Hi All,
I'm building a gainclone based on Peter Daniel's design (who, by the way, is extremely helpful and patient with all my questions). I would like to add a feedback capacitor per channel to minimize DC offset to help protect the speakers (just in case). The spec for LM 3875 form National's website calls for a 22uF capacitor. I would like to use a very good quality cap which is carried by partsconexion.com. I am also not sure as to the voltage rating and capacitor type. The cost is also an issue.
Can you help me by recommending a good capacitor from that supplier?
Thanks,
And now to my favorite hobby...
Rafal
I'm building a gainclone based on Peter Daniel's design (who, by the way, is extremely helpful and patient with all my questions). I would like to add a feedback capacitor per channel to minimize DC offset to help protect the speakers (just in case). The spec for LM 3875 form National's website calls for a 22uF capacitor. I would like to use a very good quality cap which is carried by partsconexion.com. I am also not sure as to the voltage rating and capacitor type. The cost is also an issue.
Can you help me by recommending a good capacitor from that supplier?
Thanks,
And now to my favorite hobby...
Rafal
Since the cap is in series with a good-sized resistor, ESR is not really an issue. It also won't see too much voltage, so anything rated at 2x the rail voltage or higher will work. You can bypass it with a film cap of 1 uF or so, but there are lots of good designs out there which don't bother. Fancy designer caps in that position are, IMO, a waste of money; just use a good quality nonpolar cap (or two back-to-back elytics) from a reputable source.
ummmmmmmm.........
I don't know were people come up with this stuff. The cap to ground from the inverting input feedback resistor sees nowhere near the supply voltage ( for plus and minus supplies) and I have often seen 16V caps used in this part of the circuit. ESR and capacitor quality is very important to sonics in this cap and it would be one the first caps I would (and have, in modifying many amps). Bypassing with film caps is not always a good idea. A film cap across a very low ESR cap like a Black Gate is " The kiss of death) as one of my favorite designers puts it. I am all for conservative design but voltage rating overkill and penny pinching is not even close to a good idea in this part of the circuit. Use very good resistors for the feedback loop as well. This is one of the most critical parts of the circuit for parts quality and I am quite puzzled why anyone would think outerwise.
I don't know were people come up with this stuff. The cap to ground from the inverting input feedback resistor sees nowhere near the supply voltage ( for plus and minus supplies) and I have often seen 16V caps used in this part of the circuit. ESR and capacitor quality is very important to sonics in this cap and it would be one the first caps I would (and have, in modifying many amps). Bypassing with film caps is not always a good idea. A film cap across a very low ESR cap like a Black Gate is " The kiss of death) as one of my favorite designers puts it. I am all for conservative design but voltage rating overkill and penny pinching is not even close to a good idea in this part of the circuit. Use very good resistors for the feedback loop as well. This is one of the most critical parts of the circuit for parts quality and I am quite puzzled why anyone would think outerwise.
Thanks for clarifying that Fred. I also suspected that this cap shouldn't "see" any high voltage, as normally in my amps this end of 680 resistor is connected directly to ground. So I hoped that 16V or even less should be sufficient. Your observation regarding bypassing BG caps is right on the mark as well, as I tried it before and it actually degrades the sound, IMO.
Fred, if you've got a few hundred ohms in series with the cap, why on earth should a few milliohms of ESR make any difference? The resistance of the actual series resistor will change more than that if the room temperature changes a couple of degrees.
Speaking of penny-pinching, why not use a decent voltage rating just in case? I've already pointed out the the o/p voltage is divided down, but 63V caps are not significantly more expensive than 16V, so why not go for it? Saves worry in case of fault conditions and temperature derating.
Peter, what happened to your sig? I had a joke all ready to go and you short-circuited it!
D'accord. Low voltage coefficient of resistance and inductance are the key parameters.
EDIT: A change of merely one degree C will cause the resistance of a good quality 680 ohm resistor (100ppm/degree) to shift by 68 milliohms. Puts the ESR thing in perspective, eh?
Speaking of penny-pinching, why not use a decent voltage rating just in case? I've already pointed out the the o/p voltage is divided down, but 63V caps are not significantly more expensive than 16V, so why not go for it? Saves worry in case of fault conditions and temperature derating.
Peter, what happened to your sig? I had a joke all ready to go and you short-circuited it!
D'accord. Low voltage coefficient of resistance and inductance are the key parameters.
EDIT: A change of merely one degree C will cause the resistance of a good quality 680 ohm resistor (100ppm/degree) to shift by 68 milliohms. Puts the ESR thing in perspective, eh?
