Line mixer questions....

[G. Hoffman]

I'm designing a line mixer for a parallel effects return thing...it's complicated.  Enough to say, I'm designing a line mixer.

Now, there are two questions I have.  First, how many (buffered) inputs are appropriate per summing amp?  I got an opinion from one guy who I do trust, but the last time he built a summing amp was about 50 years ago, and he said no more than four inputs per tube summing amp, which is fine, but I'm using op amps.  I've also heard from others that eight per summing amp is fine, and since this is only going to be eight (stereo) inputs, I'd sure like only having to use one dual op amp for summing. 

The other question is to do with switching.  So, there is going to be a parallel send to the effects in question, and that will be switched, but the question is: do I need to switch the inputs to the summing amp?  It would sure be a lot easier to just leave them open all the time, BUT if I'm asking for too much of a noise penalty doing and I'll have to switch them.  It's just time based effects - Delays, Chorus, reverb (maybe), maybe a couple others.  All the boxes I know will be going into it are pretty quiet, so its mostly just a question of an idle op amp's noise.  It's hard to say without actually trying it, I know, but any guesses?


Gabriel

[sault]

I think it would depend on your power supply.

If you're going for maximum headroom, then figure that line voltage is normally around a volt, and could peak even higher, perhaps 2 or 3 volts. The op amp needs a few volts to do its thing, so 2-3 volts per channel plus a few volts for the op-amp means at least 18v for 8 channels... or maybe 2 opamps with a 9v power supply. Good place to start, I think.

More voltage means being able to handle more channels per op-amp, at least that's how it seems to me.

I'm not 100% sure what you mean by switching inputs, but open inputs (inputs not being used) shouldn't add any appreciable noise to the circuit. They do that with some guitar amps (having both a Lo and Hi input, for instance) and they don't have any issue there.

Hopefully I've understood your question!


Sault

[G. Hoffman]

I think it would depend on your power supply.

If you're going for maximum headroom, then figure that line voltage is normally around a volt, and could peak even higher, perhaps 2 or 3 volts. The op amp needs a few volts to do its thing, so 2-3 volts per channel plus a few volts for the op-amp means at least 18v for 8 channels... or maybe 2 opamps with a 9v power supply. Good place to start, I think.

More voltage means being able to handle more channels per op-amp, at least that's how it seems to me.

I'm not 100% sure what you mean by switching inputs, but open inputs (inputs not being used) shouldn't add any appreciable noise to the circuit. They do that with some guitar amps (having both a Lo and Hi input, for instance) and they don't have any issue there.

Hopefully I've understood your question!


Sault

Thanks!  That makes sense to me, and with my planned ±15V power supply (I already have all the parts, and it can't be a bad thing to have that much headroom), I should probably be fine.



Gabriel

[Nasse]

http://douglas-self.com/ampins/mixer/mixindex.htm

some theory there I believe. Good luck for your project.

[therecordingart]

I'm throwing around the idea of building an API-esque mixer. It won't be cheap.

Here are the line amps:

http://www.waltzingbear.com/Schematics/API/API_325.htm

Here are the mic pres:

http://www.waltzingbear.com/Schematics/API/API_312.htm


I built a pair of the pres in the past (since sold them), and they sounded great!

[sault]

Thanks!  That makes sense to me, and with my planned ±15V power supply (I already have all the parts, and it can't be a bad thing to have that much headroom), I should probably be fine.

While I hesitate to say that 15v is enough if you plan on using 8 channels, I suppose that if you have volume pots on the inputs then there shouldn't be an issue at all. I was assuming none, for whatever reason, but if that's the case, then it seems to me that you'd have plenty... even 3v from each input could easily be attenuated a few db, and would still give you plenty of headroom to work with. (ie 3v * 8 at -6db each = 12v, and I think that's a pretty high amount of input voltage!)

It sounds like a fun project, hope it works out for you... Feel free to post on your progress if you like, I'd be interested to see how it goes.


Sault

[G. Hoffman]

http://douglas-self.com/ampins/mixer/mixindex.htm

some theory there I believe. Good luck for your project.

Theory!  Always useful. 

Thanks!  That makes sense to me, and with my planned ±15V power supply (I already have all the parts, and it can't be a bad thing to have that much headroom), I should probably be fine.

While I hesitate to say that 15v is enough if you plan on using 8 channels, I suppose that if you have volume pots on the inputs then there shouldn't be an issue at all. I was assuming none, for whatever reason, but if that's the case, then it seems to me that you'd have plenty... even 3v from each input could easily be attenuated a few db, and would still give you plenty of headroom to work with. (ie 3v * 8 at -6db each = 12v, and I think that's a pretty high amount of input voltage!)

It sounds like a fun project, hope it works out for you... Feel free to post on your progress if you like, I'd be interested to see how it goes.


Sault

±15V.  30V total, but a bit easier to bias.  I've already got a ±15V power supply board I made for another project that ended up going another way, so its easy to do.


Gabriel

[sault]

and with my planned ±15V power supply

Ugh. Would help if I paid a little closer attention, wouldn't it?   

Feel free to post a schematic!



Sault