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

It depends, but since a preamplifier might not consume much current, around 2200uF could be fine. However, the output won't be at precisely 12V, it will be a bit higher (as you'll see with a multimeter if you then measure it). This shouldn't be an issue for a preamplifier, but you may get better quality if supplied with the voltage it requires, with less ripple too. For that you could then also attach a voltage regulator like L4940D2T12-TR . Its circuit is in its datasheet. It is a surface-mount part but it's large and easy to attach to wires. A better option would be to get a small piece of copper-clad board, and just use a knife to score out some traces. This type of style basically, although it won't be intricate with the voltage regulator mentioned, it will be far easier.

It should work fine as shabaz  indicates. Just remember to put an ac coupling capacitor between the preamp output and power amp input signals.

I was scratching some stuff out, and I thought that half wave rectification would yield only 6v if fed 12ac.  With two diodes, it could be full wave, but still only 6v with some ripple.  The idea of a voltage regulator is good, but I am trying to develop more understanding of electronics, the point of the project.  The pre-amp is a cheap $12 dual tube basically a 'buffer' preamp. the preamp can be fed with a typical wall wort of 12v, suggested 1A capable.  IS there a way I can use the out of phase 2^12v outputs to rectify to 12v DC?

I will try some stuff, thanks for your feedback, I will try to update on the project.

Roger

These amps are made to work together sort of.  Each has its input and output stages set up to work with line stage voltages.  The volume control is on the pre-amp.

Roger

Hi,

You mention it is a 24V CT transformer, and that you wish to rectify it to 12V DC, so it can be wired as shown:

If you actually have access to the amplifier circuit and can identify a 12V DC rail, i.e. after rectification and filtering (through visual inspection and using a multimeter to check) then you could wire that to the pre-amp, and then you don't need the circuit above. If you have access to the 12V rail in the amplifier circuit, then there's no need for any diodes, and no need to try to connect the +12V and -12V rails with diodes (if that is what you were suggesting), because that won't work with DC rails that will be inside the amplifier. Besides, usually the preamplifier will use a very small amount of current*, so it doesn't matter if one rail has a slightly higher load than the other.

If you wish to do the rectification yourself using the circuit shown above (and even if you'd used supply rails inside the amplifier circuit) then it won't be 12V, it will be slightly higher as mentioned, plus it will have some ripple. It may not be an issue for the preamplifier (it depends how good it is), but if it is an issue, then you will need a voltage regulator as mentioned earlier. You could try it without, and if the sound contains hum, then you'll likely at least need the voltage regulator, and probably careful audio wiring.

* In your case, since you now mention it is a tube preamp, it may actually consume more current, you mention 1A which is probably 10 times more than a non-tube pre-amp, so you'll need to ensure the transformer can provide enough power for both the preamp and the amplifier.

AT the risk of teaching my Granny to suck eggs - are you quite sure the power amplifier takes 12V AC ?

I ask beacuse it's a little unusual (but certainly not impossible).

If it does take AC in it may well have  a place where you could tap off the pre-amp supply.

MK

Mike,

it uses the out of phase ac voltage from a center tapped transformer.  It calls for up to 18v*2, up to 100VA, I decided that I would try a lower voltage, it may or may not work on 12v but I wanted this transformer in any case.  

The pre-amp requires 12v DC, suggested that a typical wall wort would work.  I do not suppose they have the greatest DC power in the world.  The pre-amp has two vacuum tubes which require heaters, and a B+ of some higher voltage.  The pre-amp has a voltage multiplier circuit to perform this funtion, but requires more amperage, up to 1A.

I would not expect that the power amp would perform well with the additional current drained from only one rail.

This is  a learning project, so I do not mind tinkering as long as I gain knowledge and experience.

Roger

Thank you Shabaz, you are always so helpful to everyone.

Roger

I understand better now. Are these bought things - if so perhaps you could post a link. I'm always interested in audio novelty of any kind .

MK

these are the components

https://www.amazon.com/gp/product/B08XML9GWC/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

https://www.amazon.com/dp/B08D6CB83J?psc=1&ref=ppx_yo2ov_dt_b_product_details

and this transformer

www.hammfg.com/.../166

I just noticed that the pre-amp requires 12vAC, not DC.  I want to find a way to use the transformer to power both units.  I know they are basically junk, but I am interested in audio DIY and wanted a start.

roger