Post by thread:[TECH] Audio Mixer MixUp

A puzzle for you. I recently had occasion to attach 3 acoustic guitars and a Yamaha keyboard through balanced mic cables into a Yamaha mixer. The acoustic guitars all have internal battery-powered preamps and 1/4" Phone plug outputs, which are very common. I split each of two balanced mic channels into two single ended circuits each using one side of the balanced circuit with ground common. At the mixer end I split the signals out to 1/4" single ended and ran them into the line inputs. (Yes, I know they are line inputs. The channels have preamp level boost pots and the sound was fine with no significant hum. Yes, I also know about direct boxes.) On the first cable I connected two acoustic guitars. This worked just fine. On the second cable I put another acoustic guitar into one side, and the Yamaha keyboard into the other. My fatal mistake was that I never hooked them up at the same time until shortly before the actual performance. Well, with both sources hooked up, I could get no signal from the guitar. In the process of troubleshooting, I swapped everything in the circuit. Finally, I happened to pull the connection to the keyboard and the guitar channel sprang into life. At present, all I can think of is that the keyboard imparted some DC offset to it's channel which confused the mixer's input somehow. I'll get back to it in a couple of weeks and try to figure out what actually happened.. Any ideas what might have happened here? Thanks, Mark

On 24/05/2011 14:42, Mark E. Skeels wrote: {Quote hidden}> A puzzle for you. > > I recently had occasion to attach 3 acoustic guitars and a Yamaha > keyboard through balanced mic cables into a Yamaha mixer. > > The acoustic guitars all have internal battery-powered preamps and 1/4" > Phone plug outputs, which are very common. > > I split each of two balanced mic channels into two single ended circuits > each using one side of the balanced circuit with ground common. At the > mixer end I split the signals out to 1/4" single ended and ran them into > the line inputs. > > (Yes, I know they are line inputs. The channels have preamp level boost > pots and the sound was fine with no significant hum. Yes, I also know > about direct boxes.) > > On the first cable I connected two acoustic guitars. This worked just fine. > > On the second cable I put another acoustic guitar into one side, and the > Yamaha keyboard into the other. > > My fatal mistake was that I never hooked them up at the same time until > shortly before the actual performance. > > Well, with both sources hooked up, I could get no signal from the guitar. > > In the process of troubleshooting, I swapped everything in the circuit. > Finally, I happened to pull the connection to the keyboard and the > guitar channel sprang into life. > > At present, all I can think of is that the keyboard imparted some DC > offset to it's channel which confused the mixer's input somehow. > > I'll get back to it in a couple of weeks and try to figure out what > actually happened.. > > Any ideas what might have happened here? > > Thanks, > Mark > > I'm guessing the keyboard output impedance might be low, and the guitar output a lot higher (I know they have a small preamp built in but I think it will still be higher) so it's getting attenuated severely when you mix them together. I would check the output impedances of both to make sure anyway. Why are you splitting the inputs anyway? Do you not have enough channels on the mixer?

I don't know what mixer he has, but both of mine are stereo mixers. One is 6 stereo channels or 12 mono channels, and the other one is 8 stereo channels or 16 mono channels. Using the pan control, you can move virtually any channel to either the left or right output, or any point in between. They were designed to allow stereo signals to be input conveniently, but also, to allow single ended inputs to be utilized too. Of course, you could always use each stereo pair as one by either feeding both left and right input with the same signal, or just leave either the left or right channel unconnected, using only half of the stereo pair. But leaving an input unconnected is asking for problems. Regards, Jim > ---{Original Message removed}

