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Old 28th April 2009, 02:44 PM   #1
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Default 6 Channel Volume Control

I have just built my first amplifier , and I am very very happy with it! Here are some pictures, just for interest:

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There is still one more part to finish, that being volume attenuation. I need a way to adjust all 6 channels at the same time to the same degree. One option is a 6 gang 10k log pot, but they are expensive. I have found this device: http://www.st.com/stonline/products/...e/ds/9227.pdf, but I have no idea about digital electronics, so I dont know how to implement it. I am very willing to learn about the digital ways, so prehaps someone could point me in the right direction as to how I would gain the required knowledge to use such a chip. If anyone has any implementation ideas on how to get that 6 channel volume control chip working with a simple 5xLM3886 based amp I would be very grateful.

My amp design was based around this: http://dogbreath.de/Chipamps/ThreeRe...reeResAmp.html

Thanks for your help!

Dan
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Old 28th April 2009, 03:11 PM   #2
Bigun is offline Bigun  Canada
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Nice looking amp. I'm curious, can you describe what you have on the back panel ?

The stm link isn't working ?
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Old 28th April 2009, 10:58 PM   #3
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apologies, i added a comma at the end of that datasheet link. here it is fixed. http://www.st.com/stonline/products/...re/ds/9227.pdf


on the back panel you can see 3 LED's, and 2 voltage regulators. one at 5v and 1 at 12v. these are connected up to a molex plug, and is used to power the LED's and the fan on top! Easy to plug/unplug when you take the top off the amp. heres a closer look:


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Old 28th April 2009, 11:38 PM   #4
Fenris is offline Fenris  United States
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Nice bugsplat design! I did one for a friend with 15 amps, 7 transformers, 40,000uF of capacitance, and an active crossover in a slightly bigger case. Much cleaner than my effort.

Look in the Solid State forum for the "Lightspeed attenuator" thread. It's a pretty straightforward design that can easily be expanded from two to six channels. It only requires a single pot of no particular quality, a power supply (you can probably use the 5 or 12v you already have there), and 6 or 12 LDRs (light dependant resistors). And it sounds better than most any pot, IMHO. Also, you could look at the article on diyparadise.com "All about volume controls" for the basics of a LDR attenuator.

The LDRs will be the most difficult part to source. I thought I saw a group buy a while ago, but I'm not sure if it went through. The hardest part is matching the LDRs.

Edit: it looks like someone has already done a 6 channel volume control with LDRs:
http://www.diyaudio.com/forums/showt...82#post1545782
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Old 29th April 2009, 01:48 AM   #5
Bigun is offline Bigun  Canada
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The TDA7448 looks like a nice chip, but the distortion spec. aren't that great if you want the best possible quality. You'll need a pcb and a means to set the volume via the serial bus. It doesn't seem like a simple approach - unless I'm misunderstanding something.

The Lightspeed is currently the favoured approach for maximum sound quality. But do you need this - it's always tempting to follow the excitement of others on the forum in picking the ultimate solution. The Lightspeed requires you to match non-linear resistors that suffer from a lot of manufacturing variations. For 6 channels it might be expensive as you'll have to buy a larger batch to get matching units. Having said that, I'm going in this direction myself.

There are multiganged potentiometers. Even though they aren't cheap the price probably compares favourably with these other solutions. I believe for sound quality you need to choose a good ganged pot. There are some known brand names. I was looking at the Vishay linear ganged pots as an economical choice but with reasonable quality. To make up several channels you may have to buy several dual ganged pots and disassemble and reassemble them - but it looks trivial to do this. http://www.vishay.com/docs/51047/p9.pdf

You mention a fan. I know at my work the fan from the LCD projectors cause a lot of EMI noise on our conference phones if placed nearby. I don't know what kind of fan you are using, or if you have noticed any EMI on the sound output of your amp ?
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Old 29th April 2009, 02:11 AM   #6
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thankyou very much fenris! that is a really good solution. you my friend have just saved my *** about 100 quid lol. ill read up on all of that info and give an update on what the plans are =]
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Old 29th April 2009, 02:37 AM   #7
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Default Re: 6 Channel Volume Control

Quote:
Originally posted by dannybeckett

There is still one more part to finish, that being volume attenuation. I need a way to adjust all 6 channels at the same time to the same degree. Dan
My Lightspeed Attenuator LDR one will send you nutz trying to make all 6 channels track.
There is this one not as good sounding, also not cheap, but you need something that all 6 channels will track equally.
http://www.goldpt.com/dims.html

Cheers George
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Old 29th April 2009, 02:52 AM   #8
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I have the same "need" but haven't yet gone througn the options.

Is there any reason why we cannot simply use this:


source => single preamp/pot => active xo => amp

and why you need 6x attenuator like this:

source => active xo => 6x pots => amp


The 1st method is used in linkwitz orion etc. so they can't be that bad (??)
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Old 29th April 2009, 03:44 AM   #9
CLS is offline CLS  Taiwan
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If you use something like DCX2496, there'd be a 'problematic' addtional A/D converter in front of each channel

If all analog, it's of course no problem to have the control in front of xover.
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Old 29th April 2009, 04:02 AM   #10
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Quote:
Originally posted by gainphile
Is there any reason why we cannot simply use this:
source => single preamp/pot => active xo => amp
That's how I do it. Works great.

Quote:
Originally posted by CLS
If you use something like DCX2496, there'd be a 'problematic' addtional A/D converter in front of each channel

If all analog, it's of course no problem to have the control in front of xover.

'problematic' ?


Balderdash
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