Effect of coupling cap on touch response

[TimS]

A post in this thread on TGP describes a method of adding a coupling cap before the tone stack to eliminate DC voltage on the slope resistor:

http://www.thegearpage.net/board/showth ... p?t=192605

I understand the effect adding this cap has on frequency, but would it have any effect on the dynamic response of the amp?

[dogears]

I dislike it. I have extensively experimented with this very thing. First, attenuating the subguitar frequencies does have a noticeable effect on the guitar frequencies. The feel is different and the top end seems to be effected as well. There will be phase shift and the interaction of the stack will be subtly changed. All in all, keep it simple and stupid.

[TimS]

I dislike it. I have extensively experimented with this very thing. First, attenuating the subguitar frequencies does have a noticeable effect on the guitar frequencies. The feel is different and the top end seems to be effected as well. There will be phase shift and the interaction of the stack will be subtly changed. All in all, keep it simple and stupid.

In what way are the feel and top end affected?

Also, I don't know if you've experimented with this or not, but if you have do you notice the same effects when switching from a .1uf bass cap to a .022uf?

[drz400]

A post in this thread on TGP describes a method of adding a coupling cap before the tone stack to eliminate DC voltage on the slope resistor:

http://www.thegearpage.net/board/showth ... p?t=192605

I understand the effect adding this cap has on frequency, but would it have any effect on the dynamic response of the amp?

It is safer to add a coupling cap if you have a long wire run but dont do it for the sake of keeping voltage off the slope resistor, there is no reason for that purpose. Do it for safety concerns if the layout requires it. If there is no issue then leave it out.

Many people think they can hear a phase difference and you certainly can if it is pushed to the extreme but in this very simple addition of a large coupling cap, if the coupling cap is large enough there should be no difference. Otherwise you should work on an amp with no caps since changing or adding one cap will not make a difference when you have 20 other caps you are going through not to mention a bunch of resistors, tubes and other passive wires, shielded cable, wire, capacitance etc etc. Now if you are comparing 20 to 0 that would be different, but you are comparing 20 to 21
Here is some reading for you.

http://www.ethanwiner.com/phase.html[quote]It is indeed fortunate that humans are so unresponsive to this effect because, if we could hear phase shift, we would go absolutely nuts in everyday life. Every time a reflected version of a sound adds to the direct sound, the phase shifts are enormous, and it happens in abundance in all rooms, even carrying on a conversation across a table. Do the stand up/ sit down test while speaking. The voice changes very subtly, but our hearing system compensates immediately and, on a scale of 10, the voice quality remains a 10. Yet the transfer function between the voice and the ears has greatly changed in both amplitude and phase. I cannot help but think of all the opera recordings and film voice overs that are done with librettos and scripts on large angled (sound reflecting) surfaces between the mouth and the mic. The signal is corrupted at the source! Thank your favorite diety for human adaptability. Floyd E. Toole, PhD Vice President Acoustical Engineering Harman International Industries, Inc[/quote]

[TimS]

It is safer to add a coupling cap if you have a long wire run but dont do it for the sake of keeping voltage off the slope resistor, there is no reason for that purpose.

The reason I was thinking about adding the cap is that I was considering using a 50k resistor and a 100k pot to make the slope adjustable. I don't think I'll do it, though.

[Darkbluemurder]

It is safer to add a coupling cap if you have a long wire run but dont do it for the sake of keeping voltage off the slope resistor, there is no reason for that purpose.

The reason I was thinking about adding the cap is that I was considering using a 50k resistor and a 100k pot to make the slope adjustable. I don't think I'll do it, though.

There is no benefit in making the slope resistor adjustable. I once had a Holland Brentwood Custom amp (similar to Marshall Bluesbreaker) that had a slope control. I thought it was worthless and replaced it with a master volume.

[dogears]

This is BS IMHO. I absolutely can hear the difference when changing from cap to no cap. Not close.... Caps have tone too... Unless the author believes one can't hear that difference either. Like I said, a specific amp I have logged countless hours on uses one. I have focused many hours on not only replacing it but trying other sizes and brands. No cap easily sounds best.

A post in this thread on TGP describes a method of adding a coupling cap before the tone stack to eliminate DC voltage on the slope resistor:

http://www.thegearpage.net/board/showth ... p?t=192605

I understand the effect adding this cap has on frequency, but would it have any effect on the dynamic response of the amp?

It is safer to add a coupling cap if you have a long wire run but dont do it for the sake of keeping voltage off the slope resistor, there is no reason for that purpose. Do it for safety concerns if the layout requires it. If there is no issue then leave it out.

