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Re: And an even more indepth article [message #8222 is a reply to message #8220] Sat, 05 June 2004 18:41 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
Illuminati (13th Degree)
I have read that amplifiers clip far more frequently and at lower volumnes than we suspect. Maybe that accounts for why tube amps sound more musical. Something about infrasonic resonances and airborne feedback as well as RFI that reaches the signal regardless of shielding. How's the ASUSA el 84 doin'? Any further observations?

Re: Feedback [message #8223 is a reply to message #8207] Sun, 06 June 2004 18:51 Go to previous messageGo to next message
metasonix is currently offline  metasonix
Messages: 103
Registered: May 2009
Viscount
The most experienced engineers I've ever known--guys with experience going back 50 years or more--all seem to say the same thing: the best performance can be had with push-pull low-mu triodes, with about 10 dB of loop negative feedback. This is exactly what you see in early-30s professional PA amplifiers like the Western Electric 86 (300Bs), and most PA amps made by Thordarson/Lafayette/UTC/ etc. (usually had 2A3s or 45s).

One occasionally saw 6B5 type class-B amps, but those were in the minority. Class AB low-mu triodes in push-pull were pretty close to being universal.

The 6L6 killed all this off pretty quickly in 1936. It came down to a battle of fidelity vs. power/dollar. And the 6L6 won those kinds of battles. It was the Depression.

Another thing to disappear in the late '30s was the driver transformer. You want the best possible phase inverter/driver? There is still nothing like a good transformer. Unfortunately, electronic-type phase inverters became the norm--because they were cheaper than a good transformer. Not better. If carefully designed and run from a high plate voltage, a phase inverter is capable of great performance. (Until the tube starts to get weak, that is.)

Yet another thing you saw in the best amps (and still do) is fixed grid biasing (negative grid voltage). Cathode bias resistors were cost-cutting measures. They did have one other advantage: they offered better reliability than a negative bias supply (which sometimes fails, usually killing all the output tubes and maybe the transformer). Those old WE theater amps usually had cathode bias for this reason.

Look in any old data sheet for an audio output tube--triode or pentode or whatever. The ratings for fixed bias always show lower distortion than similar cathode bias ratings.

(Remember, WE didn't sell this stuff, they LEASED it. IBM copied this trick later. You wanna make money producing exotic electronics, you sell it. You REALLY wanna make money, you lease the stuff. Microsoft is such a money-making powerhouse for only one reason: they don't sell outright contracts for ownership of commercial software, they force big businesses to pay big, fat license fees on a yearly basis. They don't actually own the software, they are only allowed to use it on a license-fee basis. That's how to make a lot of money very quickly.)

The SE thing is pretty much a freak of the 1990s. Oldtimers look at this trend VERY askance. In the 30s, SE meant simply a low-cost amplifier for low-power applications. The infamous WE 91A was a fluke--it was WE's theater amp for small-town movie houses who didn't make a lot of money. Everything after that you can chalk up to obsessive Japanese guys, who influenced Japanophiles like Arthur Loesch and Joe Roberts. This is very much like the anime world, which is now huge outside Japan because the Japanese anime producers were taking artistic risks in the 1980s, at a time when American animators were producing garbage like "He-Man and the Masters of the Universe". The fact that most anime is also junk doesn't matter. They seized young eyeballs early on, and now they've got those eyeballs for life.

With push-pull triode amps, you don't NEED a lot of feedback. So you don't use it. A major problem with a typical feedback loop is that it includes the output transformer--the FB resistor goes from the speaker terminal back to the input stage, usually. This can magnify any tendency the transformer has toward ringing, peaking, phase-margin stability problems etc. So before WWII, one often saw feedback going only from an output tube's plate back to the input. Better output transformer winding techniques solved this.

Look at the Harmon-Kardon Citation II. It had a LOT of feedback. All over the place. But it also used a very special transformer, carefully wound to prevent peaking and provide excellent, FLAT high frequency response. A stock Citation can go out to past 100 kHz routinely--this was thought to be impossible in the 30s.

The same was true of old Dynaco tube amps. Their transformers weren't quite that good, but they were well-behaved in the audio passband. So 20-25 dB of feedback didn't result in instability. (They needed that feedback too. 6550s are ok, but EL34s have a LOT of distortion. And those simple 7199 driver circuits are barely adequate for the job. Dynaco amps were masterpieces of minimal design.)

The free advice is:
1) push-pull low mu triodes, running near the limit of plate
dissipation for lowest distortion.
2) driver transformer, NOT a phase inverter.
3) get decent transformers.

I'd watch out for some of the crazy, exotic circuits being pushed nowadays. I disagree with many of the hard-ass high-end audio designers, who feel that perfection is only possible with differential amp circuits stolen from old Tektronix oscilloscopes, plus heavy power supply regulation. If you want to see that philosophy in action, go to any high-end salon and look inside the monster push-pull 6550 amps they all sell.


