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Old 14th January 2005, 07:58 AM   #1
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Question optimizing the VBE multiplier



hello guys,

do you know of some ways to optimize the the VBE multiplier.
Does this thing affect sonic quality if not optimized, I mean
aside from thermal tracking?

what transistor parameters are involve or needed when designing an amp with VBE multipliers, becuase I think not all BJT's
are suitable for VBE multipliers.


I've got this amp here (pls. see the doc attachment).
Before finally arriving to that device (as VBE multiplier), BC546,
I've tried TIP41, although current and voltage tests were done.
The output was not that good, bass reproduction was sagging,
then Ive tried the D669, there was a little improvement,
until finally I arrived with BC546,
everything in the audio spectrum
was not compromised, bass was tight, the mids and highs were clear.

does these things affect:?

a) hfe
b) Ft

Or maybe the components around it.?

the real question here is, what if were building an amp with the different but the only available devices (for the VBE multiplier) around?


regards,


hienrich





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Old 14th January 2005, 08:11 AM   #2
Jennice is offline Jennice  Denmark
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As I see it, the quality of a Vbe stage depends on the current through the stage and the gain of the used transistor.
To make the gain less important during transients, I use a capacitor in parallel with the Vbe stage. (collector-emitter).

The current through the Vbe multiplier is supplied (often) from the voltage gain stage. This also needs current enough to effectively drive the output stage(s).

The higher the gain of the Vbe transistor, the less current is needed to supply the base, which is needed to actually create the Vbe multiplying function.

Actually, with high value resistors, you could be in a situation where your Vbe-resistors do not provide a normal voltage divider, but a current source for the transistor. If you reach this point, you could be in trouble, as this current varies with the load (current) for the output stage.

For a given base current resistor set, the output impedance of the Vbe configuration (it's tendency to vary with current through it) is depending on the beta (gain) of the transistor.
I use BD139 (or is it the 140 which is NPN - can't remember) as Vbe stage, as it has reasonable gain, and has a hole for easy mounting on the heat sink.

Jennice
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Old 14th January 2005, 09:31 AM   #3
Cortez is offline Cortez  Hungary
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hienrich !
Try to put a foil cap (1uF) paralell the 47uF, like Jennice told it.
I'am curious about the changes in the quality !

And what about series diodes as solution ?
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Old 14th January 2005, 10:10 AM   #4
Jennice is offline Jennice  Denmark
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Heinrich,

What is the current through the Vbe transistor and the voltage gain transistors?

Jennice
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Old 14th January 2005, 11:30 AM   #5
Mr Evil is offline Mr Evil  United Kingdom
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Using a Sziklai or Darlington pair instead of a single transistor decreases the Vbe multiplier's output impedance, reducing the variation in bias with current through it.
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Old 14th January 2005, 11:43 AM   #6
AndrewT is online now AndrewT  Scotland
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Hi,
Mr evil, what effect (better or worse) does the shiklai or darlington Vbe have on the sound quality?
Regards Andrew T.
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Old 14th January 2005, 11:51 AM   #7
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Talking hi sorry for the delay

hi guys,


Jennice: I think you've got the point, I was also thinking about this
after finalizing my amp.

as you could see in my schemes I'm using the BC546 which is the highest in gain and in Ft, (compared to TIP41 and D669) actually
there is an 11ma current across the voltage amp stage.

So far I didn't mount the VBE multiplier on the heatsinks! which it should be, but I'm using forced cooling fans against the heatsinks, which make me confident.

though without foil caps across (emmiter to collector),I just used
47uf electrolytic, and with the chosen VBE multiplier BC546 my best choice, the sonic output of this beast is very punchy and involving.
bass was very profound and dynamic even compared to the LEACH AMP which I've got one.

Cortez: if we are just near from each other I would enjoy listening
music with this newly built amp with you!!!!


hey guys you might have some great sounding amps with you.
may you share their schemes.

I hope I could also post some pictures



regards,


hienrich
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Old 14th January 2005, 12:20 PM   #8
AndrewT is online now AndrewT  Scotland
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Hi,
Hienrich the Vbe multi only monitors the output temperature in the darlington topology.
With shiklai, the Vbe must monitor the driver temp or predriver if using 3 deep cascade.
regards Andrew T.
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Old 14th January 2005, 12:47 PM   #9
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Andrew, Mr Evil was on about the actual Vbe multiplier being a Darlington or Sziklai. Your comment is correct if the output stage is Sziklai.
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Old 14th January 2005, 12:48 PM   #10
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Default I should be





andrewT,


hi, youre correct! but I'm using it the other way, a real contrast
to the "should be".

becuase I'm using fans for cooling the heatsinks, and I still have to redesign my PCB in order for the VBE multiplier to be closer and directly mounted to the heatsinks and the same time on the PCB.

regards,

hienrich

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