Yep, thats right. Do It Yourself Mixer. Crazy? Perhaps. There are a lot of very nice mixers out there for a very good price. Mackie comes to mind right away. However, being in Santa Barbara, there are no music stores around here that stock the kind of mixer I want. And from reading the specs of several, I cannot say for sure that they have the features that I might want. One way to get what you want is of course to do it yourself and stop bitching.
What are some of the problems with building a mixer yourself?
1. COST! Yep, it is going to cost me almost as much to build one (or more) than it would to buy one. Parts cost on a 24 in 8 out mixer will be from $1000 to $1500, depending on features. A Mackie 24x8 mixer is only $2400 or so (if discounted).
2. NOISE! A mixer project is not for bed wetters and wimps. With modern digital recording, 110dB of dynamic range is not out of the question. It is not easy to do. My ART 2408 (a very crappy mixer, and a very bad purchase on my part) has a noise floor of about -70dB, but a lot of that is 120Hz hum (buzz really). It also has a lot of crosstalk problems. The crosstalk is so bad I can actually get the system to oscilate if I turn the faders up too high. Crosstalk and hum can be delt with by doing a good design. The other problem is white noise, which comes from the components. You need to use premium semiconductors to get good low noise performance.
3. SOUND QUALITY! This is a little more subjective. A lot of mixers use op-amps. Many of these have class AB outputs to keep the power consumption down, but require a lot of feed back to keep the distortion down. Op-amps also make a lot of noise unless you get premium ones. There are several op-amps that are popular with mixer makers. Another route to take is to go descrete. Or nearly so. You can build your own amplifiers out of transistor arrays, of which there are a few premium ones (Analog Devices, THAT Corporation). You can custom build your amplifiers with Class A output stages to keep the distortion to a minimum and you can then have less feedback (which seems to be prefered by audiophiles). If you are really ambitious and don't mind a huge electric bill, tubes are another option. The console would have to be very big and would have to use a lot of tubes. This is some place even I am not crazy enough to go.
4. FRONT PANELS! Yep this is a problem. If you use faders, you need to cut a slot into aluminum. This can be done with a punch (expensive) or a milling machine (even more expensive).
So, what do I plan to do?
The vision I have for this project is to build a semi-modular system. The base module with be an 8 in 8 out module. These can be joined to form 16x8, 24x8, 32x8, etc mixers. These will then feed into a master module.
The electronics is going to be constructed using THAT Corporation transistor arrays. The THAT120 (quad PNP) and THAT140 (dual pnp/dual npn) will make up the majority of the devices with 2n4401 and 2n4403 transistors making up the less critical applications. I do not plan on having any commercial Op-Amps in the signal chain.
I am working on three basic building blocks.
1. Differential Amplifier.
This amplifier will be used for the inputs. It will be a high impeadance differential input amplifier. Hopefull, the gain will be variable from 1 to 100. This is going to be full differential (balanced) input with a common mode input range of +/- 8 volts or so. I am hoping to at least 80dB of CMRR. More would be better. The current circuit I have for this uses 1 THAT120, 1 THAT140, and 5 2n4401/2n4403 transistors.
Preliminary Schematic. It has no part values (except transistors). This is the second version. I still haven't tried it yet. My work room right now is somewhere in the 50's (10 degrees C for you europeans who use a sensable temperature scale). One of these days I have to get the furnace fixed.
2. Buffer Amplifier.
This amplifier will be a unity gain amplifier. The current circuit for this uses 1 THAT140 and a CA3046. There is a posibility that I might use an LM310. It depends on which one works better. The LM310 is not really an opamp. It does not have all that much open loop gain and has a Class A output. Depends a lot on it's noise performance. This is one circuit I hope will work out.
3. Differential Ouput Opamp.
This is one I know I have to build. It will be like a normal opamp except it will have two outputs. This will be used for summing amplifiers and driving ballanced outputs. It will consist of 1 THAT120 and 1 THAT140 and some other 2n4401/4403 transistors. Open loop gain will be low (~500) and it will have class A outputs.
So as you can already guess, balanced inputs and outputs will be the order of the day. This should help keep pickup to a minimum.
I am also working on a low noise power supply. Sure the amplifiers will have a certain amount of power supply rejection, but the more quiet everythings starts out as the less work you have to do. I plan on using choke input filters on the supply. One reason for doing this is to keep the current spike that charges the filter capacitors to a minimum.