Superphon: DC offset a serious problem?

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I was over at the AudioKarma site and in the 'Vintage Solid State' forum there is a sticky about DC offset in solid state amps.

I measured my 23 year old Superphon DM200 amp and found about 60 mv DC on each channel. Some questions:

1) Is this enough offset to be concerned about?

If yes, then:

2) Can I simply adjust the bias to reduce this offset or is it more involved than that?

3) Does anybody have a schematic for this amp? (hoping beeger sees this....)

Thanks in advance guys.

Kirk
 
kirk57 said:
I was over at the AudioKarma site and in the 'Vintage Solid State' forum there is a sticky about DC offset in solid state amps.

I measured my 23 year old Superphon DM200 amp and found about 60 mv DC on each channel. Some questions:

1) Is this enough offset to be concerned about?

If yes, then:

2) Can I simply adjust the bias to reduce this offset or is it more involved than that?

3) Does anybody have a schematic for this amp? (hoping beeger sees this....)

Thanks in advance guys.

Kirk


The offset is not enough to worrry about in my opnion.
Its hardly going to move the speaker if at all.

Many amps have a preset on the pcb to adjust the DC offset.
But be careful coz some amps also have a bias preset as well and adjusting that instead could fry the amp.
 
kirk57 said:
I was over at the AudioKarma site and in the 'Vintage Solid State' forum there is a sticky about DC offset in solid state amps.

I measured my 23 year old Superphon DM200 amp and found about 60 mv DC on each channel. Some questions:

1) Is this enough offset to be concerned about?


Not enough that its going to cause any harm but enough IMO to look into it, especially if your amp has provisions for adjustment.


2) Can I simply adjust the bias to reduce this offset or is it more involved than that?

The bias is completely different and I WOULD NOT go messing with it unless you have a clue what your doing. No disrespect but its obvious you don't.

3) Does anybody have a schematic for this amp? (hoping beeger sees this....)

Sorry can't help but I would leave well enough alone until you DO get a schematic and/or SM and then be very clear about what you are doing and adjusting.
 
The 60mV you're measuring is not ideal, but harmless to your speakers, the amp, your cat, and your cholesterol level. If there's an easy way to reduce it, go for it. If not, don't lose too much sleep (but by learning how to reduce it, you might learn something about amps :cool: ).

I started the thread at AK about offset years ago, and it was made a sticky by the mods. Don't ask why it is 100 pages long & 2500 posts. Perhaps just the concept that the non-tech types can check and maybe tweak something besides an IC cable strikes a chord with them. Still, I'd like to think that I saved a few speakers from destruction on amps without protection circuitry. Consider that my contribution to the hobby. ;)
 
Echowars-

Ah so that was your post over on AudioKarma.

OK, so if my speakers are not in any danger (or my cat)
is there an impact to the sound of the amp by having this much offset?

If so I'd like to see it can be addressed, first step is to find a schematic.

Kirk
 
kirk57 said:
is there an impact to the sound of the amp by having this much offset?

Nahh, stop worrying about that. Your speakers do not care about some mV of offset. You can adjust it if you find it interesting, informative or you're outrightly bored.

Have fun, Hannes

PS: after 20 years you could change the caps ;)
 
I've always heard that electrolytics should be replaced if the amp is old, so I did just that on a Knight tube amp.

Recently I bought an ESR meter and checked all the 40-year old caps that I had replaced, and not a single one of them was bad!

Is age itself a sufficient indicator that the cap is bad, or should the ESR meter be trusted?
 
kirk57 said:
Echowars-

Ah so that was your post over on AudioKarma.

OK, so if my speakers are not in any danger (or my cat)
is there an impact to the sound of the amp by having this much offset?
I'd say that it is a matter of whether the offset is due to falling gain from older parts, or because the input pairs are no longer matched (and thus have unequal collector currents).

These small transistors are cheap, and easy to match with the hfe check on a multimeter. When in doubt, I replace them with a new matched pair and go from there.
 
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