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Old 15th May 2012, 12:24 AM   #1
Bigun is offline Bigun  Canada
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Default why do wimpy drivers for 2A3 work as well as they do?

From reading a few web pages I find a great deal of respect for the sound of the Fi 2A3 amplifier. It is reputedly based on an old design (aren't they all) that I saw once in 'practical amplifier diagrams' by Robin and Lipman 1947. It's a 6SF5 directly coupled to a 2A3, the driver tube is biassed at only 0.4 to 0.5mA and has a rather high internal resistance.

Many people who have been building amps for many years will say that this is simply no good, that the driver is too wimpy (too weak), go try a 6C45P or something. (I personally heard a 6C45P at relatively high current driving a 6C4C and it was a tad clinical, a triode wired 6E5P was nicer)

Yet there is no doubt about the reverence that the sound of the Fi 2A3 is held.

This reminds me of the AKSA amplifier by Hugh Dean. A simple and old design that has been tweaked up by somebody who knows their engineering but values sound above all. A topology that is now bypassed by most people on the search for something better. Yet the sound is simply lovely. And I believe Hugh has played with wimpy drivers (e.g. 6SL7) in his past.

Does anybody know why the wimpy driver approach works ?
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Last edited by Bigun; 15th May 2012 at 12:27 AM.
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Old 15th May 2012, 02:26 AM   #2
Sheldon is offline Sheldon  United States
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Cold feet? The amp is the sum of its parts. The perfect driver driving an imperfect output stage may be less than the "imperfect" driver, driving the same output stage. Per Mr. D., lots of older designs used driver tubes at lower currents than current designs. No sims. All they could do was try it.

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Old 15th May 2012, 06:14 AM   #3
ColinA is offline ColinA  United Kingdom
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The trouble was that back in the day they could only listen to the quality of an amp.

We now have the benefit of computer simualtion and spectrum anylisers so we can judge the quality of a design without having to use our ears.
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Old 15th May 2012, 06:34 AM   #4
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Quote:
Originally Posted by Bigun View Post
Does anybody know why the wimpy driver approach works ?
The 6SF5 is hideously nonlinear, being that it was designed to serve as a high gain pre for AM or COMM receivers, and linearity wasn't a concern. There is always the possibility that its nonlinearities compensate for the nonlinearities of the 2A3 + OPT final. It's not something I'd want to stick in front of any DHT. I have some 6SF5s in the junk box, but if I ever use 'em it'll be as intended: receiver front end amp.

There's also the possibility of user preference.

"I personally heard a 6C45P at relatively high current driving a 6C4C and it was a tad clinical, a triode wired 6E5P was nicer".

"A tad clinical" looks to me like the driver was doing what it should: giving less distortion. If you want "nicer", you're basically wanting an FX box. Nothing wrong with that, as individual preference is, well, individual.
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Old 15th May 2012, 02:41 PM   #5
piano3 is offline piano3  United Kingdom
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I find it surprising that a 6s45p and a trioded 6e5p sounded much different if they were used appropriately.
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Old 15th May 2012, 02:58 PM   #6
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The 6F5/6SF5 doesn't look "hideously nonlinear" to me. It looks like it is quite linear for the input range used by Fi and many others. The issue is not non-linearity; the issue is whether it has the current necessary to drive the Miller capacitance of the 2A3.

http://www.mif.pg.gda.pl/homepages/f.../021/6/6F5.pdf
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Old 15th May 2012, 06:58 PM   #7
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I did some quick calculations and it appears that 2A3 needs only about .5mA in the driver to avoid slew rate limiting when driven to clipping at 20kHz. That's why is sounds okay.

The general rule of thumb in DIY is to use 5x what is needed, though because slew rate induced distortions sound really bad.

The 1947 amp probably sounds really bad when it is clipping because it will not only be clipping in the output stage but the driver will have trouble slewing as well. However, up to the point of clipping, it would sound fine.

EDIT: I did an experiment once, built an amp that had really good overload recovery and no slew problems. I listened to music which I could visibly see was clipping. It didn't sound too bad. The stuff that sounds bad is the blocking distortions, slewing distortions, IMD, etc.

Last edited by SpreadSpectrum; 15th May 2012 at 07:02 PM.
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Old 15th May 2012, 08:13 PM   #8
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Quote:
Originally Posted by SpreadSpectrum View Post
I did some quick calculations and it appears that 2A3 needs only about .5mA in the driver to avoid slew rate limiting when driven to clipping at 20kHz. That's why is sounds okay.
You have corroborated what others have calculated over and over on this board and others; yet the debate continues....
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Old 15th May 2012, 09:34 PM   #9
smbrown is offline smbrown  United States
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It would be interesting to consider also if the designs in question are direct coupled or cap coupled, from my own experience, a 6SL7 at about 0.7ma driving a 2A3 direct coupled, sounds fantastic. I have had many positive reviews by others of this amp as well.
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Old 15th May 2012, 09:40 PM   #10
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Default 2A3

One of the shocks of my life was listening to a 2A3 amplifier used by the boss of Japans FAL speaker company . The sound was not only first class it was almost home cinema in dynamics . I stopped reading spec sheets that day and trusted my ears . I am told PX25 is similar . Build it and try it . Make sure the bias is correct naturally .
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