Agreeing to disagree
ESR is not a linear resistance and low ESR is usually one of the indicators of capacitor quality. Does anyone else actually research these things or is just throwing opinions out adequate effort for the forum? Many of the things that seem to be controversial have been known for a decade or more. Spend 10 or 15 years trying some of this stuff and I will happy to discuss honest opinions based on experience.
Jocko and I both came into high end audio with healthy skepticism and degrees and work experience as EEs. Nobody has asked my degree back for trying many the mods discussed on the forum, even some that conflict with first order engineering. Looking into subtleties has made me a better engineer when troubleshooting problems in Telecom design. Nothing ventured nothing gained.
The difference in price for higher voltage caps in some of the specialty caps is substantial, as anyone who has priced them will tell you. Size is also an issue when a layout is tight or when modifying existing audio products. The penny pinching I had in mind concerning using a good cap as opposed to a cheap electrolytic cap.
I just got off the phone with Jam and mentioned your post about capacitor quality in this circuit locating. After he stopped laughing he told me that in his experience that this cap was very important to sonics. I would be quite content to agree to disagree though rather than argue about a simple circuit change that the truly curious can try without getting a second mortgage on the house.
ESR is not a linear resistance and low ESR is usually one of the indicators of capacitor quality. Does anyone else actually research these things or is just throwing opinions out adequate effort for the forum? Many of the things that seem to be controversial have been known for a decade or more. Spend 10 or 15 years trying some of this stuff and I will happy to discuss honest opinions based on experience.
Jocko and I both came into high end audio with healthy skepticism and degrees and work experience as EEs. Nobody has asked my degree back for trying many the mods discussed on the forum, even some that conflict with first order engineering. Looking into subtleties has made me a better engineer when troubleshooting problems in Telecom design. Nothing ventured nothing gained.
The difference in price for higher voltage caps in some of the specialty caps is substantial, as anyone who has priced them will tell you. Size is also an issue when a layout is tight or when modifying existing audio products. The penny pinching I had in mind concerning using a good cap as opposed to a cheap electrolytic cap.
I just got off the phone with Jam and mentioned your post about capacitor quality in this circuit locating. After he stopped laughing he told me that in his experience that this cap was very important to sonics. I would be quite content to agree to disagree though rather than argue about a simple circuit change that the truly curious can try without getting a second mortgage on the house.
This cap is as important to sonics as the series coupling cap installed at amps input (or maybe even more). That's why some people decide not to use it at all and connect resistor directly to ground, risking more DC offset in this way. In my experience, the offset is usually not more that 60mV and it's fine with me.
Looking at prices, at partsconnexions, we see that BG N 33/16 is $4.40 and similar cap in 50V range is about twice as much (47u). The second row represent current new price (which is greatly reduced comparing to previous prices)
BLKGATE- 60091 NX 0.1 50v 4 x 7 $2.75 $2.20
BLKGATE- 60092 NX 0.47 50v 4 x 7 $2.95 $1.95
BLKGATE- 60097 N 1 50v 5 x 11 $3.95 $2.90
BLKGATE- 60098 N 4.7 50v 5 x 11 $4.95 $3.15
BLKGATE- 60099 N 10 50v 8 x 11 $6.50 $4.50
BLKGATE- 60205 NX 22 6.3v 5 x 7 $3.50 $2.80
BLKGATE- 60093 N 33 16v 6 x 11 $8.95 $4.40
BLKGATE- 60206 NX 47 6.3v 6 x 7 $3.75 $2.55
BLKGATE- 60100 N 47 50v 13 x 24 $14.95 $9.95
BLKGATE- 60102 NH 68 350v 30 x 25 $47.50 $35.95
BLKGATE- 60207 NX 100 6.3v 6 x 12 $8.95 $5.35
BLKGATE- 60094 N 100 16v 13 x 20 $15.95 $10.30
Looking at prices, at partsconnexions, we see that BG N 33/16 is $4.40 and similar cap in 50V range is about twice as much (47u). The second row represent current new price (which is greatly reduced comparing to previous prices)
BLKGATE- 60091 NX 0.1 50v 4 x 7 $2.75 $2.20
BLKGATE- 60092 NX 0.47 50v 4 x 7 $2.95 $1.95
BLKGATE- 60097 N 1 50v 5 x 11 $3.95 $2.90
BLKGATE- 60098 N 4.7 50v 5 x 11 $4.95 $3.15
BLKGATE- 60099 N 10 50v 8 x 11 $6.50 $4.50
BLKGATE- 60205 NX 22 6.3v 5 x 7 $3.50 $2.80
BLKGATE- 60093 N 33 16v 6 x 11 $8.95 $4.40
BLKGATE- 60206 NX 47 6.3v 6 x 7 $3.75 $2.55
BLKGATE- 60100 N 47 50v 13 x 24 $14.95 $9.95
BLKGATE- 60102 NH 68 350v 30 x 25 $47.50 $35.95
BLKGATE- 60207 NX 100 6.3v 6 x 12 $8.95 $5.35
BLKGATE- 60094 N 100 16v 13 x 20 $15.95 $10.30
If the ESR varies by a factor of 5 (which it doesn't) over the audio band, it still doesn't vary the effective resistance of that shunt leg by more than 0.005% or so, given what actual esrs of these caps are. And that's a linear frequency response error. Now, tan delta can cause nonlinearities, and that's yet another reason not to cut the corners you suggest and use a low voltage cap; all else equal, lower voltage caps show higher tan deltas.
