Studio Cue System Project

[Potato Cakes]

Hello, Everyone,

I've been working on a custom cue mixer for the studio I've move into that has a simplified personal mixer station and a 75W headphone amp (2x 75W amps tied together for stereo headphones). The system that was in place was a Hear Technologies system which it's headphone amp would start distorting when used at levels needed for full band tracking sessions. I believe Mytek has one that is 12 channels and has a hefty amp built in it, but these are pretty expensive and require more expensive multi pair cables. I also wanted to avoid having a massive amount of cable to manage in a small space. So my thought was build a system that used a single shielded CAT5e/6A cable to send 6 unbalanced lines to a small mixer and return 2 lines for a passive stereo mix back to a mainframe that would provide make up gain and balance the signal and then send to a multi channel headphone amp and then out a cue cable via XLR back to the mixer for headphone signal. I got every up and running and it does exactly what I intended and it has plenty of power to drive 32 ohm headphones to make them much louder than I would ever want to listen to something, so for functionality it is a success. I tried to make with as minimal off board wiring as possible and the 40 pin IDC connector is directly compatible with the JLM 12PAK Blue amp.

But it is not quite as perfect as i would like. It does have a low level buzz in it the system that gets quieter depending on where the level knobs are turned. When anything signal is passed through it this is a non issues, but it is still there. I suspect having the large transformer in the unit has something to do with it. I will say that when the CAT5e cable is pulled from the unit and just the cue cable is connected, it is very quiet, so I know the noise isn't being induced from the make up gain to amp out.

There is also low level cross talk that can be heard even if the level control for the channel is turned all of the way down. I am using 15k resistors for the bus resistors which may be the less than optimal value for this application. There are 5 inputs on each summing network (5/6 are fixed stereo) so that gives me 3k bus impedance. The spec sheet for the NE5532 says its input impedance is 30k minimum and 300k typical, but I don't know if that is correct number to use when trying to match impedance. The other mixers aren't finished yet as I am waiting on the stereo jacks and I'm going to try a couple different summing resistors to see if I can better results. I have also read where the output of the summing network can be strapped with a resistor to ground to change the impedance but again I'm not sure is that is suitable in this application.

The mixer in the photo is the first one I made got mangled as I miscalculated connector clearance and had to move the XLR and etherCon around to make it fit. The upside down XLR was corrected.

I'd be curious to know if there is a fairly simple passive solution that would solve some of the signal bleed when the pots are turned all of the way down. I think the issue is related to voltage getting into the ground potential since this is unbalanced. I know I could have used another op amp after the potentiometer and then panning and summing network, but I would have had to have put them on the mixers and that would require power which I didn't want to have to deal with running power and adding to cable mess.

I'll swap out the PSU sometime this week for the SMPS and see if that helps with noise. It would be nice to at least have the buzz issue sorted. I can deal with the cross talk if I have to.

Thanks!

Paul

 

[ruffrecords]

From you description is sounds like the mixer is totally passive so you are running signals to and from it unbuffered. The is the most likely cause of your buzz. I bet it is lowest at max and min positions and worst somewhere in between. You might need to slug the bus to reduce its source impedance in order to fix the buzz. Could you post its schematic?

Cheers

ian

 

[abbey road d enfer]

You have two different issues there.
Hum may be entirely due to the power xfmr stray field. Your planned experience with a smps should tell you. There may also be induced hum in the cat5 cables; moving them should tell you how much.
The issue of crosstalk is a different subject. It's due to the (admittedly) low voltage induced in the signal return ("ground") conductor, resulting from current circulating in the potentiometers, combined with capacitive coupling between conductors.
There is not much you can do about it. Increasing the value of potentiometers would force increasing bus impedance, which in turn would increase noise and capacitive crosstalk.
Maybe you can find a cable with a less resistive shield...?

 

[Potato Cakes]

From you description is sounds like the mixer is totally passive so you are running signals to and from it unbuffered. The is the most likely cause of your buzz. I bet it is lowest at max and min positions and worst somewhere in between. You might need to slug the bus to reduce its source impedance in order to fix the buzz. Could you post its schematic?

Cheers

ian

Hi, Ian,

Schematic for project is in the first post. I am going into the mainframe balanced XLR and it is being unbalanced with INA134 and then sent to the mixer via CAT5e, mixed passively, then sent to NE5532s for make up gain and balancing, then balanced signal to the amps.

Thanks!

Paul

 

[Potato Cakes]

You have two different issues there.
Hum may be entirely due to the power xfmr stray field. Your planned experience with a smps should tell you. There may also be induced hum in the cat5 cables; moving them should tell you how much.
The issue of crosstalk is a different subject. It's due to the (admittedly) low voltage induced in the signal return ("ground") conductor, resulting from current circulating in the potentiometers, combined with capacitive coupling between conductors.
There is not much you can do about it. Increasing the value of potentiometers would force increasing bus impedance, which in turn would increase noise and capacitive crosstalk.
Maybe you can find a cable with a less resistive shield...?

