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Old 29th November 2008, 04:55 PM   #1
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Default BrianGT LM1875 feedback capacitor question

A simple question. Why would nonpolarized caps (back to back itty bitty electrolytics in one can) be used for the feedback resistor, instead of a regular electrolytic? Is there supposed to be some "magic" in that?
What effect would I hear, (unfair, not knowing me, you can't assume I could hear at all ) if I were to use larger electrolytics, like 47uF or 100uF? I did play with one channel for awhile using no cap at all, but rather jumpered to ground. Thanks from a relative newbie.
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Old 29th November 2008, 06:54 PM   #2
Igla is offline Igla  Slovenia
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If offset voltage permits, I would use no cap.
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Old 28th December 2008, 04:19 PM   #3
ide2003 is offline ide2003  Indonesia
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me too..no caps, a 'cap' called 'straight-wire'
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Old 28th December 2008, 06:06 PM   #4
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How does the value of that cap influence sound, bigger cap better or less? (or is it discernible?)
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Old 28th December 2008, 07:00 PM   #5
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by Sparky OR
How does the value of that cap influence sound, bigger cap better or less? (or is it discernible?)
That cap in the feedback is part of a high pass filter. If it operates as such, filtering the signal, then D. Self alleges it increases the distortion of the amplifier.
His simple test, for minimal distortion, was to measure the AC voltage across the cap. The closer to near zero the better.
The caps' main purpose is as a DC block to reduce the amplifier gain @ DC to ~ 0dB (1times). The audio signal should be filtered before it enters the amplifier. Now we are talking about two high pass filters. If either move the filter F-3dB frequency into the audio band then they are audible. So the question becomes: how far below the audio band do these filters go and then your question on discernibility no longer applies to the filtering effect but on whether the caps affect the audio quality of the signal. There is much argument on these interlinked effects of the DC blocking abilities of the two DC blocking caps. Omitting them is not an option for an AC coupled amplifier. The designer can design a DC coupled amplifier, but that is a very different exercise from AC coupled design.
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Old 28th December 2008, 07:03 PM   #6
Igla is offline Igla  Slovenia
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It's a haghpass roll-of. Look LM3875 datasheet page 6 (Ci and Ri);
f=1/(2*PI*Ci*Ri).
It has big influence to the sound.
Try it and if you can hear no difference leave it there.
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Old 29th December 2008, 06:15 PM   #7
ide2003 is offline ide2003  Indonesia
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guys, ve' heard the caps first hands, even the kind of caps has big influence..the best sound was marcon can-paper in oil, next same of russian made (smaller can), next MBM, of course the best sound is no cap..but did not dare

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Old 29th December 2008, 06:21 PM   #8
Igla is offline Igla  Slovenia
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''...the best sound is no cap..but did not dare''

It's better to burn out then fade away
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Old 30th December 2008, 03:07 PM   #9
ide2003 is offline ide2003  Indonesia
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Igla, no in any possible way I'll let my old trusty magnepan SMGa burn out & fade away..

i'm a newbie, they (our friends the EE) said, don't use the caps, use servo or DC null circuit..what is that?..how its works..and.. ehm.., schematic?

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TPS
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Old 30th December 2008, 06:11 PM   #10
Igla is offline Igla  Slovenia
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Here is the thing...
if you use commercial source - they all have capacitors at the output so you don't need capacitor at the input of the amplifier.
And what about the cap on the feedback to ground? Remove it and measure the DC offset voltage at the output of the amp. If the voltage is in the range of +- 100mV I would not worry. All Pass Labs amps have offset voltage it the same range. In reality the offset voltage of Gainclone amps (LM3875, LM1875 and TDA2050) is somewhere between 10 to 40mV without the cap in the feedback. I must say that this feedback cap has big influence to the sound (almost the same as input cap); but it's your call.
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