Best preamp I/O AC coupling without top $$ caps? - diyAudio
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Old 12th August 2004, 02:24 AM   #1
Optical is offline Optical  New Zealand
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Default Best preamp I/O AC coupling without top $$ caps?

Are there any tricks to use when AC coupling preamps? Ive tried electrlydics and they arent any good, ive tried various polyester and MKT caps but everything seems to colour the sound a little bit which can be very annoying..

Does anyone have any tricks for better coupling? i saw something on the LC audio website where there were 2 different caps in parallel with a series resistor on each, i guess it was to pass different frequency ranges properly but i dont know enough about this sort of thing so it would all be trial and error..

Any thoughts?
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Old 12th August 2004, 02:49 AM   #2
hughmon is offline hughmon  United States
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My personal opinion is that the only good cap is a dead cap, i.e. try to use a few as possible. Whenever possible, direct coupling seems to be the best choice (at least to my ears). There are places where caps are necessary though, and carefull selection can help, as well as paralleling multiple types as you've mentioned. Shunting the cap with a resistor can also help a little, choosing a value high enough to minimize DC errors. If there's a large DC component though, this isn't always practicle.
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Old 12th August 2004, 03:25 AM   #3
hughmon is offline hughmon  United States
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Another trick that sometimes helps is to parallel different values within a given model line of capacitor. For whatever reason, ratios of 1/3 and 1/30 of the primary value often times have a musical quality. For example, a 3.3uF cap paralleled with a 1uF paralleled with a .1uF. This is something more born out of experience then any solid textbook rational.
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Old 12th August 2004, 06:40 AM   #4
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And yet another opinion is that bypassing caps may give the impression of extended bandwidth but the results are not that great musically. It often results a mish-mash of the various sonic signatures which is not entirely pleasant.
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Old 16th August 2004, 03:04 PM   #5
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A low cost solution can be as follows.
1. Get two 47-uF electrolythic cap and a 10k ohm resistor
2. connect the output to one of the neg input lead of a cap.
3. connect the two pos leads of caps together.
4. connect one end of the resistor to the two pos cap leads
5. connect the other end of the resistor to a good positive voltage source. Say dc 12v.
6. connect the uncconect neg cap lead to output.

The above configuration relies on the fact that electrolythic cap has much better response when properly biased.

Whereas I would like to say the best cap is no cap. I like dc-coupling much better than $$$$MKP caps.
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Old 16th August 2004, 10:32 PM   #6
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If you are not going for the cheapest route, you could use a high quality small audio coupling transformer. Just be sure to match the impeadences on both sides correctly.
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Old 17th August 2004, 02:27 AM   #7
jcx is offline jcx  United States
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Cyril Bateman's Capacitor Sound series in Electronics World addresses these issues in terms of measurable harmonic distortion

he comes to a slightly different conclusion on electrolytics; use bipolar Al electros, higher V ratings and series 2 bipolar devices when blocking significant DCV
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