I have a small audio amplifier project I am working on.

The pre-amp ciruit looks very much like this:

https://www.circuitlab.com/editor/#?id=9vnehk

The power amp ciruit is on the Amazon link.

I think you can use the same transformer for both power and pre-amp.

The centre tap of the transformer will be a common earth for both ciruits.

It may be  hard to get earths right - try star connecting power amps, pre-amp and trnasformer centre tap.

Don't expect too much - neither pre-amp or power amp circuits will give low distortion.

Expect to need a DMM and a scope to get it working.

MK

I may be peeling the bark of the wrong tree here but isn't clean preamp power a must to reduce noise? I seem to recall preamp pulling very weak signal out of the mud. Any power issues can be amplified at this stage. Should there be some focus on ensuring the power is some what clean for this stage?

This is a bit rambly and you may need to read it more than once.

The key thing is that the pre-amp is designed to work from a 12V AC supply and has its own voltage doubling and regulation.

The pre-amp in question (assuming it's the one I think) doesn't have much gain 4700/200 = 23.5.

The power amp has a gain of 137.

1W into 8R needs 2.82V.

Hum and noise bigger than 50dB below 1W would be bad, (-80dB would be good). 50dB -is x316 so we can tolerate 2.82/316 = 0.009V of noise at the input of the power amplifier.

The pre-amp has no psu noise rejectionat all (which is why all its transistors are devoted to supply regulation).

We could simulate its built in psu (but I really have to some work) , might do it later.

My guess is that it'll be OK - I don't know how noisy the valve will be !

Most of the noise will be mains hum - and that won't be affected by a different transformer.

The bigger problem with a shared transformer might be the amount of unwanted coupling of output to input - and the arrangment of earths will affect that a lot.

The power amp has  again of 137 so ideally we would like any output signal coupling into the pre-amp to be less than 1/1370 of the output signal.

MK

Thanks MK for the response. My electronics is rusty and needs some WD40 like this follow-up post to loosen up. I am surprised at the low gain in the preamp. Maybe my narrow thinking of phonograph preamps is what leads me astray.

In the good old days we had get full power from 3mV input, needed a gain of 6600 on an amp I worked on. First run of them would burst into oscilation if you turned the treble and volume controls right up - boosted the system gain at 20kHz to about 25000 and that was enough. Fixed with a little aluminium screen.

The little valve amp and the power amp here, have a combined gain of 3200 which I think is too high for a line input. The preamp has its volume control on the input which is not ideal.

The pair would be much improved for use with a typical line input like CD if the power amp gain was dropped to 20 and the pre-amp gain to 5. That would give full power  out with about 80mV input. (Volume control at max). The distortion would be a lot lower. It might be hard to tweak the gain that much without adding some frequency compensation caps in the right places.

MK

I have had four different technology careers in the 40 years I worked. The magnetic retention of those earlier memories in my core memory are not as strong as the current memories. I am unable to upgrade the core refresh in the model of brain i have. There appears to be few add-on but they haven't worked that well for me. Blasts of knowledge shares like this I enjoy because for some of it, I actually understand.

the pre-amp is copied from a British amp designed to be just a 'buffer' with tube like sound with a gain of around 1 or so,

the amp calls for 18vAC*2 so giving it only 12vAC*2 will lower the gain, by how much I do not know.

Right now I am trying to find a way to combine the two out of phase 12vAC transformer outputs to power the amp and preamp, likely may not have enough power to do so very loudly.  

I was thinking that I could feed the pre-amp with 12vAC by running the two transformer outputs through an op amp.  The result without feedback would be a 24vAC output?  I could change this to a differential amplifier with a gain of 0.5 to feed my pre-amp.  Like;y there is a more straightforward approach.

Roger

The power supply voltage won't affect the gain of the power amplifier.

There are some suggestions about changing the pre-amp to have a gain of 1 here:

https://theartofsound.net/forum/showthread.php?49724-6J1-Pre-amp-modifications-(hum-removal)

The Amazon page says 12VAC for the pre-amp and I would stick with that at first.

You really don't want to start trying to feed the power through op-amps - they would need their own power supply to work.

This should work but you may get mains hum problems. Idlealy you want the zero volt (or earht) link between pre and power amplifiers from the same points as I've drawn the pre-amp out and the power amp in.

But if you have a shared transformer it connects those points as well but with tracks and wires that also carry supply current. and this can cause hum injection into the signal.

The problem could be avoided by using a second transformer 12V AC for the pre-amplifier with only one earth connection between the two amplifiers where the signals connect.

To work out how to connect the two windings of the power amp transformer.

I've marked the start of each winding with an S and the transformer documents or the part itself may be marked with dots or something. If not then just link the two windings in the most convenient way and measure the voltage between the two unconnected ends - if it s about twice the voltage of each winding alone you guessed right, if it's much less then move the link to the other end of just one of the windings.

Gough Lui, a regular in these parts looked at a similar kit:

https://goughlui.com/2019/03/24/project-diy-more-tube-based-pre-amp-kit-6j1-fever-clone/

He measured the gain of his at 18dB, I calculated the gain of yours as 27dB but Gough didn't publish a schematic so we don't know if resistor values were the same.

Or the assumptions I made in my gain calculation may be wrong.

I've just ordered one from Amazon - should arrive tomorrow - I'll report back.

MK

I have decided to ditch the center tap 12AC *2 transformer.  It will be used somewhere else.  I purchased separate simple single primary and secondary wind 12vAC transformer and a 18vAC*2 center tap transformer.  I gues lesson learned here is to read over the details several times before settling on a strategy.  I see through some correspondence linked to above, that some of these kits have options for different configurations.  I will receive my kit toimorrow for the pre-amp, the power amp is built, without a power supply yet.

Thanks for all the helpful suggestions.  

I discovered the issue with the op-amp solution while looking for some, they are not able to handle much power, and as suggested, need their own power.

I DID have one idea of using the 12vAC*2 center tap transformer.   It was to use two diodes and perform a half wave rectification on each side, but somehow invert one of the outputs to get a combined +- full wave AC signal.  Decided it might be interesting to play witrh in the future, but for now, just get on with getting it up and running.

Regards,

Roger Ayotte