Re: Feedback [message #8224 is a reply to message #8223] Sun, 06 June 2004 23:50 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
Great post, Eric, thanks! I feel like I just got an extra page or two in my latest VTV.

Re: Feedback [message #8226 is a reply to message #8223] Mon, 07 June 2004 17:19 Go to previous messageGo to next message
Henry is currently offline  Henry
Messages: 5
Registered: May 2009
Esquire
I have p-p amps that sound very good, so I understand what you are saying about
push-pull low-mu triodes with about 10 dB of loop negative feedback. Same for your
idea about coupling transformers, I agree with you.

On the other hand, simple amps using single ended triodes are a modern fad,
but might they be on to something too? The one thing they aren't good at is power
but I think some SET amps sound very good at low levels. What do you make of this?
What makes a good SET amp? What is the nature of their appeal?

Re: Feedback [message #8227 is a reply to message #8226] Tue, 08 June 2004 03:59 Go to previous messageGo to next message
metasonix is currently offline  metasonix
Messages: 103
Registered: May 2009
Viscount
>but I think some SET amps sound very good at low levels. What do you make of this?

It's still an issue of power output. Some (mind you, not all) SE amps have their design just right--by choosing the driver tube and circuit properly, it's possible to get some distortion cancellation. Because this is VERY difficult to design for (computer models and plate curve analyses aren't accurate enough), it's more a matter of finding the right combination. Trial and error.

The WE 91A is like that. I suspect that a low-mu triode, combined with a driver tube having very low distortion and a plate characteristic similar to the power tube (310A pentodes of all things), offers the best chance at effective cancellation.

One amp I've seen that does it is the DeHavilland 845 monoblock. Kara did a LOT of trial and error, and came up with something rare. (Please, don't lecture me about whatever the Sound Practices crowd is into this week, or what Bottlehead sells. That stuff is all over the place with regard to cancellation effect.) I feel that SE is popular primarily because it is easy for DIYers to build and mess around with.

The safest way is still push-pull. It guarantees cancellation of even harmonics. If you have a lot of patience and really accurate test equipment, you can try for a cancelling SE circuit. Lots of luck.


Re: Feedback [message #8228 is a reply to message #8227] Wed, 09 June 2004 01:45 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
I never really thought much about this subject, but I can sure see what you mean. The SE idea is really neat for simplicity - It's usually less than a dozen parts and can be built on an open chassis or breadboard by practically anyone. But with no feedback mechanism, it is particularly sensitive to load impedance fluxuations and without having any complementary components, there's no real way to cancel asymmetries with an "equal and opposite." I suppose you can try to counter the output amp curve with a driver amp curve, but that's not complementary-symmetry, it's more like pseudo-symmetry. So maybe it can be made close, but it's a hack. And the load impedance thing can only be dealt with by brute force, which is kind of an oxymoron when discussing Class A amps.

You mentioned the DeHavilland 845. What are some of your other favorites?

Re: Feedback [message #8229 is a reply to message #8228] Wed, 09 June 2004 05:07 Go to previous messageGo to next message
SET lover is currently offline  SET lover
Messages: 3
Registered: May 2009
Esquire
The reason SET is so special is there is no crossover distortion. Crossover distortion makes an unnatural sound and single ended triodes are more musical because of their absense of it. One trades power for presence with a SET amp. They have a magic that no other amp can quite match.

Re: Feedback [message #8230 is a reply to message #8229] Wed, 09 June 2004 06:13 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
Illuminati (13th Degree)
What exactly is crossover distortion?

Re: Feedback [message #8231 is a reply to message #8229] Wed, 09 June 2004 18:42 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
Certainly, there is merit in what you say. But it isn't like a good Class AB amp has a hard transition between positive and negative cycles. They are made to have a smooth transition where both devices are on through the zero crossing line. So I guess there are probably some amps of both types - Class A and Class AB - that are really good and some that are just mediocre. No feedback Class A (SET) amps will probably tend to have a little more 2nd harmonic distortion and a little less power. Class AB (PP) amps will have greater power and reduced second harmonics but may be susceptible to other artifacts. I can see relevance in both design types.

Re: Feedback [message #8232 is a reply to message #8230] Wed, 09 June 2004 18:48 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
On a push-pull amp, you have one component doing the work on positive cycles and another on negative cycles. Class B push-pull amps have a sharp transition at the zero crossing line and are prone to having crossover distortion. Class AB push-pull amps are biased so that both positive and negative driver devices share the load through the crossover region so that the transition is more gradual. Some Class AB amps are made so much this way that both devices are practically run as Class A amps run in parallel. There is a lot of room for performance in Class AB configurations spanning the entire bias range between pure Class A and pure Class B, so the characteristics of a Class AB amplifier are entirely determined by the specific circuit implementation.

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