keep calling it "loop gain" to avoid confusion, because a lot of people think that the gain of the forwards amp if the feedback path is disabled is the open loop gain. Certainly the amp is then being operated open loop, but the gain needs to be considered at the point where the loop is cut for the normal maths to work.

So feedback is fairly hard to do in a valve amp. A lot of feedback (a lot of loop gain) is very hard to do. What Leak did is about as far as you would want to push a conventional amplifier architecture. With a much wilder design, you could go a lot further.

Would you want to?

Why should you?

The use of a lot of feedback engineers out any 'valve sound' characteristics of the basic amplifier, and it usually introduces less desirable behaviours when over-driven.

If you want to claim an amplifier which reproduces most faithfully what is put into it, then this is good.

But If you like the characteristics inherent in relatively simple, low-feedback, valve amps, then you may not like the result. Words like 'Soul-less', 'Flat', 'Dead' can be used with some justification.

The key is in your head. Do you want your amplifier to just give what it gets, no more no less, or do you want it to change the signal in a way you consider to be an improvement?

Those are jobs for two quite different amplifiers.

Do you want your amplifier to vanish and not get in the way of hearing what was put on the recording, or do you want it to be an active part of the performance and to influence what you hear?

Not all valve amplifiers have valve sound. It can be engineered out, but that is expensive and difficult.

Not all transistor amplifiers have transistor sound. It not only can be designed out, it ought to be!

1) If you're a guitarist (or Jon Lord RIP), you want a characterful amplifier. It's part of your performance and that's how you earn your living. It has to be valves for most people.

2) If you're a hifi person wanting to claim your setup is accurate or right or some such verisimilitude, spend your money on good transistor circuitry. Really low-intrusion valve stuff isn't commercial. Radfords, Leaks and Quads are getting there, but the prices are silly.

3) If you want to enjoy a nice, warm, golden sound, like sinking into a warm bath at the end of a tiring day, simple valve circuits will do it for you. Enjoy! The Leaks may be beginning to be a bit too clinical for you.

4) If you don't actually listen to the music, but to the equipment, and you care mostly about what you are seen using, you probably won't admit it even to yourself in the wee small hours. You are trapped in a fashionista's hell and you have to be seen wearing the 'in' labels and knowing what you are supposed to say about them. Like Dante's Inferno, there is a way out, but few seem to make it. The barriers are in their own minds - they just don't believe in a world outside.


So it's all a matter of sophistication of design, not really a matter of valves versus transistors.

Keep you ears firmly ahead of your mind!

David
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Old 16th Jul 2018, 8:05 pm   #88
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Cross-posted with Steve. I'm a slow typist

By Fuzz, I meant distortion, however achieved. Oh, what a sound and performance. Have you heard the recentish youtube video of King Crimson 'Starless' Oh my!

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Old 16th Jul 2018, 8:17 pm   #89
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I wonder if anyone here has or knows someone who has built that experimental 5W amp from the Mullard transistor handbook. The spec. they published was quite impressive for the time.

Ging
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Old 17th Jul 2018, 6:44 am   #90
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I thought about it for a while circa 1963 with a view to substituting the OC22s with OC28s but the transformer put me off - my pocket money didn`t really stretch to buying expensive components.
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Old 17th Jul 2018, 1:51 pm   #91
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David makes an interesting theoretical point that feedback is frequency dependent and only reduces the error but I am still stuck in the real world of difficult loads and transients in actual music.

For those seeking audio Nirvana, it also might be worth considering what setup the engineer mixed it to be played on. The extreme of this is the aggressive compression used on FM radio to make it sound right on a car stereo.
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