quick question about coupling caps - polarized or nonpolar

[dramadisease]

can anyone tell me why in certain designs (auditronics input module mic pre, wurlitzer solidstate preamps, tons of other stuff) they use electrolytics as coupling caps and in other designs, non-polarized paper or oil or polystyrene/polypropylene is used? what is the criteria you would have in picking these values? ive been expiermenting latley with coupling caps in tube circuits, value choosing vs. freq. response - but why exactly in older solidstate gear are electrolytics used, and when ive tried to sub them for nonpolars, it doesnt work?

hope this isnt a dumb question to throw out, thanks much
-bryan sours

 

[Samuel Groner]

and when ive tried to sub them for nonpolars, it doesnt work?

If you could specify your "it doesnt work" a bit more precisely, we can help you better, I guess.

Coupling caps, together with the input-impedance of the following stage, form a high-pass filter. You can calculate the -3 dB cutoffpoint with this formula: f=1/(2*3.14*C*R). As you can see, with a 2k input impedance (which is typical for a mic pre), you need at least a 47 uF cap to get good LF performance. If you don't want an electrolytic, you need i.e. ten 4.7 uF polyprops in parallel, which is both big and expensive.

Is that what you wanted to know?
Hope so...
Samuel

 

[dramadisease]

sorry bout that - signal just doesnt pass.

makes sense to me, i guess at this point i would ask would there be any sonic difference in using 10 4.7uf in parrallel if the caps are of approx the same quality? sorry if this sounds ludicris, as im typing this im thinking about how .47uf are a little more affordable and youd need a hundred of those....

well the situation im looking at is im trying to redesign this input module for my auditronics console, and the output cap off the first micpre stage, is a 330uf electrolytic. but i think im going to just replace that whole stage with a melcor. anyway youve answered my first question quite well thanks!
-bryan

 

[Kev]

I think it is best to use external Mic-pres ... but that's just me.

People complain that Auditronics desks don't have enough power supply ... but that hasn't been my experience ?? I have a 501 with 24 channels fitted and have the other two are used as spares and experimenters. ( I also have another complete desk waiting for new life )

I do use the internal Mic-pres but I have considered converting them to line level inputs with trim ... to interface with Synths and other lower level line inputs.

Most of my channels have the 5534 conversion.
That might be just a little boring for you but it did lower the noise floor.
Then again it might just have been the tender loving care and new caps that had the most to do with the noise floor.

I don't really see why the BiPolars or other nonpolars didn't work ?
.... err ? too much DC offset going on .... ?

 

[dramadisease]

sorry, as usual im not typing enough!
when i tried to replace the electrolytic coupling caps in a wurly, i noticed that signal didnt pass - im assuming thats a standard thing, but maybe im wrong?

i just didnt really get why exactly they would be used, in an api mic pre they use a 330uf coupling cap that is non-polar (it is an electrolytic though...)

the auditronics console i have is a grandson 110b - old little guy, 18 channels but i have 2 of them. im really just wanting once console so i decided to start rebuilding this thing. the supply seems reasonable for the console - im sure ive got enough headroom for what i want to. as i sit here im depinning elcos for use as my insert cabling. its a simple micpre - modded with the 5534 - with a jensen input transformer. my plan is to have some api channels, some melcor channels, some 990 channels, and so on. next step is to rebuild the summing bus which is terrbile!! gonna do api mix buss for the stereo and a tube bus for the rear buss - so that i can slam the bus then dump it back into the stereo buss. the eq is REALLY simple in this thing. 6 bands, thats it. i need to figure a way to mod it to be able to sweep frequencies, been meaning to scan a schematic and put it up.

i just think its easier for me to rebuild this console, i mean i could do that or build all new external stuff, but ive got this console sitting here, might as well use it and make it what i want. hell, i payed for it! i dont like to eq much and i definatly want a really nice summing buss, i dont need automation - so i think this would be pretty much all i need minus compression and a little bit of gating.

my only problem is i like tube stuff so much better, so somehow im trying to infuse character into this console... jensen trannies just aint cuttin` it, but i dont have much money!

any electrolytic coupling cap brands that you feel have better sonic qualities than others? FCs?

thanks kev
-bryan sours

 

[sismofyt]

They certainly should pass sound. Plastic caps are also non polarized, btw. Non polarized lytics are just two ordinary ones back to back in the same housing. Mostly cheap ****** ones..

 

[Samuel Groner]

any electrolytic coupling cap brands that you feel have better sonic qualities than others?

Check the Vishay Roederstein EKSU-Series. www.vishay.com

Samuel

 

[PRR]

The cheapest capacitor is the electrolytic. When you need a big capacitor, you use an electrolytic.

BUT: an electrolytic MUST have a DC voltage across it. If it has zero DC voltage, it slowly turns into a low-value resistor (a metal can full lof lye). If it has reverse voltage, it goes bad very quickly. If it can get a lot of power, it will probably burst in minutes.

Film caps don't have DC polarity, so you can use them either way.

But film caps in large capacitance are very big and expensive.

You can put two electrolytics back-to-back together and get a "non-polar electrolytic" that will not burst no matter which way the DC is pointing. These are even bigger than regular electrolytics.

As a general guide: they don't bother to make electrolyics much smaller than 1 uFd. If you need something with lower capacitance, use a film. They don't routinely make film caps a lot larger than 1 uFd because they are enormous and expensive.

Your failure with the Whirly does not make sense the way you describe it.

 

[dramadisease]

the formula explains why the auditronics uses em as coupling caps - for more lf response. makes sense to me - i did run across a 10uf solen NP cap, and that thing is bigger than in input tranny..haha

the wurly thing was a bit ago and now that i read my previous post im recalling incorrectly, just ignore my rambling - thanks prr for the explanation!