Mixer Pictures from Paul
These are the best pictures Paul can get. I just received them today. But what does it all mean?
This is my appraisal.
It’s a very simple four channel mixer. It doesn’t appear on the Studiomaster website, so it must be quite old and obscure. A mention is made here on this website for a similar model.
Here’s how I see the circuit board on a point-by-point basis in no particular order, just as I come to them:
- It’s a little mixer using @9V supplied by an external “brick”, say. This limits the p-p voltage swing to something less than 9V, say 8 to 8.4V. This is only 4x line voltage but as that is p-p and the line is rms, it’s only going to give you a few dB gain. Check out the formula for comparing p-p voltage to rms of a sine wave. There’s a lot of difference.
- The half hidden sub-board has a diode close to the 9V input which will probably protect the unit from a wrongly connected power supply.
- On the main circuit board I can see four identical preamp stages. They aren’t in line with the pot controls but you can spot them as the bunches of four electrolytic capacitors. Squeezed in each group are two little black transistors (little D-shaped thingies). These are probably something like BC182s or similar. These two transistors will most likely be connected as common emitter for the first stage and common collector (aka emitter follower) for the second stage.
- Because there are only the two active components, to maintain linearity the gain will be quite low, set to be just enough to amplify a microphone signal, no more.
- It will also be quite bandwidth limited to keep the gain-bandwidth product down. Basically I wouldn’t expect a full 20Hz-20kHz range and this seems to tally with Paul’s observations.
- There is probably an extra stage or two of amplification and buffering on the little half-hidden board that holds the line inputs and outputs. This will drive the line outputs. It’ll provide a high input impedance to the four channel outputs, being fed in by a resistor network to split and take signals from the mic preamp outputs and the line inputs.
- I can’t tell from the board how the “gain” controls work. There are two main options:
- Have the pot in the negative feedback path of the preamp.
- Have the pot across the second transistor output with the slider giving a variable output to the mixer stage.
- The mixer is unlikely to be an op-amp using a virtual earth mixer. Unlikely, but not impossible as it’s really the best way to do this.
- In the preamp, the four capacitors will be providing DC decoupling, probably two in series on the input and output and one dangling off the emitter paralleled with a resistor to the 0V rail.
- The two larger electrolytic capacitors will be to smooth the main power rail and reduce any extra mains hum that will arrive with a cheap consumer “brick” power supply.
- Examining now the resistors surrounding each preamp. In a fully set-up single transistor amplifier stage, I’d expect to see a minimum of four resistors to set the bias currents etc. Maybe add a couple with a couple of low capacitance capacitors to set the operating frequency range. That is, about six per transistor. Check the picture and there are about a dozen around each preamp, so that’s about as I expected.
- The low value capacitors are the small dull orange components. As I thought, there are two in total per preamp.
- Most resistors are mounted end-on to conserve space so I can’t see the values. However, even if I could, without seeing the circuit board tracks and the type and thus pin-out of the transistors, I can’t draw a circuit diagram from it with any certainty. It’s just the general principles I can see at work here.
- It’s likely that the inputs could have high or low impedance sources thrown at them. The input impedances will probably be set as a compromise.
- A better look at the half-hidden board may provide me with more info.
It’s a little cheap mixer with no pretensions to hi-fi. You’d maybe use it to quickly mix a few mics together for a conference or garden party sort of affair. What’s needed for this is a nice flat mid-range and no bottom or top to make it easy for an unskilled person to get something going without feedback etc. This unit fulfils that purpose admirably.
If you are wondering where the 6dB of amplification has gone, I don’t think it’s gone anywhere, unless the power supply isn’t managing to chuck out 9V when it’s connected (you could check that with a meter when it’s working). It’s the way it is, from what I can see. You could always stick another amplification stage into the output. You’d only need 2 transistors and associated components and about a 2cm of circuit board to squeeze into the box. It could be a nice little project for your soldering iron!
Plugging crystal quality sound through the unit will thoroughly muddy and truncate the sound spectrum. Of course, it’s up to one’s artistic sensibilities to determine the “goodness” of it all… 😀
I’ve added a few books that might build on the knowledge for this post – SP