{Quote hidden}> > A puzzle for you. > > I recently had occasion to attach 3 acoustic guitars and a Yamaha > keyboard through balanced mic cables into a Yamaha mixer. > > The acoustic guitars all have internal battery-powered preamps and 1/4" > Phone plug outputs, which are very common. > > I split each of two balanced mic channels into two single ended circuits > each using one side of the balanced circuit with ground common. At the > mixer end I split the signals out to 1/4" single ended and ran them into > the line inputs. > > (Yes, I know they are line inputs. The channels have preamp level boost > pots and the sound was fine with no significant hum. Yes, I also know > about direct boxes.) > > On the first cable I connected two acoustic guitars. This worked just fine. > > On the second cable I put another acoustic guitar into one side, and the > Yamaha keyboard into the other. > > My fatal mistake was that I never hooked them up at the same time until > shortly before the actual performance. > > Well, with both sources hooked up, I could get no signal from the guitar. > > In the process of troubleshooting, I swapped everything in the circuit. > Finally, I happened to pull the connection to the keyboard and the > guitar channel sprang into life. > > At present, all I can think of is that the keyboard imparted some DC > offset to it's channel which confused the mixer's input somehow. > > I'll get back to it in a couple of weeks and try to figure out what > actually happened.. > > Any ideas what might have happened here? > > Thanks, > Mark > > > Is the keyboard battery powered or line powered? (I'm thinking ground loop) -Denn

> I'm guessing the keyboard output impedance might be low, and the guitar > output a lot higher (I know they have a small preamp built in but I > think it will still be higher) so it's getting attenuated severely when > you mix them together. I would check the output impedances of both to > make sure anyway. > Why are you splitting the inputs anyway? Do you not have enough channels > on the mixer? > > > > > It was all about the wiring between the stage and the mixer. It's not a snake in the common sense of the word; there is dedicated wiring inside the building. The inputs appear at stage boxes in the floor. I could also have used direct boxes (probably a better solution in retrospect.) But there are only 4 mic inputs in a stage box, and I wanted to use one for a monitor send. It was just a choice with unforeseen consequences. I should have tried the whole rig together beforehand. I'm confused about your comment about mixing them together. I ran them into separate channels at the mixer end.....I just used the balanced microphone cables as two channels with a common ground. I am at a loss to figure out why impedance would have any affect on that. Unless the low side line inputs at the mixer are not actually common. Which I find hard to believe.

Mark E. Skeels wrote: > I just used the balanced > microphone cables as two channels with a common ground. I haven't really been following this and didn't read all of your original message, but this sounds wrong. A ballanced mic cable may have two signal wires, but they represent a single signal differentially. The two wires could very well go into the primary of a transformer at the mixer. They are not two independent channels you can send separate signals over. The circuit at the other end will detect the difference between the two voltages and ignore the average. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

part 1 944 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable) > I haven't really been following this and didn't read all of your original > message, but this sounds wrong. A ballanced mic cable may have two signal > wires, but they represent a single signal differentially. The two wires > could very well go into the primary of a transformer at the mixer. They are > not two independent channels you can send separate signals over. > > The circuit at the other end will detect the difference between the two > voltages and ignore the average. > > I only used the balanced cables...I didn't use the balanced inputs on the mixer. This mixer has an unbalanced input on each channel, as well as a balanced input. I used the common ground in the 3 conductor mic cable as common for both sources, and used the + and - (labeled) conductors as independent signals for each channel. See attached: maybe this will make it clearer. part 2 10695 bytes content-type:image/png; name="Hookup.png" (decode) part 3 181 bytes content-type:text/plain; name="ATT00001.txt" (decoded base64) -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at mailman.mit.edu/mailman/listinfo/piclist

Hi Mark, The only thing I can think might be wrong is maybe there is a short between the two signal lines in one of the builtin cables, or else a signal and ground line are swapped. Bob P.S. guitar pickups feeding that cable setup, even if it's wired correctly, are not going to have their optimum fidelity, but you know that :) On Tue, 24 May 2011 11:19 -0500, "Mark E. Skeels" wrote: {Quote hidden}> > > I haven't really been following this and didn't read all of your original > > message, but this sounds wrong. A ballanced mic cable may have two signal > > wires, but they represent a single signal differentially. The two wires > > could very well go into the primary of a transformer at the mixer. They are > > not two independent channels you can send separate signals over. > > > > The circuit at the other end will detect the difference between the two > > voltages and ignore the average. > > > > > I only used the balanced cables...I didn't use the balanced inputs on > the mixer. This mixer has an unbalanced input on each channel, as well > as a balanced input. I used the common ground in the 3 conductor mic > cable as common for both sources, and used the + and - (labeled) > conductors as independent signals for each channel. > > See attached: maybe this will make it clearer. -- http://www.fastmail.fm - Access all of your messages and folders wherever you are

