sreten said:
Hi,
Ft or hfe it doesn't matter, the first post is drivel.
Correct biasing is not related to Iq (Iq is a consequence of biasing and very dependent on the output stage details, not the devices) and there is no optimum bias current for particular BJT devices.
There is no such thing as an output transistor specifically developed
to be used with low Iq, you simply don't understand the problem.
(May be developed specifically for audio output stages......)
/sreten.
there is no optimum bias current for particular BJT devices.
Thank you Sir. This is important for me to understand.
Is the performance of a BJT in some way related to the bias current?
Is there an optimum bias for a complete amp (not the single device)?
I would like to understand better that issue.
I think is very fundamental.
Thank you very much indeed.
Kind regards,
beppe
pooge said:
In this case i'd say Self is correct, i experimentally proved the 24mv in sims, studying the unlinearities in the outputstage... (GM-doubling)
I second sreten, Class-aB has the overall lowest distortion, Class-AB has the lower distortion only at low power levels.
But, this is only valid if the output BJTs do not suffer from crossover distortion at high freqs at optimal bias due to beeing too slow.
Mike
beppe61 said:
Hi,
It does not stay the same. Even if it did the bias conditions will be
different when an amplifier is warm compared to when it is cold.
Accurate aB biasing is about the point the two halfs hand over
output current on the devices Vbe curves, and Vbe is very
temperature dependent , hence accurate biasing requires
knowledge of the output devices temperature, Iq impacts
Vbe via the drop across the emitter resistors value, Ve.
The bias network produces a voltage which sums to 2 x Ve +
an even number x Vbe depending on output stage topology.
Accurate aB biasing is about the point the two halfs hand over
output current on the devices Vbe curves, and Vbe is very
temperature dependent , hence accurate biasing requires
knowledge of the output devices temperature and
compensating for the changes in biasing conditions.
An amplifer will be adjusted for optimum bias when warm. When
cold it will have non-optimum bias. If the temperature compensation
of the amplifer is poor its bias will be further out when cold compared
to an amplifier with good temperature characteristics.
Doubling bias current is not a good idea.
/sreten.Hi,
regarding Self versus Leach :
Self decribes a phenomena he calls "gm doubling", it is very real.
Leach describes a fallacious situation that he says leads to the
concept of "transconductance doubling" which does not exist.
They are not talking about the same thing, Self is talking about overbiasing
whilst Leach is talking about misconceptions of optimum biasing.
They both agree optimum biasing gives the flattest transconductance.
Leach then goes on to ignore the overbiasing case and in
this sense is wrong, because this does illustrate a real case
of "transconductance doubling".
Self has a much better grasp of all the issue involved.
/sreten.
regarding Self versus Leach :
Self decribes a phenomena he calls "gm doubling", it is very real.
Leach describes a fallacious situation that he says leads to the
concept of "transconductance doubling" which does not exist.
They are not talking about the same thing, Self is talking about overbiasing
whilst Leach is talking about misconceptions of optimum biasing.
They both agree optimum biasing gives the flattest transconductance.
Leach then goes on to ignore the overbiasing case and in
this sense is wrong, because this does illustrate a real case
of "transconductance doubling".
Self has a much better grasp of all the issue involved.
/sreten.It should be noted that some high Ft low beta droop BJTs are actually of cell based design, in essence a large number of smaller transistors in parallel, with an emitter resistor integrated in the structure, for each cell (LAPT structure). This is frequently disregarded, even when seen in the datasheets - in form of Vbe ~~ 1.5-2V at maximum Ic, and correspondingly higher saturation voltage. The built-in degeneration should actually be included when exploring optimum bias.
beppe61 said:
As sreten has pointed out it does not stay the same, but it usually does not vary a lot, unlike what I had presumed you would be doing if you fiddled with the bias current.
As well as the bias current, there is also the issue of semiconductors behaving slightly different when hot.
Continue with your planned test
sreten said:
Hi, It does not stay the same.
Even if it did the bias conditions will be different when an amplifier is warm compared to when it is cold.
Accurate aB biasing is about the point the two halfs hand over
output current on the devices Vbe curves, and Vbe is very
temperature dependent , hence accurate biasing requires
knowledge of the output devices temperature, Iq impacts
Vbe via the drop across the emitter resistors value, Ve.
The bias network produces a voltage which sums to 2 x Ve +
an even number x Vbe depending on output stage topology.
