Beppe, you're right, the emitter follower arrangement is the most common. But there are also:
- Transconductance output
- Quasi complementary (fore sure not the case in your amp) Meeeeow....
- Sziklai (CFP)
- more exotic ones
About the bias pot, as long as it is capsuled, the risk of loosing contact is minimal. If the pot is the cheap open construction, it gets risky. But only some amps have this flaw. Depending on how the vbe multiplier is built, loosing the contact will completely open the multiplier (giving zero bias) or the multiplier completely closes, supplying the full voltage rail as bias. (bang)
Mike
- Transconductance output
- Quasi complementary (fore sure not the case in your amp) Meeeeow....
- Sziklai (CFP)
- more exotic ones
About the bias pot, as long as it is capsuled, the risk of loosing contact is minimal. If the pot is the cheap open construction, it gets risky. But only some amps have this flaw. Depending on how the vbe multiplier is built, loosing the contact will completely open the multiplier (giving zero bias) or the multiplier completely closes, supplying the full voltage rail as bias. (bang)
Mike
Beppe, you're right, the emitter follower arrangement is the most common. But there are also:
- Transconductance output
- Quasi complementary (fore sure not the case in your amp)
- Sziklai (CFP)
- more exotic ones
Thanks for the explanation.
About the bias pot, as long as it is capsuled, the risk of loosing contact is minimal.
If the pot is the cheap open construction, it gets risky.
It seems to me that all these pots so important should be of the incapsulated type (I think like the blue Bourns and similar).
They are a very delicate component in an amp and some care should be used.
The old amp I was referring to has indeed open type pots.
Better not to play with it.
Thank you very much again.
Regards,
beppe
Babowana said:
The statement was: "Amp design is a lot of compromises" (or similar). What do you think, after reading stuff on this forum?
Jan Didden
janneman said:
I have lived, thinking (noy only amp design) that many designs are lot of compromises. I don't mean compromises in quality itself, but in process to get the target quality.
me, time to go to bed . . .aaaah . . . hmm . . .
"No compromise" is just advetising smoke. There is always somthing, if only cost, mass or practicality.
Hi!
beppe 61,
I have seen some internal pics of Behringer A500 and as I can see there is only two supply leads from transformer to each channel (no center winding ground lead), can you comfirm this?
If that is the case, together with Behringers behaviour of using schematics from QSC implies that we may have "flying rail" topology implemented here like QSC uses on several of their amps.
Maybe your schematic is simular to this:
http://www.qsc.com/support/library/schems/Discontinued/USA Series/usa370.pdf
beppe 61,
I have seen some internal pics of Behringer A500 and as I can see there is only two supply leads from transformer to each channel (no center winding ground lead), can you comfirm this?
If that is the case, together with Behringers behaviour of using schematics from QSC implies that we may have "flying rail" topology implemented here like QSC uses on several of their amps.
Maybe your schematic is simular to this:
http://www.qsc.com/support/library/schems/Discontinued/USA Series/usa370.pdf
4fun said:
Hello Mr. 4fun.
I have seen some internal pics of Behringer A500 and as I can see there is only two supply leads from transformer to each channel (no center winding ground lead), can you comfirm this?
Firstly I declare my complete ignorance in electronics.
Then I asked Behringer for a service manual getting a resounding no. I will try through a service lab anyway. But I feel it will be a difficult task.
From what I can see YES. There are only 2 wires coming out of the transformer and going to each PCB/channel.
They are connected to the PCBs with thermally glued connector.
I do not know how to remove the glue for measuring the AC voltage in the secondaries.
So YES. There are no center winding ground lead.
And this was the first thing that surprised me. A very uncommon scheme.
If that is the case, together with Behringers behaviour of using schematics from QSC implies that we may have "flying rail" topology implemented here like QSC uses on several of their amps.
It is highly possible. Thank you very much for your advice.
The filter caps are indeed 3300uF like in the schematic.
It should be also the same of the Alesis RA300.
Very similar aesthetically.
Maybe your schematic is simular to this: http://www.qsc.com/support/library/schems/Discontinued/USA Series/usa370.pdf
Thank you very much again.
I will try to understand if the schematic is correct, maybe with the help of a technician.
I fell this amp has some unexpressed potential.
For now it is hard on bass and harsh in the mid.
Not very pleasant at all.
But the toroidal is large (about 600VA) and the output devices 2 pairs of 15A Toshibas.
I would like to try to add capacitance in the power supply, just to start. But the space on the PCB is minimal (the 3300uF caps have 25 mm diameter and there are a lot of components surrounding them).
Do you have any advice about any mods that could be performed?
Thank you very much indeed.
Kind regards,
beppe
janneman said:
Huh ? "25mV across RE" as a blind rule you show is wrong.
So "a strong case that it IS applicable to ANY (bjt) output config,
be it EF or CFP ", a strong case for what ? What is this "it" ?
🙂/sreten.
Mr Dean,
The main attraction in my family is coming up with an original way to do yourself in.
One of my uncles succeeded in convincing a Samoerai gentleman to cure his headache during the war years in Indonesia.
My very creative kid sister just did drugs and received an A+ by hopping into a coma.
Up till now i've been a blatent disgrace to my heritage.
After a pityfull electrocution attempt i displayed my lack of talent 4 years ago by copying my sister.
Unfortunately my skills are so deplorable that i've only managed to remain a comatose vegetable for a week and a half.
The few historical exceptions i can think of are my grandfather and my great uncle, the latter seems to have been the only classical music composer Belgium ever had.
Gramps definitely tried by smuggling meat during WWII. Thanks to the miscommunication of our occupying neighbours grandpa came out of the war filthy rich and married a baron's daughter.