I do have some Mil-spec CAT6A wire that are heavily shielded that I just need to put ends on them and try. When I swap out the PSU for the SMPS I will report back.

Thanks!

Paul

 

[ruffrecords]

Hi, Ian,

Schematic for project is in the first post. I am going into the mainframe balanced XLR and it is being unbalanced with INA134 and then sent to the mixer via CAT5e, mixed passively, then sent to NE5532s for make up gain and balancing, then balanced signal to the amps.

Thanks!

Paul

OK, that makes sense. So, to be clear, the NE5532 is definitely acting only as a make up amplifier, it is not configured as a virtual earth?

As others have said, your crosstalk probleem is certain due to the combination of the resistance of the CAT5 cable screen plus the off resistance of the pots. With regular rotary 10K pots you rarely get better than 60dB of 'offness'.

Cheers

ian

 

[Potato Cakes]

OK, that makes sense. So, to be clear, the NE5532 is definitely acting only as a make up amplifier, it is not configured as a virtual earth?

As others have said, your crosstalk probleem is certain due to the combination of the resistance of the CAT5 cable screen plus the off resistance of the pots. With regular rotary 10K pots you rarely get better than 60dB of 'offness'.

Cheers

ian

Correct. The NE5532s are providing make up gain and then balancing the signal. I used 10k pots for level and pan because that I what I have seen used in consoles for panning and faders. Obviously those same consoles have another amplifier for the insertion loss of the fader. In my application, would higher value pots provide a bit more separation? Say something as high as 100k?

Thanks!

Paul

 

[Potato Cakes]

Also, thanks Ian and abbey road for the assistance. This project is one of the last things I need to complete before I can put down the soldering iron and get back to focusing on making records.

Thanks!

Paul

 

[abbey road d enfer]

Correct. The NE5532s are providing make up gain and then balancing the signal. I used 10k pots for level and pan because that I what I have seen used in consoles for panning and faders. Obviously those same consoles have another amplifier for the insertion loss of the fader. In my application, would higher value pots provide a bit more separation? Say something as high as 100k?

Thanks!

Paul

The problem with 100k pots is that you'll end up with a bus impedance too high, which results in HF loss and increased hiss.

 

[Potato Cakes]

Could this be compensated by strapping a 10k resistor to ground after the summing network before make up gain?

 

[abbey road d enfer]

Well, yes, but this will also create attenuation of the signal, so in the end you get a somwhate better frequency response, but worse S/N.
The actual bus impedance of your system is about 10k. Putting a 10k in parallels would drop level by 6dB.
What is the capacitance of your CAT6 cables, and what length?
Actually, for musician foldback, you don't strictly need 20hz-20kHz, since most of them are partially deaf.

After more thinking, you may try 100k pots without changing the bus resistors. It wouldn't change much the bus impedance but may improve x-talk.

 

[Potato Cakes]

Ha! I definitely don't need 20Hz-20kHz!

I'll change out the pot values when I get back from traveling. I still haven't changed out the power supply to see if that improves the buzz noise level. As for the CAT6A cable capacitance I will have to find the spec sheet. I'm using varying lengths, the longest being 30ft.

Thanks!

Paul

 

[dmp]

You may be interested in looking at the HDS-6 and HR-6 cue mixer schematics. It also sends unbalanced signals for four mono + a stereo feed to the cue boxes, on 2 ethernet cables.
I have this system and it is a little hissy but it is totally fine for its purpose.
I don't know why they didn't send balanced over the ethernet since it has twister pairs and two ethernet cables is enough for 6 balanced lines +22v/-22v and ground.

 

[Potato Cakes]

Got the SMPS in the unit and now it is very quiet. If turn the headphone volume all the way up and nothing else is going on, I can hear the faintest of buzzes. I never plan on having the headphone volume past 1 o'clock. Even at full volume there is no hiss. Other studio cue headphone amps I've used have a hiss well before this level. Very pleased with it indeed. I will see about getting some 100k pots to mitigate minor issue of crosstalk. The only problem is that the 100k audio pots readily available in the 9mm format that will fit these boards do not have bushing, which is how I'm attaching the mixer PCBs to the top panel. I may just have to drill some holes and add some stand offs.

Thanks!

Paul

 

[Whoops]

I hope you solve your issues with the unit, you got really nice advice here.
I too started to build something like that in the past, actually a quite similar concept to yours, this was around 2011/2012.
I had a complete Avion personal mixer system (16 channels) in the studio I worked, musicians loved it and I couldn't be happier by not having to to make all their individual mixes and monitoring needs, so when I left the studio and got freelance I wanted to DIY a poor man's Avion.
As I remember the idea was to have 2x shielded CAT5 cables going to each box, yielding 7 balanced inputs plus 2 wires for + and - voltage to feed the headphone amp.
I even started to build it and do some prototypes, but then it got complicated into many details and it would be expensive in the end, cost of the parts, cost of all the metal work and the many many hours of work involved.
Then Behringer came with the Powerplay P16 mixing system, an affordable Avion type system, it has 16 channels, EQ and limiter and its unbeatable for the price, so I completely ditched the DIY idea and went for the Behringer systems, I'm glad I did as it gave me much more time in my life to concentrate in music and I even think it was cheaper in the end.




