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Just reading a post on another site and the bias went up when blah blah but the amp sounded better..
Hmm. Now can i adjust the bias on a solid state amp myself? Increase it to more class "A"?
My amp currently is only slightly warm to the touch while playing.. I could have it be a lot warmer and no problem.
So Just a foolish question.
It is a Bryston, and I would ask over on Audio Circle, but i bet James the moderator over there would rather not open this can of worms...
Also, what sort of tools would i need to make both channels the same? Saying I did find the bias pots??? (and had the guts to try)
Edits: 05/02/12Follow Ups:
One really needs to be careful here. I converted a pair of Threshold S300 (150 WPC Class AB) to a SA/3 (50 WPC Class A), and yes, it does sound (subjectively) better. However, the conversion involved the following:
• Changing the output rails from the power transformer to a lower voltage
• Changing the rail fuses to accommodate the higher current
• Adjusting the bias and DC offset to match the new parameters
In general, the designer of the amp takes all of this into account when the amp is manufactured. So, just bumping the bias up could have a number or unintended consequences as a result (mostly failure from too much heat).
A safer method here may be to simply see if you can borrow a Pass Labs XA amp for a listen. That way, you can hear the effects of high bias first hand, and hear for yourself.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Be careful with this as others have said there are limits to the design. I'm sure you can do it but you may not like the results. Not only does it drive up the heat generation but also the power consumption.
I did this to a couple of pro Hafler amps I had but only after a lot of research and getting the specific instructions and setting limits from the company techs. IIRC they biased them conservatively on purpose. That was several years ago before I went all tubes.
At the time I was using them bridged mono and felt one was a bit off and it did turn out one was measurably off from the other. I also could watch the power consumption increase as I raised the bias up toward the upper limit.
I eventually settled on a happy medium slightly higher than stock but at a power draw I was more comfortable with as they were on 24/7.
They also did run noticeably warmer and needed a couple of fans to keep the circulation going and help with the heat dissipation. I did enjoy the sound more once they were matched but honestly cant say it was because of that or the increased bias that made the difference. Honestly if it didn't need to be done it probably isn't worth the effort
There is only so much heat "dissipatable" into the circuit and its heat sinking to take quiescently and then also dynamically (possibly). Dialing the class AB bias and DC output are two of the most dangerous operations to be done, and probably needs re-biasing and re-DC-output-adjusted after each heat soaking (long re-warm period).
It's iterative if you don't just have the exact specs of current to measure against. And if you're experimenting, you are playing around outside the exact specs known to be safe. Heat sinking isn't just an art, it's a calculated science to it. It's about not frying silicon with too much power dissipation and unknown cooling factors.
If a DIY designer, it's best to have a lot of extra output transistors on hand for this process. (: One is never sure of exact quality of heat sinking contact sometimes.
If you have never done such a change yourself and are posting this kind of a question here, do yourself a favor. DON'T DO IT! Refer it to a good local technician that has a bench and the proper tools to make such a change. Unfortunately few are probably willing to make such a change since the available heat sinking may be insufficient to dissipate the increased in heat. I have a buddy that jacked up the bias of his amp and it sounded quite good after the change. For a while.
2, 3, or 4?
Do not exceed 150°F heatsink temperature.
Measured with an IM analyzer, there is a definite 'sweet spot' for distortion, and it's not all that high. Harmonic distortion does not seem to change much with bias, but IM distortion does.
Proceed with caution, and you can always change it back if need be.
On a Bryston 2B the optimum distortion was at about 50mA, or about 20mV across the emitter resistors. On a 3 or 4 this would be 100mA total, or about 20mV for each device's emitter resistor.
And, you should reset the DC offset at the same time.
Note that a post in response is preferred.
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Getting those old pots exactly to spec is usually a PITFA!
Boy, if you don't know what your are doing or have the proper equipment to do checks... leave SS amps alone. I don't know how many fried 1980-2000's SS amps I've seen, after a so-called "tech" worked on it.
Often, after being "tweaked" by a tech with years of tube amp experience.
This also goes for those tube amps with a lot of SS components in them. Like same vintage CJ or ARC's. Basic output tube biasing and changing tubes is all that an owner should attempt. Don't try tweaking stages, unless you really know what you are doing. Following something posted on the Net is dangerous, at best. IMHO.
I find TNT's advice very dangerous, given the wide range and number of output devices used in sold state amps.
IMO this is a very slippery blanket statement:
"it's very likely that your initial per transistor value is something like 10-20 mA for Japanese units, 40- 60 mA for European units, with an odd unit or two at around 100 mA."
I suggest that anyone that is not fully equipped to measure current, temperature and distortion disregard the whole article.
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