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Old 28th July 2005, 04:00 PM   #51
sam9 is offline sam9  United States
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I would disagree about putting a cap across the bias pot. I also used to think this would be good but some simulations I have done show that this method causes bias drop problems when the amp clips.
What value cap did you use in your sim and how long is "over time"?

I tried it with a 1uF cap and found a drop of 0.002V after one scecond. The results seemed to have bottomed out at about 0.6s. In any case the curve looked asymptotic. I expect less of an effect with higher cap vlues such as the standard 47uF, and in fact that showed a drop of 0.0005V.

At least for my part, I'm leaveing the cap in my amps. Perhaps the results are topology dependent.
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Old 28th July 2005, 07:04 PM   #52
edl is offline edl  Hungary
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I'm using small 100nF-63V-MKT cap paralel with the Vbe multiplier.
It improves the stability of the whole amp, too.

(My commercial made PA amp uses 47uF elko there)
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Old 1st August 2005, 07:20 AM   #53
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I made a simple simulation and found with a 47u cap a drop of 30 mv plateauing at about 4 ms. There is more drop the longer the time spent clipping and the driver base current, but the time constant depends on the voltage amp collector current and the size of the cap.

I guess the drop is smaller than I had remembered or perhaps it has to do with the topology I was looking at.

I suppose it might help stability some but I dont like the idea that the audio signal can influence the bias.

Doug
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Old 1st August 2005, 10:47 AM   #54
ilimzn is offline ilimzn  Croatia
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Unless the Vbe multiplier is bypassed, audio WILL influence bias, because the Vbe multiplier, taken as a voltage source, has a finite and not very linear impedance (which has to do with nonlinearity of gm vs current). You have to keep in mind that you get variations of Ib of the output stage, hence also variations of current thorugh the Vbe multiplier. At clipping, this problem is the worst. Also at clipping, with the capacitor, you get rectification effects of suddenly increasing Ib, for some topologies. Same thing when the output current limit activates. All in all, it is worth analysing these effects but not overanalysing as neither mode of operation is really 'normal' for amps.
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Old 1st August 2005, 11:06 AM   #55
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ilimzn, you are right that audio signal does influence bias because of current source impedence and changes in bias current. However, I think these kind of effects are straightforward to recognize because they show up on even simple simulations. They also disappear immediatly whenever the signal returns to normal levels.

In the case of clipping and having a bias capacitor the bias remains reduced (admittedlty only slightly) for a short time after the signal has disappeared. Therefore it can affect other signals than occur after the clipping. And not many people actually look at changes in bais due to clipping so the effect might be overlooked in simulation.

Of courcse, if you look at the problem psycho-acoutically (?) then it would be worst when large clipping is followed by a quiet passage. If the bias recovery time is short enough then you probably cannot hear it. Oh well.

Doug
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Old 25th August 2005, 07:55 PM   #56
vAD is offline vAD  Russian Federation
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All hello! I ask the help, comment.
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Old 25th August 2005, 08:01 PM   #57
vAD is offline vAD  Russian Federation
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Here IHD with help Microcap
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Old 5th November 2005, 05:55 PM   #58
vAD is offline vAD  Russian Federation
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Look, how to you, such circuit?
At 100, on a maximum level, distortions < 0,004 %
Horizontal 2SK, 2Sj, for improvement stabilize IRF.
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Old 28th March 2007, 08:17 PM   #59
vAD is offline vAD  Russian Federation
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Whether such projects can here, work. What, lacks.?
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