AMP without DC coubling

JensRasmussen · 16th September 2003, 08:09 PM

Hi all,

I'm wondering, why not make an amp where the different stages are seperated by a cap.

The reason I ask is that this would be an easy way controlling the Q-point in the different stages (eg. IN, VAS and OUT) and thereby look closer on effects of different working currents and voltages.

Perhaps this could also be used to try "NO GLOBAL NFB" because it's no longer needed to hold the output DC in place.

Please be gentle, thanks for your time

sam9 · 16th September 2003, 09:21 PM

Just a couple of thoughts (though negative):

1- There are enough caps in the signal path of amps already. Adding two more sounds a bit dubious.

2-
"Perhaps this could also be used to try "NO GLOBAL NFB" because it's no longer needed to hold the output DC in place."
This is not the sole function of NFB.

sajti · 17th September 2003, 06:58 AM

There was power amplifier with capacitor coupling. But this was 40-50 years before. Good quality capacitor is expensive, ans you need large one to coupling the output devices...

Sajti

PMA · 17th September 2003, 07:04 AM

Try it, but keep the cap's and res. low frequency limit (-3dB) of every stage well below 1Hz - just to prevent cap's distortion and changes of phase characteristics in the audio band.

peranders · 17th September 2003, 08:35 AM

Think once again why caps are necessary!

If you can't control the DC-levels between the stages you must use a cap but otherwise not.

If each stage has good DC-precision you won't need any caps.

JensRasmussen · 17th September 2003, 08:46 AM

The reason for my question is inspired by this thread:

Unstable VAS current in amp from Slone book

I figured it would be nice to have each stage in a design totally DC-independent of the others because it offers the advantage of fredom of Q-points in each stage.

Maby it's a bad idear, ill toy around in spice and see what I can find out.

Thanks for your time

\Jens

Solid Snake · 19th September 2003, 01:26 PM

Remember that every cap shifts phase and changes resistance due to frequency. Every cap you add compounds this effect. The last amp I built used only two caps on the input to filter DC. The amp I'm building now uses ZERO caps. It will be 100% phase accurate and should be able to amplify a signal from DC upto several hundred Khz

JensRasmussen · 19th September 2003, 04:38 PM

That would make you a rich man

\Jens

sam9 · 19th September 2003, 10:04 PM

Apart from whatever benefits accrue to being phase accurate, it also makes the feedback loop easier to deal with as each shift brings you closer to 180 deg and to oscillation.

JensRasmussen · 20th September 2003, 07:55 AM

Well the idear was to make the phaseshift (cross overfreq) occur long after the openloop gain has fallen to 0 dB.

This way the caps scholud not give any problems with stabiblity

I don't have a working cuircuit yet. I'm busy on an other project also

\Jens

17th September 2003, 08:35 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Think once again why caps are necessary! If you can't control the DC-levels between the stages you must use a cap but otherwise not. If each stage has good DC-precision you won't need any caps. __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

17th September 2003, 08:46 AM	#6
JensRasmussen diyAudio Member Join Date: Jul 2002 Location: Denmark - Jutland	The reason The reason for my question is inspired by this thread: Unstable VAS current in amp from Slone book I figured it would be nice to have each stage in a design totally DC-independent of the others because it offers the advantage of fredom of Q-points in each stage. Maby it's a bad idear, ill toy around in spice and see what I can find out. Thanks for your time \Jens

19th September 2003, 04:38 PM	#8
JensRasmussen diyAudio Member Join Date: Jul 2002 Location: Denmark - Jutland	$$$ That would make you a rich man \Jens

20th September 2003, 07:55 AM	#10
JensRasmussen diyAudio Member Join Date: Jul 2002 Location: Denmark - Jutland	Phaseshift Well the idear was to make the phaseshift (cross overfreq) occur long after the openloop gain has fallen to 0 dB. This way the caps scholud not give any problems with stabiblity I don't have a working cuircuit yet. I'm busy on an other project also \Jens

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16th September 2003, 09:21 PM	#2
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	Just a couple of thoughts (though negative): 1- There are enough caps in the signal path of amps already. Adding two more sounds a bit dubious. 2- "Perhaps this could also be used to try "NO GLOBAL NFB" because it's no longer needed to hold the output DC in place." This is not the sole function of NFB.

17th September 2003, 06:58 AM	#3
sajti diyAudio Member Join Date: Apr 2003 Location: Vác, Hungary	There was power amplifier with capacitor coupling. But this was 40-50 years before. Good quality capacitor is expensive, ans you need large one to coupling the output devices... Sajti

17th September 2003, 07:04 AM	#4
PMA diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Try it, but keep the cap's and res. low frequency limit (-3dB) of every stage well below 1Hz - just to prevent cap's distortion and changes of phase characteristics in the audio band.

19th September 2003, 01:26 PM	#7
Solid Snake diyAudio Member Join Date: Feb 2003	Remember that every cap shifts phase and changes resistance due to frequency. Every cap you add compounds this effect. The last amp I built used only two caps on the input to filter DC. The amp I'm building now uses ZERO caps. It will be 100% phase accurate and should be able to amplify a signal from DC upto several hundred Khz

19th September 2003, 10:04 PM	#9
sam9 diyAudio Member Join Date: Jun 2002 Location: Left Coast	Apart from whatever benefits accrue to being phase accurate, it also makes the feedback loop easier to deal with as each shift brings you closer to 180 deg and to oscillation.

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