DIY rotary DJ mixer

[efinque]

Another cause for the mono summation could be the mic channel (otherwise it's only audible in either L or R) but I was just eyeing through the circuitry quickly and I recall solving it by omitting the trim/vol grounds and relying on the taper providing enough resistance across the stereo channels (in practice this means turning the mic on creates a mono sum, the trim needs to be set as high as possible without feedback and keeping the mic volume pot at 11-12 o clock max, which is the exact opposite of how the phono/line channels are supposed to be operated)

I believe some mixer manufacturers have solved this simply by putting a balance pot in each channel which provides the necessary shunt resistance.

EDIT : however, with the summing bus I built it's very easy to switch component values (or even put caps/diodes in the mic channel L/R summing buses in place of resistors, this depends on where it is used; for live work it's in theory possible to use diodes because of ambient/background noise forming the ~0,7 forward voltage, ie. threshold but for broadcast I'd go for electrolytic caps since the mic channel could gate the signal with diodes as there's no background noise present although it could high-pass the signal somewhat)

[light-o-matic]

Nope.

[light-o-matic]

What's the reason behind this exactly? Touching the ground shouldn't do anything in theory, in some cases it even attenuates the hum.

If touching the ground does anything, that's an indication that your grounding is incorrect. The only thing to do then is to figure out what you did wrong and fix it.

In your case, things are a little more complicated because you mixed modules from different manufacturers. But the first thing to try is to run just one ground wire from each module and have them all connect together in one place. You don't want your modules connected together through more than one ground path. Just connect the hot wire of each signal from one module to the next. The problem with jacks where the ground touches the chassis is that, although the chassis is ground, it allows multiple paths...

The schematic you posted is technically correct but once you go stereo with a mono PFL you need summing (or output) resistors in the cue, otherwise you'd need to be cueing only left or right since the mono bus creates a shunt between the stereo channels (think of having hot and cold water pipes and mixing them prior to the tap)

To mix left and right to mono, you use the same scheme as mixing different inputs: You start with a low impedance source (output of an amp stage), one for left and one for right. You connect each one of those through a large resistance then the other side of the large resistors you tie together and connect into your headphone amp input (high impedance). This allows the left and right to mix into one channel with relatively little loss.. but very little signal leaks between the left and the right bus.

I understand the basic theory of operation but resistors generate noise and the larger values you use the more amplification you need after the summing, ie. gain staging.. while in a headphone bus it doesn't matter (and you could probably even use electrolytic capacitors) you're still left with L/R+mono where the underlying total resistance of the mono signal across the two channels is formed via the resistors (in other words you get a quiet mono sum in the background)

Every circuit has a little bit of noise, if you want a mixer with zero noise, you have to leave ALL the parts out.. but that wouldn't work very well as a mixer.
If you design the circuit correctly there will be very little noise and it won't be a problem. After all, the professionally built mixers have hundreds or even thousands of resistors in them and they work fine with very little noise! If you use the resistors correctly you won't have a noise problem.

And you will not lose that much signal.. because.. let's do the math. Let's say the input impedance of your amp stage is 100K, and you use a 220K resistors to mix your signals.. you will lose 10db, if you use 100K resistors, you will lose 6db.. which you can easily make up in the amp stage.

But the point is, if you want your mixer to work, that is how you do it.

EDIT : to further elaborate the intercom project started off as a "joke" as I've built one about 15 years ago.. I refined the idea a bit further though so it's closer to a PBX/telephone exchange with an automatic call handler but I haven't gotten around to building it yet.

Consumer and pro audio which both have different technical needs aren't like "enterprise" solutions where very impressive things are done with as little as 1-5W, sometimes within a narrow frequency range (400-4kHz) over long distances, main concerns being effeciency, speech intelliglebity and ease of use whereas pro audio is mostly concerned about balanced transmission, wide frequency response and large power handling capability.

Phone lines (analog ones) are balanced! That's why phone signals can travel for miles with very little noise.

The first "audio" systems were phone systems, and many of the ideas used in pro audio came from phone systems.. including the signal levels we use, balanced lines, amplifiers, microphones, the decibel.. were all invented for telephones.. NOT for pro audio, which came along much later.

So yea of course phone systems don't need wide frequency response or high power.. but the principles are the same.

[SWS Productions]

If touching the ground does anything, that's an indication that your grounding is incorrect. The only thing to do then is to figure out what you did wrong and fix it.

