a few practical questions...

hugobross · 28th August 2002, 04:27 PM

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.

tiroth · 28th August 2002, 04:33 PM

PCB trace width calculator:

http://www.geocities.com/CapeCanaver...TraceWidth.htm

hugobross · 29th August 2002, 10:25 AM

hey thanks, it's a very usefull calculator!

HB.

yeti · 29th August 2002, 10:45 AM

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

hugobross · 29th August 2002, 10:59 AM

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.

djk · 29th August 2002, 11:18 AM

C4=220nF? Surely you jest? 220pF would be a HUGE value for C4

hugobross · 29th August 2002, 11:20 AM

no sorry for that: those values are in pF, just a stupid thing in the program I use for the pcb designing.

HB.

janneman · 29th August 2002, 02:07 PM

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

hugobross · 29th August 2002, 02:34 PM

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.

janneman · 29th August 2002, 03:27 PM

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

29th August 2002, 10:45 AM	#4
yeti diyAudio Member Join Date: Nov 2001 Location: Ewersbach	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

29th August 2002, 02:07 PM	#8
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	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

29th August 2002, 03:27 PM	#10
janneman diyAudio Member Join Date: May 2002 Location: Where Germany, The Netherlands and Belgium meet Blog Entries: 6	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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28th August 2002, 04:33 PM	#2
tiroth diyAudio Member Join Date: Dec 2001 Location: Pittsburgh, PA, USA	PCB trace width calculator: http://www.geocities.com/CapeCanaver...TraceWidth.htm

29th August 2002, 10:25 AM	#3
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	hey thanks, it's a very usefull calculator! HB.

29th August 2002, 10:59 AM	#5
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	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.

29th August 2002, 11:18 AM	#6
djk diyAudio Member Join Date: Feb 2001 Location: USA	C4=220nF? Surely you jest? 220pF would be a HUGE value for C4

29th August 2002, 11:20 AM	#7
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	no sorry for that: those values are in pF, just a stupid thing in the program I use for the pcb designing. HB.

29th August 2002, 02:34 PM	#9
hugobross diyAudio Member Join Date: Jun 2001 Location: Flanders, Belgium	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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