My simple class a approach

Most who look at the schematic will see right away that this arrangement has been used with tubes for quite some time.It is very simple with no global nfb, it uses only local feedback in the first voltage gain stage then feeds a phasesplitter for the push pull output. Will build it soon and pass on the word. Under sim i get a nice 10 watts into 8 ohm with a sensitivity of 3volts so a preamp is a must for full output.

Yes about 2 amps idle current so of course I will put a couple in parrallel for the output when I build. This isnt any larger of a bias than any other class a I've seen in the same power range.

I couldn't agree more with you on the JLH amps as I have built a stereo pair and love them. It is true that the output isnt exactly push pull but rather more like single ended with a varying ccs load. Resistor values for the phase splitter outputs will have to be determined by measured transistor gain to equalize the current at each emitter of the output trans.

Thanks so much for kind words Destroyer I always enjoy reading your posts. As for my personal info I'm mainly just a music junky that has toyed with stereos most his life. The only background I have in electronics was a rather poor 2 year course in electronics repair at the local vocational school. I find building simple circuits and testing and toying with them great fun, who cares if those cheap ratshack transistors go to pot.

As of about 30 mins ago I started listening to this amp (yes it does work,better than i could have thought) and it makes some nice music, very compareable to my JLH's. The treble is very "crisp" so to speak and the mids are smooth no glaring and the bass sounds tight not muddy at all.


ps. I did however dig up a 30v supply to use with it as the argument against the input trans is very true and didnt want it to self destruct before I had the chance to listen. The only resistor values changed were the emitter resistor on the input trans to 220 and of course the ones at the outputs of the phase splitter both at 1k.

The only real dismay I find is the very poor ripple rejection but I am only using a very basic supply using +/- 30v center tapped trans into a large flat pack rectifier with 2 ~ in and a center + only out and a single 22000uF 63v cap. I will have to make a decent supply for it when I get more time.

After some time listening it still sounds pleasing without harsness but the hum is getting to me will have to futher evalute when a much quiter supply is made.

After making a simple choke filtered supply and getting rid of the hum, I hooked this amp to my main listening speakers. You can believe me or not but the sound is something different than what I had expected. Having always heard people say that they could hear the air between the recording mikes, I had thought it was just bs that people talked. I heard what they were talking about for the first time last night. This amplifier has something special, I had not heard a ride symbol hiss for 2 measures after it was struck. The JLH's were my reference to good sounding amps but this circuit has something more, the treble is amazing and the difference between loud and soft sounds are huge.

Getting the bias center is a bit tricky though. Refering to the schematic I posted, it is a balancing game between r6 and r2 with r4 being kept constant. When the current is increased through the output stage, current must also be increased through the voltage stage to balance the output voltage back to half supply. Also the amount of power dissapated by the phase splitter trans and input trans becomes much higher with higher output current. I changed Q1 to a tip42a and Q3 to a tip31a as the heat on these 2 were a bit much when i got the output to settle at 1.6 amps at half supply voltage.

For those that love high effiency speakers ran on small class a's this circuit may have something that you will enjoy.

For those out there much more knowledgeable than me, I would love some tips on how make a suitible bias arrangement for this circuit.

Just wanted to post up the schematic at the values I am using and listening to. tschrama I'm not sure exactly what you mean but if you are saying as a way to balance the bias it will not work that way. Q2 of this schematic is the varying ccs load and supplys a larger amount of ac current than Q4, actually with some current gain over the idle current.

Thanks for the links lineup some interesting reading there. As for the circuit not working very well I would have to disagree. I've been listening to it for 3 days now and it works quite well. Sound quality rivals the JLH, the JLH has a little "fatter" bottom end in the bass region but this circuit surpasses it in the treble department. Dont get me wrong it has very tight good sounding bass all the way down. This circuit also will reproduce almost perfect square waves on my oscope. The major issue is balancing the bias because of the dependency on trans beta, I spent much time measuring and changing values and remeasuring.

I now have a second circuit up and running. Using the same values of the latest schematic I posted it started up and is playing perfect with 1.7 amps idle current. I did not have a pair of outputs that were very closely matched but it doesnt seem to have an audible effect.

For any of you who builld and play with this circuit dont be disappointed if it is very low volume, a preamp able to swing 3v into 10k load is a minimum to use. This circuit has intentionally a very low gain.
Although I'm not real knowledable I do understand how this circuit works. Refer to the disscussion on the Class-A website discussing the output stage for the JLH, single ended or push pull. That is where I derived this circuit, only instead of a vas driving the output mine is working in current mode. The top output trans and the phase splitter form a complimentary pair for current gain while the lower output trans is the current sink.