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In Reply to: RE: Nope, no coupling cap with the battery bias posted by Triode_Kingdom on July 27, 2014 at 18:50:55
Hi tk. Just put off curiosity have you ever heard an Altec 1570?
Or comments on other a2 amps you've listened to?
Builder of MagneQuest & Peerless transformers since 1989
Follow Ups:
Hi Mikey, long time no see. Hope you're doing well. If I'm not mistaken, the 1570 uses zero-bias triodes in Class B. I'm not a big fan of this topology, but I admit its been many years since I've heard them. As I'm sure you know, Class A2 is very different. In an A2 amplifier, operating conditions at most power levels (including "the first watt") are essentially Class A. The signal may not be exactly centered in the curve, but it's usually close. This means nothing untoward happens (grid current) until power levels are very high, well into the area where nuance and detail are usually masked anyway by the higher dynamics of the sound. More important, being IT or direct coupled, the A2 input circuit creates minimal recovery issues. The design is enhanced from this perspective if the cathode is also connected to ground with a low impedance and no bypass cap. The fact that an A2 design can not only resolve the recovery issue, but also eliminates the audible liability of grid and cathode caps, makes it my favorite topology. ;)
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Buy Chinese. Bury freedom.
Edits: 07/30/14 07/30/14
Hey TK, should just re-iterate that you are talking about A1 designed to transition seamlessly into A2.
FWIW, I've been building amps that way for many years and would never go back to cap coupling for output stages and even gain stages where possible, though for slightly different reasons.
Naz
"you are talking about A1 designed to transition seamlessly into A2."I consider that to be more nearly the classic definition of A2, essentially a single-ended version of AB2, the latter used to similarly describe PP amps that don't draw grid current when idling. OTOH, I tend to think of the positively-biased designs (which draw grid current regardless of signal level) as a more contemporary topology, and as such, being the class subject to a need for supplemental notation. I'll try to remember to be clearer on this point in the future. :)
What are the "slightly different" reasons you're referring to?
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Buy Chinese. Bury freedom.
Edits: 07/30/14
I wasn't having a go at you TK ... more for the sake of those unfamiliar with these classes. I tend to agree that there should be a class specifically describing the operation we are discussing for the sake of clarity.
To answer your question, for output stages I seek to ensure that my driver will provide sufficient drive to ensure that the OP tube transitions smoothly into grid current up to the tube's saturation level without a significant increase in distortion.
For gain stages the grid is generally sufficiently negative that it will never become positive with respect to the cathode even at the highest signal levels it will ever experience. Therefore designing to drive the following stage into grid current under overload conditions is normally irrelevant.
However, I do see value in direct coupling and whilst this element of the design may normally be common to driving into A2 (normally being the operative word) I do it to take the coupling cap directly out of the signal path wherever it makes sense. Where it doesn't (and this is relevant to the original question in this thread) I try to use a high quality coupling cap, generally as low a value as I can get away with. More often than not I use Schottky diodes for bias because I like their sound or lack of it, eliminating the need for a bypass cap without the performance loss associated with an unbypassed cathode R (gain, internal resistance, OP impedance etc).
Hope this covers it.
Naz
I think that's a worthwhile design philosophy. I've been thinking lately about various ways to implement this at all stages within an amplifier. As you say, the idea is to eliminate as many coupling caps as possible, and simultaneously - where possible - to install power drivers capable of pushing grid current. This will probably be my goal for all future designs.
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Buy Chinese. Bury freedom.
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