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Old 6th April 2004, 03:18 PM   #1
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Question dc offset question?

hi there, is 21 0r 22 millivolts dc offset going to be too much for inputs on adcom gfp 565 preamp? dc offset from preamp, leading to power amp, is 3.3/4.4 millivolt. the onkyo cdp is using ad825 op amps well decoupled, and bypassed are jfet class a biased to 1.70 and 1.84 milliamps. could this biasing be causing such high offset? if i remember correctly, the jfet source resistor is 470. ohms and theres a 100.ohm between drain, and op amp output. the cheap, 22mfd. lytics. output coupling caps when not bypassed reduce offset to about .1 mv. dc both channels which is great but, i sure hate having lytics, for output caps. could there be maybe, be a resistor change i could make etc. or other circuit mod i could make to reduce this? or could i bypass original lytics. with better film caps. or order some 20 mfd. solens to replace the original lytics? don't want to risk damage to preamp, wasn't sure if that's too much offset going into it or not so i haven't played it much. if not, i would rather have no cap if at all possible if there's a way, and i'm capable of doing it. any advice appreaciated! thanks, crippledchicken
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Old 7th April 2004, 10:59 AM   #2
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Default Re: dc offset question?

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Originally posted by crippledchicken
hi there, is 21 0r 22 millivolts dc offset going to be too much for inputs on adcom gfp 565 preamp?..... the cheap, 22mfd. lytics. output coupling caps when not bypassed reduce offset to about .1 mv. dc both channels which is great but, i sure hate having lytics, for output caps. ...
Most likely you have an input DC blocking cap on the preamp too. So the offset does not matter, except possibly for turn-on and switching thumps. Double check your preamp, but it's a rare preamp that does not have DC blockers on inputs.

In the real world, almost all consumer cd players use electrolytic capacitors for DC blocking on outputs. You seemed to think they are causing some problems and took them out or brought the output around them? I'm curious what problems. In any case, now you have to be willing to accept the consequences, slight DC offset, or having to replace the caps with film caps.

I use some compact WIMA film caps for this purpose, Over the years, electrolytics dry out and go open, or leak. So I dislike them for that reason (more for long term reliability than any presumed sonic benefit).
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Old 7th April 2004, 03:01 PM   #3
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hi slowhands, thanks for your response. i just try to do alot of reading here, and at the audio asylum and most of what i have read, the majority seem to think that the best cap, is no cap at all for output coupling. i'm pretty new at the field of diy audio tweaking, have some basic understandings of electronics but, still lots to learn. my actual trade was new home construction for about 25 years so, due to unfortunate accident, i now just enjoy trying to learn all i can about this fun hobby. got lots to learn yet but, enjoy every moment of it. i've removed bypass jumpers, and bypassed the original caps, with some .01mfd film/foil and seems to sound more detailed with better transient response than the original lytics. also i can't measure any dc offset now. by the way, what size wimas do you use for that purpose? thanks abunch! crippledchicken
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Old 7th April 2004, 06:46 PM   #4
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The size of the output cap needed depends on the input impedance you will be driving in the preamp. A high pass filter is formed by the RC network of your output cap and the preamp input resistor. You can safely assume the input resistor is at least 10k, and most are at least 22k or 47k. Given that, 22uF is OK for the low end, and better still if bypassed with a film cap in parallel of 0.1uF so the hhighs get through too.

This appears to me to be a reasonable compromise. There is probably a smaller cap on the input side of your preamp, so this is not the only highpass you have to worry about in the system, but at least your CD player won't make things worse.

Components are all non-ideal. Electrolytics give high capacitance in a small package, but their properties vary, depending on their construction (primarily dielectric absorption and effective series resistance). Most have poor performance at frequencies well above the audio band, so bypassing them with film caps is advised and works quite well, as you observed.

If you pass large AC signals through electrolytics, most experts agree you get some distortion at low frequency. One school of thought says just use a large value, another says the distortion is inherent in the construction of electrolytics. I recall several articles that blasted them, due to inherent distortion at all frequencies.

I was not convinced, but I don't pretend to be an expert nor have I investigated this personally. Bottom line, I hate them for low long term reliability. They are some of the most frequent failed parts in gear I repair. So I replace input caps with compact WIMA caps, 10uF or 4.7uF/63v, that I found a trove of at a surplus place. Sorry I can't give you a part number, but it must be in the WIMA catalog.

Larger value film caps are expensive and huge, compared to comparable value electrolytics. So they are not always practical to replace electrolytics, in my opinion. The usual large electrolytic DC blocking cap (bypassed) on the feedback net stays for now, until a practical alternative is developed.
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Old 7th April 2004, 10:37 PM   #5
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hi slowhands, great load of info there, appreaciate it. i looked at the service manual i have for the preamp, and your right the high level input is-22k ohm and phono says 47,000/100pf. i'll keep searching, and order a couple of electronics books also, to study up on. thanks again, crippledchicken
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