<fred imitation>Do you ever bother to do order-of-magnitude calculations, or do you think it's just fine to throw out onto the forum any sort of audio urban legend that you have chosen to believe?</fred imitation>
I also think that the quality of the cap in the feedback loop is very important (the feedback network has to transfer the output signal to the -in without any distortions in order for the amp to get proper feedback signal). That's why I don't use one (20mV DC offset on my amp). If you decide to use one use as reasonably large value as possible. That way the voltage across the cap would be smaller, thus less ditortions (based on some tests on cap. distortions). As Peter said if you're going to use one use 100.0 for 680 ohm FB resistor.
/Greg
/Greg
Amp Feedback
There are two basic feedback networks for reducing output offset. They both have unity gain at DC, so the intrinsic input offset of the amp is passed on without amplification, assuming resistor values are used that are appropriate for the amp in question (bipolar or FET input). The circuit in the attached file labeled "A" is the one discussed in this thread so far. The disadvantage of this approach is that C1 must be relatively large, and it does see some reverse bias when the output swings negative. I would try using an Oscon for C1 in circuit A, as they are supposed to tolerate substantial reversal without ill effects.
I first learned about the Circuit labeled "B" over 20 years ago in Audio magazine. It appeared in an analysis of a Nakamichi amplifier. In this circuit, unity gain is provided by feedback resistor R3, and the gain network(R1-2) is coupled to the summing node via C1. For FET input amplifiers, R3 can be about 1M, and the coupling capacitor can be a high quality film unit of 1-2uF. Bipolar input amps will reqire an R3 of several 10s of k to reduce interaction with the input bias current. I have tried 33-47k and ~5-10uF with good results. I use this circuit ("B") in my projects almost always instead of the conventional one ("A").
There are two basic feedback networks for reducing output offset. They both have unity gain at DC, so the intrinsic input offset of the amp is passed on without amplification, assuming resistor values are used that are appropriate for the amp in question (bipolar or FET input). The circuit in the attached file labeled "A" is the one discussed in this thread so far. The disadvantage of this approach is that C1 must be relatively large, and it does see some reverse bias when the output swings negative. I would try using an Oscon for C1 in circuit A, as they are supposed to tolerate substantial reversal without ill effects.
I first learned about the Circuit labeled "B" over 20 years ago in Audio magazine. It appeared in an analysis of a Nakamichi amplifier. In this circuit, unity gain is provided by feedback resistor R3, and the gain network(R1-2) is coupled to the summing node via C1. For FET input amplifiers, R3 can be about 1M, and the coupling capacitor can be a high quality film unit of 1-2uF. Bipolar input amps will reqire an R3 of several 10s of k to reduce interaction with the input bias current. I have tried 33-47k and ~5-10uF with good results. I use this circuit ("B") in my projects almost always instead of the conventional one ("A").
Attachments
Re: Amp Feedback
Thanks for sharing it. Interesting aproach. I quite like it. I gues the advatage is that you don't have to use a high value for the FB resistors in order to decrease the value of the FB cap. In the clasic case of 1k/22k AC FB res of the NIGC and 200k DC FB res. you can use just 1uF or even less. And the res from +in to GND have to be also 200k (to keep the output DC offset low) which means that the input impedance of that thing can be pretty High, thus your input cap can be also very small (1uF or less). Nice. I'd like to try it out of curiosity (though no caps is the best).
/Greg
wrenchone said:
Thanks for sharing it. Interesting aproach. I quite like it. I gues the advatage is that you don't have to use a high value for the FB resistors in order to decrease the value of the FB cap. In the clasic case of 1k/22k AC FB res of the NIGC and 200k DC FB res. you can use just 1uF or even less. And the res from +in to GND have to be also 200k (to keep the output DC offset low) which means that the input impedance of that thing can be pretty High, thus your input cap can be also very small (1uF or less). Nice. I'd like to try it out of curiosity (though no caps is the best).
/Greg
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