Inline volume control for high-end sound system

[jaem]

Hey all,

Several years ago, a friend of mine started work on designing and building a pipe organ in his house. He ended up building it with electric valves, using an organ synthesizer as a console for it. He also bought a few very high end speakers, hidden behind the pipes, so that he could play the synthesizer if he wished, as well as using them to play his collection of ~150 Bach CDs. The problem was that he wants to use a 5-disc DVD player that he already has on hand, to play the CDs, but of course, there is no volume control on the player, and without it, the music makes the neighbours complain. What he wants is an inline attenuator (variable) for the RCA-style audio connection from the DVD player. Now, I could just look something up, but given the expense he's put out on speakers and other equipment, I need a design that won't noticeably degrade the audio signal *at all*. I know it's probably not that complicated, but while I possess some knowledge of electronics, I don't have the requisite experience to be sure of the quality of any design I make.

I'm looking for pointers, links, circuits if possible... Money isn't too much of a concern, but I'd like to keep it as low as I can.
Thanks,
Jeff

[MrAl]

Hi there,

Many times you can get away with just a pot. Would you like
a circuit to try (just a pot)?

[dacflyer]

ya a simple inline pot between the RCA connections will do the trick, i think i use a 10K on my set up here when i test and repair audio equip. etc..

i'd post a schem, but i have no way to do it here..

[jaem]

I know it's probably not that complicated...

wow... talk about overthinking things!
*slaps forehead*... yeah, I don't imagine a Pot would introduce much distortion, now would it?

I honestly think I've gotten stupider this summer...
Thanks,
Jeff

[MrAl]

Hi Jeff,

Hee hee. There is a tiny bit more to consider in that the value of the
pot comes into question also. The value is chosen so that it does not
load the signal source too much when at full volume, and the top
of the pot goes to the signal source, bottom to ground, and arm
to the input of the next amplifier.

For example, a guitar i have uses a 250k pot for the volume while
the pickups impedance is about 9k ohms. Since the 250k is always
in parallel with the 9k pickup, its value is high enough so that it
does not load the 9k pickup too much. I would think 100k would
not be too bad either and would be a little less noisy, but would
cause a little more signal loss.

[jaem]

Hi Jeff,
--SNIP--
For example, a guitar i have uses a 250k pot for the volume while
the pickups impedance is about 9k ohms. Since the 250k is always
in parallel with the 9k pickup, its value is high enough so that it
does not load the 9k pickup too much. I would think 100k would
not be too bad either and would be a little less noisy, but would
cause a little more signal loss.

Okay, so I guess I'll need to figure out the DVD Player's output impedance then -- or is it standard for RCA jacks on such a device? As for signal loss, that's not really an issue. At the moment, it's so loud that you can hear it quite well outside with the doors closed!

[Dean Huster]

He ended up building it with electric valves, using an organ synthesizer as a console for it.

Sounds to me like he didn't build a pipe organ at all and that the pipes are simply decoration. A true pipe organ of any decent size would be a challenge to put in most homes, considering that "small" 16-foot stopped pipes are still a maximum of 8 feet long and unstopped 32-foot pipes are a maximum of 32 feet long -- and there's something just not right about installing the pedal clavier pipes horizontally just to get them to fit the room height!

I know of only one person who ever intended to build a pipe organ and his was going to be a very, very modest 2-rank, tracker-action organ that would require only a very small wind chest and a comparatively-small blower. This guy was building it from scratch, hand-making every component save for a set of used pipes and the blower.

Dean

[MrAl]

Hi Jeff,
--SNIP--
For example, a guitar i have uses a 250k pot for the volume while
the pickups impedance is about 9k ohms. Since the 250k is always
in parallel with the 9k pickup, its value is high enough so that it
does not load the 9k pickup too much. I would think 100k would
not be too bad either and would be a little less noisy, but would
cause a little more signal loss.

Okay, so I guess I'll need to figure out the DVD Player's output impedance then -- or is it standard for RCA jacks on such a device? As for signal loss, that's not really an issue. At the moment, it's so loud that you can hear it quite well outside with the doors closed!

Hi Jeff,

I am not sure about the DVD players output impedance, but one way
to see if your pot is what you want it to be is to try one or parallel
a resistor with a high value pot. The idea is to use a pot that has the
lowest value that the application can stand without too much loss at
full volume. I say at full volume because that would be the test,
although loss at other volumes will occur too.
Starting with a 250k pot for example, you can parallel a 10k resistor
with the whole pot and see if you loose any volume at the full setting.
If you dont loose too much (perhaps none even) then you know
you can use a 10k pot, and that should work ok for most apps.
The problem with using a pot that is too high is that you dont get the
full value of the adjustment range, so that when you turn it a little
you loose lots of volume, turn it the other way and it's back up to
full volume, and for 80 percent of the turning range it's super low
volume. With the right pot you should get a decent adjustment
range so you can set it without having to be real careful how far
you turn it.
You can even try different pots altogether if you have some around,
such as 1k, 5k, 10k, 25k, 100k, and 250k. Which ever one makes
the control 'feel' right is the one you can use.