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[bmaxwell]

OK, so I've run into this a bunch;

I'm helping someone record in there home studio, they have a full blown pro tools rig and they're running the main outs(stereo 1 and 2) to a mackie or something just so they can have a volume knob to the monitors and a send for the headphone amp.  I hate the idea of the mix output going through all those op amps just to get to a volume knob.

I've seen some pretty expensive bells and whistle boxes out there(multiple input selectors, talk back, etc), but it seems like it would be a simple thing to make a stereo xlr in/xlr out volume box that doesn't fuck with the sound too much.  The only other thing I'd maybe want on it would be a copy of the left/right input(non volumed) that I could send to a headphone amp.

I have some basic soldering skills, probably all the equipment to get the job done, and a digikey catalog.  I also enjoy little projects like this.

Anybody out there have any design/schematic ideas?  Am I totally underestimating how hard this would be?  Is it something that can be done for relatively cheap(parts wise)?  

Any input or help would be much appreciated.

Thanks
Bruce

[Gunnar Hellqvist]

I am no real expert on this when it comes to balanced signals. But I will tell what I generally do on unbalanced signals first.

I have done several volume boxes of this kind for unbalanced signals. I use them connecting the phone out from various stuff to an unbalanced line in on just about anything.

The simplest version only needs a stereo potentiometer. Generally a logarithmic 10 kOhm potentiometer is a good choice. Any value really goes as long as it is not too low, because this is the new loading the output will see. These can probably be found in just about any radio parts store all over the world. If possible get one with a plastic resistor instead of carbon as they tend to be less noisy.

In this case each channel of stereo has two connections, live and ground. Basically you can tie all the grounds (both inputs and outputs) together at one point. You may end up in a groundloop situation if there is a lot of mains signals around, so you might select to leave one of the input grounds unconnected.

Next connect the input signal to the "high" input of the potentiometer. Generally this would be the right connector when you view the pot from the front. Ground goes to the other signal, and the wiper or middle connector is the output signal.

I have not done it on a balanced signal, but I can see some possible variations. First of all, the grounds all go together as before, but check for ground loops.

a) a fourgang potentiometer. I have seen them, but they are not as easy to find as a stereo pot. Connect each of the four in the same way as for the unbalanced.

b) separate left and right volume

c) a little more elaborate thing using two resistors and one potentiometer per channel. Take the two input signals (hot and cold) and run them through a resistor, say around 2 kOhm or a bit higher. Now connect a pot between the two signals after the resistor, in effect shorting the hot and cold to each other in higher or lower degree. On the pot one end goes to the left terminal (as seen from shaft) and other to middle. Experiment a bit with different size resistors.


So good luck, heat up your soldering pen and start experimenting. Put the speakers on low volume first, and be sure to not shortcircuit the output signals. In general professional audio equipment is well protected by it does pay with a bit of caution.

[John Klett]

I do a 1:1 transformer in front of a dual 10K audio pot.  The output of the pot can go straight to a power amp or through a booster amplifier to another transformer to rebalance the output and provide a low source impedance for longer cable runs.

You can go with a 10K:10K line input transformer.  You want something that will handle +18dBm - ideally you'll invest in good transformers and a good stereo pot.

I use TKD conductive plastic pots - about $70 or so from

http://www.partsconnexion.com/catalog/controlsswitches_files /sheet003.html

and for transformers - I have a bunch of old ones on the shelf but for new I'd go with something along the lines of a

Jensen DIN-PB repeat coil

http://www.jensentransformers.com/iso_aud.html

or

Jensen JT-11P1

http://www.jensentransformers.com/ln_in.html

You don;t have to go with Jensen but they'll make a good reference point in your search

[Bill Park]

You might consider an op amp/stepped attenuator solution... I used a Forssell amp with a TKD 40 position attenuator.  Unbalanced, because as far as I know, there is no TKD 4 gang, and I didn't want to buy 4 op amps.  The nice part about something like this is the ability to calibrate the system and have repeatable levels.

On the low budget side, there is the NHTPro piece, about $150.  Or the Mackie Big Knob, for about $325.

 Bill

[hollywood_steve]

I'll second the vote for an NHT PVC; street price closer to $100.  For anyone monitoring without a console, its a great choice for a budget solution.