Mark E. Skeels wrote: > I only used the balanced cables...I didn't use the balanced inputs on > the mixer. This mixer has an unbalanced input on each channel, as > well > as a balanced input. I used the common ground in the 3 conductor mic > cable as common for both sources, and used the + and - (labeled) > conductors as independent signals for each channel. Are you really really sure there are no transformers in the path or some other circuitry? You mentioned something about these cables being built into the stage, so how do you know they are really just cables? All in all this sounds like a bad idea. Even if you do just have cables with no electronics, you still end up with two separate signals squished next to each other in a common run. I expect you would get some crosstalk. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

> All in all this sounds like a bad idea. Even if you do just have cables > with no electronics, you still end up with two separate signals squished > next to each other in a common run. I expect you would get some crosstalk. > > Interesting you should mention that. I noticed if I left one channel disconnected there would be crosstalk across channels in the cable, but when both sources were connected, it went away.

part 1 328 bytes content-type:text/plain; charset="iso-8859-1" (decoded quoted-printable) > Seriously bad idea unless you know there are no transformers and it's > only wire. > > Hmmmm. I had no idea that this was a (seriously) bad idea. I know it's not optimal. I think I do know that there are no transformers and it is only wire..... see input #2. part 2 11311 bytes content-type:image/png; name="YamahaSmall.png" (decode) part 3 181 bytes content-type:text/plain; name="ATT00001.txt" (decoded base64) -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at mailman.mit.edu/mailman/listinfo/piclist

Mark E. Skeels wrote: > Interesting you should mention that. I noticed if I left one channel > disconnected there would be crosstalk across channels in the cable, > but > when both sources were connected, it went away. That makes sense, since the unconnected channel was at higher impedance then. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000

On 5/24/2011 11:28 AM, Bob Blick wrote: > Hi Mark, > > The only thing I can think might be wrong is maybe there is a short > between the two signal lines in one of the builtin cables, or else a > signal and ground line are swapped. > Yea, you would think; but I swapped the entire signal path between source and mixer. Still, it's probable I did something dumb here.......I just don't know what it is.......I thought maybe somebody else had already seen this symptom and could point out the error.

On 24/05/2011 16:27, Mark E. Skeels wrote: > I'm confused about your comment about mixing them together. I ran them > into separate channels at the mixer end.....I just used the balanced > microphone cables as two channels with a common ground. I am at a loss > to figure out why impedance would have any affect on that. Unless the > low side line inputs at the mixer are not actually common. Which I find > hard to believe. I was a little confused with your description of the connections, but I thought even with your description there may be something you are unaware of so I thought it worth a mention. With two separate channels there should be no problem at all (assuming no problems or "special" inputs with the mixer), so the fact you have one and it changes with the removal of one input suggests there is not complete independence in this case. Also the fact that it works with two guitars but not with keyboard/guitar would seem to be more evidence for this. I would check the entire run all the way to the mixer, including the stage box (could have something unexpected in there) and the mixer input circuitry.

I was a little confused with your description of the connections, but I > thought even with your description there may be something you are > unaware of so I thought it worth a mention. > With two separate channels there should be no problem at all (assuming > no problems or "special" inputs with the mixer), so the fact you have > one and it changes with the removal of one input suggests there is not > complete independence in this case. Also the fact that it works with two > guitars but not with keyboard/guitar would seem to be more evidence for > this. > I would check the entire run all the way to the mixer, including the > stage box (could have something unexpected in there) and the mixer input > circuitry. > > True. I can substitute a separate mic snake the construction of which I am sure of and see if I get the same result. I twill be difficult to check every foot of the built in cable, since it is "built in." But if there was something other than three wires it would be unconventional compared to what I have seen in other installations.

