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I've got a solid state preamp with +/-15v rails (so no danger of a large dc on the output) and 100uF output coupling capacitors. My power amp has high quality 50v electrolytic coupling caps. My instinct is to just remove the preamp coupling caps as "the best cap is no cap" as they say. Any reason to not do this?
Follow Ups:
Before removing the caps, measure the dc present (other lead to ground) at the preamp side of the output coupling cap. Sometimes there is a deliberate application of dc to bias the electrolytic so that it will sound "better" passing ac signals. This is common in Sony cd players for example.
I would not eliminate them though. By doing so you will undoubtedly get a severe and nasty pop if you turn off the pre before the power amp.
I've replaced countless electrolytic coupling and interstage caps in my gear since I read the famous capacitor article in Audio years ago. In all cases, even where I judged it safe to not use a cap at all, the sonic differences were inaudible or subtle at best.
doggy
I replaced in my power amp the 4,7 uF cheap lytic coupling caps with a wire after checking that I only had a few mV DC on preamp output. works fine and sounds better.
Duomike
get some 10uf 150v paper in oil caps ....you are likely to be bowled over by the sheer musicality of these things ...the down side of course is the physical size ....you can pic up some interesting ones of ebay [ex soviot military stuff ...generally of a high quality]
its not a good idea to run without dc blockers
Thanks for the reply. I might try some different caps when I get the time and money. My question was really: isn't it overdoing it having two sets of dc blockers, one on the output of the preamp and one on the input of the power amp? If I would be bowled over by paper in oil caps, wouldn't I be even more bowled over by eliminating a cap altogether?
Hi.
As I suggested below, try it without any couple caps & see. If it works safely, who needs to spend redundant bucks to buy exotic caps?
c-J
You might also consider wiring up a Lundahl or some other type of solid state output transformer to get rid of that DC. Also, measure the DC at the output and see what you get. 100uf film caps will run you at lest 20 bucks each, and they are freakin huge, so they may have to be outside the chasis. Also verify that the caps in the amplifier are at the input, not somewhere upstream. If it was my setup, I would take the caps out of all of them and put some high quality films inline where they would fit.
-Paul
Using excessively large capacitance values may create a bandwidth problem due to capacitor self-resonance.
All capacitors have some parasitic inductance that resonates with the capacitance to create an impedance maximum. The resonant frequency scales with the inverse of the square-root of the product of inductance and capacitance. You want to keep this resonant frequency well above 100 KHz to avoid phase shift in the audio band.
For reference, I measured some 0.01 microfarad capacitors for self-resonance and got results as low as 15 MHz. If the inductance were the same, a 1000 microfarad capacitor would have a resonance at 47 KHz. Of course, a larger capacitor is going to have more inductance, so the actual value would be lower.
Get rid of ALL electrolytics in the signal path.
Replace with bipole, poly-whatever, just not electrolytics.
here
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=37523&highlight=remove+coupling&r=&session=
" I INCLUDE THE FOLLOWING FOR COMPLETENESS ONLY:
You could remove the cap entirely, BUT THIS CAN BE DANGEROUS to upstream equipment if the equipment does not contain an AC coupling capacitor at its input or the CD player ouputs significant(?)levels of DC. Also if you try the CD player on other equipment you may forget to check. SO PROBABLY leave this one alone."
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=55062&highlight=remove+coupling+caps&r=&session=
" DC coupling eliminates the capacitor, but this means DC voltage levels must match between the stages that are coupled together. Also the DC-drift of preceding stage will be induced into the following stage.
The outputs of preamps will usually have a DC-offset(not zero volts) and is therefore coupled to the output connectors with capacitors. To protect amplifiers from DC offsets, most amps have capacitors at their inputs. That means the preamp and amp are AC-coupled. (A tweak: If you know for sure a preamp has output caps, you can safely remove the pwramp's input cap). "
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=65835&highlight=remove+coupling+caps&r=&session=
Plastic film caps are ususlly superior to electrolytics for coupling applications (voltage and capacitance requirements permitting). Plastic film with metal foil electrodes (rather than metallised film) are even better still. These are, however, very difficult to find above say 1uF. Polystyrene caps use this construction. Use these when possible due to their low dielectric absorption.
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=71660&highlight=remove+coupling+caps&r=&session=
I've also removed all electrolytics, and replaced them with motor runs, other polypropylene caps and teflons for coupling. Very big improvement - I'll never go back. I agree - choose better caps. I don't bypass, don't need to. Yes, size is a problem, solution is build bigger amps, or use a subsiduary power supply which can sit under the existing amp as a base. If you do that you can fill it with chokes too.
http://db.audioasylum.com/cgi/m.mpl?forum=tweaks&n=108717&highlight=remove+coupling+caps&r=&session=
DC coupling (no input or output coupling caps) is the best way to go for best sonics accross the board. However, you MUST be sure that all of the devices in the chain (signal source, preamp, amp, etc.) are not outputing DC current (called 'offset') when idling with no signal being output. Offset may be either + or - voltages. With the unit in question powered up and no signal being output check each channel's output with a DVM starting with a scale of about 5-10 v DC and then going to the lowest scale. Be sure to check for offset AHEAD of any output coupling cap. Any DC output from the DUT will be amplified by an average 10x by the next unit in the chain. If your offset is below 2-3 mV you should be okay and can safely remove the next unit's input coupling cap. If it's above this try slight adjustments in the bias pot for that channel while monitoring offset on the DVM's lowest scale. Repeat for the other channel.
LOL myself....You've never heard Black Gate NX HiQ then...they do have a burn in problem, but can actually sound absolutely amazing. They are very different to most electrolytics and not just because they are bipolar. I've heard some people even prefer them to poly whatever.
Thanks for the reply though.
Hi.
As long as there is I/P coupling cap in yr upstream power amp, it should not cause major issue.
If it sounds better & you want to carry out OCL, you may add an active DC offset sensor like those speaker protection sensor board to trip the O/P signal circuit in case of DC offset runaway.
c-J
PS: Likewise, I removed the O/P coupling cap of my tube phonostage
as I see no DC voltage around there.
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