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Old 28th August 2002, 04:27 PM   #1
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Default a few practical questions...

Hi all,

I've been testing for a while the amp I've build some weeks ago; and I'm very pleased with the results of it... But there are still a few questions left in my mind.

The amp produce a very low sound (maybe because of the HUGE PSU); I thought I could help this by pulling up the higher and the medium frequencies by bypassing the bias diodes with a cap; but this had no effect on the sound.
So what value do you recommend for this cap?? Are there any other solutions? (schematic below)

I'm also building a pcb layout for my further amps, how thick should the tracks be? Are there any guidelines for this ?
(i.e. ?? ampère => ?? mm thick)


thank you all,

HB.
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Old 28th August 2002, 04:33 PM   #2
tiroth is offline tiroth  United States
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PCB trace width calculator:

http://www.geocities.com/CapeCanaver...TraceWidth.htm
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Old 29th August 2002, 10:25 AM   #3
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hey thanks, it's a very usefull calculator!

HB.
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Old 29th August 2002, 10:45 AM   #4
yeti is offline yeti  Germany
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Default low sound

When you mention "low sound" i think you mean the absence of high frequencies.
You can try to decrease the value of C4 (220 nF) ,because this one seems to be responsible for the negative feedback on high frequencies.
But be careful.... if it's too low it may cause oscillations!

regards

Arne
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Old 29th August 2002, 10:59 AM   #5
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Yes, that's what I mean with the low sound; the sound is good when I turn (on my mixers equaliser) the low freq. to about a quarter of the range (of that potm.), the mid freq. turned to normal (= centre) and the higher freq. at full range.

If there's one thing left what I don't understand about the schematic, it's the function of C4; it's still in the design because this is in fact a modified amp (the design failed when I build it for the first time). I often don't see this cap in other designs, can I leave it away?? Because I left it away in the schematic for which I'm designing the pcb.

thanks,

HB.
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Old 29th August 2002, 11:18 AM   #6
djk is offline djk
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C4=220nF? Surely you jest? 220pF would be a HUGE value for C4
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Old 29th August 2002, 11:20 AM   #7
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no sorry for that: those values are in pF, just a stupid thing in the program I use for the pcb designing.

HB.
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Old 29th August 2002, 02:07 PM   #8
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Default practical questions

Hugo,

Still, 220pF is rather high, I would try 100pF or lower.
Another thing I noted is the cap from the bottom of the bias diode string to the base of T5 I think it is. How large is that one? It does roll of the closed loop gain with frequency and could cause the effect you describe, but I can't read the value so don't know if it is significant.

Jan Didden
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Old 29th August 2002, 02:34 PM   #9
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Jan,

I think you mean the cap between the collector and the emittor of T6, this one is 100pF. The value of the cap in parallel with the diode string is 1µF.

HB.
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Old 29th August 2002, 03:27 PM   #10
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Default practical questions

Hugo, no,

I mean the cap from the bottom of the diode string to the base of the right-hand transistor of the input LTP. Because the bottom of the diode string is for AC (almost) identical to the output voltage, this cap is effectively in parallel with the (10k?) feedback resistor, and therfore rolls off the closed loop gain with rising frequency.

Jan Didden
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