If you disconnect all boards and just have the PSU sitting on the table but plugged in, it is delivering negligible power as the electrons are going through the air to get from one rail to another. Air is a very good insulator, so only one or two make it. (they sound a bit like sperms in this description) Not enough to register on most equipment.

The thing looks clean and tidy from the pix. I’m surprised it’s so noisy. As you’ll see, everything, all the pots and switches are circuit board mounted. If there’s any strain, then it’s nearly always because of wire pulling the boards around a bit against the resistance of the pot pins. This makes the boards move to and fro against the pot pins and they can work a hole around the pins when they are supposed to be nicely soldered. I’ve seen a few like that. Re-solder the pots into their holes solves that. Hopefully, any movement hasn’t moved the copper tracks on the boards. That’s what usually goes through my mind when I’ve seen it. I can see little angle brackets supporting the boards at the bottom and pop riveted to them(which is good). This probably helps earthing as well with the front panel.

It should be a pleasure to work on and pretty simple. Well done!

It’s actually quite hard to get any more routing and stuff from it because it’s all done on each board after seeing the manual thingy. If you need any extra break points you have to cut the track and insert your own, which is incredibly fiddly. And you also need some box real estate to come out on. You’ve probably got enough options to keep you going.

They quote -85dB roughly for the noise on mic amps. This is good, but nowhere near the capability of your EMU, which is something like 120dB down if I recall. So if you define analogue noise as good, then that’s your prerogative. The figure that you measured -35 I think, is really bad. Does it actually sound like a blizzard? Because if not, then the test setup is a bit cronky. This was my gut feeling when you said you’d done that.

I did a lot of measuring in Physics and it’s a devil to make sure you know which bit your are talking about and what it’s related to!. Everything to do with noise and amplification is related to something else and the standards try to get it so that like is compared with like.

I’d still ditch the phones amp if you can’t stop it humming. It’d save a separate power supply connection and you can always get one cheap to stick across the monitor outs anyway. With an outboard amp, you can monitor each aux feed etc as well with the phones as they are switched in/out. Basically, you can attach the phones to any source you’d want. Think of the phones as an amp/phones combo and you’ll see what I mean. It looks like the mixer phones amp has little connectivity as it is; unless I’ve missed something! ;-)

AS for the clip, there aren’t that many filters and stuff in the box for you to pump your Hammond through! It’s still mutton dressed as lamb. You’ve probably got more than enough filters on the mixer to keep you going! The actual sounds it’s producing are about the same, and about the same controllability as the guitar effects processor I made. The filter you are after is a steep Chebyschev probably, anything higher is hard to implement stably. You can make one for a quid or two. It’s the next one up from a Bessel and has a peak before the slope. This is what you can hear when he sweeps it around. If you were really creative, you could set aside a couple of mixer channels and modify the tone controls/filters on them for the required functionality. You’d still have a dozen to play with normally and it wouldn’t affect anything. Check the circuits round the tone op-amps and compare to here say:

logic: http://en.wikipedia.org/wiki/Chebyshev_filter

Op-amp implementation: http://people.seas.harvard.edu/~jones/es154/lectures/lecture_1/filters/filters.html at the bottom. It wouldn’t take much as most of the circuit is already made on the channel board anyway, you’d only need one or two flying leads and change a component or two. The part of your circuit to look at is the two op-amps at middle left on page 1 of your circuit diagram. The far left one does the swept mid EQ and the right one is a standard Baxendall tone control.

Happy soldering and scoping!
Rees

I think it’s pants. It’s worth a fiver. Just use your PC stuff like that moog emulator etc.

Cool thanks for that, the thing is the digital world doesnt sound as good as the analog world. i trust my ears and feelings so if you know anyone or come across anything that would work for that clip i sent you, let me know
Thanks for all the breakdowns on the mixer. Apart from me drowning it in servisol, what do you think are its weaknesses and what do i do about them?
On the final mixdown i use all of my outs from EMU, therefore i cant use output 5/6 for headphones. this is why i need to use the headphones on mixer. so i have to get around this im gonna strip her back down in a bit and have another go and a friend of mine is gonna bring over his oscilloscope on monday
thanks again paul

I’ve checked the circuit diags. Like I thought, there’s not much actual electronics as it’s all done in the op-amps. Nearly everything is switches and pots, which are mechanical, collect dirt and wear out!

But of course, this is the whole point of a mixer. It’s easy to make an amp to get a signal up to line level. Two transistors can do that. It’s what happens afterwards that gives the mixer it’s power and flexibility. And the more flexibility you want, means more routing and busses to choose and thus more switches and level controls and thus more mechanical things to go wrong.

You should see your dad for a bit of help or something – he’s mechanical, and that’s what most of your problems stem from (the mechanics, not the dad!).

Like I said, about a gallon of Servisol and loads of jiggling and wiping up! So happy squirting Studiotoe.

