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Old 14th October 2009, 06:59 PM   #1
scottw is offline scottw  United States
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Default Highpass filter via feedback cap

I was wondering if anyone has used the FB cap (Ci in the National datasheets) to apply highpass filtering to their chimpamps.

Does this have the potential to work as well as a first order input (line level) filter?

Would you think the quality of the cap in the feedback position (parallel to the FB path and series to the gain setting resistor to ground) will be as critical to sound quality as a cap placed in series with input signal?

To help clarify my thinking here, I consider caps in the signal path as necessary evils, best avoided when possible. My chimpamps so far (non-inverting) have omitted the input caps (AC coupling via pre) and feedback cap (just put up with the few mV's offset on the outputs).

Thanks,

Scott
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Old 14th October 2009, 07:46 PM   #2
ratza is offline ratza  Romania
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Any film capacitor should do the job (MKT, MKP, MKS), even a non polar electrolytic. Caps in the signal path are usually avoided in general by those who do not understand how the whole circuitry works. The signal DOES NOT go from input to output. Seriously, it doesn't. I can put my hand in fire that you won't make a difference if I swap your cap with a different type, same value.
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Old 14th October 2009, 08:02 PM   #3
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Quote:
Originally Posted by scottw View Post
Would you think the quality of the cap in the feedback position (parallel to the FB path and series to the gain setting resistor to ground) will be as critical to sound quality as a cap placed in series with input signal?
Hint- to the opamp it is exactly the same.
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Old 15th October 2009, 12:01 AM   #4
scottw is offline scottw  United States
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I appreciate the replies.

And I really should know better but.........

per ratza:

"Caps in the signal path are usually avoided in general by those who do not understand how the whole circuitry works. The signal DOES NOT go from input to output. Seriously, it doesn't."


Yes, thanks....(and for the derision too)........But, I trust you will allow that the signal (or "a" signal whose well being I care for) does pass through the capacitor (electron clouds and quantum mechanics aside) to which I refer.

And my "whole circuitry" for several years has worked fine and not included these peripheral capacitors.

And....(don't do it, leave it alone.....oh no here I go....)....do I really detect from your post that you don't hear a difference between different types of capacitors? If that is true, I have to tell you that at some level, I am envious. I'm sure no golden ear but I am "burdened" by being able to hear some differences sometimes, some subtle and some more distinct, between caps. I know, I know, next it's imagined/placebo/psychoacoustics, fine, whatever, been there, nobody expected the Spanish Inquisition,etc.


Scott
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Old 15th October 2009, 06:12 AM   #5
ratza is offline ratza  Romania
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It wasn't a derision at all, this is the truth, otherwise you shoud hear something in your speakers without having the amplifier powered (through "signal path"). Yes, the signal passes through a capacitor or more and not all the capacitors are the same. Honestly, I cannot hear any diferences between diferent types for the same value and I don't care if other people hear differences or not. It matters if they feel OK with their systems. Afterall, we build them to listen music, don't we?

My opinion on the cause of this different sounding might be that we normally use different capacitor values and types in signal path and NFB. It's very obvious that the input impedance and NFB impedance do not match, therefore the timbre modifications. The changes in sound are minor though, I never said that they do not exist. If the music plays fine, then I don't care what's inside.
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Old 16th October 2009, 02:33 AM   #6
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A feedback cap does something no input cap can do: it prevents DC on the output caused by clipping an asymmetrical waveform. Without a feedback cap, pushing an amp to clipping may cause the amp to generate DC.
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Old 16th October 2009, 03:08 AM   #7
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I seem to post links to Walt Jungs picking capacitors articles a lot lately, but here I will again. Services anyone who thinks that capacitors cannot sound different should have a read of picking capacitors parts one and two from the above linked page. It has been written by an engineer, with rational explanations as to why people can hear differences, and measurements to show that there are indeed differences.

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Old 16th October 2009, 04:16 PM   #8
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Quote:
Originally Posted by wintermute View Post
I seem to post links to Walt Jungs picking capacitors articles a lot lately, but here I will again. Services anyone who thinks that capacitors cannot sound different should have a read of picking capacitors parts one and two from the above linked page. It has been written by an engineer, with rational explanations as to why people can hear differences, and measurements to show that there are indeed differences.

Tony.
Yes, written in 1985, how many types of caps did people have to choose from then?
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Old 16th October 2009, 07:29 PM   #9
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About the same as now, with probably better supply of Polystyrene types. About the only new caps I can think of in that time are PPS dialectric, X-Y types, and maybe reliable multilayer ceramics. Very little has really changed IMO apart from the move to lower-inductance formats (SMT)

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Old 17th October 2009, 12:45 PM   #10
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Yes, written in 1985, how many types of caps did people have to choose from then?
The point is that there are differences, and they are demonstrable I don't know how many times I have read people say that there isn't any difference between caps, because a) they can't hear it, and b) it isn't measureable!

The article is about the various dialectrics, and as Martin says, PPS is about the only (widely available) new one out there and it is from what research I have done it better in almost all respsects than polycarbonate for which it is widely regarded as a replacement)..

Just because something is old, doesn't mean it isn't valuable. Would you say that using Thiele/Small parameters (when designing loudspeakers) was a waste of time because they were originally published in the 1960's?

edit: I can't say one way or another whether any particular design you will hear a difference with a particular cap in the feedback position. Some people don't use them at all (at the expense of dc offset on the output) because they feel that they have a detrimental effect on the sound... if you think about it though, what that cap is doing is modifying the feedback signal which will affect the gain of the amplifier at certain frequencies. If you read the section on dielectric absorbtion in the walt jung article, I think it is possible to say that it is feasible that caps with higher DA would be more likely to have an affect on the signal... in general coupling caps will be the ones that will show the greatest differences, but they do mention that using ceramic caps as bypass caps can increase distortion significantly, so not everything needs to be "in the signal path" in order to make a difference.

My approach is to try and choose something appropriate for the circuit position (without getting rediculous on price or size) and then rest easy that any improvement that I could make is probaly so small an increment that the rest of my system (or my ears) problably wouldn't be able to pick it up anyway

Tony.
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Last edited by wintermute; 17th October 2009 at 12:56 PM.
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