On Tue, May 24, 2011 at 1:55 PM, Mark E. Skeels <spam_OUTmskeelsTakeThisOuTcompetitionelectronics.com> wrote: > But if there was something other than three wires it would be > unconventional compared to what I have seen in other installations. That's true, but don't rule it out. Bad installation practices can result in cables being pinched or compromised in various ways. Then of course there are rodents out for a snack. Josh -- A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools. -Douglas Adams

At 10:53 AM 5/24/2011, Mark E. Skeels wrote: > > All in all this sounds like a bad idea. Even if you do just have cables > > with no electronics, you still end up with two separate signals squished > > next to each other in a common run. I expect you would get some crosstalk. > > > > >Interesting you should mention that. I noticed if I left one channel >disconnected there would be crosstalk across channels in the cable, but >when both sources were connected, it went away. Actually, although what Mark did is generally regarded as a 'bad idea', there are mitigating circumstances. 1) The guitars all had built-in preamps. That means that the output impedance presented to the interconnect cable was fairly low impedance, and probably significantly higher level than normally comes out of a guitar pickup. 2) The keyboard most likely has a very low output impedance (22R to 100R in any of the keyboards that I've worked on). In other words, it should have worked very well. Mark - I strongly suspect that one of your adapter cables has a short between pins 2 & 3 of the XLR connector. Either that, or one of the cables in the stage box has those pins shorted. Here's why I think that: even though the guitars have built-in preamps, most of the guitar preamps that I've worked on are built using discrete transistors rather than op-amps. The output impedance in often in the range of 1K to 10K. The keyboard most likely has a very low output impedance. Thus: if the guitar signal were shorted to the keyboard signal, the low output impedance of the keyboard would attenuate the guitar signal significantly. One way to have confirmed this would have been to have seen if the keyboard signal showed up in two channels on the mixer. That's my best guess based on what you have said so far. dwayne -- Dwayne Reid <.....dwaynerKILLspam@spam@planet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax http://www.trinity-electronics.com Custom Electronics Design and Manufacturing

On 5/24/2011 1:40 PM, Dwayne Reid wrote: > Mark - I strongly suspect that one of your adapter cables has a short > between pins 2& 3 of the XLR connector. Either that, or one of the > cables in the stage box has those pins shorted. > > Here's why I think that: even though the guitars have built-in > preamps, most of the guitar preamps that I've worked on are built > using discrete transistors rather than op-amps. The output impedance > in often in the range of 1K to 10K. > > The keyboard most likely has a very low output impedance. Thus: if > the guitar signal were shorted to the keyboard signal, the low output > impedance of the keyboard would attenuate the guitar signal significantly.. > > One way to have confirmed this would have been to have seen if the > keyboard signal showed up in two channels on the mixer. > > That's my best guess based on what you have said so far. > > dwayne > I will check the cables at the earliest opportunity.

On 24/05/2011 19:40, Dwayne Reid wrote: > The keyboard most likely has a very low output impedance. Thus: if > the guitar signal were shorted to the keyboard signal, the low output > impedance of the keyboard would attenuate the guitar signal significantly.. Yep, this is my guess still too, and thinking about it a problem with the cable does seem like a good bet. I can't think of another reason other than the output impedances that would account for the symptoms at present. > One way to have confirmed this would have been to have seen if the > keyboard signal showed up in two channels on the mixer. That's a good point - I think the OP mentioned he did try this and mentioned "crosstalk" across both channels. Trying again and checking the levels would be a good idea (e.g. whether it's a short and the levels are pretty much the same or just the small amount of coupling you would expect from a long run of cable passing unbalanced signals) Another quick way would be to use a multimeter if one is handy.