[shrimants]

PREAMBLE:

Skip this part unless you are interested in development reasons and such. Questions are at the very end in color.

I have 2 active devices, A5 bookshelf speakers and a 250 watt subwoofer. I am feeding them input from my Gamma 2 Full++. The gamma2 has no software volume control ability, so I have to manually control its output via youtube and such. This is a gigantic pain in the butt because every single app has a different volume curve and setting, which changes based on if i'm using headphones or the speakers+sub.

Right now I've managed to put the sub and A5's to a matched volume and now I control both through the software control of the app itself. The main problems with this:
1) no ability for ipod or other device output. There is no y-splitter configuration, so I can either not use my subwoofer with the A5's or I have to use specifically the laptop and transfer media to it in order to play through the entire system.
2) Software volume sucks and is REALLY touchy. like 2 pixels on youtube is more than enough.
3) No easily accessible control available for each. I have to get up to change the A5 volume or reach behind the sub to change is volume and then wait to adjust and see if it is the correct volume.
4) A5's are not even close to their 50% max efficient volume. Audioengine recommends using the A5's at 50%-75% volume for maximum performance, otherwise the amp has to do a bit of a balancing act. I can barely get it past 30% because the A5's want a headphone-level input instead of line-level like the gamma2 provides. Conversely, the subwoofer is at ALMOST 50% and it is accepting line level output currently.

I'd like to be able to control volume in a preamp stage with a buffered y-splitter (true signal duplication, not a simple Y-splitter). I have 2 50k alps pots for the volume controls, but I dont know how to go about the signal duplicator.

Questions:

[COLOR=FF00AA]Should the preamp as an entire device present itself as a high impedance load to the Gamma 2 F++ or should it present itself as a low impedance load? I have 470uF output caps on it currently for driving headphones.

Should the preamp have a high output impedance or a low output impedance? It will be going to the subwoofer and the A5's amplifiers.

Should the potentiometers be used AS one of the gain resistors, used to attenuate the signal before the gain resistors, or used to attenuate signal after gain resistors? Should I even bother putting a unity gain buffer in this circuit or will it actually help performance to not simply have a 50K pot before the A5/subwoofer amplifier?

The only way I can think of to duplicate a signal is to first double the voltage (opamps or something) and then double the current (transistors). Is there a better way? Is there any way?[/COLOR] I dont want design ANSWERS, just design hints or even some baseline circuit theory knowledge if possible. I could obviously go out and buy 2 passive attenuators and simply use those but this is as much a learning and hobby experience as it is a simple necessity for me.

The old design was just the potentiometers between a bunch of RCA jacks but now i'm thinking i'll start revising and upgrading the design. First revision is a single input that splits between 2 outputs without creating a voltage/current divider situation. Second revision is buffering the potentiometers.

 

[Avro_Arrow]

In general, you want a high input impedance and a low output impedance.
 
You can go just as simple as a stereo input jack followed by a volume
control and two (or three) output jacks wired in parallel. This will give
you no extra voltage increase. Input impedance will be the volume
control value and output impedance will vary with the position of the
volume control. This is know as a passive preamp.
 
You can add some op amps after the volume control.
They can be either unity gain or have some voltage gain
if you need it. One dual (or two single) op amps for each
output is best. This will give you a low output impedance
that does not vary with volume.
 
If you take a page from my Adcom preamp, it has buffers
on the inputs, then the volume control, then buffers on the
outputs.
 
I would strongly recommend a proper dual rail power supply
rather than a CMoy style rail splitter power supply.

 

[shrimants]

I have 2 volume controls. I need 1 input feeding 2 outputs, but I need each output to have its own dedicated volume control. From your above tips it seems that the only way to accomplish this would be to hook a single stereo input to the 2 50k pots. However, doing this (in my head) seems like it wouldnt properly control the volume because it would just create 2 resistors in parallel and screw with voltage/currents.

