0.300 VDC (from soundcard) on LM3875 GC input !!!

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Hi gentlemen,

I’m currently assembling a LM3875 gainclone from audiosector.com.

Problem : checking for DC presence at the output of my Xfi Extrememusic PCI soundcard, I unfortunately found out a stable 0.300 V on the stereo channel (the other channels are clean)

It seems not to be a “static” voltage, my controller is a very old one with a needle. Strangely, it's the same value on left and right channel…

I checked the card's electronic circuits without succes, too small and complicated. Nevertheless I like this card and don't plan to change it :)

Have anybody already got the same phenomena, not necessarily on a XFI card ?

Is that dangerous for my FE168EZ drivers, as the GC kit is passing the DC ?

If yes, what do you think is better at the input of the gainclone, a capacitor or a tranny ?

Thanks for your advices...
Thanks Nordic. Simple and direct answer… you just killed my problems in 1 sec, so it let me a little bit frustrated ;)

My questions are because the kit is originally designed for a direct entry, so I would like to get some advices or experiences about adding a capacitor or a tranny at the input and the possible impact on the sound quality…
Just use a big capacitor on the input to keep the bass. Instead of the standard 2.2 or 4.7 uf caps on the input, try 10uf, 22uf, or bigger depending on your input impedance. I used 10uf/100V cap on my 63K impedance input on my subwoofer amp and get bass that goes really low.

Don't go too big, so it don't take forever to charge the input cap, and you get a short DC offset in your speakers on startup.
Sorry for my English… and thx to Systran :)

Hi Eworkshops1708… 22uf, really ?!

I just done an extensive search in the forum (better to be done before…hmm..) and you’re right, usual values are 2.2 or 4.7uf, depending of the impedance of the CG.

Lower values don’t pass the LF, and phase error increase… but higher values present a risk of DC escape (the price escape also :))

In my circuit (50K pot + 20K shunt + LM3875), depending of the volume setting, I estimate the input impedance to be 20K to 25K, so a 4.7uf would pass 1,5Hz @ –3dB (16Hz @ 0dB) with a low phase error...


4.7uf seems correct as the amp is dedicated to 6” FR drivers, not subwoofers.

But I keep in mind the idea of increasing the value to boost the bass if necessary… with cheap parts, before breaking the bank for that :

Remember that a lot of PC stuff runs single supply, and may already have caps. If you measure with a high impedance meter or scope, you might see a significant offset. Try loading the thing down with about 10kohms. I've also seen this on inputs, where if you don't sink the leakage current, there will be a large offset and even shut down the input circuitry. If you really do have 300mV, follow the cap advice.
I have that very same sound card.

Panasonic ECW-F2335JB is a 3.3uf polypropylene capacitor that will let everything through except the DC.
Even an electrolytic 3.3uf can work (+ goes towards the amp, - towards the source).

I believe that, as illustrated, the "load" is the 22k resistor that came with the kit. So, 3.3uf allows the amplifier to attempt to amplify non-audio signals.

So, a cap at 0.68uf will be "down by 3" at 10hz. Of course, that's misleading. Unlike engineers, most people would draw the line before it has any audio effects. That's flat to 21hz, and its the same 0.68uf cap.

If related to speaker designers, the "speaker" (load) is 22k (or whatever you're using for input load) and the cap is to decrease bass below 10hz.

A mighty fine layout for Soundblaster X-Fi in combination with LM chips is a layout of entirely 100k and 2.2k resistors (not including speaker zobel about 4 ohms).
According to National's design spreadsheet, this matches every part for 100k layout with 100k pot, with the Rb and Ri at 2.2k.
Somebody please correct if that's not right.

See my latest tinkering of LM1875 over at its thread, because here's a design that works great with X-Fi--and currently in service rocking down the house. ;)

EDIT: My main reason for suggesting that layout is the 2.2k in-series resistor (Rb) provides additional isolation (armor) for the amp to use an X-Fi source. That will protect the new input filter cap as well.
Conrad Hoffman, as per your advice, I tested with a 10K shunt : it doesn't kill the 0.300 VDC !

The Xfi has a very low output impedance (+/- 50 Ohm, I’m not sure) so you can plug in directly a headphone.

There is a lot of polarised caps near the female plugs but I was unable to find any continuity between one of them and the stereo output plug. Are some caps plugged in reverse ? Anyway I’m afraid that is too much sophisticated for my 40W soldering iron :)

The really strange thing is this concerns both channels, exactly the same DC on right and left ! Does it mean this is a "per design" phenomena !?
Regardless of wether your sound card is DC or AC coupled, it was not designed to have such an excessive offset at the output. The usual culprit for elevated DC offset is poor grounding of the PC assembly, case and power supply. Connecting my PC power supply to the right earth has solved this and some other problems, so it might be worth a try in your case.
And, of course - check your instrument, too.
Hi danielwritesbac, I have some old electrolytic caps for testing, no problem !

I read your posts... but that's a little bit complicated for me (due to the English, also).

You know, as I bought a “Premium kit” with nice resistors as Riken and Caddock :cool: , I would prefer to keep them as per the Audiosector design, with the addition of a 50K Alps potentiometer I already own. The only changes I can afford is a cap or tranny at the input.

Just a question : on your x-fi card ouput, would you please check for me if you have the same DC presence ?

Many thanks
Give better grounding a try, but if that doesn't do it, you need a big coupling cap. Try to use a large film, but if you can't get that, use a large electrolytic. A larger electrolytic than initially seems necessary will result in almost zero AC across the cap, and very good performance. Personally, I don't like electrolytics, but sometimes they're the best choice. Since you have 300mV anyway, just use a regular polarized part and install it so it's correctly biased.
Hi Willi Studer, are you THE real Willi Studer ?
Don’t you know, I own a Studer/Revox Modell 40 – Ecl86 push-pull amplifier –incredible making quality !


Thanks for your advice, I checked my controller – ok – and also the ground : independent ground line to PC/Hi-FI area, 225VAC ground/phase, 1VAC ground/neutral, it is perfect, for us, in France.
All the parts of the PC are at the potential of the ground and 0R between them, so no (measurable) problem...
The real Dr. Studer is not with us for some years now.
At this point I might suggest you to check eventual voltage between computer chassis and amp chassis - try to measure sound card DC against amp chassis - you might be getting ground loops.
If this shouldn't help, in your place I would fit two 10 uF polypropylene caps on output and solve the problem.

Beautiful restoration work on your M40. Just keep it always on for optimum performance.
Willi Studer said:
... At this point I might suggest you to check eventual voltage between computer chassis and amp chassis - try to measure sound card DC against amp chassis - you might be getting ground loops.

Arggg, the killing question :)

My amps : M40, NAD 3150, AIWA P30, are vintage items, no ground in the power cables.
Yes, it is bad, but there is no humm at all, so...

Now, this would be a valid explaination, a floating grounding at the amp (i.e. M40 and his hight VDC) to damage electronic parts on the soundcard ?! I will have to re-install the M40 to check...
I have always considered DC-coupled inputs to be a risky proposition and therefore have never done it in any of my audio amplifier circuit designs, especially when the output is also DC-coupled and doesn't have DC protection circuitry.

I prefer to always stay on the safe side, don't want a faulty source device taking your speakers along the ride of destruction!
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