The phones amps are a simple tap off the main stereo monitor mix, not independently from each channel from what I can see. They aren’t particularly good, the one I made from the ETI circuit is better (it can drive very low loads whereas the mixer one says 8ohms minimum or pop!) So you could easily ditch them and have that functionality in your next stage (PC or EMU presumably)

The circuit also shows a 1kHz tone generator for setting up, which is good, as well as a studio talkback mic and switch. This is good but no use to you, I think.

They’ve started to use FET switching for some of the busses. This is also good as it stops clicks going through the signal. This is used to mute each channel, so if you get a click there, than it means that something is duff in that area as they should mute smoothly without a thump. They haven’t used it on the EQ cut switch….it’s a cost thing and also, they probably had a load of old boards to use up.

The pan controls are ganged. These don’t always sweep at the same rate. Basically it’s like having two volume controls wired back to back across the stereo outs. My design just used a single. There are pros and cons to any panning design, cost being one of them of course.

Each board seems to include bypass and smoothing capacitors.

Bypass is also included in the negative feedback loop of most op-amps to stop local rf runaway. This is good. You’ll see this as 22pF or 220nF capacitors around the op-amps.

Each stage seems to be properly DC decoupled. They seem to use 33uF throughout. These are probably tantalums.

Hope this helps. It’s pretty well designed but lacks some of the outs that I included on mine that made life pretty useful. I never used busses much. Our system never extended that much and the “buss” would be what we called a mix down bounce – so we had to plan well ahead.

Like I said, the flexibility is the routing, but that means more mechanical bits to break. Reaper does all that so much better, and without the Servisol!

Rees

You’ve really got to pay attention to the circuit diagram. I’ve just checked the stuff so far that I put on my website here: http://strangelyperfect.tv/1291/studiomaster-16-4-8-restore/

Two things:

Check the circuit here: http://i390.photobucket.com/albums/oo345/paulrichards7/PSUschem16-8-16.jpg

You’ll see that there are three earths in the connector! Follow the heavy black lines and it even says that it’s connected to the chassis! What they are doing is making sure the return power feed has enough capacity by trebling up the wiring. It’s all the same 0V (earth)!

Second, check the photos you put up of the PSU and look carefully at the wiring? What I mean is, it’s quite fat! Now compare to the Maplin 8-pin pins. It says it’s 5A rated. I don’t know what that means, is it the pins individually or the whole connector? I can’t tell. But what I can tell is that the pins are thinner than the wiring coming from the PSU. This is bad!

Try this, which is what I suggested in the first place, and with good reason. http://www.maplin.co.uk/Module.aspx?ModuleNo=44123&DOY=9m10 or use two 3 pin ones (better) or even better, use the Neutrik power connector or power type XLR connector.

If you check the circuit diagram again, you’ll see that you only need five wires if they are fat enough! That’s 0V (earth), 15 & -15 for the op-amps, +48V for the phantom and a separate 15V feed for the headphone amps. Do away with phones and you only need 4 wires. They can be nice and fat and use a good quality connector as well. If you did this you could use a 4 pin Speakon or other speaker type connector. These things are designed to take heavy currents.

If you take the phantom powering out of the equation, that’s a 3-pin connector! The phantom could come in on it’s own 2-pin connector in that case. Lots of the mods people recommend a separate ready made supply for this anyway as the current demand is so low.

Don’t muck around when you’ve got large currents floating around. You need a connection and wiring capable of at least 10 times the current you expect. The electricity in your mixer is like a river with tributaries all feeding in. The tributary streams are small like the current from each op-amp but when they all join at the river mouth (PSU), there’s a lot of water and the river must be wide enough to take it all with room to spare for all the wear and tear expected. Think how big a standard 3-pin house plug is and the big oblong pins? Right! They are only rated at 13A! Your PSU chips can deliver in excess of 1A. So that means the pins must have 1/13th of the cross-sectional area of a UK mains pin to have an equivalent current or power rating.you see what I mean about safety margins? Remember, current is the electrons whizzing along the wire. The electrons don’t know that it’s only 15V pushing them along and not 220V! They’re electrons, after all! But the heating effect in a wire comes from the square of the current; remember,

Power in watts W = I squared R

So if there’s any increased resistive points in the current flow (like a kink in a wire or a bad joint in the connector or rust on the connector), that’s where it very rapidly heats up, so that’s why you ALWAYS over-engineer for power connections. Especially ones with idiots wandering around. It’s so easy to trip over cables etc so the connections MUST be physically robust with connections having a cross-sectional area well capable of carrying lots of electrons.

Put it another way; I never had power supply problems but I’ve seen smoke come from other people’s gear on stage followed very rapidly by silence! Who looks the twit then?

As for your bid, I hope you get it. I hear the Pink’s have a vacancy for a keyboard player now!. From boy-band to studiotoe to psychedelic! Wahay!

Rees