Also, usually a headphone splitter will add 2 headphones in parallel to a regular stereo output. This causes a current split between the two and yields uneven amounts of power going through each assuming they are not of the same or similar impedance. This makes volume a pain in the butt to control, especially if there is a low impedance IEM and a higher impedance full size can connected. I dont want this to happen when with the volume pots acting like the "headphone impedances". It wouldnt matter if they were each fixed impedances but since they can change their impedance i feel like that will screw with the way they control volume.

I was definitely thinking of the dual-rail PSU.

I have made some notes during a rather boring day in class. Ignore the stuff about balanced audio unless you see me doing something right or wong.

Also, I cannot find your adcom preamp or your DIY website/projects, but the buffer before and buffer after is pretty much what I came up with too, if you look at the notes underneath. Thanks a bunch for your help.

 

[Avro_Arrow]

I ran across this unbalanced to balanced converter in my schematics.
You can use it for inspiration if you wish. I didn't draw it and I don't
remember where it's from.
 

 

[Avro_Arrow]

For your volume control I was thinking you wanted something
like a master volume with individually adjustable outputs.
 
You can used volume controls in parallel, your DAC won't mind.
 
I'll draw some rough examples later when I have more time...

 

[Avro_Arrow]

Here is what I was thinking...
 

 

 

[shrimants]

I only have 2 potentiometers and I dont want to have a single master control with 2 individual controls. Technically, I already have that because both sub and speakers are active and the software acts as the "master". If I had to use a single pot for this i'd have the pot having 2 stereo outputs in parallel and use the pot to control volume going to the amp and sub.

I want this design to simply add a preamp to both the subwoofer and the bookshelfs. The schematic you proposed with the single opamp wont really cut it because for one thing I dont have a third pot and for another thing i'd never use it because I just want to be able to control the volume individually.

The splitter section I have in my drawing was set up like that hypothetically. I figured I could feed line input to the opamp, make it have a gain of 2, to turn the ~1-2 volts into ~2-4 volts. That would go through 2 seperate buffers to keep the pots "seperate" from the circuit. My reasoning behind this was a simple circuit in parallel. If you have 2 resistors of 10 ohms each in parallel, each sees, lets say, 1 volt and 1 amp going through. However, if you change one of those resistors to a 5ohm (analogous to cranking the pot to 50%) it will cause one resistor to see a decrease in current while the other will actually see an INCREASE in current. In my head, this means that lowering the volume of output 1 will cause an increase in volume of output 2 without the presence of those buffers.

As for the final unity gain buffer stage I think it is unnecessary. I dont think the input of the amps on the subwoofer and speakers cares how much of an input load they see. I just read that amps should have very high input and very low output impedance, so i put the buffers there. As an added benefit of the buffers being there, I could design a further extension to the circuit that lets me flip a switch such that the speaker/sub stops getting input while a third output, headphone, will get the output from what used to be the speaker or subwoofer output. Either that or I can simply unplug one of the devices, turn volume to 0 for the other device, and plug my headphones in to the first device output.

I hope that made sense.

Also, I dont know how I feel about that unbalanced to balanced converter. If theres one thing i've learned from my DIY experience it is to never put capacitors in the signal path, and I see capacitors all over the signal path of that schematic.

 

[Avro_Arrow]

If you get rid of the master volume pot from my schematic, it will do what you want.
Adjusting the volume of one pot will not affect the other.
 
You could probably do away with the caps from the unbalanced to balanced converter.
It depends to some extent on the op amp you use.

 

[shrimants]

yeah the caps are only there because there was this one lab i did in analog electronics where I had a TON of noise from the opamp. Adding a cap there made it go away somehow. I dunno, I have no way to measure it and RLC circuits are something i'm godawful at.

Why will 2 pots in parallel not affect the volume control to each individual device? Is it because the pots are only "concerned with" voltage and the driver itself will control current flow based on voltage it "sees"?

EDIT: also, is it just me or does your schematic have the opamp +/- terminals reversed?