100% passive monitor control ideas

[dagoose]

I'm in the middle of designing a totally passive balanced monitor controller and i'm thinking of some stuff that i'm not sure about or might some revision maybe.

The main volume is going to be a stepped attenuator (4 deck 24 position elma of 10k load in total) expensive one, but it think it's worth it, it's allready in order and the resistors are waiting in line
Next to that, i going to do 3 balanced inputs with a 3x4 lorlin switch, pretty straigth forward.
I'm going for a left, right and stereo mute, so 3 mute buttons in total, relais-switched.

I need at least 2 monitor outputs with a seperate subwoofer out that can be switched off and on. (relais)
That is one of the things i'm thinking about, can i just parallel the monitor out with the sub out and switch the sub on and off (balanced)? or do i need to do anything for the right impedance/volume changes when switching sub in/out?

Next up, polarity switches, (left and right so 2 switches) i think it's enough to reverse the balanced signal + and - polarity or do i need more? don't think so but not sure if there is a better way.

Then, a mono summing switch, paralleling left and right will give me mono so i think that is straigth forward as well, only thing i'm thinking about is maybe the impedance, do i need anything else then just a left + right?

The case will be a 1u 19inch case and the relais will be switched with nice lightend switches.

Like i said, i'm going 100% passive, maybe i'll add a led VU meter if i can find a decent one with at least 20 leds per channel (anyone?) but not really a must.

Any ideas or maybe some people who did this before and have some stuff to think about?

 

[martthie_08]

some ideas:

• if you already have relays in your box, you might as well do the input switching with DPDT relays and get a nice one pole switch with a good feel for the input selector instead of the crappy lorlin which will sooner or later fail you.
• will your subwoofer be parallel to your main speakers or does it have a crossover network to which your stereo speakers are attached to?
• are you sure you want polarity switches on a monitor controller?
• mono summing will not be that easy, look here: http://www.groupdiy.com/index.php?topic=7351

that said, I built myself an unbalanced passive monitor controller that works splendid. Unbalanced may or may not be an issue for you.

 

[radiance]

Yeah...as always...the mono summing....

 

[matthias]

I will build a very similar unit next week...

2 stereo inputs (relais switched)
2 stereo outputs (relais switched)
active mono summing circuit (relais switched)
mute switch (stereo, relais switched))
headphone amp (based on ti TPA6120A)
external switching power supply (+12V and +5V)

instead of a rotary switch, I use a digital controlled 64 step-relais circuit controlled by an rotary encoder (-> www.dantimax.dk) load is 5k (half of the input impedance of my monitor speakers)

NYD posted a nice schematic for an passive monitor controller...
I think this is a very good starting point...

I also thought about a led peak meter but I think it doesn't make much sense here, because the signal only gets attenuated and not amplified...

for what reason do you need a phase switch?

 

[dagoose]

[quote author="martthie_08"]some ideas:

• if you already have relays in your box, you might as well do the input switching with DPDT relays and get a nice one pole switch with a good feel for the input selector instead of the crappy lorlin which will sooner or later fail you.
• will your subwoofer be parallel to your main speakers or does it have a crossover network to which your stereo speakers are attached to?
• are you sure you want polarity switches on a monitor controller?
• mono summing will not be that easy, look here: http://www.groupdiy.com/index.php?topic=7351

that said, I built myself an unbalanced passive monitor controller that works splendid. Unbalanced may or may not be an issue for you.[/quote]

Maybe i'll do that with the input switch, now i need a nice 3 position switch instead of the lorlin.
But is the lorlin that bad then? i use 'm a lot here.. what is better in this case? i think elma?

About the mono summing, i have a passive monitor controller (forgot the brand but i posted it with some pics here somewhere in the other monitor control topic) right now and the mono summing is done with just paralleling left and right (just a switch, no resistors or whatever) and it works fine i think but i'll check with the active summing and maybe combine it.

The sub has it's own x-over and the mains are in fullrange so actually they just go parallel but i think i'm going to have some impedance problems if i just parallel 'm and switch the sub in/out which means instant impendance change.
I think i need some kind of resistor based summing for this, but how is this done best?

I want to go passive and balanced all the way, i know it's not really necassery but i like it balanced a bit more and the elma 4 pole is allready on it's way

 

[dagoose]

OK, the Elma 24 steps 4 pole switch is in, time to start putting 4x 24 resistors to it
Before i start soldering everything and later find out that the values aren't exactly what i needed i'll post it here for review (if anyone wants to)
I want the elma to control the main level of the monitors with a 83db calibrated point (marked 0 on the panel) at about 2/3 of the scale.
I found a, what i think, good 10k log 1db steps att (marchand electronics attenuators) schematic and modified the resistors just a bit to these values:

820r R22 R23 R27
750r R20 R21
680r R18 R19
560r R16 R17
430r R14 R15
330r R12 R13
270r R10 R11
240r R8 R9
180r R6 R7
130r R4 R5
100r R2 R3

What do you think? will this be the attenuator i am looking for?

