Increasing bias in amps.

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#61
anatech said:
Hi sam9,
I have seen an amplifier sound a little better after 5 min, and another after 20. In both cases the bias currents were perfectly stable.
I think this has to do more with the front end stages than the output stages.
Then again I've seen more than my share of amplifiers that have horribly unstable bias current and do take a good half hour to become listenable.
The early Bryston 3 and 4 B come to mind here as perfect examples of this.
So don't focus on bias current as the cure all for fidelity.
There are several other parts of an amplifier that impact this.
-Chris
Click to expand...

Thank you Mr. Chris.
So, as you very clearly state, the bias should not be seen as the cure of all problems.
Other things, like for instance an undersized power supply, are much more detrimental to sound.
Taken the "25mV across RE" rule as a reference for setting the bias then it would be better to adress other parts of the unit for upgrading.
I will have the bias checked in that sense.

Thank you very much and kind regards,

beppe
 
#62
beppe61 said:


Taken the "25mV across RE" rule as a reference for setting the bias
beppe
Click to expand...


This is only applicable to a particular output configuration,
if the actual bias current is 10mA it is highly unlikely that this
rule is applicable as its likely the output configuration is some
sort of of CFP stage, which has much lower standing current.

🙂/sreten.
 
#63
sreten said:

This is only applicable to a particular output configuration
if the actual bias current is 10mA it is highly unlikely that this
rule is applicable as its likely the output configuration is some
sort of of CFP stage, which has much lower standing current.
🙂/sreten.
Click to expand...

So to get a schematic is mandatory before proceeding with any action.
I think that it should be very similar to the Alesis RA 300.
Actually they seems twins.

regards,

beppe
 
#64
anatech said:
this has to do more with the front end stages than the output stages.
Click to expand...

Build stuff with separate PS's for front end and driver/output stage.
Switch off the power for the output stage but keep the front end alive.
Adds only a few pennies to the electricity bill, but heat-up time is dramatically reduced. Even on big Class A amps, additionally keeping the output stage alive but switched to a low bias setting and you''ll have 99% performance in a few minutes.
 
#66
This bias business, at least as presented in the TNT article is sympyomatic of a certain tendancy thought in audio (and probably other areas of human endevor as well). Namely, if a little more (or less) of something is helpful in a specific instance still more will will aways be helpful regardless of circumstance.

If one particular manufacturer uses far too little filter capacitance in the power supply then it is reasoned that all amplifiers can be improved by increaseing the size of filter caps without limit.

If one amplifier comes from the factory with bias set too low then it must be true that all amps everywhere will benefit from increasing the bias.

If someone finds speakers hooked up with 50 feet of 28AWG and finds it helpful to replace it with16AWG, then it follows that a 6 foot run of 12GA should be replaced by 6GA

ECT.

At least thi approach is easier than learning and thinking.
 
#68
sreten said:



This is only applicable to a particular output configuration,
if the actual bias current is 10mA it is highly unlikely that this
rule is applicable as its likely the output configuration is some
sort of of CFP stage, which has much lower standing current.

🙂/sreten.
Click to expand...


Doug Self makes a strong case that it IS applicable to ANY (bjt) output config, be it EF or CFP. It has to do with the relation between the voltage across Re and Vbe.

There is of course a difference in Ibias for the different output stage configuration when the Vre is the same. The EF stage will have almost an order of magnitude larger Iq than the CFP for optimal Vre bias. Your quoted 10mA is Dougs recommendation for Re=0.22 ohms (actually, 11.5mA at Vbias=1.297V).

Jan Didden
 
#71
Just to state again an important conclusion:
"An optimum bias setting exists but it is not specific of the device used but both of the device and the type of circuit in which the device is used".
Is it this way ? Have I misunderstood something ?
In my complete ignorance I though it were specific of the device used, indipendently from the circuit.

I would lke to thank all of you Sirs for these very valuable lessons.
I start to learn, I think.
Regards,

beppe
 
#73
AndrewT said:
Hi, I believe that the general rule: that FET output stage benefit more from higher bias in ClassAB than BJT.
Click to expand...

