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
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
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
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
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.
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.
As in the progress of making the DC offset as low as possible, I have tried to changed the Q5/Q7 pack with matched pair down to 0.01v. Not only that, I also matched my R/23/R25 and all the Q2/Q11 mosfet. However, by swapping the Q5/Q7 many times (and changed many pairs), I can only goes down to about 30 mv. By adding my X-BZLS or A-P, the will rise to about 70-80 mv. This is not bad actually; however, I just feel more comfortable if it stays lower than this!
After re-study the wiki page and the schematic again, I decided to use the resister tweak. I changed R25 with a 500 ohm VR and adjust it. At about 402 ohms, I bring the DC offset down to about 12 mv. By adding the pre-amp, it stays stable at about 26 to 30 mv. Perfect for me!
I then test it with different music. I did not sense any degradation in the sound quality. I am wondering by using un-matched R23/R25, will it cause any problem (or sound change) in the long run?
Any idea is appreciate!
Thomas
After re-study the wiki page and the schematic again, I decided to use the resister tweak. I changed R25 with a 500 ohm VR and adjust it. At about 402 ohms, I bring the DC offset down to about 12 mv. By adding the pre-amp, it stays stable at about 26 to 30 mv. Perfect for me!
I then test it with different music. I did not sense any degradation in the sound quality. I am wondering by using un-matched R23/R25, will it cause any problem (or sound change) in the long run?
Any idea is appreciate!
Thomas
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