 

[rob_gould]

[quote author="dagoose"]maybe i'll add a led VU meter if i can find a decent one with at least 20 leds per channel (anyone?) but not really a must.
[/quote]

Have you seen this LED meter that forms part of the pico compressor website?

http://www.picocompressor.com/PicoComp/PCB-Kits/Documents/GR_Meter_20/GR_Meter_20.htm

It comes in 20 or 10 segment versions, it's based around the LM3914 chip so easily available, plus Wayne who sells this stuff is a super nice guy and there's even a forum over there to ask questions.

Definitely worth a look I'd say if you're interested in LED metering.

Are you going to be releasing build notes or a schematic when you've finished putting this together? This looks like just the kind of thing I'm looking to put together myself at the moment :wink:

 

[Harpo]

[quote author="dagoose"]OK, the Elma 24 steps 4 pole switch is in, time to start putting 4x 24 resistors to it
Before i start soldering everything and later find out that the values aren't exactly what i needed i'll post it here for review (if anyone wants to)
I want the elma to control the main level of the monitors with a 83db calibrated point (marked 0 on the panel) at about 2/3 of the scale.
I found a, what i think, good 10k log 1db steps att (marchand electronics attenuators) schematic and modified the resistors just a bit to these values:

820r R22 R23 R27
750r R20 R21
680r R18 R19
560r R16 R17
430r R14 R15
330r R12 R13
270r R10 R11
240r R8 R9
180r R6 R7
130r R4 R5
100r R2 R3

What do you think? will this be the attenuator i am looking for?[/quote]
Hi dagoose,

from your posted values the step next to off-position is -21,5 dB, the remaining 21 following steps vary from 0,75dB to 1,2 dB for your total of 9800 ohms. You may change some values to get a more linear dB attenuation. Maybe this excel sheet can help. Just edit the red cells to your needs. Other calculators show up in the meta.
Your 'about 2/3 of the scale' doesn't make much sense, as you have a switch with defined step positions, so pick one step. The posted scale is showing above 0dB values that would call for an (no more passive only) amp stage to get there. I'd scale it from -xxdB to 0dB to avoid confusion and may put in an additional dim switch. YMMV.
-Harpo

 

[Freddy G]

I'm building a passive controller as well. Does anyone know if there's any problem with using a switching power supply in the monitor controller's enclosure? I heard they are not good for audio, I've never used one...always just made linear supplies. But all this supply needs to power are the relays, so it's not for audio signal or anything. The signal will remain completley passive. Just wondering if there might be problems with the proximity of the supply to the unbalanced signal.
Any advice appreciated,
Freddy :sam:

 

[Tubefreak]

Good luck with your project. You might want to think about a ladder-type attenuator instead of a serial-type. Here's a nice link: http://www.penguinlovers.net/audio/Attenuator.html

Veel succes
Maarten

 

[dagoose]

OK, i have some progress here!
Thanks for the excell sheet with the resistor values, that is just what i needed
I'm going for a xx > 0db scale, makes more sense indeed since i'm going passive i can't go up to above 0db.
The only non passive thing i'm going to use is the mono summing.

I have a small question, i have these really nice buttons with lights in them which are actually a to color led right now, i think it might look nice when the switch is green, it's not muted and when it's red the channel is muted or is mono, whatever.
When the polarity of the led is reversed you have the different color.
I was thinking of doing the polarity reversing with a relays but you'll need 1 relais extra per switch to reverse ploarity, i think that could be done much more easy with some kind of opamp or whatever.
So what i basically need is a circuit that is flipping polarity of a 3v line to drive the 2 color leds.
Anyone?

(the swithes i'm talking about)

http://www.dagoosemusic.nl/diy/monitor/monitorcontroller.pdf (the design of the (schaeffer) frontpanel)

I'm also not sure about the extra input pot since i want to go balanced all the way i need a 12 pos 4 pole or a 4 pole potmeter for these.
Input 3 is just going straigth to the attenuator (which will be a real big aluminium knob on a 4 pole 24 steps elma switch which is allready here)

 

[matthias]

When the polarity of the led is reversed you have the different color.
I was thinking of doing the polarity reversing with a relays but you'll need 1 relais extra per switch to reverse ploarity, i think that could be done much more easy with some kind of opamp or whatever.

I think you will need a dpdt relais for this... it would be easier if the switch had enough poles... but I think you will need some relais anyway...

do you think you need the in1 and in2 level controls..??
if you use a 4pole potentiometer you are all the way balanced, but you don't benefit from the low tolerance stepped attenuator... I think you should adjust the output level of the source...

 

[Harpo]

[quote author="dagoose"]OK, i have some progress here!
Thanks for the excell sheet with the resistor values, that is just what i needed
I'm going for a xx > 0db scale, makes more sense indeed since i'm going passive i can't go up to above 0db.
The only non passive thing i'm going to use is the mono summing.

