Barebones Monitor Volume

It has come to my attention recently that if I expect to hear small differences in audio circuits I'm working on then I need a clean signal path to my monitors.  I'm reluctant to admit that my monitors are plugged in to a Ma ck ie dfx 12, so I've set off to quickly make something better. 

I just want two balanced inputs, a volume knob, and two balanced outputs with the cleanest possible signal.  I'll make it with SMD components, and I'd like to power it with batteries as an option.  Check out the schmeatic, but basicically it's an AD797 on the input and an LM6172 balanced output, with a pot in between.  Two channels means 4 8 pin chips, one dual pot, and I guess an on off switch to save batteries since it will pull 40 or so mA.

My DAC outputs around 1.8Vrms and my monitors want 0.5Vrms so really all I'm trying to do is attenuate and buffer.

I've got the AD797 wired up and it's working fine, the LM6172 will follow and then I'll hopefully get a sense of whether or not I'm barking up the right tree here.

millzners said:

My DAC outputs around 1.8Vrms and my monitors want 0.5Vrms so really all I'm trying to do is attenuate and buffer.

Click to expand...

So why place any active electronics in the way at all? If you really want to hear subtle differences in sound make it passive. Just a 4 pole 23 way switch in a screened box and 4 XLRs.

Cheers

Ian

ruffrecords said:

millzners said:

My DAC outputs around 1.8Vrms and my monitors want 0.5Vrms so really all I'm trying to do is attenuate and buffer.

Click to expand...

So why place any active electronics in the way at all? If you really want to hear subtle differences in sound make it passive. Just a 4 pole 23 way switch in a screened box and 4 XLRs.

Cheers

Ian

Click to expand...

Yeah I originally wanted to go the passive route, but then ended up thinking about how I would get one knob for two balanced channels.  The only thing my feeble mind could muster was getting down to an unbalanced signal and using a dual pot or 2 pole rotary switch before going back to balanced outputs.  I hadn't considered a 4 pole rotary switch, but it sounds like the cleanest way to do it.  If it's not too expensive I may try it.

I've attached the schematic I'm working off of, nothing special since it's just ripped from the datasheets.

millzners said:

Yeah I originally wanted to go the passive route, but then ended up thinking about how I would get one knob for two balanced channels.  The only thing my feeble mind could muster was getting down to an unbalanced signal and using a dual pot or 2 pole rotary switch before going back to balanced outputs.  I hadn't considered a 4 pole rotary switch, but it sounds like the cleanest way to do it.  If it's not too expensive I may try it.

I've attached the schematic I'm working off of, nothing special since it's just ripped from the datasheets.

Click to expand...

To save cost you can do it with a 2 pole switch. Just connect input hot and cold across the attenuator, and take the output hot from the wiper and the cold from where the input cold is connected. You could do it even more cheaply with a dual gang pot - a 1K audio taper would do. However audio taper pots are not well controlled so it might be better to use a lin pot slugged to give an appropriate law. If you slug a lin pot with about half its value from wiper to ground you get close to -10dB at the mid position so a 2K2 pot slugged with 1K would do it. Neve used this techinigue when I was there in the 70s for studio loudspeaker level controls.

Cheers

Ian

I've got a 4 pole rotary attenuator in my monitor controller.... you can buy the switch on ebay for about ten bucks or so.  It just
fits in a 1u... although you may have to do some creative bending of solder tabs/resistors

1K input and 30K output impedances??

from AT issue 78. About 2/3 the way down the page you'll find the link to the article pdf

http://www.proharmonic.com/articles.htm

PRR said:

1K input and 30K output impedances??

Click to expand...

Oops.  The 30k was supposed to be just 30r. As for the 1k i had enough 0805 in that value so that's what I started with.  The monitors are supposedly 40k input impedance... seems like a lot.

squib said:

from AT issue 78. About 2/3 the way down the page you'll find the link to the article pdf

http://www.proharmonic.com/articles.htm

Click to expand...

Awesome, thank you!  This in combination with what Ian said about slugging a dual lin pot may just make this economical enough to try out.  I'll shelf the active design I drew up above because the passive approach now seems achievable and clearly better. 

