Help a newb - Passive I/O box

Howdy DIYers.

Any links/suggestions/orientation or constructive criticism corrections always welcome!

Objective:

Create a passive I/O project box that will serve to split a hot balanced stereo TRS output into Left and Right XLR channels as follows:

Source: RME Headphone DAC output - 75ohm (+13db) stereo TRS (female 6.3mm)

Target: 2x balanced line-level Active speakers/mid-fields.


Preferred Extras:


  • Add a stereo level attenuator (presumably before splitting stereo to L+R)
Luxury Options:

  • An additional (source) input (stereo TRS 6.3mm or 3.5mm), that sums/mults to the same single output pair (honestly, just to patch in my TV line output! I can control the level from the TV)
  • An additional summed to mono output (for re-amping to an amp for a passive Sub-woofer) - happy to build separate re-amper, so can keep this output balanced
I do have an XLR Y-split cable (the original cable that was sold to me on the basis that it contained the necessary resistors in the circuit to prevent the phasey nonsense that prevailed) that I could simply modify (add the in-line resistors and I guess a fuse like bridge to minimise connection pops etc.), not that I have any idea which pins to connect them to nor the appropriate values.


The box just seems like a more enjoyable project and a better overall solution.
Basically just want to use off the shelf components (I don't mind drilling/fabricating the box etc.), but I am not up to customising PCBs etc.




Fire away!


Cheers,
Dylan
 
> Source: RME Headphone DAC output - 75ohm (+13db) stereo TRS (female 6.3mm)

Unlikely to be stereo *and* balanced on one TRS.

What specific model is it?

How long are the runs to the monitors? How awful is the room electrical?

Thanks for the reply, PRR. The Audio Interface is exactly one
RME HDSPe MADI FX.
Analog Monitoring Output
The HDSPe MADI FX provides an analog stereo output in reference quality with RME's proven low-latency converter technology. The output is ideally suited for high- and low-impedance headphones.

Thanks to the powerful monitoring features of TotalMix, any input channel, recorded tracks, or complex sub-mixes can be selected for listening. An advanced Cue function allows each of the 98 possible stereo sub-mixes to be monitored with just a single mouse click.

HDSPe MADI FX - RME Audio Interfaces | Format Converters | Preamps | Network Audio & MADI Solutions

The run from the DAW output to where the split to L and R would occur could range from 1.5 to 6 metres (as the layout may change just because it's now going to be possible to rework things to more ideal positioning), but that's not including the distance from the junction to the speaker - that is around 3m each (to allow for tidy cable rigging).

Cheers
 
Two runs of 1.5 meters could lead to noise pickup, 6 meters very likely.
I'd go active instead of passive.
The XLR requirement for drive makes a commercial mixer difficult unless you buy/build 1/4" TRS to XLR adapters. The summing requirement could be handled by such as a PV8. That also has good TRS line drivers, 1/4 phone (main monitor control) and RCA jack (tape). I found a broken one for $45 including freight on e-bay and replaced the master volume pot and found 1 bad via causing left outputs to be dead. New ones, PV10 have autotune, which is weird and digital. PV6 might still be simple, there was a new one on e-bay here last month for $149 + freight.
If you do build something, I recommend the power supply stay outside the box in a wall transformer, to reduce chance for hum. I like 33078 op amps for fitting on dip boards which can be soldered by ordinary mortals with $60 irons. for the long line driver Peavey uses 4580 which is dip, or 4565 which is smt (in the PV8). All 3 are very quiet, but require close PS bypassing & 22-33 pf caps around the feedback resistor to not oscillate.
I've found recipe file boxes useful for this sort of small project, but you may prefer something more expensive & stylish. When cutting holes with power tools use safety glasses. The boards can be on standoffs. I could build this point to point on $2 dip project boards from farnell, but use berylium copper plate sockets for the op amps to avoid overheating them soldering.
Anything involving a speaker requires a power amp. Those require larger power supplies and ideally should be in a separate steel box from the mixing/line driving parts.
Good fortune on your project.
 
