Dead on Arrival?

Isn’t that what “DOA” normally means? Fortunately in this case nothing has died (yet!) and “DOA” instead stands for “Discrete OpAmp” 🙂

The background is that I have noticed that quite a few clones of various Bryston amps have started cropping up over the last year or two – not least the excellent BP26 clone I have built myself. On one of my PCB-searches I was looking into these clones and found a small dedicated headphone amp that looked interesting. However, looking at the PCB layout I honestly wasn’t particularly impressed and a small “I can do this better” thought started forming at the back of my mind…

Some more digging revealed that Bryston is one of the (few) companies that publish schematics of their older models which would explain the surge of clones – these schematics are obviously a great resource. I started looking through the schematics and found that a lot of these products are build around the same “DOA33” discrete opamp building block. That’s obviously interesting since Bryston’s commercial products tend to be well-regarded and that gave me the idea to do a version myself, but with the modular “opamp” concept intact. As a replacement for an integrated opamp this is probably a stupid idea, but as a way of building compact and modular discrete amplifiers it’s not so bad I guess.

I started doing some layout work and eventually ended up with an all through hole design of around 45 mm squared that I was quite happy with. As a “mini-evolution” of that design there I also did an SMD version (with SMD passives) which I still need to test. The idea being that it can be smaller (I could have done more here), that SMD passives can actually be had better/more accuarate (and for less money) and that it would be possible to get the SMD pasives mounted professionally and relatively cheaply by one of the PCB companies that also offer assembly service. Then it would be possible to have a drawer full of “building blocks” and then simply mount the leaded parts (mainly matched transistors) when needed. I didn’t get to that just yet, but I might explore the idea again later.

To accompany the DOAs, a series of “baseboards” would be required to provide space for the external components required. The baseboard shown here (and the only one I have done so far) is the most simple circuit to build a basic preamp, i.e. similar to a CMoy-style amp with an integrated opamp, and is probably most accurately described as a “test bed”. to confirm that the opamp circuit works. The baseboard just includes the various gain setting resistors, some PSU caps and an output capacitor to get rid of any DC on the output.

What I am looking into next is one of the more “advanced” circuit variations that Bryston describe, namely a headamp (with a DC-servo to get rid of the impractically large output cap for headphone impedances), a RIAA (with two modules per channel) and also a specific preamp with the volume pot isolated between two gain/buffer blocks (similar to Bryston’s own designs) etc.

I may end up more or less “full circle” back to the BP-26 design I originally came from, but then at least I will have had some fun along the way 😀

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