5-watt Class A push pull SS amp

coolnose

Member
2009-09-12 9:14 am
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You might want to find a substitude for the VAS transistors nowadays, works fine as per schematics since 2009 for me...

More interesting designs from the designer here:

2A & DIY

EDIT: The CCS jfet is obsolete as well, but hey, it's only a CCS
 

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Post #2 has 3 unicorns you can't buy in the US from a distributor. 2sk170, 2sa162, 2sk1058.
There was a guy in California that would sell you 3 ea 2sa162 2sk1058, 6 transistor for $100, but I didn't write it down.
Push pull is class AB. Post #2 is class AB. 25 W AX6 can be done with MPSA56 input , MJE340 VAS, MJE15028/29 drivers, & BD243 outputs. $8 in transistors. All from Newark. If you drop the rail voltage from 50 (36 v transformer) to 33 (24 v transformer) you can get down to 10 or 15 watts. You can get 24 vac 1 amp transformers at the home store, they are called a doorbell transformer. Link of APEX 25 W Retro amp is in the last posts of this thread: Retro Amp 50W Single Supply - Page 22 - diyAudio
Which I'm linking you to the middle of. I used heatsinks on VAS drivers in addition to the one on the output transistors, and can get 70 W out of "50W" AX6 for 5 seconds at a time at 70 v rail voltage. Don't forget to put heatsinks & compound on the output transistors.
Post #2 is split supply, which is fine as long as your soldering is perfect. Mine is not. Peavey's is not, I've bought 2 amps with bad soldering joints from the 1993 factory. If the wrong joint pops loose, a split supply amp with center tap connection will burn your speaker or rip the cone out of the suspension with DC voltage. AX6 is a speaker capacitor amp, where a $3 capacitor protects your speaker from DC voltage. Those capacitors are pretty reliable for 5 or 10 years, especially if you buy the 3000 hour service life grade, or better.
 
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I am about to make a 39w version of this amp called the Alpha Nirvana. Some quick LTspice sims shows that replacing the outputs with IRF610 and IRF9610 and dropping PSU to +/-15v and bias current to 500mA should get 5w into 8ohms. There are no unobtanium or even expensive parts. You probably have these in your bench drawers already. Although technically a SE Class A output, there is a reactive CCS that will make it behave like a push pull.

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I can get you new resistor settings if interested. Would make a fine headphone amp for power hungry HE-6 cans too.
 

maton00

Member
2012-02-20 8:25 pm
MÉXICO
10WMT.gif


You might want to find a substitude for the VAS transistors nowadays, works fine as per schematics since 2009 for me...

More interesting designs from the designer here:

2A & DIY

EDIT: The CCS jfet is obsolete as well, but hey, it's only a CCS

Thats a good practice amp, I had constructed one with J113, MJE340/350 and for outputs BUZ90X with no problems, however you can't get the same gain without the risk of oscilation with that kind of FET (J113) you will need to reduce the gain experimentally. The J113 is nowhere as good as the 2SK30/170
 
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SA and SK can easily be replaced by Exicons which are readily available, .
The only "exicon" that newark, digikey, mouser stock in US are terminal block, wire, & cable.
Buy US stock. I ordered TI CMOS in stock in UK 10/27, have been between farnell UK ship dept 10/28 and newark US receiving dept today 11/14. So who got them, a farnell employee or customs? I saw what the son of a US customs agent was driving & wearing in 1974, I'll bet on customs.
 

coolnose

Member
2009-09-12 9:14 am
That's where I did buy mine, absolutely reliable seller!

" Post #2 is class AB"

It's a 2x15V secondary biased at 1A, I wouldn't consider that AB...

The current soure shouldn't be too much of trouble and some members have found satisfying substitutes for the 2SK170 with modern available parts, agreed with the need of parrelleling 2 of them... but they are 40 cts parts...

That said, I am not selling that amp in any way, but the OP asked for a 5W PP amp which to me is very specific as a request, the amp I showed is given for 8W, suggested speaker are from 96dB on IIRC, I guess the OP is having something similar from his request.
 
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