simple passive monitor switcher/attenuator

[plumsolly]

I was looking at the schematic for the control room monitor module in my console and it seems like it has a whole lot of stuff in there for just switching between line level sources and controlling volume (it does other things, just not things that I use for the most part). And in addition to all of that junk the signal has to pass through, in order to monitor from my converters or other sources, I have to plug them into line amps in my console - so more opamps and faders. So I was thinking about a simple passive monitor switcher/attenuator that I could mount in my console, like this:

I wasn't sure what to use for the attenuator. The input impedance of my powered monitors is listed as 20k, the output my DA is not published and the output of my console is specd at 65 ohms actual, 600 ohms nominal load, and acceptable range of 600 ohms or higher. So I guess I want the input impedance of the attenuator to be 15k or so, and the output to be 600 ohms or so. Does that sound right? I could order a stereo balanced bridged T attenuator in that configuration from shallco, but i have a feeling it would cost an arm and a leg, so i don't know what I'll do. Any suggestions would be greatly appreciated. The other question i had was in regards to clicks and pops - what should i expect and will it be tolerable? Thanks so much - Ben

 

[nielsk]

Don't you want to be able to feed the master machine and monitor at the same time?
Correct me if I am wrong, but using a passive attenuator will affect the mix out level, and possibly the sound. This is why there are buffer circuits in monitor sections.... I think a 1:1 transformer may work to isolate the source from interaction with the attenuator, but that seems expensive
I'm still trying to wrap my head around why anyone wants to use a passive monitor controller. The same thing with the passive mix buss... they pretty much suck (although it is a good way to get a few $100s for a few cents in resistors!). This is what was used before better ways were figured out to accomplish the job.
I'm not trying to 'dis you in any way, just attempting to see if I am missing something here. It is pretty easy & cheap to build a very good sounding active monitor section that works with no problems or signal degredation (the same with a mix buss...)

 

[plumsolly]

Hey Niels - I would monitor from the DA outs when im printing a mix. (flip 2-buss to AD in and the monitor selector to DA out.) I actually would have liked to be able to work it out so that i could put the attenuator in between the 2-buss and the AD when I wanted to in order to drive the 2-mix a bit, but it would be to complicated. If I need do that I'll just have to patch around a bit. -Ben

 

[ognam2]

I just built a little box for this:

I used 10k for R and for the attenuator as well, so I have 6dB of attenuation with full Volume, but I never put the Volume all the way up anyway. It is still clipping my KRK´s when I go with full Volume...
I used a nice alps stereo pot for 13 eur and a cheap Lorlin swith (4x3) which fits perfectly for 2 x unballanced stereo and 1 x ballanced stereo.
Works fine and the sound that is coming out of my active speakers is great.

Here is the thread where we discussed this (german only):

http://www.nrgrecording.de/nrg/viewtopic.php?t=962&postdays=0&postorder=asc&start=0

 

[plumsolly]

thats a very nice looking box - neat wiring too. so the lowest impedance input your sources will see is 10k (this would be at total attenuation anyway) and the highest source impedance your monitors will see would be 10k (but this would be at the highest vol - which would rarely if ever be used anyway) does 10k seem a bit high for an output impedance of the box, as your monitors are 10k input? - but this rarely if ever happens. If I used 5k for the shunt pot and 10k for series R, I would have ~9.5db attenuation at the highest volume setting, That would probably be fine, and 5k would be the highest output impedance of my box. Thanks so much for the input, you've got me thinking. - Ben

 

[3nity]

I like this idea...cheap and clean!
But what if we need headphones?
Is it okay to feed in parallel this passive monitor switcher and a Headphone amp?

Keep the good work....

I do really want to experiment on this project....

Believe me or not i have a C-contro who totally sucks!

Thanks

 

[plumsolly]

hi 3nity- for headphones, I was going to use the control room monitor module or studio monitor module on my console - I'll do a parallel output from the box mix into a stereo channel on my console and send the crm output to my headphone amp. - Ben

 

[Gold]

So I guess I want the input impedance of the attenuator to be 15k or so, and the output to be 600 ohms or so. Does that sound right? I could order a stereo balanced bridged T attenuator in that configuration from shallco, but i have a feeling it would cost an arm and a leg, so i don't know what I'll do.

