P3A power supply

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Hi.

I'm planning to build a class ab amplifier. I've got my hands on some mj21193/4 transistors and feel that a bridge tiled p3a is the way to go.

For this I was looking for a 800VA 25-0-25 transformer, and that's when I came accross this cheap 800VA 42-0-42 toroidal.

I know that 35V rails are max for the bridged p3a, but is there any way to make it work with the too-high-voltaged-transformer? Is there a trick for splitting the 60V dc in half or anything like that?

My alternatives is to pay twice the price when I buy a transformer elsewhere or to build a different amp. Suggestions on other projects using the transistors I've got and 60V rails are by all means welcome. The design should be simple and easy to build. That's why I wanted to build Elliots amplifier in the first place.

-truls
 
I have a similar problem in that I'm trying to use existing transformers in a pair of powered loudspeakers and replace the amps. A further problem is that it uses 4-ohm speakers. I have tentatively decided to adapt the ESP3a to a single supply arrangement and run it from the +50V supply (ignoring the -50V rail). This means, among other things, adding an output capacitor which for many people here is a mortal sin.
 
change of plans

Hi all.

I just realized another problem with my original plans. My room-mate's fairly heavy self-made speakers use parallell bass-elements. This means that the speakers have 4ohm resistance, even though all elements are 8ohms.

The bridged p3a doesn't like 4ohm speakers, which would be a bit of a problem. Therefore, I'm looking into building the Leach Amp instead. I'm not an experienced amplifier builder, so I'm a bit afraid this will be a more complicated project.

Does anyone have any suggestions/experience/guidelines for building this amp with mj21193/4 devices and +/- 60V supply rails? I'm aiming in the general direction of 150W in 8ohm / 200W in 4ohm.

-truls
 
The reason for building Leach instead of p3a was to make use of the transformer and also having enough power without BTL.

Is it straight forward to build the amp with the transistors and transformer I mentioned? I'd like the rest of the components to be as cheap/easy to get as possible. Like I said, I have very little experience with these things.

-truls
 
I have a 20 year old Leach with three pair of MJ15011/15012. It runs on +/- 63Vdc from a 45-0-45 1KVA transformer and will do 300W per channel two channels driven 20hz~20Khz into 4 ohms. For the last 10 years it has been driving a 2 ohm load with no problems.

If driven real hard into clipping at 2 ohms the foldback current limiter will engage and make the typical flyback or 'popping' sounds.

The 22µF power supply bypass caps across the main filters need to be replaced every 7 years or so, these have very high ripple current. The 100µF front end bypass caps dried up after 15 years or so as did the cap in the feedback loop too. The 20,000µF 75V main filter caps are still going strong.
 
Other transistors

From Leach's Website:

"From a correspondent on 2/8/00 - I built your amp and it sounds great. I read in your FAQ-section, that one person asked you if he can use the European types MPSA06/56. I used the BC550/BC560 types and it sounds great. I also used BD243/BD244 instead of the MJE15030/MJE15031 and it works perfectly. But the Power of my amp is not 240 W, it is 310 W!!! Thank you for your perfect amp!!"

I don't know if those transistors you listed are similar, but they should work if they are.

On a side note...I got to listen to the original Leach amp while testing my boards in Leach's lab last night. I was excited to say the least.

Austin
 
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