Many people think they can hear a phase difference and you certainly can if it is pushed to the extreme but in this very simple addition of a large coupling cap, if the coupling cap is large enough there should be no difference. Otherwise you should work on an amp with no caps since changing or adding one cap will not make a difference when you have 20 other caps you are going through not to mention a bunch of resistors, tubes and other passive wires, shielded cable, wire, capacitance etc etc. Now if you are comparing 20 to 0 that would be different, but you are comparing 20 to 21
Here is some reading for you.

http://www.ethanwiner.com/phase.html[quote]It is indeed fortunate that humans are so unresponsive to this effect because, if we could hear phase shift, we would go absolutely nuts in everyday life. Every time a reflected version of a sound adds to the direct sound, the phase shifts are enormous, and it happens in abundance in all rooms, even carrying on a conversation across a table. Do the stand up/ sit down test while speaking. The voice changes very subtly, but our hearing system compensates immediately and, on a scale of 10, the voice quality remains a 10. Yet the transfer function between the voice and the ears has greatly changed in both amplitude and phase. I cannot help but think of all the opera recordings and film voice overs that are done with librettos and scripts on large angled (sound reflecting) surfaces between the mouth and the mic. The signal is corrupted at the source! Thank your favorite diety for human adaptability. Floyd E. Toole, PhD Vice President Acoustical Engineering Harman International Industries, Inc

[/quote]

[TimS]

There is no benefit in making the slope resistor adjustable. I once had a Holland Brentwood Custom amp (similar to Marshall Bluesbreaker) that had a slope control. I thought it was worthless and replaced it with a master volume.

Yeah, it seems to me that anything you could do with a slope control could be done just as easily with a switchable mid cap and adjusting the tone controls.

This is BS IMHO. I absolutely can hear the difference when changing from cap to no cap. Not close.... Caps have tone too... Unless the author believes one can't hear that difference either. Like I said, a specific amp I have logged countless hours on uses one. I have focused many hours on not only replacing it but trying other sizes and brands. No cap easily sounds best.

What difference do you hear/feel?

[drz400]

This is BS IMHO. I absolutely can hear the difference when changing from cap to no cap. Not close.... Caps have tone too...

They are not saying there is no difference just that in the audiophile world no one has been able to hear 1 capacitor in a double blind test, providing the reactance allows full bandwidth. These are some pretty serious audio engineers in case you dont know who they are.

I know you can be credited with good ears but do you really think that with 20~30 other capacitors in your circuit you could hear a correct value cap (one that you like) added in this one spot? Do you have proof? Spectrum Analysis or anything? Did you try and insert a switch to A/B the difference? What value and brand cap did you use? Could you record the difference for us? Do you think it is extreme enough to pass double blind tests or to hear in a recording? Not doubting you, just a bit skeptical :wink

[jelle]

I hear this difference too and the composition of the cap (polyester, polypropylene etc) matters. What I'm hearing is that the highs get softer in a way...less marshally. Maybe this is because the ceram treble cap is not exposed to DC when this cap is in circuit.

Jelle

[drz400]

I hear this difference too and the composition of the cap (polyester, polypropylene etc) matters. What I'm hearing is that the highs get softer in a way...less marshally. Maybe this is because the ceram treble cap is not exposed to DC when this cap is in circuit.

Jelle

I dont know what you guys are hearing, maybe something maybe imagination but..... some empirical proof would be nice

[Funkalicousgroove]

Forgive me for my ignorance, but what is the piont of a coupling cap to block DC on the slope resistor? and wouldn't this effectively change the value of the rest of the caps in the tone stack? product/sum of the values involved?

[dogears]

Absolutely. The cap is in series with the stack and does have an effect, more or less based on stack settings. My post about hearing caps was referring to leaving this cap out versus putting any size/type there. Without the cap has more life, more zing, better tonestack operation, all around superior..... For what I am listening for. This is a Dumble circuit and the cap changes things.

Forgive me for my ignorance, but what is the piont of a coupling cap to block DC on the slope resistor? and wouldn't this effectively change the value of the rest of the caps in the tone stack? product/sum of the values involved?

[TimS]

Forgive me for my ignorance, but what is the piont of a coupling cap to block DC on the slope resistor?

Well, if you wanted to make the slope adjustable by using a small resistor in series with a pot, using a coupling cap to block DC would eliminate a lot of noise. But from what I can tell there are better ways of achieving the same results than adjusting the slope, so the question is essentially moot.

[Funkalicousgroove]

Why not put your 3 favorite flavors of slope on a rotary switch, then you don't have to worry about the cap?

DC on the pot won't hurt anything, so long as the pot is rated for the voltage and current, although I'd probably use a plastic knob!! The presence control on a Plexi has DC across it, that's why it sounds all scratchy as you turn the control, same would apply for your slope resistor.