circuit boards for 6-channel gain controls

[charlielaub]

I have been poking around the net now and then each time I get the urge to try and find a good quality, affordable solution to multi-channel gain controls. Specifically I want to use a 6-channel analog gain control after my Behringer DCX2496 crossover so that I can use a direct digital input to it (e.g. from a CD player) so that I can avoid one set of A-D/D-A conversions. There are a couple of off the shelf solutions I know of: GoldPoint or DACT 6-gang stepped attenuators ($$$) and a company in France makes a 6-channel control (also $$$). Neither of these are really all that affordable...

On the other hand, a set of VCA-based volume controls using the Analog SSM2018T chip would do the trick. You gang together the controls using a single pot supplying a control voltage to all SSM2018Ts. I think that the simple circuit design shown in the link could be used for this purpose.

Unfortunately I don't have any experience in circuit board layout or printing. I am guessing that some of you out there have this knowledge and experience and I am wondering if we could all team up to design and print a run of 2-channel board (to keep the board size small). We could then all go in on a group buy for a bunch of the boards, the IC and other components to keep costs down. It would not be difficult to build up a nice 6+ channel volume control unit just like Dave Reite did (author of the link). I haven't contacted him, but I will shoot him an EMail now in case he still has the circuit layout and design that he used available.

So how many of you out there would be interested in going on and how many board would you want to purchase?

-Charlie

[vasyachkin]

just run the volume control upstream of the digital gear

quantization noise should stay below thermal noise in your amplifiers unless you use some sort of ultra-high-end $10,000 amps

i'm just using the volume control in my soundcard ( which acts as a digital source over SPDIF ) and it seems to work fine

[Jonathan Anspach]

I would be interested if it's possible to gang, or slave, multiple boards together. I'm using 2 DCX's to run my 3-way LCR speakers. I need to be able to control all 9 channels synchronously.

[charlielaub]

I would be interested if it's possible to gang, or slave, multiple boards together. I'm using 2 DCX's to run my 3-way LCR speakers. I need to be able to control all 9 channels synchronously.

Yes, that is the best part, really. With this approach you can gang together as many 2-channel board as you would like and control them all from one knob.

[charlielaub]

just run the volume control upstream of the digital gear

quantization noise should stay below thermal noise in your amplifiers unless you use some sort of ultra-high-end $10,000 amps

i'm just using the volume control in my soundcard ( which acts as a digital source over SPDIF ) and it seems to work fine

There is no "upstream of the digital gear" (at least for what I want to do) because the digital out of the CD player will be fed to the digital input of the DCX2496 crossover. The signal only becomes analog after the crossover so there is only one D to A conversion, and it is at 24 bits. There is one D to D conversion in the crossover (or in a Behringer SRC2496 that I have, that would then feed in to the DCX).

[philiparcario]

this works

http://cgi.ebay.com/ws/eBayISAPI.dll...m=160289769396

it is mine. it is 5 channels I have a one channel in a smaller case. use the 1 channel for the sub and the 5 channel for the sats. if you want it buy it now and i will give you the 6th channel for free. they have high quality noble pots. note the sixth channel is in a smaller black case. all rca's

[charlielaub]

this works

http://cgi.ebay.com/ws/eBayISAPI.dll...m=160289769396

it is mine. it is 5 channels I have a one channel in a smaller case. use the 1 channel for the sub and the 5 channel for the sats. if you want it buy it now and i will give you the 6th channel for free. they have high quality noble pots. note the sixth channel is in a smaller black case. all rca's

I want to adjust all 6 or more channels together, at the same time, with very close tracking, from one knob. It looks like your system is 5 independent pots, right? That would not work, since each time you adjust one knob the balance between channels (each channel feeds a different driver in a multi-way speaker in my case) would be thrown off.

[AJ]

There is no low cost way to do what you are asking unless you can do your own layout and printing. Check over at diyaudio.com if anyone there already has a board design, and then learn how to print it. Or it may be possible someone is selling boards or completed 6ch controls.

Seriously though, its not worth the effort to save one DA conversion. Just use a volume control after the analog outputs of your cd player.

[Fred T]

I believe Alps makes 2,4 and 6 gang motorized conductive plastic pots.The electronics to operate the remote control section could likely be purchased from the far east cheaply(ebay'ed).As an option you could buffer the outputs of each section of the pot with a hi quality op amp.

[philiparcario]

I want to adjust all 6 or more channels together, at the same time, with very close tracking, from one knob. It looks like your system is 5 independent pots, right? That would not work, since each time you adjust one knob the balance between channels (each channel feeds a different driver in a multi-way speaker in my case) would be thrown off.

I had a few six gang pots none of them tracked together closely. I ended up using this 5 separate pot with a sixth stand alone for the sub. it allowed total separation and dead equal levels. The 6 gang dact works close enough but cost too much.
If you adjust a lot this system is a little time consuming. see if you can find a used audio refinement preamp pre 5 it would do the trick and cost less then a dact. phil

[philiparcario]

http://www.audiorefinement.com/handb...e-5-Manual.pdf

I had one of these for a few years. they show up for 300 to 400 on audiogon and ebay. it will do what you want.

[Fred T]

I had a few six gang pots none of them tracked together closely.

Yes this can be an issue as variances between sections can exceed 10%.

