Aleph X DC offset

[tomchaoda]

My 60W A-X, on one channel, the relative DC offset is about 30mv when standalone. After hooking up to the pre-amp (XBZLS) and speakers, the relative DC offset is hovering between 60mv and 89mv. Compare to my Aleph 5, Aleph 5's DC offset is very stable.

Is my A-x constant changing dc offset due to the Active source gain actively ajusting the gain? or is it something else?

Is this normal?

Thomas

[wuffwaff]

Hi,

I think this happens when you don´t use input caps on your X. With the X-BOSOZ you don´t have 0 volts DCat the output because of the feedback loop including the output caps. So if your source has some dc this will show up at the output of the X-BOSOZ.

William

[GRollins]

Expect some drift in usage. As long as the DC offset remains small, I wouldn't worry about it.
That said, yes, caps at the inputs can help if there's any DC coming from upstream.

Grey

[tomchaoda]

Originally posted by wuffwaff
Hi,

I think this happens when you don´t use input caps on your X. With the X-BOSOZ you don´t have 0 volts DCat the output because of the feedback loop including the output caps. So if your source has some dc this will show up at the output of the X-BOSOZ.

William

William:

Any suggestion for the capacitance value for the input cap on X?

Thanks,

Thomas

[wuffwaff]

Thomas,

this depends on the chosen input impedance. Normally 10uF should be enough and maybe you can use even less (as seen in the XA200 pictures around here somewhere).

I´m still not shure if I will use them in my (future) X and if I will go for foil or Black Gate N or NX caps.

William

[tomchaoda]

William:

Thanks.

Is it the C12, C13 in Hifizen's design? It is using 47uF. (I was using 4.7uF)

Thomas

[tomchaoda]

Originally posted by GRollins
That said, yes, caps at the inputs can help if there's any DC coming from upstream.

Grey

Grey:

If the DC is not coming from the upstream, any way I can find it out and to minimize it? I found out just now, by disconnecting the pre-amp, it shoots up to 140 mv and stays there. This is never happening before!!

Thomas

[tomchaoda]

After a careful exam of my pre-amp, the problem is in the source leaking DC from pre-amp to A-X. I tried both XBZLS and A-P and they both add up about 30 mv to the A-X.

Is adding input cap (or DC blocking cap) a good solution? I remembered it will chop off some frequencies mentioned somewhere!

If yes, any schematic done by anyone before?

Thanks,

Thomas

[GRollins]

If you're passing DC from your preamp, it will show up at the output of your A-X. You can drive yourself crazy trying to decide whether to zero out the DC in the amp on the bench, then deal with the DC passed on from the preamp when the amp is inserted into the system--or-- zero out the DC with the amp in the system. With 20dB of gain, you'll see 10x the DC that comes in the input. So, if you were thinking that 5mV at the preamp was pretty good, bear in mind that it'll come out the back of the amp closer to 50mV.
The DC will drift a little with temperature no matter what.
A decent film cap in series with the input will take care of DC from out in the real world. Digikey has a 10uF 100V cap, actually I think they've got two, made by Panasonic. Polypropylene, perhaps? Don't remember. Anyway, that's a good bet, and not all that expensive. Naturally, you can go with the expensive boutique caps if you want. Bigger is better in terms of low frequency response, but costs more.
Unless you want to use the cap to purposely roll off the response in a biamp system. (Who, me? Trying to put ideas into your head? Nah...not a chance.)

Grey

P.S.: If you're using the amp with an unbalanced input and really want to tweak things, try a little resistance to ground at the unused input. What you're shooting for is to match the Zout of the preamp as seen by the driven input.

[GRollins]

Incidentally, I believe the "real" XA models all come stock with DC blocking caps, so if you go that route, you'll be in good company.

Grey