I have a small question, i have these really nice buttons with lights in them which are actually a to color led right now, i think it might look nice when the switch is green, it's not muted and when it's red the channel is muted or is mono, whatever.
When the polarity of the led is reversed you have the different color.
I was thinking of doing the polarity reversing with a relays but you'll need 1 relais extra per switch to reverse ploarity, i think that could be done much more easy with some kind of opamp or whatever.
So what i basically need is a circuit that is flipping polarity of a 3v line to drive the 2 color leds.
Anyone?

(the swithes i'm talking about)

http://www.dagoosemusic.nl/diy/monitor/monitorcontroller.pdf (the design of the (schaeffer) frontpanel)

I'm also not sure about the extra input pot since i want to go balanced all the way i need a 12 pos 4 pole or a 4 pole potmeter for these.
Input 3 is just going straigth to the attenuator (which will be a real big aluminium knob on a 4 pole 24 steps elma switch which is allready here)[/quote]
for your polarity flipping led this quick and dirty switch hitter may do the trick.
For the sub out (assuming mono) build another active summer (like the mono part)

 

[dagoose]

[quote author="Harpo"][quote author="dagoose"]OK, i have some progress here!
Thanks for the excell sheet with the resistor values, that is just what i needed
I'm going for a xx > 0db scale, makes more sense indeed since i'm going passive i can't go up to above 0db.
The only non passive thing i'm going to use is the mono summing.

I have a small question, i have these really nice buttons with lights in them which are actually a to color led right now, i think it might look nice when the switch is green, it's not muted and when it's red the channel is muted or is mono, whatever.
When the polarity of the led is reversed you have the different color.
I was thinking of doing the polarity reversing with a relays but you'll need 1 relais extra per switch to reverse ploarity, i think that could be done much more easy with some kind of opamp or whatever.
So what i basically need is a circuit that is flipping polarity of a 3v line to drive the 2 color leds.
Anyone?

(the swithes i'm talking about)

http://www.dagoosemusic.nl/diy/monitor/monitorcontroller.pdf (the design of the (schaeffer) frontpanel)

I'm also not sure about the extra input pot since i want to go balanced all the way i need a 12 pos 4 pole or a 4 pole potmeter for these.
Input 3 is just going straigth to the attenuator (which will be a real big aluminium knob on a 4 pole 24 steps elma switch which is allready here)[/quote]
for your polarity flipping led this quick and dirty switch hitter may do the trick.
For the sub out (assuming mono) build another active summer (like the mono part)[/quote]

Thanks!
That might also do it tough i decided to use a dedicated relais to do that so i can parallel de relaiscoils on the switch and it's easier to build since i go PTP on veroboard and the switches only have one contact.

The sub is stereo but i think i found a way how to do it, resistor (about 2k) in series with + and - and after the resistor (the input of the sub) a resistor to ground on the relais-contact when not connected to the sub.
So when the sub is on it uses it's own impendance and when switch off it uses the resistor to ground (10k?).
Or is that not correct?

 

[dagoose]

The led circuits are done, 6 relays in total for 6 switches.
It's fun to see the knobs go from green to red when you push it, i feel like a kid again
I decided to add a headphone section as well, the circuit in the other topic is cool but it needs + and -15v and is SMD so i was searching for another one that might be worth checking out, it's based on a TDA2004 and just uses +15v (can also be powered with +12v)
This is it, what do you think?
Ofcourse R5 and R10 are replaced for a stereopot.

R1=120Kohm
R6-9=1.2Kohm
C6-15=3.3nF 100V MKT
R2-13=330 ohm
R7-8=1.2 ohm
C7-8-13-14=100uF 16V
R3-12=2.2Kohm
C1-4-5=10uF 16V
C9-12=470uF 25V
R4-11=27 ohm
C2=100uF 25V
IC1=TDA2004 in Heatsink
R5-10=22 ohm
C3-10-11=100nF 100V MKT

 

[Harpo]

you know this amp is optimized for car stereo ? input saturation 300mV, 35Hz-15kHz with -3dB rolloff, ...
better have a look in the meta http://www.groupdiy.com/index.php?topic=25091

 

[matthias]

In my monitor controller I only get +5 and +12V from my external power supply, but for the +/- voltages I simply use an 3W dc/dc converter

 

[dagoose]

removed

 

[dagoose]

OK, different approach for the headphone amp, i just build one using a 5532 opamp and that one works great, no noise, LOADS of level and good sound using my beyer dynamics DT770.
And it's even a lot smaller and simpler then the one i tried before.
It's running on + and - 7.5 volt which is working ok with just a bridge rectifier, devider with 2x 4k7 and some caps.

 

[dagoose]

OK, allmost done!! Just waiting for the frontpanel to come in.. i hope to get it in this week so i can finish the rest of the work.
Will drop some more info later, in total 20 relais used.
Totally passive and balanced :roll:

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