Building my own 10 Channel Preamp

[dannybeckett]

Hi Guys.

I am trying to put together a plan for a preamp I want to build in the near future. I require 2 stereo line level inputs and another input which caters for 5.1 (6) channels to connect to my PC. I have put together a little block diagram schematic of how I think I can implement this design relatively easily, but I have made some assumptions which I would like validated.

Firstly I will talk you though my design. I am adapting the design from High Quality Audio Preamp

If PC input is selected, the 6 op amps in the inital +6dB amplifier section are powered on (the ones with the small '3' on them). These 6 channels get passed straight through into the second amplifier stage, and then out. the op amps labeled 1 and 2 are not powered.

If either line 1 or line 2 has been selected, those respective op amps are powered. Lets imagine line 1 has been powered. The +6dB stereo signal is fed into 2 sets of buffers:-
-One set is wired so that the outputs are combined, to give a mono output from a stereo input. This is then fed into an adjustable low pass filter and then connected to the subwoofer channel before the 'main amp' part.
-The second buffer set duplicates the stereo signal over to the rear left and righ speakers, as well as the front left and right speakers. This is again fed into the main amp section.

I have chosen to alternate power between sets of opamps for an input selection method - as this seems to be a very simple and easy way to achieve switching of such a large number of channels 2/2/6.


Assumptions-

When an op amp is off, will any signal seen at the output get fed back into the circuit in any way? An assumption that I have made is that signal will not be fed back - that when op amps are off, they may as well not be there at all.

To downmix a stereo signal to mono, simply 2 buffers can be used and the outputs tied together. Is this correct?

To upmix a stereo signal to 4 speakers, 2 buffers are used to feed 2 channels each. Is this ok to do?

I have attached the block diagram/schematic lovechild I have come up with.

Thanks a lot

Dan

[dannybeckett]

noone? lol

[Mooly]

You can't connect opamp outputs together as you have done... the output impedance is so low that you are just shorting the two together.

When you power off an opamp all bets are off... it may well present a low and non uniform impedance at it's input, the output will be undefined, and what about switch on/off noises.

To combine signals you need a virtual earth mixer using an opamp, with electronic switching of the inputs etc using either FET's or even CMOS 4016 or 4066 IC's.

[dannybeckett]

I understand. You have given me a lot to think about. I will be able to redesign all this without connectiong the opamps in this strange way (I had a feeling this wasn't going to work anyway).

[Mooly]

The 4066 CMOS IC is pretty good if used correctly. That means ensuring it doesn't "see" any signal voltage across it, which is easy if used at the input to a virtual earth mixer. Use a series shunt arrangement for even higher "off" attenuation.

I use FET's but if you replaced them with 4066 switches and the appropriate logic it would work pretty much the same.

[dannybeckett]

I had no idea these kinds of switches existed! This makes channel selection quite a bit easier lol. I think i will be able to cut out 4 op amps as well (2 from the upmix and downmix sections).

[Mooly]

4016 and 4066 are pin compatable... 4066 has lower on resistance and is the preferred one of the two.
http://www.datasheetcatalog.org/data.../206766_DS.pdf

The 4053 is useful
http://www.datasheetcatalog.org/data...4/149194_2.pdf

and so to the 4051
http://www.datasheetcatalog.org/data...lips/4051B.pdf

[dannybeckett]

OK I have a couple more questions. I have attached a picture illustrating what I plan to do. All I want is a way to get a mono output from a stereo input, without tying the stereo signal together (to go off to a subwoofer out).

Also, is it even necessary to include the set of buffers in the centre of the schematic thing I drew? The only reason I included them is so that there would be a completely even load distribution, but now I am thinking that this doesn't even matter.

Upon studying the schematic futhur, I have realised that I stupidly shorted front left to rear left and front right to rear right permanently, within the bos which contains the opamps in question. More logic switches here I think!

[dannybeckett]

Heres an updated and simpler version of the schematic

[Mooly]

Originally Posted by dannybeckett

OK I have a couple more questions. I have attached a picture illustrating what I plan to do. All I want is a way to get a mono output from a stereo input, without tying the stereo signal together (to go off to a subwoofer out).

Also, is it even necessary to include the set of buffers in the centre of the schematic thing I drew? The only reason I included them is so that there would be a completely even load distribution, but now I am thinking that this doesn't even matter.

Upon studying the schematic futhur, I have realised that I stupidly shorted front left to rear left and front right to rear right permanently, within the bos which contains the opamps in question. More logic switches here I think!

You need a virtual earth mixer, so that means adding 10k resistors to the output of those first opamps, then make the third an inverting stage with 10k feedback resistor. Essentially the same as in the circuit I posted earlier.
Only drawback is that inverts overall phase so you can either add another inverting stage to correct that, or make those input buffers inverting as well... easy