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Old 29th August 2002, 02:45 PM   #11
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well that's C4 you mean, it's value is 220 pF. So this value is rather high? I've asked this before, do you think I can leave C4 away?

HB.
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Old 29th August 2002, 03:14 PM   #12
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Default practical questions

Ok, now I get it.

The 220pF with the 10k gives a 3dB point of the closed loop gain of something like 65kHz (f=1/(2.pie.R.C)) so that cannot be the problem.
220pF is high here though, normally you see here small values up to 50pF to correct the phase shift, but if it works with 220pF it's OK.
What's the Ccb on that Vas stage below the diode string?


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Old 30th August 2002, 11:32 AM   #13
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what do you mean with Ccb? I guess the cap between the collector and the base of T6 => 100pF.

Other cap values:

bootstrap-capacitor: 220 F
cap parallel over the bias diode string: 1F
cap below R10 = 470F
input cap = 10F (in combination with 10K resistor), maybe I can solve the problem with lowering this cap, but this wouldn't solve the main problem.

(sorry for the unclear picture)

thanks,

HB.
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Old 4th September 2002, 11:15 AM   #14
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Default problem solved

Hi all,

I've just did another test; this time without C4. The sound is perfect now .

Thanks again for all your input!!

HB.
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Old 4th September 2002, 09:50 PM   #15
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Hugo, a few more tips for your consideration:

Put a 470pF polystyrene cap from T4 base to 0V (for stability)
Replace R1 R2 C11 D6 with a CCS (for better CMRR)
Increase C12 to 100uF (to reduce impedance of the diode chain)
Add bias resistors between T8 emitter and T11/T12 emitters. Repeat for the neg half. Use 100 ohms. (for linearity)
Remove C9 and replace R5 and R6 with a CCS. (for linearity)
Remove C4 altogether - but make sure the amp is stable. (for linearity)

BAM
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Old 5th September 2002, 09:49 AM   #16
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BAM,

you've just described what my new design looks like.

I didn't like bootstrapping at all; so the solution was to add a current source instead of the two resistors.
I've increased the cap C12 to 100F, but there was no difference in sound...

Can you explane the thing about the 'bias resistors'?? I've also read about a resistor between the emittor of T8 and the emittor of T7 (and a cap placed in parallel). It's said to improve the sound, but I've never read why.

thanks,

HB.
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Old 5th September 2002, 03:30 PM   #17
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The idea is to run the drivers in a reasonably linear region most of the time...to reduce distortion. It will be less important if the output devices are run at high standing current, but bear in mind that unless the betas of the output devices are the same then the drivers will be biased differently from one another at 0V output.

Using resistors between driver emitters and output emitters is one way. Another is to run a single resistor between driver emitters. The effects on distortion depend on the deviced you are using and how you match your output devices and drivers. Measure the distortion in each case if you have the equipment.

In any case I would not introduce a capacitor across the bias resistor. This is because the Vbe of the two drivers will not always be required to be the same - because the output devices are non-linear. If you use a capacitor to force the voltage to be the same then the drivers will be pumping non-linear current to charge the cap up and down with a non-linear voltage. The impact is frequency-dependent too. I suppose if the output devices are not matched well then it might make a bad job better, but normally I wouldn't do it.

Tip: think hard about the effects of capacitance in non-linear circuits. Where in the circuit is it ok and where not?

BAM
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Old 6th September 2002, 10:26 AM   #18
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Thanks for the explanation!

Unfortunately, I don't have all the necessary testing equipment; but in two weeks I return to school where I can test anything I want...

Thanks for helping me,


HB.
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