I don't want in any way to criticize your goals or demoralize you, I wish you all the best for your project, but just consider that in the end you will only have 6 channels, which I think is a bit short for studio needs and also consider the amount of money and time involved (and time is also money) in trying to achieve something that has more limitations when compared to an affordable and readilly available (already done) commercial system.

Wish you all the best and congrats for the work you already done

 

[Potato Cakes]

I am very aware of the P16 style mixers, which require the use of their console. The purpose of my project is to keep the monitor chain analog. This is important when recording vocals, as even with 2ms of latency, that small amount of delay combined with the chest resonance starts causing disorientation with a singer. And since I record everything at 192kHz, I don't have to take the DAW with monitor mix routing and keeping up with capturing audio. Plus, I wanted to provide more than enough power for headphones, which is a big complaint of some of Aviom/Hear Technologies systems. Also, the noise floor results I am getting with my system are almost nonexistence compared to the P16 systems.


Thanks!

Paul

 

[Whoops]

The purpose of my project is to keep the monitor chain analog. This is important when recording vocals, as even with 2ms of latency, that small amount of delay combined with the chest resonance starts causing disorientation with a singer.

I understand most of the points you talked about but as far as the very very small latency of a systems like Avion, Hear Back or P16 what I can say from personal experience of using those type of systems (specially the Avion) for a tremendous amount of records and clients over the last 17 years is that I never had anyone complain, never, no musician or singer. Neither I ever had any disorientated singer at the studio because of the monitoring system.
Actually I had the oppositive, clients being really happy that they can have their personal mixes and do their own mix.

I don't like the Hear back systems that much, it's limited, only 8 channels without Pan, and I don't like it's sound. But Avion had 16 channels with individual PAN and a simple EQ in the master, it was very flexible, no complaints at all of the sound or the marginal latency.
P16 is a knock off of the Avion, it's very flexible and it's much cheaper, it seems to me the "Hiss" is higher than the Avion, but I guess I need to try it again to be sure.

The photos of the work you have already done look pretty good, congratulations for your work and wish you achieve your end goals. In no way I wanted to criticize you I just wanted to show an alternative in case you got stuck with your project like I did.

Good Work and have fun while doing it

 

[isophase]

Very nice project Potato Cakes and hat off for all the work!
I get your point Whoops and I do agree with you about the time/cost balance of starting such a project.
I am familiar with all the headphone mixer systems mentioned here, I also really love the Aviom system but this unit is quite too expensive for me, I also like the Berhinger system and most probably the system I will buy at some point fir my personal studio..
I also used to like very much the Furman system for its very simple/ergonomic design, but I find they often lacked some juice.
Looks like Potato’s system has « fixed » all the flaws of the Furman system

Many years ago, I was thinking of building a four channel passive headphone mixer system (with one stereo and two mono channels) with power attenuators and power amplifiers. The goal was to minimise the cables on the studio floor and use a single 8 points speakon connector..
there should be a subject about it here on the forum.
As whoops said, in the end the time/cost balance thing wasn’t really worth it.

Cheers

 

[Potato Cakes]

So I just swapped out the 10k potentiometers with 100k but I have the same level of bleed. The only difference is the reduction in the resolution of the fader level as most of the gain is in the last third of the rotation. I even changed the bus resistors from 15k to 47k. I'm wondering if I isolated each channel's potentiometer connection to ground with a 100R resistor parallel with a 100nF cap. Probably would be easier and smarter to do this at the input XLR. I can make some XLR cables with the ground connection removed on the mainframe same to confirm this theory.

Thanks!

Paul

 

[abbey road d enfer]

So I just swapped out the 10k potentiometers with 100k but I have the same level of bleed. The only difference is the reduction in the resolution of the fader level as most of the gain is in the last third of the rotation. I even changed the bus resistors from 15k to 47k.

Have you also changed the pan-pots to 100k?

I'm wondering if I isolated each channel's potentiometer connection to ground with a 100R resistor parallel with a 100nF cap.

Waste of time IMO.
You should take some time for analyzing the true cause of leakage.
Does leakage depend on volume settings in the other boxes?
Does leakage happen with one box only or does it increase with the number of bowes connected?
Does leakage increase with length of CAT5 cable?
My understanding is that leakage ciomes mainly from current circulating in the volume pots AND pan-pots.
Replacing just the volume pots changes only fractionally.