[steinchen]

Unfortunately one of my audio sources (Creative Audigy 2 ZS) has a quite high dc offset (some tens of millivolts). With a gain of 10 in my headamps this would result in some hundred mV on the output of the amp. Therefore I got to use coupling capacitors in my amps.

On my PIMETA i soldered (large) Wima MKP 0.22uF with short wires under the pcb (C1L, C1R).

For my PPA I'm planning to put the coupling capacitor between the pot and R1, too. Means cutting the traces on the bottom side of the pcb and mounting the caps directly to the pot bridging the cut. Hope this is correct so far. I'm planning to use large high quality caps (Wima MKP or Mundorf).

With R2=1M the corner frequency (-3dB) of the resulting high-pass should be 0.5Hz with a 0.33uF cap. Would this be overkill ? The PIMETA reference from tangent is 0.7Hz with a 0.22uF cap. Would a smaller (even if higher quality) cap result in degraded sound quality due to phase distortion ? Should I use the largest cap possible ?

Furthermore I thought about putting a switch parallel to the cap, my CD-player has almost no dc offset, so I could bridge the cap depending on the source, but I'm afraid of noise pickup etc.

Are there other ways to deal with dc offsets from the source ? Maybe a high-pass-filter between source and amp outside of the amp ? Or an active circuit between R1 and the + input of the opamp making caps obsolete ?

 

[morsel]

You could put coupling caps on the output of your sound card instead of altering your amp.

 

[steinchen]

That is the option I ment with "between source and amp". With the biggest MKP cap available (Mundorf MCap63 10,0 uF $10 per piece) and the 50K impedance of the pot the corner frequence of the resulting high pass is 0.3Hz which is quite acceptable. I don't even mind that it is an expensive solution. First of all I want to avoid caps in the signal path, if possible.

I've seen some designs with op27 to eliminate the dc offset (called servo ??? I'm not sure): output of amp with 1M to op27 -input, op27 out with .47uF to -input an +input to ground. I wondered if it is possible to simply connect such a circuit to PIMETA or PPA. would be the perfect solution ...

 

[morsel]

Your source is not worthy of such heroic measures. Just use coupling caps.

 

[Garbz]

Your souce should have many caps in the signal path anyway. Most likely big and cheap electrolytics.

Morsel is right, save your money, buy some cheap caps, and put the $9 difference asside for a new soundcard.

 

[steinchen]

agree, I'd better spend the 2 * $9 = $18 for a new soundcard than for expensive caps. Can I be sure that the M-Audio Delta 2496 will have a much lower offset than my Audigy 2 (30mV) ?

I need the microphone of my headset for online-gaming. Due to the lack of a mic input on the M-Audio I need the schematic of a simple and mid-quality mic-preamp. Any recommendations ?

The active dc offset correction is still on my mind since I got two op27 and 0.33uF MKP caps lying around, would cost me nothing, just need to know how to build.

 

[cosmopragma]

Quote:

Originally Posted by steinchen I need the microphone of my headset for online-gaming. Due to the lack of a mic input on the M-Audio I need the schematic of a simple and mid-quality mic-preamp. Any recommendations ?

`The computer mag C't published a schematic some time ago (21/2002).Might be overkill, it's stereo and quite good.They are even selling a PCB, look at page 165 of the current C't (02/2005).

 

[tangent]

If you replace the Audigy with the Audiophile 2496, you won't get positional audio in gaming. If I were going to upgrade the sound card with gaming in mind, I'd only consider a Turtle Beach Santa Cruz, since it sounds better and still has some positional audio features.

A better solution would be to put one of aos's DACs between the sound card and the PPA. This should get you 100% of the sound quality possible from the Audigy's DSP engine while avoiding noise and DC offset. PM aos to see what he can do for you.

 

[Garbz]

Actually considering the apparent flaws in the Audigy2 DSP chip I'd be more tempted to run 2 soundcards side by side. Things get even easier if one is external like the M-Audio Transit. Windows should give you a choice of which to use as a primary, and any decent media player app should give you the option to override.

If worst comes to worst simply unplugging the usb card should re-activate the audigy.

 

[Vadi]

I have the same card and I want to plug it into my hi-fi and from that the headphone amp gets its input. Would this solve the problem?

 

[silvervarg]

It depends if your HiFi has caps on the input (or output, but it is not likely that it has caps on the output).

 

[Garbz]

You can almost be certain that it a hifi will have caps on the input or some other form of DC protection.

Whether it has a zero DC offset on it's output is another question entirely

 

[Okkler]

I also have an Audigy 2 but Im really afraid to plug my K501s in. If theyre amplified with my CMOY which have .22uF input caps, will I be protected from the DC offset from the Audigy or any other source with DC offset for that matter?

Or do I need to put caps in the source too?

 

[jefemeister]

The DC offset from your source will not be increased in the amp, at least not by the gain factor. The signal itself gets larger, the relative DC level will stay the same. Of course, the amp can always add it's own offset but it would be minimal.

 

[steinchen]

Quote:

Originally Posted by Okkler I also have an Audigy 2 but Im really afraid to plug my K501s in. If theyre amplified with my CMOY which have .22uF input caps, will I be protected from the DC offset from the Audigy or any other source with DC offset for that matter? Or do I need to put caps in the source too?

yes, with the input caps your cans are protected, no further caps needed. I got input caps in my cmoy47, too, but want to avoid these in pimeta / ppa, rather have them DC-coupled.

The last days I thought my problem over and realized that switching from Audigy to M-Audio would sacrifice EAX etc and therefore is no solution for me. I thought about an external DAC before but discarded the idea due to the high costs.

Comparing some DACs I got disappointed that a $350 CIA VDA-1 simply uses an rather old PCM1716 (96kHz, 106dB) and "jellybean" opa2134. So I decided to order an AOS Piccolo (PCM1793, 192kHz, 113dB) the next days.

Nevertheless I am curious how much difference an input cap (and the type it's of) makes. I'm planning to build a little switchboard to find that out ...

Tnx a lot to all for your comments. Tnx to cosmopragma for the mic-preamp hint, as a subscriber of c'T I got the schematics @home and will probably build it although I don't really need it. What would a long, dark, muddy winter be without projects ?