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Thread: DIY rotary DJ mixer

  1. #81
    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)

  2. #82
    Nope.

  3. #83
    Quote Originally Posted by efinque View Post
    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...

    Quote Originally Posted by efinque View Post
    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.

    Quote Originally Posted by efinque View Post
    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.

    Quote Originally Posted by efinque View Post
    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.
    Last edited by light-o-matic; 03-28-2020 at 07:33 PM.

  4. #84
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    Quote Originally Posted by light-o-matic View Post
    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!

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