Excuse me, when you say FET output stages you mean power MOSFETs ?
I have read the MOSFETs work well only when used in class A mode.
Anyway even when used highly biased they tend to stabilize their temperature, so they are quite suitable for class A designs, differently for instance from bjts.

Thanks and regards,

beppe
 
#74
Hi Beppe, i think you mixed up something about the mosfets. There are 2 different types of mosfets, the expensive lateral mosfets, these are designed for audio and can be well used in ClassAB and have the self stabilizing bias with temperature, and the vertical mosfets (IRFs for example), these are not so suitable for ClassAB and are not thermally stable, but very cheap.

The "An optimum bias setting exists but it is not specific of the device used" is about the fact that the nearly same optimal bias exists for all bjts.
But, if these bjts are too slow, the crossoverdistortion created by their insufficient speed dominates and needs to be biased higher. I personally think of these devices beeing not adequate for use in ClassAB outputstages.

Mike
 
#75
MikeB said:
Hi Beppe, i think you mixed up something about the mosfets. There are 2 different types of mosfets, the expensive lateral mosfets, these are designed for audio and can be well used in ClassAB and have the self stabilizing bias with temperature, and the vertical mosfets (IRFs for example), these are not so suitable for ClassAB and are not thermally stable, but very cheap.
The "An optimum bias setting exists but it is not specific of the device used" is about the fact that the nearly same optimal bias exists for all bjts.
But, if these bjts are too slow, the crossoverdistortion created by their insufficient speed dominates and needs to be biased higher. I personally think of these devices beeing not adequate for use in ClassAB outputstages.
Mike
Click to expand...

Thank you Mr. Mike.
So the fundamental statement really is Nearly same optimal bias exists for all bjts being that one that gives about 25mV across RE.
I think it is a very fundamental point, isn't it ?
Actually I am mostly interested in power amps with output bjts having my amp two 2SC5200/2SA1943 couples per channel.
Hoping that are not "fake" Toshibas of course.
I have read a lot about fake Sanken (maybe I am silly but I love Sanken bjts).
The REs are 0,22 ohm.
I am sure the actual bias setting on my amp is much lower that this optimal, remaining the amp completely cold even when left on continuously.

Thanks again and regards,

beppe
 
#76
I forgot to mention, this 25mv rule applies only to emitter followers using bjts.
The drawback is, with 2 pairs biased at 110ma you have a total of 220ma. Assuming 48v rails you have idle dissipation of 21watts per channel. The question now is, does the amp sound that much better at the higher bias to justify a total idle dissipation of 42watts ?
Building/designing amps is always a big collection of compromises.

Also, many amps are designed in a way that the amp blows up when the wiper in the bias pot looses contact. (Stupidity, not compromise)
When amp is older, the bias pot gets dirty, guaranteeing that the amp blows up when trying to adjust the bias...

Mike
 
#80
Originally posted by MikeB

I forgot to mention, this 25mv rule applies only to emitter followers using bjts.
If I am not wrong most of the output bjts are arranged this way.
Am I wrong?

The drawback is, with 2 pairs biased at 110ma you have a total of 220ma. Assuming 48v rails you have idle dissipation of 21watts per channel.
I think that this amount of dissipation requires very large heatsink. So I feel it is not applicable in my case without changing the box. The heatsinks' surface is just normal.
So a compromise is needed.

The question now is, does the amp sound that much better at the higher bias to justify a total idle dissipation of 42watts ?
Nice question. Not easy to answer I guess.

Also, many amps are designed in a way that the amp blows up when the wiper in the bias pot looses contact (Stupidity, not compromise).
When amp is older, the bias pot gets dirty, guaranteeing that the amp blows up when trying to adjust the bias...
I did not know. Very very risky.
Now I understand why some audio "restorers" change firstly trim pots with new and better multiturn ones (and the PS filter caps as well).
I noticed in an old amp very bad trimmer on the PCB.
I do not play with bias any more.

Thanks and regards,

beppe
 
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