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Influence of DC offset on bias

Hi everyone. I'm usually an observer here and hopefully learn something along the way. Can someone shed some light on my question of what if any influence or affect dc offset has on the bias of an amp. I know how to check both and have never come across an amp with identical offset on both sides; well within acceptable levels on both sides, but never the same. Also impossible to adjust them to the same level. Does this render identical bias on both sides incorrect in terms of channel balance as it affects your ears? I would greatly appreciate some insight into this.

Thank you
csontos
marakanetz's avatar
marakanetz
5,473 posts
 
Are you sure that you can play around with bias adjustment in SS amp?
In most of the SS cases I would strongly recommend not to adjust.
csontos OP
1,311 posts
 
The pots are there for that very purpose. Channel balance depends on quiescent current being equal in both channels. But my question addresses exactly that issue. Must it be equal or adjusted relative to offset for each channel in order to achieve an equal end result at the speakers?
hifihvn
943 posts
 
I rescind my post. It's in error for the purpose of this thread.
marakanetz's avatar
marakanetz
5,473 posts
 
Hifihvn,
In transistors it's the same story only make plate equivalent to collector(assuming connection with common emitter). There's a purpose to adjust the offset so the output is measured the same across collector and ground on both opposite junction transistors pnp,npn pair (I assume that it's a class B amplifier, but I might be wrong) and another channel accordingly. So the offset might be different in transistors (they're way harder to match than tubes and have larger parameter tolerance) but bias current will be the same and 'sound' the same.
Can you tell what kind of amplifier we're talking about?
Brand? Class of operation? Must be a very vintage one. Most of the current ones would have an auto-bias which is a piece of cake to implement.
csontos OP
1,311 posts
 
The amps in question are a pair of LSR&D Superamp monos with no offset pots, AmpzillaII which is a dcservo amp., Son of Ampzilla with both offset and bias pots, and some Bedinis also with no offset pots, a pair of early Boothroyd Stuart Meridian 105 monos with no offset adjustment. I recently sold a Meridian 559 with auto bias because IMO, it had no high-end characteristics at all. The 105's were fabulous and thought it would be more of the same but it sounded very average with no real redeeming qualities; not particularly fast, no great extension or linearity, imaging, sound stage etc.. I wondered if it was because it is not a fixed bias amp? When offset and/or bias is off, these are exactly the characteristics an amp displays. I guess my question is how much tolerance is engineered into auto bias?
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