As previewed in an earlier post, this is a preamplifier designed by Shaan and offered for sale in a group buy at DIY Audio. It is fully discrete, Class A with an integrated power supply. The twist is that it offers adjustable (from less than 0.001% to over 1%) single-ended, predominantly second harmonic distortion. In this build, the harmonic signature is adjusted in ‘real time’ by a front-panel mounted potentiometer. The rationale is that, much like the H2 by Nelson Pass, you can choose the distortion level – ranging from neutral to a tube-like ‘warm’ sound. Shaan suggests that the different settings will work better for different music types – a lower setting for ‘busier’ music and a higher setting for more minimal music. Much more about the circuit can be gained by reading the first post in the group buy.
Although there is a relatively high component count for this board, there is nothing particularly exotic in the parts selection, with commonly found bipolar transistors (BC546/556B and BD139/140) used. The onboard power supply element is a CRC + regulated capacitance multiplier type and I used a 15V 30VA encapsulated toroidal transformer. If you’re unsure of how to connect transformers, follow this link to a really useful site. While you’re there, take the time to have a look at his LM3886 project, probably one of the best documented builds I’ve seen!
Once the components are soldered, the board has an initial set up. This is done in two stages, first adjusting the rail voltages to be symmetrical and then setting the bias to 20mA. I thought I had a problem with my build as I couldn’t get a voltage reading when I was trimming for bias, it turned out the trimpots needed a lot of turns to get a reading, upwards of 30 in my case. As ever, Shaan was extremely patient and helpful in helping me with this.
The circuit uses two potentiometers, one for volume, the other, labelled signal, is for varying the amount of harmonic distortion. I usually use an Alps RK27 for volume, but as I went for some generic 16mm 50K pots this time (log and linear tapers for volume and signal respectively), I modified the board designed by slowdiyer to accept these smaller versions.
If you don’t have access to CNC or drill presses and you don’t want the trouble, or more to the point, the expense of having your own panels machined, there are a few options, each coming with their own pros and cons.
Buying an enclosure with a pre-machined front panel.
These are particularly helpful when you have relatively thick front panels and you want to use potentiometers which don’t have a long thread, or short headphone jacks. There are a couple of downsides; you are restricted in the placement of things like pots and switches, and they often have text on them that may be inappropriate for your use (or even spelled incorrectly).
Enclosures with no machining.
Often (but not always) the cheapest option, these allow for more flexible layouts. However, if you opt for the thick front panels, it can be difficult finding suitable components to allow fixing to the panel, mounted potentiometers are one example. You also don’t get the nice recess for your control knob. Drilling large diameter holes for illuminated switches can be a challenge, with cutouts of up to 19mm diameter required. I bought a step drill bit to deal with these and the results are much better than when I used standard bits in my hand-held drill.
One of the ways to get around the potentiometer problem is to use extension shafts; these can be a bit of a pain and I have used them in the past with varying degrees of success. There are kits available but the threaded bush provided for the front panel often isn’t long enough, but hopefully I have found a slightly better solution for this build. I used a 6mm diameter rod and found some suitable couplings to attach the rod to the potentiometer. Nylon insulated screw inserts come in a variety of lengths and internal diameters and can be used as a bush for the cutout where the rod goes through the front panel.
For my enclosure I had a scan through the offerings from the places I usually look, but nothing really appealed. The options with two rotary controls all had text for a source selector to go with the set up for standard volume. I ended up finding a nice, plain chassis and drilled the control holes, RCA holes and power switch myself. I used Decc vinyl lettering for the front and back labelling.
Initial tests were undertaken in my workstation setup using both a phone and my DIY DAC/streamer coupled with the ACA.
Now the wiring is completed and it’s all photographed, I’ve moved it to its final home, coupled with its big brother the PeeCeeBee V4. Although the harmonic distortion can be altered on the fly, I don’t really notice any immediate difference. Shaan suggests some prolonged listening with the signal set to different positions for different music types to find your individual preferred settings for these. I’ll be exploring this during the next few weeks or so, something I look forward to as I haven’t listened to much music over the last month.
My thanks go to Shaan for delivering another superb design and for his help with this project.