The Great B1-binge…

I’m obviously a fan of the Pass B1 design but the last couple of weeks have seen the arrival of no less than three more B1s which is a bit much, even by my standards… 😀

Not sure how this really came about, but it must be something like this: A while ago a saw an ad on a forum for a B1 clone board (the original type which is pretty much the same as the original pasty board). It was quite cheap and so I bought it. Since I now use both analog and digital I thought I could build one with two inputs (because my previous one only has a single input).

Shortly after I received this board and had started populating it, I realised I already had a partially-assembled board of another clone design that was basically only missing the input and output caps in order to be ready (yeah I know, I should keep a list or something… 😀 ). Because of the size of the onboard electrolytic caps, the board I had would fit nicely in a 40mm high enclosure, whereas the new board would require a bit more internal height, i.e. an 80mm chassis.

More or less the only thing I don’t like about these B1 boards is the fact that you have to “air-wire” the input switch, which means criss-crossing the inside of the chassis with long wires carrying the input signals. The obvious solution is to use a relay, so I went ahead and made a couple of small adapter boards to accomplish this. With a 24V relay all that is needed to switch the relay is the supply voltage to the B1 so it makes for very simple connections.

The last design was prompted by a reader email asking if I knew of a B1 with more than 2 inputs. The answer was “no”, but I then decided to build a source selector to match my own B1-board. The selector is a simple relay-based type with four inputs but it should work just fine. As I had some PCB mounted RCAs I made a board version for those, but also a more universal one without onboard connectors (not shown).

I’ve tried to put all of these in fairly nice cases, but it’ll still be a bit of a challenge to decide which one to keep as my “personal reference” 🙂

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Cloning a classic…

I have been looking at class AB amp designs for a while, trying to find a “compromise” between my low-power (but very large) class A and high-power (and compact) class D builds. For some reason very few among the class AB designs managed to “stand out” to me with the right combination of simplicity, compactness and reputation. It’s not that I really had anything specific in mind, I just kept looking at stuff and thinking “naah, that’s not what I want” 🙂

However, one design that did keep cropping up was Rod Elliot’s “Project 3A” (or just “P3A” for short). This is a discrete amplifier with a reasonable power level for normal use and a very simple design. There’s plenty of evidence out there to support that the performance is good and even a way threads to evolve the design (search the “solid state” forum on diyaudio.com)

Rod sells PCBs for the P3A and that would of course have been the easy route, but because I had a specific form factor in mind I decided to “roll my own” 🙂 The end result is app. 70mm square (flat mounting on a 75mm heatsink were one of the key design criteria) and quite compact. My only concession over the original is that I removed the input capacitor. Well, I didn’t actually remove it on this version since there is space for a bipolar electrolytic from Muse or Blackgate, but the main version is intended for having the input cap off-board.

As I wanted a matching power supply I “recycled” the last Gainclone PSU I did but added a second capacitor bank (which just fit on a board that is still constrained by me using the free version of Eagle). Since the P3A runs on 35V rails it is possible to use 50V capacitors and then a reasonable capacitance is still feasible with this PSU footprint – especially in a dual-mono setup. Of course the board has space for normal 35mm snap-in caps as well, but that’s so boring 😀

I have only done this test version of the amp so far and confirmed that it works and that it plays music. (This is also the reason for the transistor pins not being trimmed properly – bias adjustment). That said, I do have a couple of case ideas in mind for this one where the lower heat of a class AB amp will be welcome (or should I say “required”) 😀