[vasyachkin]

There is no "upstream of the digital gear" (at least for what I want to do) because the digital out of the CD player will be fed to the digital input of the DCX2496 crossover.

so you don't have philosophical objection to running volume control in the digital domain.

then you can just buy a digital mixer and insert it between your source and the behringer. my soundcard for example can be used in this way ( theoretically, haven't actually tried ) and controlled either through computer or using onboard knobs.

in fact there should be adjustable gain on the INPUT of the behringer that you could use to control the volume but i don't remember if it can be adjusted all the way down to silence.

[charlielaub]

so you don't have philosophical objection to running volume control in the digital domain.

then you can just buy a digital mixer and insert it between your source and the behringer. my soundcard for example can be used in this way ( theoretically, haven't actually tried ) and controlled either through computer or using onboard knobs.

in fact there should be adjustable gain on the INPUT of the behringer that you could use to control the volume but i don't remember if it can be adjusted all the way down to silence.

I don't think that the gain control on the input of the DCX2496 is functional with a digital input. I will have to check. It can only boost or cut by 15dB max and this is not a very large range. Also, I would think that this approach would possibly impact headroom and so on in the input stage or in the outputs.

Am interested in the idea of a digital domain volume control, but I thought that doing it this way would lead to some degradation because of errors arising from the multiplication process that would take place as part of the attenuation operation. Not sure as I am not familiar with exactly how signal amplitude adjustment would be achieved on a digital bitstream.

[Fred T]

Am interested in the idea of a digital domain volume control, but I thought that doing it this way would lead to some degradation...

I believe that doing any attenuation in the digital domain results in a reduction of resolution.


Here is an inexpensive remote 4 gang pot.A 6 gang Alps could be used in place of the 4 sectioner. Although it wouldn't fit on the board.http://cgi.ebay.ca/Preamplifier-IR-V...p3286.c0.m14.A


If posting the link breaks the rules let me know and I will delete it.

[charlielaub]

I believe that doing any attenuation in the digital domain results in a reduction of resolution.


Here is an inexpensive remote 4 gang pot.A 6 gang Alps could be used in place of the 4 sectioner. Although it wouldn't fit on the board.http://cgi.ebay.ca/Preamplifier-IR-V...p3286.c0.m14.A


If posting the link breaks the rules let me know and I will delete it.

Thanks, but I want to avid ganged pots because the tightest tolerances that you can get are typically only about +/- 10% unless you go with stepped attenuators and those are very expensive (hundreds of dollars I think) for a six channel version.

[Fred T]

Thanks, but I want to avid ganged pots because the tightest tolerances that you can get are typically only about +/- 10% unless you go with stepped attenuators and those are very expensive (hundreds of dollars I think) for a six channel version.

Yeah just throwing some ideas out there.Stepped attenuators are the cats meow to be sure and the previously mention DACT's are superb but spendy.I have seen 2 and 4 sectioned units utilizing Dale resistors on epay that were very reasonable.

[vasyachkin]

I believe that doing any attenuation in the digital domain results in a reduction of resolution.

you could be an economist. you already know how to count money that doesn't exist

yes resolution will be reduced. from 24 bit to lets say 20 bit depending on how much attenuation you use. problem is you had only 16 bits to begin with so you didn't necessarily lose anything.

i prefer to look at it this way:

scenario 1 ( digital volume adjustment ) :

lets say the behringer has 100 db SNR and you hook it up to an amp with 100 db SNR. now you reduce the level on the behringer by 50 decibels - you're left with only 50db SNR.

scenario 2 ( analog volume adjustment ) :

you reduce the level on the amps by 50 decibels by using an analog 6 channel preamplifier. you're still only left with 50 db SNR because your amps still make the same amount of noise and your signal is down by 50 db.

so you don't really win anything by going analog as long as your analog electronics don't have more SNR than your digital electronics. now if your amps had 120db SNR it would be different but such amps are uncommon.

in terms of digital 24 bit is 144db but in reality you only get about 100db - 120db depending on the quality of your DAC. so the limitation of digital gear is really analog ... sort of. in the end you simply want to build a signal chain out of cleanest analog stages and DAC is just another ANALOG stage. SNR of the DAC will degrade at lower signal level but the same is true for any other analog piece of electronics. using analog volume control only makes sense if your DAC is the bottleneck in terms of SNR and you want to make sure it operates at its best.

well, thats what i think, i can be wrong

[Fred T]

you could be an economist. you already know how to count money that doesn't exist

In these scary economic times being an economist might be benificial.

yes resolution will be reduced.

I guess the real question is will it be audibly significant?Maybe? Maybe not.

[neildavis]

The best audio quality will be from the TI PGA4311 series or similar Cirrus volume control chips. About 5 years ago people were asking about 6 channel volume controls so I added some PGA4311 chips to a circuit I already had in progress. These chips work great--quiet, low distortion, perfect tracking, expandable, etc.

The preamp I built has a lot of extra stuff that makes it expensive, but you could pick apart the circuit and just use the pieces you need to build a simple, expandable multi-channel volume controller. However, you would need to adapt the board design and write some code. I have the code written for a Freescale 68HC08 micro to control the PGA4311, but you would probably want to come up with new code for a different display and different controls.

The schematic and PWB files are buried in this Powerpoint file:
http://home.comcast.net/~neilandbarb...kers/N-way.ppt