An integrated amplifier

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Greetings everyone,

I'd like to start off my first post here with a few pictures of something you don't see too often: an integrated amplifier. I built this about 10 years ago to move with me across the big pond. Since I didn't want to carry two heavy boxes, and didn't know how much space I'd have, I decided to make it compact (well, sort of).

In this first picture you see the preamp section on top, the power amps left and right at the heat sinks, and the transformers in the middle. Hope this is enjoyable and provides some inspiration for someone. I'll be posting more pictures and details in this thread shortly.


- B


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I'm back, talking to myself. This picture makes the front look worse than it is in reality, but here it is anyway. I got a solid mahagony board from a furniture maker and glued the copper strip on it. I could not figure out a good way to get letters or labels on it, so I convinced myself that no labels are so much more classy ;-). The knobs are: input selector (drives relays on back), tone control bypass switch, tone controls and balance. Yes, I know that the last three are not really ideal, but given my target application (unknown) I opted to have them anyway. The big knob is of course the volume, with a decent Alps pot behind it.

More to follow ...


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One more before everyone starts with their suggestions ... :)

This is the rear. All connectors are gold plated for looks and maybe even quality. I tried to keep the preamp and power stage completely separate inside, and use an external cable to connect the pre-out to the power amp input. The idea was that this would allow me to use this thing with an external pre or power amp, but I have never actually done this.

You can also see the highly sophisticated ;) but useless shielding I added around the transformer section in the center.

- B


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Sorry, no shcematics, but another picture

Schematics - I wish I could ... don't mean to cause any undue stress, but most of it is based on circuits published in Elektor, thus copyrighted and I can't really post them here.

The preamp is a fairly standard op-amp based affair: relays select the input, unity-gain buffer drives tape-out and tone control or volume pot (if tone control is bypassed). Tone control is based on an Elektor circuit that I had used before and liked. It has the most effect at the lower and upper end of the frequency range and does not affect the mids too much, compared to some other circuits.

The output buffer is a plain non-inverting opamp. The head phone amp also uses opamps with discrete BD139/140 transistors to higher current capabilities - but I never use it.

The power amp is the 'Medium Power AMP' from Elektor, a fairly straight-forward bipolar design using three parallel BD911/912 transistors. As far as I remember, it has two voltage amplifier stages built around matched MAT02/03 pairs (I used SSM2210/2220 for cost reasons).

Any more questions, just ask, I'll try to describe what I can. To make up for the lack of schematics, here is another view of the preamplifier section, with the trwo channels stacked on top of each other. The power supply is on the third board in the bottom, while the regulators are on the preamp boards. I layed everything out by hand on paper (remember, this was 10 years ago) and etched the boards in the kitchen.

- B

Edit: spelling, added subject


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Joined 2003

'Medium Power AMP' from Elektor... Can U describe the sound or make some parallel betwen some "well known" designs and this amp ;)l

I have some SSM matched pairs and Elektor schematic and PCB (on paper only)... Need to know how this thing sings :smash:


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