This is why I love this site, here I was marching down the wrong path and now I've got some great advice to set me straight.  Thanks!  I'll post my progress here once I've ordered some parts.

Have you you checked out New York Dave's design... might be worth searching the site for NYD Passive Monitor Controller. Similar to the design above apart from the mono sum uses an op amp...

If your cable lengths are below 50 feet total you can simply run unbalanced through a 10K or 100K dual log pot wired as a fader.  With the usual length of cabling for monitor runs you will hear zero difference between balanced and unbalanced.  You will have to get a few pots to get one that tracks L-R properly at the lower volumes, but it is really that simple.
Mike

sodderboy said:

If your cable lengths are below 50 feet total you can simply run unbalanced through a 10K or 100K dual log pot wired as a fader.  With the usual length of cabling for monitor runs you will hear zero difference between balanced and unbalanced.  You will have to get a few pots to get one that tracks L-R properly at the lower volumes, but it is really that simple.
Mike

Click to expand...

You can still do it balanced with a dual pot. See my earlier post:

http://www.groupdiy.com/index.php?topic=47779.msg602105#msg602105

Cheers

Ian

I got the idea that I could rumage through some old crap and rip out the rotary switches for this project.  That worked just fine, but so far the switches are next to worthless; they must have been custom designed for the eqiupment the were in.  I've got a couple more I'll look at before I turn to digikey. 

Of course as soon as I set out on the simplest possible design I get a 2nd pair of monitors as a gift so now I'm looking at putting in the input selector switch.

Thank you for everyone's input, as soon as I can get some usable parts I'll slap a schematic together and post an update.

ruffrecords said:

You can still do it balanced with a dual pot. See my earlier post:

Click to expand...

I checked a 2 of 3 drawing and see what you are talking about.  How is signal attenuated fully CCW with the output to the monitor connected to cold?  Isn't the pot just wiping between two polarity reversed signals?
Mike

sodderboy said:

ruffrecords said:

You can still do it balanced with a dual pot. See my earlier post:

Click to expand...

I checked a 2 of 3 drawing and see what you are talking about.  How is signal attenuated fully CCW with the output to the monitor connected to cold?  Isn't the pot just wiping between two polarity reversed signals?
Mike

Click to expand...

Yes and the balanced input it is fed to takes the difference between the two. When the pot is fully CCW the hot and cold are connected together so the difference is zero.

Cheers

Ian

ruffrecords said:

sodderboy said:

ruffrecords said:

You can still do it balanced with a dual pot. See my earlier post:

Click to expand...

I checked a 2 of 3 drawing and see what you are talking about.  How is signal attenuated fully CCW with the output to the monitor connected to cold?  Isn't the pot just wiping between two polarity reversed signals?
Mike

Click to expand...

Yes and the balanced input it is fed to takes the difference between the two. When the pot is fully CCW the hot and cold are connected together so the difference is zero.

Cheers

Ian

Click to expand...

Is there any disadvantage to this arrangement?  It's basically what that article linked earlier suggests too: taking a variable amount of reistance between the balaned inputs in order to get attenuation.  I was looking at a ladder style arrangement but it's twice as many reistors and it's unbalanced.  Seems like a no brainer but I thought I'd double check. 

No there is no disadvantage, especially within the context of a Barebones Monitor Volume project.

It's funny.  I was looking at the pot in relation to the source and could not see attenuation.  When you look in relation to the load, it IS a no-brainer. 

Just keep overall cable lengths source to load under 50 feet.

Mike

millzners said:

Yes and the balanced input it is fed to takes the difference between the two. When the pot is fully CCW the hot and cold are connected together so the difference is zero.

Cheers

Ian

Click to expand...

Is there any disadvantage to this arrangement?  It's basically what that article linked earlier suggests too: taking a variable amount of reistance between the balaned inputs in order to get attenuation.  I was looking at a ladder style arrangement but it's twice as many reistors and it's unbalanced.  Seems like a no brainer but I thought I'd double check.
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None at all. Neve did it often enough in their monitor loudspeaker controls so if it's good enough for Neve it's good enough for me.

Cheers

Ian