Last edited:
Two runs of 1.5 meters could lead to noise pickup, 6 meters very likely.
I'd go active instead of passive.
The XLR requirement for drive makes a commercial mixer difficult unless you buy/build 1/4" TRS to XLR adapters. The summing requirement could be handled by such as a PV8. That also has good TRS line drivers, 1/4 phone (main monitor control) and RCA jack (tape). I found a broken one for $45 including freight on e-bay and replaced the master volume pot and found 1 bad via causing left outputs to be dead. New ones, PV10 have autotune, which is weird and digital. PV6 might still be simple, there was a new one on e-bay here last month for $149 + freight.
If you do build something, I recommend the power supply stay outside the box in a wall transformer, to reduce chance for hum. I like 33078 op amps for fitting on dip boards which can be soldered by ordinary mortals with $60 irons. for the long line driver Peavey uses 4580 which is dip, or 4565 which is smt (in the PV8). All 3 are very quiet, but require close PS bypassing & 22-33 pf caps around the feedback resistor to not oscillate.
I've found recipe file boxes useful for this sort of small project, but you may prefer something more expensive & stylish. When cutting holes with power tools use safety glasses. The boards can be on standoffs. I could build this point to point on $2 dip project boards from farnell, but use berylium copper plate sockets for the op amps to avoid overheating them soldering.
Anything involving a speaker requires a power amp. Those require larger power supplies and ideally should be in a separate steel box from the mixing/line driving parts.
Good fortune on your project.

Thanks for the detailed response. Just to clarify, I'm I total novice - I haven't designed or built any audio circuits before, nor do I really know the fundamentals. I can interpret and follow circuit diagrams/instructions and solder things together though haha

Regarding cheaper/consumer mixers as a solution - I've looked into it and have another project I'm working on (concept stage) that is more "console routing" in nature (though Pro-Audio in purpose).

The situation is probably confusing as the Audio Interface in question (and the monitors) are professional grade gear, but my intended use of the Line/Phone out is certainly more consumer-level... this is because I had a make-shift pro-audio studio a number of years ago but already parted with my monitor controller (which I had connected to the Interface by AES (Crane Song Avocet has built in mastering-grade DAC via AES) - and I actually used the Line-Out just for headphones. I could just hook up 16 channels of MADI ADA and route-away till the cows come home, but I'm trying to be minimalistic.

Now I have the DAW and monitors in more of a "bedroom entertainment" layout, so rather than solve the basic problem by buying a pro-sumer monitor controller (or mixer), I'm opting to get my DIY hands wet on this.

I could just buy or build a TRS to TS+TS cable with XLR adaptors (or TRS to XLR+XLR, wired appropriately), but I thought it'd be more fun to build a box-based solution as my first DIY project, also the lack of a centralised stereo level control is quite annoying when multiple sources of varying levels are inputed (and my partner is not able to predict level differences when changing source, so a hands on single level pot would solve that issue).

The specs are as follows:

Code:
[B]Stereo Monitor Output (Phones):[/B]
Signal-to-Noise (DR): 110 dB RMS unweighted, 113 dBA @ 44.1 kHz
Channel separation: > 100 dB
Output: 6.3 mm TRS
Output impedance: 75 Ohm
Output level at 0 dBFS: +13 dBu


I thought It'd be clever/tidier to run a 1.5m or so stereo TRS cable from the PC to the "junction box" and then split left and right from there to each speaker (both active).

The subwoofer I mentioned, is actually a 15" vintage EV full range driver in a cabinet I had built to the original EV specifications, so it's not actually a sub haha It pumps out really creamy mids too - but I'll just use a cross-over. I already have several amps from which to choose to power the speaker. It's a luxury option, becuase the 8" drivers in my nearfields produce thunderous bass already. (Also, I could easily connect ADA converters and just dedicate one channel to send a balanced line out to a re-amper.

I get all these concepts, just not what is going on under the hood!

RME state that the output does not provide any speaker protection (ie the PC being turned off/on generates noise/pops etc.) - is this problem easy to solve too?


By the way, I'm not inherently against Active (I just figured that this line output should be capable of getting a line level signal to the speakers on a transparent/passive circuit). I really want to have the most transparent/simple solution possible.

Cheers!
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
...Anything involving a speaker requires a power amp.

In the speaker:
Target: 2x balanced line-level Active speakers/mid-fields.

The only analog output on this card is a hot headphone output. Stereo UN-balanced. While any decent active monitor can be fed un-balanced (and it greatly simplifies the VOLume-knob wish-item), I expect objection.
 

Attachments

  • HDSPeMADIFX-42.gif
    HDSPeMADIFX-42.gif
    23.6 KB · Views: 19
In the speaker:


The only analog output on this card is a hot headphone output. Stereo UN-balanced. While any decent active monitor can be fed un-balanced (and it greatly simplifies the VOLume-knob wish-item), I expect objection.