I regard these two as evolutionary mistakes.
The motorsport routine is a trifle boring, i can think of at least 2 of my family members who took that easy escape route.
Chris drives a FORD truck, can you believe that !
The main attraction in my family is coming up with an original way to do yourself in.
One of my uncles succeeded in convincing a Samoerai gentleman to cure his headache during the war years in Indonesia.
My very creative kid sister just did drugs and received an A+ by hopping into a coma.
Up till now i've been a blatent disgrace to my heritage.
After a pityfull electrocution attempt i displayed my lack of talent 4 years ago by copying my sister.
Unfortunately my skills are so deplorable that i've only managed to remain a comatose vegetable for a week and a half.
The few historical exceptions i can think of are my grandfather and my great uncle, the latter seems to have been the only classical music composer Belgium ever had.
Gramps definitely tried by smuggling meat during WWII. Thanks to the miscommunication of our occupying neighbours grandpa came out of the war filthy rich and married a baron's daughter.
I regard these two as evolutionary mistakes.
The motorsport routine is a trifle boring, i can think of at least 2 of my family members who took that easy escape route.
Chris drives a FORD truck, can you believe that !
Jacco,
Interesting, colorful story - but we would expect no less.
Wrestling with ones intellect, addictions and curiousity are what makes us human, and I wouldn't be dead for quids..... only two days ago on a visit to Peter Brock home turf on my ZRX I briefly thought I too was at the MotoGP - you really know you are alive when you challenge the laws of physics, or convention, or medicine, or pretty much any rule......
Cheers,
Hugh
Interesting, colorful story - but we would expect no less.
Wrestling with ones intellect, addictions and curiousity are what makes us human, and I wouldn't be dead for quids..... only two days ago on a visit to Peter Brock home turf on my ZRX I briefly thought I too was at the MotoGP - you really know you are alive when you challenge the laws of physics, or convention, or medicine, or pretty much any rule......
Cheers,
Hugh
Beppe,
on a side note:
Besides the difference of parallel component number, heatsink capability and bias between class A and class AB output stages there's also a variance in driver stage layout.
Many seem to focus their eyes on the output stage only.
The output stage bias determines the zero crossover point for the output devices, but there's also a crossover point for the driver stage.
At higher output levels not only will the output stage have changed to class B operation, at a lower impedance level the driver stage will also reach the crossover point.
For an example: the output stage of a fully biased KSA is good for full class A operation into 8 Ohms, the driver stage does full class A in 4 Ohms. At higher output levels the output stage reaches class B mode, while the driver stage is still in the class A area.
Mr D.,
A few days ago my g/f and I talked about Stephen Timoshenko after I dropped the father of mechanical engineering's name here. In her opinion hardships are often catalytic converters for brilliance to excell. http://histsoc.stanford.edu/pdfmem/TimoshenkoS.pdf
On the other hand, Q. regretted his utter foolishness on his dying day in Don Q. revisited.
on a side note:
Besides the difference of parallel component number, heatsink capability and bias between class A and class AB output stages there's also a variance in driver stage layout.
Many seem to focus their eyes on the output stage only.
The output stage bias determines the zero crossover point for the output devices, but there's also a crossover point for the driver stage.
At higher output levels not only will the output stage have changed to class B operation, at a lower impedance level the driver stage will also reach the crossover point.
For an example: the output stage of a fully biased KSA is good for full class A operation into 8 Ohms, the driver stage does full class A in 4 Ohms. At higher output levels the output stage reaches class B mode, while the driver stage is still in the class A area.
Mr D.,
A few days ago my g/f and I talked about Stephen Timoshenko after I dropped the father of mechanical engineering's name here. In her opinion hardships are often catalytic converters for brilliance to excell. http://histsoc.stanford.edu/pdfmem/TimoshenkoS.pdf
On the other hand, Q. regretted his utter foolishness on his dying day in Don Q. revisited.
jacco vermeulen said:
I think I understand your point: both the driver and output stages must be biased for minimum distortion.
So it seems very sensible to separate as far as possible the two stages.
I remember an amp where the ouput stage had separate supply and acted as unity gain buffer with the whole voltage gain given by the driver stage powered by an independent power supply.
In that way could it be easier to bias the two stages individually?
It seems to me a smart solution: isn't it ?
Thanks and regards,
beppe
Not exactly, my last post was more than vague.
The switch-off point of an EF driver is at an output voltage that is only a little bit higher than the output voltage at which the output devices go to class B.
Remember that the Vbe multiplier keeps the voltage difference between the bases of the NPN and PNP drivers constant.
If the voltage on the output node raises by 1 Vbe drop the driver stage will also go to class B.
The switch-off point of an EF driver is at an output voltage that is only a little bit higher than the output voltage at which the output devices go to class B.
Remember that the Vbe multiplier keeps the voltage difference between the bases of the NPN and PNP drivers constant.
If the voltage on the output node raises by 1 Vbe drop the driver stage will also go to class B.
Hi Jacco,
Was Timoshenko a civil engineer?
I wish you had read my enquiry on the Leach thread. No one confirmed my suspicion that Leach was wrong, although a few thought he is right. Would you care to comment?
Was Timoshenko a civil engineer?
Something very interesting: http://www.diyaudio.com/forums/showthread.php?postid=1009145#post1009145
Chris (anatech) was adjusting bias in his symasym with a distortion analyzer connected, at 22ma crossoverdistortion was lowest... 😕
I wish i had such a tool.
Mike
Chris (anatech) was adjusting bias in his symasym with a distortion analyzer connected, at 22ma crossoverdistortion was lowest... 😕
I wish i had such a tool.
Mike
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