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Old 23rd November 2012, 01:38 PM   #21
bigwill is offline bigwill  United Kingdom
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I just measured the Iq it is running at, which was determined by watching the crossover "hop" disappear from the error signal. It abruptly disappears at 13mA.
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Old 23rd November 2012, 01:38 PM   #22
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I just don't like junctions turning on and off at arbitrary points on the transfer curve. It will certainly cause high order harmonics, and induce spikes into the rest of the amplifier. And probably will not improve distortion at all, compared to an ordinary EF which has more current gain and where the drivers never turn off.

If you still want to use it, it is your call, I will simply relate my experience with this sort of thing. I am not telling you not to.
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Old 23rd November 2012, 01:41 PM   #23
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by keantoken View Post
If you still want to use it, it is your call, I will simply relate my experience with this sort of thing. I am not telling you not to.
I can totally see your point of view on this, but in practice it seems to work well. The little circuit on my bench right now has no visible crossover glitch in the error signal . It's obviously still there somewhere, it's apparently quite small. I have not yet connected the circuit to any FFT analysis.

Edit: I have been scoping the error signal with a MUSIC source, I will try a sine later, that will probably reveal any less obvious crossover artefacts.

Last edited by bigwill; 23rd November 2012 at 01:52 PM.
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Old 23rd November 2012, 02:16 PM   #24
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Well then, let's have a look. Until you draw the schematic, can you tell me how many transistors and resistors?
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Old 23rd November 2012, 02:34 PM   #25
bigwill is offline bigwill  United Kingdom
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This represents what's actually on the breadboard. I'm actually using a single 20v rail supply, and it's not an ideal circuit, just built from bits at hand and what's practical to lay out on the breadboard.

Click the image to open in full size.

Last edited by bigwill; 23rd November 2012 at 02:39 PM.
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Old 23rd November 2012, 02:41 PM   #26
bigwill is offline bigwill  United Kingdom
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I was sampling the error signal from the collector resistor of the beta-enhance transistor, which I realise isn't a true error signal, but it gives you an idea of the drive going into the VAS.
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Old 23rd November 2012, 02:58 PM   #27
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Two of these in a mint tin?
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Old 23rd November 2012, 03:29 PM   #28
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What is the output bias current through the 1R resistors?

What is THD at 9V into 6R, at 1KHz and 20KHz?

I feel this circuit is more complex than it needs to be. Q3 and Q6 will not perform as well as a simple bootstrap I think (reference VAS bootstrap to Q12 emitter).

Use BC5xx for Q9/10 for high CM output impedance. Use BC5xxC for Q11 for more VAS gain. Use BC5xxC for Q1/Q2 for lower distortion and more linear input impedance.

Your CFP output will show Gm doubling. While both outputs are conducting Ro will be .5R, at excursions 1R. In order to bring Ro back to normal at crossover the CFPs each must have an Ro of 1R at the crossover point. The Ro of a CFP is the Re of the driver divided by the current gain of the output. At 13mA the output has a current gain of no more than 26, and the driver an Re of 4.5, so output impedance is about 170mR. If you don't have serious Gm doubling, then something is broken, or you are simply not drawing more than 26mA from your load.
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Old 23rd November 2012, 04:10 PM   #29
bigwill is offline bigwill  United Kingdom
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By tweaking the bias while scoping the error signal, optimal bias appears to be 9mA. Above that results in gm doubling, below that results in crossover distortion. Wouldn't you get gm doubling with any other over-biased output stage?

The usage of the 2n390x was just due to me having lots on hand, this was never intended to be a super-refined design - it's on a breadboard - I can't physically fit any niceties on there without it getting more haywire than it is already!
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Old 23rd November 2012, 04:27 PM   #30
bigwill is offline bigwill  United Kingdom
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Also, regarding the issue with the emitter resistors, while admittedly high in value, (but what I had nearby), wouldn't the gain reduction around the crossover point counteract any gm doubling effect those resistors would have? There will be a curved turn-on slope which you attempt to geometrically mirror with the other half.

If I had lower values at hand I would have used them, of course.
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