In your case, things are a little more complicated because you mixed modules from different manufacturers. But the first thing to try is to run just one ground wire from each module and have them all connect together in one place. You don't want your modules connected together through more than one ground path. Just connect the hot wire of each signal from one module to the next. The problem with jacks where the ground touches the chassis is that, although the chassis is ground, it allows multiple paths...

1) To mix left and right to mono, you use the same scheme as mixing different inputs: You start with a low impedance source (output of an amp stage), one for left and one for right. You connect each one of those through a large resistance then the other side of the large resistors you tie together and connect into your headphone amp input (high impedance). This allows the left and right to mix into one channel with relatively little loss.. but very little signal leaks between the left and the right bus.

Every circuit has a little bit of noise, if you want a mixer with zero noise, you have to leave ALL the parts out.. but that wouldn't work very well as a mixer.
If you design the circuit correctly there will be very little noise and it won't be a problem. After all, the professionally built mixers have hundreds or even thousands of resistors in them and they work fine with very little noise! If you use the resistors correctly you won't have a noise problem.

And you will not lose that much signal.. because.. let's do the math. Let's say the input impedance of your amp stage is 100K, and you use a 220K resistors to mix your signals.. you will lose 10db, if you use 100K resistors, you will lose 6db.. which you can easily make up in the amp stage.

But the point is, if you want your mixer to work, that is how you do it.

2) Phone lines (analog ones) are balanced! That's why phone signals can travel for miles with very little noise.

The first "audio" systems were phone systems, and many of the ideas used in pro audio came from phone systems.. including the signal levels we use, balanced lines, amplifiers, microphones, the decibel.. were all invented for telephones.. NOT for pro audio, which came along much later.

So yea of course phone systems don't need wide frequency response or high power.. but the principles are the same.

1) It's called summing.

2) Fact.

You're a pretty smart dude!

[light-o-matic]

1) It's called summing.

2) Fact.

You're a pretty smart dude!

You're sweet

But really, I've just been at this stuff for a minute. I built my first DJ mixer age 13, made mistakes.. learned these lessons.. made it work
I had some good mentors.

[efinque]

If touching the ground does anything, that's an indication that your grounding is incorrect. The only thing to do then is to figure out what you did wrong and fix it.

Many electric guitars do this.. I'm not saying you *should* touch the ground or any wired parts that can conduct electricity, it's just something that you'll notice.

In your case, things are a little more complicated because you mixed modules from different manufacturers.

Is there a problem with that? Fyi only the power supply is from another manufacturer.

But the first thing to try is to run just one ground wire from each module and have them all connect together in one place. You don't want your modules connected together through more than one ground path. Just connect the hot wire of each signal from one module to the next. The problem with jacks where the ground touches the chassis is that, although the chassis is ground, it allows multiple paths...

The chassis is plastic and they all have the same ground point.

Every circuit has a little bit of noise, if you want a mixer with zero noise, you have to leave ALL the parts out.. but that wouldn't work very well as a mixer.
If you design the circuit correctly there will be very little noise and it won't be a problem. After all, the professionally built mixers have hundreds or even thousands of resistors in them and they work fine with very little noise! If you use the resistors correctly you won't have a noise problem.

One cause of resistor noise has been tracked down to the thermal characteristics. The other is an excessively large R value in an already low signal which is then amplified unnecessary high.

And you will not lose that much signal.. because.. let's do the math. Let's say the input impedance of your amp stage is 100K, and you use a 220K resistors to mix your signals.. you will lose 10db, if you use 100K resistors, you will lose 6db.. which you can easily make up in the amp stage.

The trick is to provide enough gain for long cable runs when needed.. consumer gear rarely needs the same signal levels as FOH.

Phone lines (analog ones) are balanced! That's why phone signals can travel for miles with very little noise.

Afaik telephony is (from an audio engineering viewpoint) usually very "dirty" as the trunk lines are used to carry DTMF signals and even DC voltages (landline phones are getting rare these days though) but they're simple to build and maintain, ie. they're "dirty" however they work.

[light-o-matic]

Many electric guitars do this.. I'm not saying you *should* touch the ground or any wired parts that can conduct electricity, it's just something that you'll notice

Electric guitars are high impedance and use unbalanced connections, plus they are often wired to get a specific sound but that wiring is not always strictly "correct".
But, there are guitars that use balanced connections and that have low noise. Anyway, we are talking about mixers here, not guitars.

Is there a problem with that? Fyi only the power supply is from another manufacturer.

