Another year, another DJ mixer. This rounds out my ‘Greek Trilogy’ series. The design for this one started in October last year and took months to get to the point where anything physical was in my hands.

I don’t usually jump into projects with overly narrow goals or design ideas, but I knew from the outset this would be a 2 channel discrete mixer with full metering as compact as I could make it without SMD.

This article is about the input stage of each channel.

I had been lusting after a Revox B750 receiver for a while, initially because of misty lensed advertising material as pictured above, but eventually the unreal quality of (all) old Studer/Revox documentation had me hooked. Their service documentation reads like a novel for audio electronics nerds.

I had been simulating the phono preamplifier and equaliser sections, both discrete stages, for a while to get a sense of it. The circuitry for both revisions of the original amplifier are shown, only differing in the active devices and a couple of passive values. Later units used commonplace BC series transistors, something I have in droves. Outside of this screenshot is an adjustable input impedance switch, something that isn’t often found in DJ mixers. I wanted to include this in my build but without any free rear panel space I decided to put it on the bottom of the PCB via a PCB mounted DIP switch. This trend continued with the rest of the design and almost all of the calibration and adjustment of settings can be carried out by simply removing the rear panel. I used the quite expensive (unfortunately) C&K BD2 series, I could have sourced something like this for significantly less than the $4.50 per unit I paid at Digikey by heading out to aliexpress but my experience is that switches are a problem always solved with money. I used a BCY79 VII as the input and then BC550C in the rest of the positions.

The equaliser used in the receiver is fairly extensive for a Hifi amp. In addition to high and low pass filters, the unit essentially has the same Baxandall high/ low plus a ‘presence’ section as the 169 series and 961 series Studer mixing desks, save for a discrete gain amplifier – which of course only made it more attractive. In Studer talk ‘presence’ refers to a mid band bell eq with adjustable frequency. I have used the 169 design quite a few times, in an 8 channel 3U enclosure for a studio and also in the Logos DJ mixer channel strips so building it again with a discrete amplifier was appealing.

One common criticism of rotary DJ mixers is the lack of a gain trim control. Whether valid or not I won’t dive into, but nonetheless I chose to include one (will become a little more obvious why later). I was after a -6dB to +6dB range for the gain trim so rather than just use a potentiometer as an attenuator, I decided (again) to use the balance amplifier from the B750 with the potentiometer controlling the level of negative feedback.

The B750 also contains a neat two transistor unity gain line input buffer which I used for the line stage. Each channel has two line inputs, switched before the single line stage by a relay. Another relay is used to select between the line stage and the phono stage. Since the relays I used are a DPDT configuration I had a left over switch and so also use the phono/line relay to mute the signal going into the line stage when not selected. This means that when selecting the phono inputs the line inputs run through -absolutely- no circuitry theoretically offering greater resilience to crosstalk.

With the Revox B750 serving as the basis for each channel the mechanical design needed some thought. Being fully discrete, each channel was fairly component heavy (125 pieces in each) so a mono input card design was created. I allowed for some fairly generous component ratings so the board wound up quite dense, without compromising the size of each component. The RCA inputs are a RJ-140 (or Deltron 43-028). These are a high quality right angle PCB mounted connector with gold plating on the metallic surfaces and teflon dielectric. A pair of these cards are lashed together via a front panel PCB containing the controls, power distribution and relay drivers. The Input card and Front Panel PCBs are show below. Overall dimensions are 127mm x 65mm for the squishy input card and 180mm x 50mm for the front panel PCB.

Designing anything with front panel mounted components tends to be an exercise in either routine, by using the same stuff you did in the past, or in either pouring over datasheets/databooks for days finding the right part combinations so that everything lines up on the same plane. I haven’t actually done a huge number of projects with parallel front panel PCBs as its hard to find dual gang potentiometers and rotary switches that align without spending a fortune. In the end, the solution was simply to spend that fortune on Grayhill 56 series rotary switches and pair them with dual gang Alps RK09 series dual gang pots. The pots were actually fairly reasonable, though I did have to modify the original circuits lightly to work with the limited values and tapers I could purchase without a custom order directly with Alps Japan.

Switching is performed by NEC EC2-12NU relays, these are driven off a 12V rail generated by a LM7812 on the front panel PCB, the Grayhill 56 switch simply routes 12V to actuate the correct relays. Each relay is driven by a textbook NPN with a couple of diodes and a resistor, these components are mounted off the input PCB due to lack of space, with exception of the flyback diodes which are appropriately next to the relays themselves.

These PCBs constitute the ‘Input Module’ of the Ethos mixer. At the rear are the pairs of RCA jacks. The other I/O on the module is a power connection for +20V GND -20V and the stereo output, both on IDC ribbon cables. All power connections in the mixer were made on grey IDC connectors for easy identification. In this particular layout, the channel volume and cue control has been separated from the input stages.

PCBs arrived and mostly worked. Some routing required adjusting on the front panel and I had mis-transcribed a portion of the phono stage. With these corrected the modules are calibrated and sounding great.

Both channels with the centres spaced 5″ apart looks something like this:

See Also:

Part 1

Part 2

Part 3

Part 4

Part 5

Part 6

Part 7

Part 8