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Old 19th February 2012, 05:44 PM   #21
DIYGEEK is offline DIYGEEK  United States
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Join Date: Jan 2012
(I hope you realize that this hobby is very addictive)

Too late... I've already got bitten by the amp bug.. I'm infected for sure....

After I built the first amp my wife was so amazed that someone could build something like this out of bits and pieces... she immediately exclaimed excitedly "Wow that’s amazing... build another one...!"
This is something she will truly regret saying for sure...

And yes I’m not too happy with my bias adjustment, it was the only 500 ohm model that I could find, good enough for the prototype but I’ll order some sealed ones for the next build.

Good advice on the emitter resistance and matching, this is the kind of stuff I’m looking for.


PS I have TWO 65 volt x 65 volt 2000VA transformers, both have separate dual mono windings for left and right… I desire to work my way up to some very high wattage amps later on… It produces 91 volts after regulation so I’m thinking MKII Ledged Stage Master amps would suit the build, it takes 95 volts in.

I will create another thread when I start the THREE transistors DX Blame builds; some may be interested in duplicating the design should it work well.


Pat
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Old 19th February 2012, 07:06 PM   #22
jaycee is offline jaycee  United Kingdom
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Location: Norwich, UK
Treat your 2SA1943/2SC5200's as if they were TIP2955/TIP3055. Thats pretty much what the fake die is. I'm not sure what parts supplier you used, but if they were selling fake 2SA1943/2SC5200 they don't sound trustworthy. I would recommend Digikey or Mouser in your part of the world.

Your adapted PCB should be fine, but do make sure you use the recommended MJE15032/3 drivers, or some decent ones such as Toshiba 2SA1837/2SC4793. If it's for PA duty, i'd go with 4 pairs.

Alternates for transistors - On semi MJL21193/4, Fairchild FJL4215/FJL4315. Official Toshiba transistors are no longer branded 2SA1943/2SC5200 - they are now TTA1943/TTC5200. The Fairchild transistors are also sold as clones of 2SA1943/2SC5200 and they are good. You may also see "Magnatec" or "Semelab" brand 2SA1943/2SC5200 - these are also OK.
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Old 28th March 2012, 02:30 AM   #23
DIYGEEK is offline DIYGEEK  United States
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Join Date: Jan 2012
Thanks for that great info… I’m certainly taking notes…

Here is a final wrap up to this Blameless amplifier and my results should anyone stumble across this thread…

My Blameless DX amplifier works perfect, it has lots of gain and is super quiet with no music playing, I do not have any hiss or hum from this amp even when cranked wide open.

This amp was just a prototype to see if I could do it and how it would sound. In the future I would use output transistors MJL4281A \ MJL4302A due to their high volt / amp rating and the low beta droop of these devices.


Some interesting facts:

I replaced the output transistors, now it’s running with MN2488 – MP1620 and the drivers are MJE15034G - MJE15035G with the bias transistor being MJE340

It sounds WONDERFUL in this configuration, the base is really tight, mids and hi’s are well controlled.

So if you are wondering “What can I do with MN2488 – MP1620 transistors?
Well you can certainly build a Blameless amplifier out of them.


Wattage Measurements (my first try ever)

31.5V Rail supplies with 20k uF caps
1.2v RMS sine wave at 1Khz input
8.3 ohm 70w dummy load

Amplifier achieved 11.5 volts output before the sine wave became distorted on the oscilloscope. FULL power with distortion was measured at 13 volts (at this point the rail voltage collapsed to 27.6 volts.

W = V2 / R

11.5 volts squared / 8.3 ohms = 15.94 Watts output without distortion.
13.0 volts squared / 8.3 ohms = 20.00 Watts output max peek to peek with distortion.


DOES this look right? Did I do this correctly?

Right now I’m getting ZERO heat from my MN2488 – MP1620 output transistors, my power supply is not enough to push these by any means… But 15 watts of REAL power is quite a bit. It pushes both the book shelf speakers at 16 Ohms to their full max state.

NEXT... I will build one good enough to put in a case and add larger power supply.

Pat
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