It CAN be a problem if for example one module uses a unipolar and the other a bipolar supply, or one is balanced and the other unbalanced.. also you need to watch out for levels coming out of one module being suitable for the input of the next.

The chassis is plastic and they all have the same ground point.

That should be ok.

One cause of resistor noise has been tracked down to the thermal characteristics. The other is an excessively large R value in an already low signal which is then amplified unnecessary high.

YET, somehow people manage to build mixers with them that aren't too noisy. So I think you are worrying about things that are not really a problem for you, and your mixer would work much better if you would go ahead and use the resistors the way they are supposed to be used.

The trick is to provide enough gain for long cable runs when needed.. consumer gear rarely needs the same signal levels as FOH.

Sure. But we're talking about the inside of a mixer, the signal only has to go a couple of inches.

Afaik telephony is (from an audio engineering viewpoint) usually very "dirty" as the trunk lines are used to carry DTMF signals and even DC voltages (landline phones are getting rare these days though) but they're simple to build and maintain, ie. they're "dirty" however they work.

I'm not sure your point here. Analog phone signals are (were) VERY GOOD QUALITY considering how far they had to travel, the number of other signals that travelled next to them in the same cable, the complexity of the switching equipment and all that. It's AMAZING that, in let's say, the 1970's.. you could turn a dial and in a few seconds get a connection from New York to San Francisco, and talk to someone there.. analog audio signal travelling thousands of miles.. and it sounded pretty good!

A lot of very sophisticated engineering went into telephone systems.

But.. telephone systems have nothing to do with why your mixer does not work right. Your mixer does not work right because you are not using the correct type of circuit to mix your signals. If you take your amplifier modules, and connect them together using a proper circuit.. using resistors for summing as I explained, then your mixer will work. If you don't, it won't.

[efinque]

However, there are a few things I learned from my mistakes (in case anyone's building or thinking of building one)

1) Staying organized and considering ease of service from the beginning (this makes it easier to troubleshoot as you don't have to tear the whole thing apart) ; I don't mean you should build a modular unit as they require very competent designing, machining and engineering skills but it's one way to do it (unless you do it Eurorack/500-series compatible which is another story)

2) Being careful how and where to sum stereo to mono

3) Populate the PCBs and wire everything blindly trusting it works

4) Making "last minute" modifications usually results in disaster; messy PCBs/wiring schemes and jumper wires, see pt. 1

5) Running out of chassis space and/or components

6) It could help to keep a moderately detailed schematic at hand and double-check & update it whenever you modify parts of the circuit (I think they do this in architecture with floor plans/blueprints)

7) VCA opamp supply voltage fed through headphones/monitors could destroy them and fry parts of the circuitry as well so if you're using/modifying modules like I did check the schematic (I actually wired them a little different first and once I did a breakout connection had they been VCA this exactly could've happened)

[efinque]

Anyway, due to the mono sum post trim to the PFL I just soldered 10k summing resistors so there's a total 20k resistance between L/R.. unfortunately I don't have access to turntables at the moment but I'll continue developing and tidying up the circuitry, as I noticed a few other mistakes I did.

The reason for this is the cue/pgm split I was planning (basically a voltage divider from the HP amp between master and cue) and the fact that I don't have enough wire. This also makes it easier to switch resistor values for cue (otherwise I'd run L/R to the cue select and take the ground sum from the summing bus, but adding the pgm/split cue later on becomes a bit more complicated)

[light-o-matic]

Well, 10k summing resistors to mono will give you some isolation, you will lose some stereo separation but maybe not that much. Like I said it depends upon the output impedance of your amplifier stages.

Basically, you put a signal into.. let's say.. the left side only.
Measure the level coming out the left side.
Now measure the level coming out the right side.

Hopefully you would get at least 30dB.. more is better. A vinyl record is lucky to get around 25dB of separation but keep in mind that let's say you are playing a record with 25dB separation, then that is the MOST separation you can get if your mixer is perfect. If you have leakage in your mixer, your separation only goes down from that 25dB where you started.

Or for an easy test, just switch the plug back and forth and listen to the difference.. if it's nice and loud on the correct side, and barely audible on the wrong side.. then that's probably good enough for DJ uses.

But if you used a bigger resistor you could get it to where there's barely any leakage at all. Like I said, if your output stage has just 6-10dB of gain then you can use much larger resistors.. 100K or more, for much better isolation, more like a professional mixer.