johnny1 said:
Indeed, I stand corrected. I was *thinking* of Hfe by wrote Ft.
But I still maintain my general reasoning.
One issue not mentioned for instance is gain doubling at the bias point because both transistors contribute to the gain, whereas in the class B area only one contributes. This is shown in the gain versus output voltage curve where the gain flattens to half outside the class A area. So, biasing in an area where Hfe is half the value of the higher current region may actually be quite beneficial as it compensates for the gain doubling.
Jan Didden
There is an "optimal" bias for ClassAB EFs (using BJTs), the point where GM-doubling and unlinearity of vbe for low currents cancel out each other. (not 100%, but nearly)
This value is ~24/25mv drop over the emitter resistor. Above and below this bias the distortion rises. Of course pure ClassA has the lowest distortion. But at some point the ClassA operation is left, and the ClassB operation takes over, and these distortions are there again.
This gives ~109ma each pair using 0.22ohm emitter resistors. (or 53ma using 0.47ohm for 25mv, greetings to Hugh !
)
On the other hand, higher bias leaves more time for the ouput devices to open/close. But modern high speed bjts like the MJL0281/0302 are fast enough...
Mike
EDIT: Jan was 1 minute faster...
This value is ~24/25mv drop over the emitter resistor. Above and below this bias the distortion rises. Of course pure ClassA has the lowest distortion. But at some point the ClassA operation is left, and the ClassB operation takes over, and these distortions are there again.
This gives ~109ma each pair using 0.22ohm emitter resistors. (or 53ma using 0.47ohm for 25mv, greetings to Hugh !
)On the other hand, higher bias leaves more time for the ouput devices to open/close. But modern high speed bjts like the MJL0281/0302 are fast enough...
Mike
EDIT: Jan was 1 minute faster...
bepee,
The behringer A-500 is a pro amp without fan.
The designer knows that it will probably work 24h/day, 7days/week etc...
What he does not know is how the customer will use this pro gear.
And pro gear should always work, and never fail.
So, don't say that the design is not optimal.
It is OK for what it is intended to do.
Personally, if the amp was mine, (for home use) i would probably make some tweaks on it.
BUT
You need the schematic, you should know what you are doing, and no one in this site can guarantee that everything will be OK.
So, in other words you are on your own
The behringer A-500 is a pro amp without fan.
The designer knows that it will probably work 24h/day, 7days/week etc...
What he does not know is how the customer will use this pro gear.
And pro gear should always work, and never fail.
So, don't say that the design is not optimal.
It is OK for what it is intended to do.
Personally, if the amp was mine, (for home use) i would probably make some tweaks on it.
BUT
You need the schematic, you should know what you are doing, and no one in this site can guarantee that everything will be OK.
So, in other words you are on your own
MikeB said:There is an "optimal" bias for ClassAB EFs (using BJTs), the point where GM-doubling and unlinearity of vbe for low currents cancel out each other. (not 100%, but nearly)
This value is ~24/25mv drop over the emitter resistor. Above and below this bias the distortion rises. Of course pure ClassA has the lowest distortion.
But at some point the ClassA operation is left, and the ClassB operation takes over, and these distortions are there again.
This gives ~109ma each pair using 0.22ohm emitter resistors. (or 53ma using 0.47ohm for 25mv, greetings to Hugh !
On the other hand, higher bias leaves more time for the ouput devices to open/close.
But modern high speed bjts like the MJL0281/0302 are fast enough...
Mike
EDIT: Jan was 1 minute faster...
Thank you Mr. Mike. Very interesting.
Regards,
beppe
johnny1 said:bepee,
The behringer A-500 is a pro amp without fan.
The designer knows that it will probably work 24h/day, 7days/week etc...
What he does not know is how the customer will use this pro gear.
And pro gear should always work, and never fail.
So, don't say that the design is not optimal.
It is OK for what it is intended to do.
Personally, if the amp was mine, (for home use) i would probably make some tweaks on it.
BUT
You need the schematic, you should know what you are doing, and no one in this site can guarantee that everything will be OK.
So, in other words you are on your own
Thank you very much Mr. Johnny1 for your kind and sensible advice.
All my ramblings start from the feeling that the amp sounds better when warm.
At idle without signal the amp stays cold.
Only after some playing music on it the sounds get better.
In my silly mind, a slight increase in the bias current should give a increase in the amp temperature and consequently a better sound.
I understand perfectly well that the temperature of the amp must be kept under control during this "experiment".
My idea would be to try with a new bias current = 20mA that from what I have got here is a very common value for commercial amps.
