I had some niggling issues on the VCF and Master cards which I recently sorted out.
As mentioned in the first post on this mixer, the VCF card is based on the ThatCorp Design Note #130. This article outlines a 6db/Oct voltage controlled filter and works very well indeed. I have a 12dB/Oct version in my modular synth, which is just 2 of these in series fed from the same control voltage. Most DJ mixers use a baxandall style shelving EQ but I was keen to try building something with a high pass/low pass EQ. Why not? The whole idea behind the modular mixer was that cards can be changed for different features at any point with a minimum of fuss so this can be altered in the future if it doesn’t suit.
The card is essentially a stereo version of the schematic outlined in figure 2, minus everything ahead of the EC- pin. As mentioned in the article, the opamp used in position U3 should be something with a low bias current, I used the LT1169 as I had some left over from another project. I also used an 80’s ceramic NE5532 in the other position. I have quite a few of these, I believe they were taken from a Harrison console. The date code on these is 8406, so start of 1984. I don’t recognise the brand but it looks like XP, does anyone have any information on this manufacturer? I purchased a big bag of 5.11nF polystyrene capacitors from a surplus dealer and used those as the filter capacitor. I included a jumper to set the card to either low or high pass modes.
As far as generating the control voltage for the filters, I used the same circuitry as the VCA card as shown above. I can’t remember where I came across this design as I’ve had it scribbled in a notebook for a few years now but used it many times over in my modular synth (I’ll post something about that at some point). It is a control voltage ‘attenuversion’ (variable gain from 1 to -1) and offset (+10V to -10V) element. In this setting, it shifts and inverts the +/-10V signal generated by the potentiometer on the front panel to the -120mV to +360mV signal required by the VCA chip. As shown in the schematic I’m just using a cheap and cheerful TL072. I’m looking at using a precision voltage reference instead of the main rails for the generation of this voltage, something to report on later.
As far as setting this circuit up there are a number of trimmers that need to be uhm… trimmed with the assistance of the test points. TP1 allows you to confirm the +/-10V range coming from the potentiometer, TP2 allows you to set the overall range and TP3 allows you to adjust the offset. Easy.
Onto the Master Card. I changed the discrete opamp on this board from the NTP M100c in version 1 to the Melcor 1731 – for no other reason than that I didn’t want to keep buying 2N1711 and 2N2905 transistors which are expensive and use quite a bit of PCB real estate. I have used the Melcor 1731 in some other projects and it sounds fantastic. This discrete opamp is used to drive the output transformers on the main outputs. As far as balancing the Cue/Booth outputs I’m just using a THAT1646. The datasheet notes that you can either simply tie the sense pins to the appropriate output -or- make this connection through a medium value (generally 10uF) bipolar electrolytic cap for lower levels of common mode offset. I have used the chip in both configurations in the past but honestly haven’t made any measurements on the effectiveness.
Anyway, the whole things works quite well. I have been testing the cards in the motherboard lately and it seems hunky dory.
I haven’t gone into the pinout for each card yet, so will now. These are standardised across all cards as below:
Signal or analog grounds are kept separate from the power decoupling grounds for obvious reasons. On most cards the grounding for the left and right channels are separate as well. The need for two inputs is due to the summing card. Dimensions for the cards are 3.1 by 3.9 inches. They all have mounting holes on either side of the connector. Here is a screenshot of my template card:
Anyway, that’s all for today.