Ah you got the image attachment to work!


So it's UNbalanced.


In this case, should I not just connect a very short TRS to TS(L)+TS(R) cable to an Unbalanced to Balanced converter box then just run balanced XLR cables from there to each monitor?


In which case, I'm looking at creating an unbalanced to balanced box with level pot?
 
The subwoofer I mentioned, is actually a 15" vintage EV full range driver in a cabinet I had built to the original EV specifications, so it's not actually a sub haha It pumps out really creamy mids too - but I'll just use a cross-over. I already have several amps from which to choose to power the speaker. It's a luxury option, becuase the 8" drivers in my nearfields produce thunderous bass already. (Also, I could easily connect ADA converters and just dedicate one channel to send a balanced line out to a re-amper.

By the way, I'm not inherently against Active
Cheers!
Old R&R albums often had the bass on one channel & the treble on the other - as Beatles, Beach Boys and Mama & Papas. So to hear the bass on the sub, you're going to need to mix the two signals together with two resistors, 1k to 10 k. That is to not burn up your 75 ohms active drive. After the attenuation, it is going to require a lot of gain in your power amp to power the sub. Some PA amps have it, some will take a dynamic mike input, but not many. Plus if there is a 6 m run from passive mixer to power amp, with 1k source impedance, there is likely to be a lot of hum & AM radio pickup. Thus an op amp 10x to 30 x circuit after the mixing resistors. One op amp, 6 capacitors, 4 resistors, no big deal.
I've run single ended signal 4 m in my living room (RCA cable) from RIAA mixer to power amp, and had some hum. Putting the PV8 mixer between the RIAA mixer and the power amp, and running twisted pair TS cables, helped both hum & AM radio pickup. Didn't add any hiss noise, oddly. Your results may vary on your 6 m run, but I don't think so. If in a tent out back, with no AC generator, maybe you'll get away from the 50 hz EMF field of house wiring. Probably not.
Do the passive experiment, just do it in a box big enough to squeeze in a project board with a 2 channel op amp gain later. You'll won't waste any drilling & mounting of connectors that way. You'll need room for one volume pot, and a barrel type power connector on the back panel.
I handle the thump of turn on by 1. turning on the mixer(s) 2. turning on the power amp 3. turn on turntable, FM radio, or CD player. To turn off I reverse the procedure. It is a defined path around the room, not all that annoying to me. What is really cool about a mixer, I don't have to walk to change the selector switch on the preamp, every time I change music source. With 33078 (RA-88a) and 4565 (PV8) op amps, they don't add audible hiss. (RIAA mixer did add hiss & hum until I got rid of the internal power transformer and 4558 op amps).
One caveat about no hiss with 2 mixers: I'm deaf above 14 khz. If you're male older than 10, you probably are too. I've used ear plugs aggressively since ROTC camp 1969 trained me on them.
 
Last edited:
Old R&R albums often had the bass on one channel & the treble on the other - as Beatles, Beach Boys and Mama & Papas. So to hear the bass on the sub, you're going to need to mix the two signals together with two resistors, 1k to 10 k. That is to not burn up your 75 ohms active drive. After the attenuation, it is going to require a lot of gain in your power amp to power the sub. Some PA amps have it, some will take a dynamic mike input, but not many. Plus if there is a 6 m run from passive mixer to power amp, with 1k source impedance, there is likely to be a lot of hum & AM radio pickup. Thus an op amp 10x to 30 x circuit after the mixing resistors. One op amp, 6 capacitors, 4 resistors, no big deal.
I've run single ended signal 4 m in my living room (RCA cable) from RIAA mixer to power amp, and had some hum. Putting the PV8 mixer between the RIAA mixer and the power amp, and running twisted pair TS cables, helped both hum & AM radio pickup. Didn't add any hiss noise, oddly. Your results may vary on your 6 m run, but I don't think so. If in a tent out back, with no AC generator, maybe you'll get away from the 50 hz EMF field of house wiring. Probably not.
Do the passive experiment, just do it in a box big enough to squeeze in a project board with a 2 channel op amp gain later. You'll won't waste any drilling & mounting of connectors that way. You'll need room for one volume pot, and a barrel type power connector on the back panel.
I handle the thump of turn on by 1. turning on the mixer(s) 2. turning on the power amp 3. turn on turntable, FM radio, or CD player. To turn off I reverse the procedure. It is a defined path around the room, not all that annoying to me. What is really cool about a mixer, I don't have to walk to change the selector switch on the preamp, every time I change music source. With 33078 (RA-88a) and 4565 (PV8) op amps, they don't add audible hiss. (RIAA mixer did add hiss & hum until I got rid of the internal power transformer and 4558 op amps).