Thank you sincerely for preventing me from some "insane" action.
Kind regards,
beppe
Beppe,
Have you tried letting the amp play into a dummy load then connecting your speakers for a listen and seeing if the sound is better? My reasoning is that you think it sounds better when music has been playing a while, is due to your ears getting used to the sound in the 'warm up' period.
I must add that I have experienced this phenomenon of sounding better after a time, but I think changing bias current is not anything to do with it.
Have you tried letting the amp play into a dummy load then connecting your speakers for a listen and seeing if the sound is better? My reasoning is that you think it sounds better when music has been playing a while, is due to your ears getting used to the sound in the 'warm up' period.
I must add that I have experienced this phenomenon of sounding better after a time, but I think changing bias current is not anything to do with it.
richie00boy said:
Thank you for your interesting suggestion.
Actually I have two 5ohm/200W resistors packs to use as dummy loads.
I will try what you suggest as soon as possible.
But let me be a little unscientific.
I have gathered a lot of favourable response about the nice effects consequent to bias current increase that this practice intrigues me more and more.
I understand it could be risky.
Nevertheless if the bias current is extremely low I understand that it is a very uncommon situation.
Regards,
beppe
Basically I'm saying that the bias current and the need to warm up are two totally different things.
The bias current does not usually change drastically when going from cold to warmed up, so I do not believe that you changing bias current is the way to achieve what you seek.
However I do appreciate that in addition to the warm up issue, the bias current could also possibly be adjusted to optimise the sound.
The bias current does not usually change drastically when going from cold to warmed up, so I do not believe that you changing bias current is the way to achieve what you seek.
However I do appreciate that in addition to the warm up issue, the bias current could also possibly be adjusted to optimise the sound.
janneman said:
I'm not saying you are right or wrong, but there is some controversy over this. For example, Self swears by this phenominon, but Leach says it doesn't exist. I don't know who is correct. I tried to engage each in a discussion about it, to no avail. Self said he read Leach's arguments, but said he hadn't analyzed it fully, and would have to study it some more. There may be a difference in semantics clouding the disagreement; but if so, I couldn't get them to resolve it. Anyhow, Leach's arguments are posted here, following Figure 15:
http://users.ece.gatech.edu/~mleach/lowtim/output.html
And here:
http://users.ece.gatech.edu/~mleach/papers/classab.pdf
Don't flame me. I'm just the messenger. I'd like to know the answer to this.
richie00boy said:
I will report of course.
Nevertheless after your question the more I think about it the more I cannot understand why a warm-up amp should sound sound better than cold.
It seems very difficult to explain.
Expecially in considering that the bias current stays the same.
Anyway the answer is in the test.
Thanks and regards,
beppe
johnny1 said:
janneman 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.
Hi All,
The fact remains that we are dealing with BJTs that do not require a high bias point, and that each circuit is different. I have seen amplifiers where the total THD does not change from a couple mA to 20 mA (except there is more supply noise). I have also seen power amps that show a steady improvement as the bias current is increased. So, it depends on the design.
In each case, regardless of bias level, there is normally an improvement in all parameters that may take 30 sec to 1/2 hour to settle in. Remember, this is independant of bias level. This with a distortion analyser hung on the output across an 8 ohm dummy load. I agree with BlackCatSound here too. Measure the differences as your bias is increased in steps. We humans are terrible at judging sound when it's our work we are judging.
-Chris
The fact remains that we are dealing with BJTs that do not require a high bias point, and that each circuit is different. I have seen amplifiers where the total THD does not change from a couple mA to 20 mA (except there is more supply noise). I have also seen power amps that show a steady improvement as the bias current is increased. So, it depends on the design.
In each case, regardless of bias level, there is normally an improvement in all parameters that may take 30 sec to 1/2 hour to settle in. Remember, this is independant of bias level. This with a distortion analyser hung on the output across an 8 ohm dummy load. I agree with BlackCatSound here too. Measure the differences as your bias is increased in steps. We humans are terrible at judging sound when it's our work we are judging.
-Chris
Hi,
Class aB is attempted optimal biasing for low distortion.
Class AB is deliberately designed to operate partly in class A.
At powers where both the above operate in class B for similar circuits
the latter always has more distortion. But the the latter will have less
distortion over the range it remains in class A.
/sreten.
Class aB is attempted optimal biasing for low distortion.
Class AB is deliberately designed to operate partly in class A.
At powers where both the above operate in class B for similar circuits
the latter always has more distortion. But the the latter will have less
distortion over the range it remains in class A.
/sreten.- Status
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