Accurate aB biasing is about the point the two halfs hand over
output current on the devices Vbe curves, and Vbe is very
temperature dependent , hence accurate biasing requires
knowledge of the output devices temperature and
compensating for the changes in biasing conditions.
An amplifer will be adjusted for optimum bias when warm.
When cold it will have non-optimum bias.
If the temperature compensation of the amplifer is poor its bias will be further out when cold compared to an amplifier with good temperature characteristics.
Doubling bias current is not a good idea.
Thank you very much Sir for your valuable and extremely thorough explanation.
I have a long way to go before I can understand all these things.
As I understand the full procedure of bias adjusting is time comsuming my feeling is that when the equipment is so cheap like in my case the manufacturer does even not bother with it.
I will try to actually check it as a first action.
Only in the case it were extremely low and outside of what I understand is the quite normal range (i.e. 15-30mA) I will proceed.
Thank you very much and kind regards,
beppe
sreten said:Hi,
regarding Self versus Leach :
...
They both agree optimum biasing gives the flattest transconductance
...
So a optimum bias condition does exist.
This is fundamental for me.
Thank you very much for this information.
However I still wonder if this statement refers to the single device (bjt) or the amp on the whole.
Anyway I think it is a very fundamental statement.
Thank you very very much.
Regards,
beppe
This is about outputstage only. See my posting:
http://www.diyaudio.com/forums/showthread.php?postid=1006066#post1006066
I already gave you the information about "optimal" bias 2 pages ago...
You have to set bias for each stage seperately, depending on topology and targeted power output. These ClassAB informations do not apply to the rest of the amp, none of the devices there should ever leave ClassA in normal operation.
Mike
http://www.diyaudio.com/forums/showthread.php?postid=1006066#post1006066
I already gave you the information about "optimal" bias 2 pages ago...
You have to set bias for each stage seperately, depending on topology and targeted power output. These ClassAB informations do not apply to the rest of the amp, none of the devices there should ever leave ClassA in normal operation.
Mike
MikeB said:
Please excuse me Mr. Mike.
Now I realize better your advice.
Please correct me if I understand badly.
An optimum bias current for all bjts working in Class AB is 109 mA (i.e. 3-4 times the values chosen by manufacturers for the majority of commercial amps).
So the majority of commercial amps having around 15-30mA are misbiased.
Is this correct ?
If so how this "misbiasing" translates in terms of sound?
I have a further question.
I suspect that 100mAs of bias current generate quite a bit of heath and large heatsinks are mandatory.
Unfortunately this is not my case.
I think I will be forced to opt for a lower current anyway, even if this is not the optimum value.
Thank you very much.
Kind regards,
beppe
FYI:
LAPT, Linear Amplification Planar Transistor
i've tried the bias tweaking stuff too with consumer stuff, at the time when MF made models with higher bias setting.
imo, for AB stages you are better off with the 25 to 35mV rule.
If you go higher, go much higher or full class A.
Either, Or, not something inbetween. Unless you change the number of parallel devices and/or are able to change emitter resistor values.
Tweak a 100 watts class AB amplifier, go the Marantz way.(PM78-PM94-PM7200)
LAPT, Linear Amplification Planar Transistor
i've tried the bias tweaking stuff too with consumer stuff, at the time when MF made models with higher bias setting.
imo, for AB stages you are better off with the 25 to 35mV rule.
If you go higher, go much higher or full class A.
Either, Or, not something inbetween. Unless you change the number of parallel devices and/or are able to change emitter resistor values.
Tweak a 100 watts class AB amplifier, go the Marantz way.(PM78-PM94-PM7200)
MikeB said:
All right Mr. Mike.
So I have to act on the bias trimmer in order to get 25mV across RE. I could always try while monitoring carefully the temperature on the heatsinks.
What are LATP transistors ?
Anyway I do not think is my case.
I am attaching a view of the interiors when the REs can be seen.
Thank you very much again.
Kind regards,
beppe
Attachments
Jacco, my fault...
Beppe, regarding the size of the heatsink and the "complexity" of your amp... Don't touch it... Did you see any trimpot for bias ?
Beppe, the REs mentioned by Ilimnz are if the transistor is made of several smaller transistors, in this case these are needed to share current among these.
Mike
Beppe, regarding the size of the heatsink and the "complexity" of your amp... Don't touch it... Did you see any trimpot for bias ?