Thanks so much for all the information and guidance! It's hard to make logical sense of things or draw sensible conclusions when your version of the "big picture" is a 2 year old's finger-painting. :D


I didn't know that some older albums were mixed as treble/bass like that! Though I have noticed a lot of Motown recordings have bass (guitar) very hard left (rather than in the centre), but that is probably just a sound-stage mixing decision (like drum-kits were often stereoscopically positioned to emulate a live concert listener perspective - off-centre placement of drums on live stages was more common of bands of that era and its dominant genres).





Regarding the sub channel -all the 2.1 monitor controllers I have checked out appear to split off the source signal in parallel and sum it to mono (perhaps with two resistors in series, as you said?). Would this technique get the same result?


So to hear the bass on the sub, you're going to need to mix the two signals together with two resistors, 1k to 10 k. That is to not burn up your 75 ohms active drive.

This is probably what it is that I have seen cable techs do for DJs ... (adding resistors inside their cables to stop them from "burning up" the source).


There are several placement options for the power amplifer, passive "sub", and the original source itself (the DAW), so I have some flexibility in terms of defining which cables are long and which are short.


For example,

- I could place the power amplifier right next to the "monitor controller" (centre of the studio monitors under the TV [as I have long Speakon leads]) or;
- I could have the monitor controller on the other side of the room next to both the "sub" and the PA and have longer runs to the Active Monitors or;
- Probably plenty of other options.



The current distance from where the "monitor controller" would be (under the TV) to the power amplifier (and sub) is around 4 - 5 metres at a glance (the length of the room +/-).


I handle the thump of turn on by

Oh, it's not a problem for ME - I essentially do the same as you (or at the very least turn off the active speakers before anything else). I'm just trying to make this a bit more user-friendly for the co-habitant of this "audiophile" bedroom :D
 
Going unbalanced into active studio speakers / studio amps from a computer is just asking for trouble with ground loops - BTDT. Had to crank the input level trim way down back then. (A Behringer HD400 eventually came to the rescue.)

While you can always convert unbalanced to (impedance) balanced without too much of a problem, this requires constant output impedance - and if you need a volume control as well, that won't be the case in an all-passive setup. Like ol'd Einstein said - as simple as possible, but no simpler. Your idea qualifies as "too simple", I'm afraid.

Even if you do decide to go active, it seems quite hard to justify going DIY for this. Decent compact mixers, especially used, are cheap. You literally can't make 'em for what they cost. I would look for a decent mid-level Mackie or Yamaha or whatever (the kind with an external power supply). I almost can't imagine that there isn't anything of the sort floating around in Melbourne of all places. (Maybe a broken one if you insist making it a DIY project, but fixing stuff usually works much better when you're not totally green any more.)

Then you could concentrate your DIY ambitions on making some cables as far as still needed - 1x stereo TRS to 2x whatever unbalanced inputs the mixer takes (coax or shielded twisted pair with TP tied together), 2x TRS to XLR male balanced. I would make the unbalanced cable only as long as absolutely required, the balanced side would be far less critical.

Sorry to say, but sometimes the obvious solution is in fact the best one. People in the DIY realm have a certain tendency of trying to shoot themselves in the foot as hard as possible at times - this is definitely not my preferred approach.
 
Going unbalanced into active studio speakers / studio amps from a computer is just asking for trouble with ground loops - BTDT. Had to crank the input level trim way down back then. (A Behringer HD400 eventually came to the rescue.)

While you can always convert unbalanced to (impedance) balanced without too much of a problem, this requires constant output impedance - and if you need a volume control as well, that won't be the case in an all-passive setup. Like ol'd Einstein said - as simple as possible, but no simpler. Your idea qualifies as "too simple", I'm afraid.

Even if you do decide to go active, it seems quite hard to justify going DIY for this. Decent compact mixers, especially used, are cheap. You literally can't make 'em for what they cost. I would look for a decent mid-level Mackie or Yamaha or whatever (the kind with an external power supply). I almost can't imagine that there isn't anything of the sort floating around in Melbourne of all places. (Maybe a broken one if you insist making it a DIY project, but fixing stuff usually works much better when you're not totally green any more.)

