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Old 27th June 2002, 03:44 PM   #21
Duo is offline Duo  Canada
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Geoff Which transistors are you mentioning that have a vceo of 80?


Here's the fixed diagram for everybody to see
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Old 27th June 2002, 03:45 PM   #22
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Yes, the VAS can be grounded at the emitter but NOT at the BASE.

The data sheets on BD139 states 80 V max.

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Old 27th June 2002, 03:50 PM   #23
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Too late for that remark
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Old 27th June 2002, 03:52 PM   #24
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Yeah, I think the data sheets I've been looking at are quite bad...
I really like the ones at onsemi, curves and everything!

Well, now that I knw I had the wrong transistors in there, I'll go an pick some better ones,, Do you have any suggestions on transistors I could pick for the drivers and diff stage and diff stage current sink?
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Old 27th June 2002, 04:05 PM   #25
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actually, I think I'll use MJE340/350 rated at 300Vceo and 1 amp, more than enough to handle all kinds of OT pairs and lot's of room with the 120v pwr supply! Trouble is, I'll still have to find some small pnp's rated at maybe 80 volts but I need about an hfe of 100 for them too

EDIT: Dang just look em up again, only.5 amps, I need to idle
the OT's a .1 amps, which means, .2 amps collector current for the MJE340/350 which will only take .5, no real safe operating range there...
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Old 27th June 2002, 04:26 PM   #26
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I need some suggestions on good transistors to use with low gain, high voltage and moderate current (max Ic should be I guess about 4 or 5 amps for this driver set for such high bias)
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Old 27th June 2002, 08:35 PM   #27
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With R2 at 100 ohms, you will want VR2 at abour 220 ohms,
not a 10K pot. I suggest a 500 ohm pot with the wiper
attached to the + side so that the amp doesn't suffer
catastrophic failure when the wiper gets flakey.

Same with the bias pot. Use a fixed resistor from C to B
and a 5K pot from B to E.

Get rid of d3 and d4 and use some flyback diodes on the
output stage.
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Old 27th June 2002, 09:01 PM   #28
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Mr. Pass,



Thanks, it was by accident that I wrote a 10k pot on the end of the diff amp, I meant 1k, but If you say 500ohms is good, i certainly believe you... By the way, what do you mean by flyback
diodes, I've never heard of them before... Also, I figured out that if I expect 400 watts out of this thing, I'll have to put double the transistors in the OT stage to comensate for temp derating and kick back current from the speaker... I suppose you might have meant flyback diodes as in a way of preventing speaker back-current??? Please tell me more... I'll go and update the schematic with your ideas right now...
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Old 27th June 2002, 10:53 PM   #29
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Quote:
Originally posted by Duo
By the way, what do you mean by flyback
diodes, I've never heard of them before... I suppose you might have meant flyback diodes as in a way of preventing speaker back-current??? Please tell me more...
Think the following:

8 V DC out to the speaker, shut off the current.

What happens? The speaker has 8 ohms resistance, just for calculation

1 A flows despite the fact that the transistors are shut off. You can't turn off current in notime. Compare, you can't charge a capacitor in notime.

Where goes the current?

I can tell you that, through the transistors and they can (or will) brake. If you have diodes across collector-emitter current can go through them for a while. MOSFET's have diodes by nature but they have a little bit higher losses then regular ones but in audio amps it's no big deal.

As others have pointed out BD139 has too low voltage rating. You must have 200 volts at least. I recommend also that you put a res between the emitter and -60 V. This creates a local feedback and you will get a more temperature stable stage and lower distortion. You will also improve the phase margin (less possibilities to oscillation) and get a more predicable gain. Voltage across this res should be at least 0,5-1 V. The same goes for Q1 and Q2. 47-220 ohms are normal values. You won't improve thermal stability (not a problem here) but you lower the dist. You will also get slightly higher noise but that's not a big problem, usually only critical in mic and phono amps.
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Old 28th June 2002, 02:30 AM   #30
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Does anyone know of a good site with lots of transistor specs and curves? Also, as most of you have seen, I have transistors of insufficient Vceo for this amp, can anybody suggest anything that'll work in here?
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