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Old 13th April 2012, 12:02 PM   #1841
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Well, I wanted to be able to control each channel individually and all six channels as a group - though it is possible to control the output of each channel from my current DVD/SACD player. I have considered each pot as a receiver and a transmitter, but I know practically nothing about electronics design (it shows, I know) so I don't really know how to improve on the DACT layout. I'll keep the cables between short as possible, for one thing. Perhaps different values for the pots would work better? I'm under the impression that inputs should exhibit higher impedance than the outputs connected to them, so maybe I should use 10k then 20K? Salas cautioned against using more than 20K in series - is this 20K total (including the signal shunted to ground) or does only the resistance actually in the circuit at any one setting (disregarding resistance to ground)? Or maybe I should just can the individual channel controls and control them electronically from the DVD player? After all, once they're set they can be left alone.
I do want this to be well planned, but not being any kind of electrical engineer I appreciate help from all sources.

Last edited by schubert; 13th April 2012 at 12:10 PM. Reason: typo
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Old 13th April 2012, 12:32 PM   #1842
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Are the 6 individual pots for trimming the channel voltages to match sensitivities or similar?

Are your sources sending 6channel information?
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Last edited by AndrewT; 13th April 2012 at 12:40 PM.
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Old 13th April 2012, 01:28 PM   #1843
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I'm using the six analog outputs from my DVD player - two main (first DCB1 board), center, sub/LFE (second DCB1 board) and two surrounds (third board). The six individual pots are to adjust the relative levels of each channel for my room. My current A/V preamp does this automatically - that's nice but the overall sound quality, while quite good, could be better - that's why I'm thinking of this project.
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Old 13th April 2012, 02:20 PM   #1844
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Quote:
Originally Posted by AndrewT View Post
Are the 6 individual pots for trimming the channel voltages to match sensitivities or similar?

Are your sources sending 6channel information?
Sorry to be so wordy - I meant to say "yes" and "yes".
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Old 13th April 2012, 04:56 PM   #1845
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then I suggest you use inverting opamps as your trimming adjustments. these can be set up for gain or attenuation. You can also easily restrict the total range of adjustment.

These "inverting buffers" present very low impedance to the inputs of the attenuators. This ensures that the 6gang attenuator attenuates equally on all 6 channels.
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Old 13th April 2012, 05:00 PM   #1846
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OK, thanks - I'll try it but I'm a little unsure how to set something like this up - can you suggest a reference where I can bone up on this?

Any reason it should be inverting rather than non-inverting?

Last edited by schubert; 13th April 2012 at 05:09 PM. Reason: addition
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Old 13th April 2012, 05:30 PM   #1847
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How about something like this (see attachment, figure A)? I assume that in order to make this adjustable one would put a trimpot where R2 is - or should I use a pot in place of R1 and R2? I suppose the total resistance should remain the same. So that would mean just associating the individual level pots with an opamp rather than sticking them in there by themselves - is that correct?
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Old 13th April 2012, 08:42 PM   #1848
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Quote:
Originally Posted by AndrewT View Post
then I suggest you use inverting opamps as your trimming adjustments. these can be set up for gain or attenuation. You can also easily restrict the total range of adjustment.

These "inverting buffers" present very low impedance to the inputs of the attenuators. This ensures that the 6gang attenuator attenuates equally on all 6 channels.
I alsways learn something from you Andrew!
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Old 14th April 2012, 11:03 AM   #1849
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post1847
The left most diagram, inverting, is the one you should use.
R1 & R2 set the gain or attenuation.
With both equal to 10k the amplification factor is 1 or +0dB.

You can alter the gain by simply increasing R2. 30k will give ~+10dB.
or
You can increase attenuation by reducing R2. 3k will give ~-10dB.

That range for a trimmer should be adequate for any sources you have or plan to buy in the future.

You can replace the resistors with a pot. The wiper goes to the -IN. But you must fit some protection in case the wiper goes momentarily open circuit.
You also need to add a fixed resistor before the pot at the input and a second fixed resistor after the pot on the leg going to the output feedback tapping. These resistors limit the range of adjustment.

You can choose any range of adjustment that suits your gear.
+0dB to -20dB or +12dB to -12dB or whatever.

You must use a unity gain stable opamp, or a unity gain stable discrete opamp.
I suggest you use a socket and a cheap & good {NE5534an+compensation}, to get you started. Later you can decide to go swapping or go discrete. Or 5532 for two channels and without added compensation.
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Old 14th April 2012, 11:25 AM   #1850
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Quote:
Originally Posted by schubert View Post
How about something like this (see attachment, figure A)? I assume that in order to make this adjustable one would put a trimpot where R2 is - or should I use a pot in place of R1 and R2? I suppose the total resistance should remain the same. So that would mean just associating the individual level pots with an opamp rather than sticking them in there by themselves - is that correct?

It's normally consdiered bad practice to put a variable resistor (as opposed to a trim pot) in the feedback loop of an op-amp.

In the feedback position, this can result in quite noisy operation. (I'm talking about a volume type pot. that is constantly being adjusted.)

The normal practice is to attenuate at either the input or the output.
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