Then you could concentrate your DIY ambitions on making some cables as far as still needed - 1x stereo TRS to 2x whatever unbalanced inputs the mixer takes (coax or shielded twisted pair with TP tied together), 2x TRS to XLR male balanced. I would make the unbalanced cable only as long as absolutely required, the balanced side would be far less critical.

Sorry to say, but sometimes the obvious solution is in fact the best one. People in the DIY realm have a certain tendency of trying to shoot themselves in the foot as hard as possible at times - this is definitely not my preferred approach.


Oh, I agree with Einstein (and you). Any attempt at finding a solution to this problem that may look like an over-simplification or over-complication is just an extension of my general lack of understanding of things.


I have started studying up on the fundamentals of electronics, but having no prior scholastic experiences in the sciences (at all), it is a minefield of new concepts - nothing too complicated for me to understand on a conceptual level so far, but it takes me a long time to properly absorb each layer of information before I can move forwards with less likelihood of becoming confused immediately haha


Perhaps I picked the wrong problem to try to solve as a DIY project huh?


I had thought it would be a little more simple ... from the layman's perspective, connecting an output to speakers should be pretty straight forward - adding a box in between that allows volume control seems like it should be simple too.




Am I correct then in stating that unless I'm plugging in headphones, I shouldn't use the 75ohm output to interface with speakers? It just seems like it shouldn't be so hard to do something that many users would inevitably consider doing.
 
Am I correct then in stating that unless I'm plugging in headphones, I shouldn't use the 75ohm output to interface with speakers? It just seems like it shouldn't be so hard to do something that many users would inevitably consider doing.
You can do it, but it is likely to be noisy. A cheap experiment except for all the hole drilling & sawing. Instructive. Don't try to parallel the two channels for centered bass without 1k resistor in series with each hot. Use a recipe box from charity resale shop, shouldn't cost you more than $30 except for tools. I don't buy $$$ punches for big or square holes, I drill many 1/8" holes and connect the dots with a Stanley carbide hacksaw blade.
To sgskloss point, you'll get good music quicker with a new mixer, but repairing a used one may be above your training. PV6 & PV8 have TRS cable drive (ring is phantom but that works quietly into twisted pair) I don't know about the cheaper mixers out there are the same. I changed the volume pot on the PV8 in only 8 months, most of it staring at it and wondering what to do next. Finding the reason for a dead channel took another 3 months, probing with an analog VOM to follow the music flow. Bit subtle for a newbie. I'm experienced enough to not blow up IC's with the probe.
Read the analog devices tutorial on single op amp gain circuit, the inverting one is fewer parts. Or another electronics tutorial. It's $10 from turning a passive splitter box into an active drive. Plus $100 in test equipment, either a scope or a good analog VOM with 20 vac & 2 vac scales. DVM make up random numbers on music, are great for DC analysis.
BTW, I do experiments that fail. Trying to do CCS (constant current source) this month, JFET single part solution was a big failure. Actual Idss was 7 times the "typical" on the datasheet, WAAY too big, and current out was sensitive to the voltage it pumps into, also.
 
Last edited:
Am I correct then in stating that unless I'm plugging in headphones, I shouldn't use the 75ohm output to interface with speakers? It just seems like it shouldn't be so hard to do something that many users would inevitably consider doing.
Then you would need some speakers that are a bit more accommodating to the consumer audio realm though. (Home) Studio gear has a habit of being IEC Class I (mains earth referenced), just like your PC - not a good combination with unbalanced audio connections. You'd need something floating (IEC Class II appliance, double insulated), as common in consumer audio and traditional hi-fi.

Your headphone output ultimately is much the same as a consumer line/headphone output, just a bit louder. So you can expect to encounter the very same problems that people have faced trying to connect studio gear to those for years - and the same solutions.

If you didn't need an external volume control, it wouldn't be that bad. You could use an external line isolator like the aforementioned Behringer HD400 and enjoy your silence. But when you are looking at kludging together that plus a monitor controller, the (technically better) mixer starts looking like the more attractive solution, especially if you can also make some "ground lift" (shield disconnected) balanced cables for the run from mixer output to speaker amp input (otherwise ground loop currents could still take the route PC output ground --> unbalanced audio cable shield (the problematic part) --> mixer --> shield of balanced connection --> speaker amp ground).