I got a 10k/5k full T from Shallco. Apparently I was the only one who ever asked for an unequal impedance variable attenuator. I asked for a bridged T but they couldn't figure out how to do it so they made a full T. They stuck to their quote which I found admirable but now they know better. It's a 12 deck switch. It will cost both arms and both legs and be close to 18" long.

 

[NewYorkDave]

15k in, 600 out? A bridging transformer followed by a regular 600-ohm attenuator will accomplish that.
It won't be "transparent" but... what is, really?

 

[plumsolly]

It's a 12 deck switch. It will cost both arms and both legs and be close to 18" long.

Yikes.
Im thinking Im going to do the variable u-pad. 10k for R series and 5k pot for R shunt. 5k out will be ok feeding 20k input monitors, right? Could anyone recommend a super high quality 2 gang 5k log pot? I don't mind paying a premium for something that will track as well as one could hope for with a pot, feels nice, and is reliable. Thanks for everyone's input, Ben

 

[Gold]

Mine is an H pad so yours would be half the decks. It would still be expensive though.

UPS just showed up with the Shallco 45pos 2pole switches for the line amp gain and the 5pos 8pole switches for transfer source selection. Whoo Hoo.

 

[ruairioflaherty]

Mine is an H pad so yours would be half the decks. It would still be expensive though.

UPS just showed up with the Shallco 45pos 2pole switches for the line amp gain and the 5pos 8pole switches for transfer source selection. Whoo Hoo.

Tell us more Paul.  Did you buy straight from Shallco?

 

[Gold]

Tell us more Paul.  Did you buy straight from Shallco?

Yes, directly from Shallco. I think the minimum order is $100.  But I wouldn't want to be responsible for having a bunch of people call with $100 orders. It wouldn't be very nice to them. I've placed two orders that were round about $1000 each.

The first one was for the monitor attenuator. This order was for the switches that will be attenuators for the Input amplifier and the Output amplifier. Both will be +/- 11dB in .5dB steps. The other switch is to select the input to the A and B paths of the console. It is an A/B path lacquer mastering console.

 

[plumsolly]

Mine is an H pad so yours would be half the decks. It would still be expensive though.

I think I'm going to go with ognam2's approach, I just don't think I can afford a passive solution that maintains a constant impedance in and out. So I'm going to to do a u-pad with a variable shunt R. like this here:

with this arrangement, there is a 10.5db loss even at max volume, right: k = 1 + (Rseries/Rshunt) so k = 1 + (2k/1k) so k = 3 which is a ~10.5 loss?
I think Im going to use 10k series R and a 5k pot for shunt R - any input on that choice would be much appreciated. Thanks, Ben

 

[Gold]

FWIW I consider mine to be complete overkill and a little embarrassing. It sure looks impressive though.

I don't know how to calculate impedance with the pot between legs of a balanced connection. You are not changing impedance to ground directly, are you?

I don't think there is anything wrong with it I'm just curious to know more.

 

[plumsolly]

hey Paul - it's all here: http://www.uneeda-audio.com/pads/
P.S.

lacquer mastering console.

you are my hero. - Ben

 

[plumsolly]

holy mackerel Paul - I just checked out your discography - you are my hero. Espers, Sundazed, LCD Soundsystem, Drag City, a heap of other awesome artists and labels... If you ever need an intern for the summer... Sorry to get off topic- Ben

 

[Gold]

Aww, shucks. Thanks. I started my own place a couple of years ago. My goal is to keep it a one man operation. I'm a cranky old bastard.

 

[3nity]

Shouldnt the potentiometer reflect the monitors input impedance?
so lets say my speaker have a 20K imput impedance should i use a 20K potentiometer as an attenuator?

Thanks

 

[Gold]

The general rule of thumb is a 1:10 ratio minimum for output impedance to input impedance. For a 20k input impedance you wouldn't want to have more than a 2k output impedance. If there is a significantly higher output impedance you risk high frequency roll off. With short cables it won't be much of a problem. If you use a 5k pot and it's usually halfway up that's perfect for a 20k input impedance.

That's assuming the pot is used normally with the bottom of the pot tied to ground. I'm not sure how the impedance works in the above configuration.