Beppe, the REs mentioned by Ilimnz are if the transistor is made of several smaller transistors, in this case these are needed to share current among these.
Mike
jacco vermeulen said:
Thank you sincerely Mr. Vermeulen for your kind advice.
1 - thank you for the info. Never seen them.
2 - so another vote for the 25mV value. Do you think that the temperature could increase dangerously?
I will monitor it anyway.
3 - I record the advice.
4 - do you mean that the mentioned models are biased at about 25mV across the REs and so higher than other commercial amps?
Interesting because the PM94 in particular was one of the Marantz most prestigiuos amp if I remember well.
Thank you a lot.
Regards,
beppe
MikeB said:
Actually looking inside I can see 3 pots.
I have to disassemble the amp to expose them.
I am so resolute to try the up-biasing that I could have my amp serviced by a lab I trust completely.
I am pretty sure the present bias is very very low.
I can leave the amp on 24/24 and the heatsink remain completely cold, like the sound actually.
If I had the actual schematic, that is unobtainable, I would try myself.
beppe
If you have a high bias, the amp sounds fine when cold.
In the case of larger amps, they take a long time to get hot anyway.
My Sub amp has 46mV across 0.15 ohm emitter resistors, 306mA when fully warm, about 280mA cold. However, I have large heatsinks and low RPM fan cooling. Heatsinks barely warm to the touch. Because of the bias, bass notes at low volume sound tight and controlled.
Either way sounds fine. I see no problem with increasing the bias on an amp as long as the transistors, and heatsink can handle the heat output of the amp.
I also am a fan of lowering bias on an amp that has it set high, or if the amp has a small heatsink. Some amps just run way too hot at idle IMO.
A hot running Sony amp usually runs much cooler with a small decrease in bias.
Just play with the bias, if the amp sounds good with high bias and doesn't get too hot, then go with it. However, if you don't hear much difference, and/or the amp gets too hot, then use a low bias.
It depends on the amp and the heatsinking.
Have fun
In the case of larger amps, they take a long time to get hot anyway.
My Sub amp has 46mV across 0.15 ohm emitter resistors, 306mA when fully warm, about 280mA cold. However, I have large heatsinks and low RPM fan cooling. Heatsinks barely warm to the touch. Because of the bias, bass notes at low volume sound tight and controlled.
Either way sounds fine. I see no problem with increasing the bias on an amp as long as the transistors, and heatsink can handle the heat output of the amp.
I also am a fan of lowering bias on an amp that has it set high, or if the amp has a small heatsink. Some amps just run way too hot at idle IMO.
A hot running Sony amp usually runs much cooler with a small decrease in bias.
Just play with the bias, if the amp sounds good with high bias and doesn't get too hot, then go with it. However, if you don't hear much difference, and/or the amp gets too hot, then use a low bias.
It depends on the amp and the heatsinking.
Have fun
Two observations:
1- Regarding the observation that an amplifier may sound better after ~30 of warm up. Certainly plausible. The bias was probably set once a stable thermal cvondition was reached, i.e., after ~30 minutes. Presumably this wouldn't occurr if there were perfect thermal tracking or even "good enough". I can't say I've ever heard a change that I would notice by listening although I've seen a fft spectrum improve a little.
2- Regarding the benefits of AB (the partial Class A definition). I'll buy Self's position that the point of A-to-B transition introduces additional distortion. At the same time, in actual use many people listen at moderate levels and to program sources that may keep the amp[lifier in the Class A range nearly all the time. For them, this may be a good configuration. Maybe a 20W true class A amp would be better still. Decades ago a 20W NAD reciever seemed to me to be more than adequate.
1- Regarding the observation that an amplifier may sound better after ~30 of warm up. Certainly plausible. The bias was probably set once a stable thermal cvondition was reached, i.e., after ~30 minutes. Presumably this wouldn't occurr if there were perfect thermal tracking or even "good enough". I can't say I've ever heard a change that I would notice by listening although I've seen a fft spectrum improve a little.
2- Regarding the benefits of AB (the partial Class A definition). I'll buy Self's position that the point of A-to-B transition introduces additional distortion. At the same time, in actual use many people listen at moderate levels and to program sources that may keep the amp[lifier in the Class A range nearly all the time. For them, this may be a good configuration. Maybe a 20W true class A amp would be better still. Decades ago a 20W NAD reciever seemed to me to be more than adequate.
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
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
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