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rtill 18th January 2009 10:48 PM

elliot sound project 27b question
 
In the power amp section, I would like to use a 30-0-30 3 amp transformer. Is this possible or is it limited to the 25-0-25 5 amp noted?
Also, would one recommend heatsinks on the bd139/140's?

Thanks
http://sound.westhost.com/project27b.htm

wakibaki 18th January 2009 11:35 PM

The short answer is no, the voltage is too high and the current too low.

The simplest thing is to follow Rod Elliott's recommendations. You will have enough problems just getting that right without adding to them.

The cost and effort of building an amplifier mean that there's little point in cutting corners.

w

lineup 19th January 2009 01:05 AM

2 Attachment(s)
30-0-30 VAC transformer with generate somthing like
30 x 1.4 or even maybe 30 x 1.5 = 42-45 Volt
2 x 42-45V = 84-90 Volt

For BD139/140 this is the critical voltage, MAX 80 Volt. They may survive, they may not.
Different exemplars of any transistor can take a bit more than absolute max rating.
Others will be destroyed.

Trafo 25-0-25 will generate 2x35-2x38 Volt, and this is within safety margins for BD139/140.
I would advice to not experiment, but use the recommended 2x24 or 2x25 VAC trafos.

There will be a couple of Watts in BD139/140, maybe 1-3 Watt, when Higher output levels.
I should cut from some 1mm thick aliuminium pieces.
Size ~ 20-25x40-50 mm (like 1" x 2") and screw them onto those BD139/140.
I usually bend the wings of those plates to make the whole thing not to wide to fit.
This will be enough to keep them TO-126 from getting too hot.

Not at all as big as in attachment. Those are for bigger TO-220 Output transistors.
But I made them out of 1 mm Alumiumsheet.

ostripper 19th January 2009 02:02 AM

2 Attachment(s)
Quote:

By Rtill - In the power amp section, I would like to use a 30-0-30 3 amp transformer. Is this possible or is it limited to the 25-0-25 5 amp noted?
What a simple, stable amp. For the same price , just use better
trannies (see attached schema) this one is the 27B with
45-0-45v operation and could run nicely with over 50v.
Dont expect it to be a Hi-fi amp , but it will do a guitar real
good. Inputs (Q1/2) run at 1 and 3mA, Q3 runs at 10mA (small
radioshack to-220 heatsink), Q4/5 also run at 10mA (I would
also put these on separate small to-220 heatsinks, contrary
to rod's recommendation).

To really make a durable amp the mj15003/4 would be
the best for outputs.
OS

wakibaki 19th January 2009 08:25 AM

Quote:

Originally posted by ostripper
this one is the 27B with 45-0-45v operation and could run nicely with over 50v.

You still can't use the 3A transformer.

w

AndrewT 19th January 2009 11:42 AM

Hi,
change the VAS from 340 to a low Cob transistor.
Add an emitter resistor to the VAS. In ESP schematic Re~30r
In Os schematic use ~56r for Re.
Change input DC blocking cap (C1) to 470nF.

Os,
do you really want 1r0 for output emitter resistors?
Would you recommend input emitter resistors? Maybe 100r to 330r?

AndrewT 19th January 2009 11:46 AM

25-0,25-0Vac 3Aac will power upto 100W into 4r0.
If you go to 30Vac then use ~4Aac.
For 35Vac use ~4.3Aac.

ostripper 19th January 2009 01:27 PM

Quote:

change the VAS from 340 to a low Cob transistor.
This is not a hi-fi amp, just a crude "wire with gain".
Look at Cdom , it is 220p, that will "swamp" any amount of Cob
regardless of device (even use a mje15032,won't need a HS).

This amp was designed to be a basic instrument amp with
ultra low parts count ,you could mount the diodes on one
of the drivers for more thermal stability (CFP) in
simulation it seems a little underbiased (50mA)but I think that was intended seeing it's purpose.

It also triangulates a sqaure wave at 15khz ,which backs
my opinion of it..not a Hi-fi amp.

A 200R trim in series with the diodes would give a nice touch
to the amp. As far as degenerating the input diff. this amp
does not offer much OLG (50DB) , so I think rod intended for
that as well.

OS
Quote:

You still can't use the 3A transformer.
yes ,you could, but you would run out of current ,amp
would clip like hell (good for guitar).

wakibaki 19th January 2009 07:03 PM

Got all that?

You live in Wisconsin fer chrissake, you can get any transformer you need.

Cut yourself some slack.

w

rtill 20th January 2009 02:54 PM

for the original design, would this be the correct transformer to use?

167P50

If I am correct, since it is using 5 amp fuses in the power supply section, it would use a 5 amp transformer. It isn't really specified in the project.
http://www.hammondmfg.com/pdf/5c0020-21.pdf


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