Input/Output Caps?

[Moontan13]

While waiting for a cmoy kit to arrive, I was browsing the sellers instructional webpages. From the illustrations it became apparent this amp has no big caps. I'm intending to use this amp with a 5.5 iPod LOD, then with a 4G DIYMod.

I know that with a DIYMod, you use capacitors on the output, so why not just stick those caps on the amp's input? Also, without bypassing the stock caps, does it do any good to add caps?

 

[tomb]

Quote:

Originally Posted by Moontan13 /img/forum/go_quote.gif While waiting for a cmoy kit to arrive, I was browsing the sellers instructional webpages. From the illustrations it became apparent this amp has no big caps. I'm intending to use this amp with a 5.5 iPod LOD, then with a 4G DIYMod. I know that with a DIYMod, you use capacitors on the output, so why not just stick those caps on the amp's input? Also, without bypassing the stock caps, does it do any good to add caps?

Capacitors block DC current. They're used on the input to a CMoy because once upon a time, many sources had noticeable/measureable DC offset (DC voltage on the output). Most of the time, this voltage was very small and the use of headphones directly from the DAP/CDP/whatever was not harmful.

However, once you add an amplifier on the output of the DAP/CDP/whatever - that small voltage gets amplified and becomes a big voltage - big enough to harm your headphones. As a result, it was common practice to use input caps on the CMoy to block DC offset.

If I'm not mistaken (not personally familiar with the iPod LOD, etc.), when you use the LOD on the iPod, you are pulling current directly from the DAC. There's DC offset usually present on the output of a DAC chip, so it needs blocking. The small opamp/whatever amplifier on an iPod is enough to filter this out. However, if you bypass it, there's nothing to stop the DC. So, I think that's why people use caps on the output. Just guessing on this, though.

As for your statement, "it became apparent this amp has no big caps" has no meaning in this context. It doesn't take a big cap necessarily, unless you're interested in some sort of boutique cap (which many of the iPod LOD people use, I think).

The CMoy should have power rail electrolytic caps, but there's little to no value in bypassing them with a film cap - the opamp has a very high PSRR, so the differences would not be heard as with something like an output/input coupler - and those are already film caps.

I don't know if that answered your specific questions, but maybe it helped.

 

[Moontan13]

Yes, your reply was very helpful Thanks!

 

[suicidal_orange]

Just to clarify - a stock ipod has tiny output caps on the PCB, which are bypassed when you DIYmod it. Usually on a 4g you hide some Black Gates in the ipod, but there is no reason not to put the caps as input caps on a cmoy if you'll be modding the ipod soon.

Standard ipod + extra caps sounds worse than standard ipod alone, but it won't do any damage. In an ideal world there would be no caps, but as tomb said DC offset will kill things and that is indeed why we (imod/DIYmod users) have them.

If the cmoy already has input caps avoid adding another cap and just upgrade the input ones to something nice, how big/expensive they are is between you, your wallet and whatever you plan to carry it in

 

[eruditass]

can i forgo input caps with my line out from e-mu 0404? what about for my other amp that is connected to the headphone out ? (of a clip) ?

also, i was considering forgoing the pot with the 0404 because it has a (high quality) knob on the line out that goes from 0 db to -infinity. good or bad idea?

and for the clip cmoy, would it be worse to control the vol with a crappy radioshack 100k pot or digital clip vol?

 

[eruditass]

how much dc voltage will damage headphones?

i will be measuring the dc offset off the source, multiply it by the gain of my amp, and add the dc offset of the IC's in the signal path to compare