Line Mixer question...

[G._Hoffman]

I asked this on another board, but since I didn't get any responses, I'll try here. 


I'm kind of in the middle of building myself an overly complicated guitar rig, and one of the things I'm doing is getting all my time based effects after my amps.  I've tried adding effects loops, and they didn't do it for me - it just sounded kind of sterile to me.  So, I'm going this rout.  It has the advantage of letting me run the choruses, delays, etc. in stereo.  This all returns to an little stereo solid state amp and a couple extra speakers.  Kind of fussy, not particularly practical, but I'm having fun, which is all I'm really worried about. 


SO, one of the things I want to do is run those time based effects in parallel, which means I need a line mixer.  I've looked up a couple designs on the internet, and talked to a few people about it, but what I'm not entirely clear on is how many input stages I can comfortably sum into a single summing amp.  I had one friend tell me four, but he was specifically talking about a tube summing amp (which he made for Motown records about 50 years ago!).  He felt an op amp (which is what I'm using) might do more, but he had never built one.  I've seen other people say eight is fine, and since I want eight stereo inputs that is what I would prefer, as I could do the whole thing with 9 dual op-amps (eight for the inputs, and 1 for summing), instead of 11 (eight for inputs, two for sub-summing, and one for the final summing).  Op-amps are expensive enough for this to matter a bit (to me - I'm poor!), but also it saves space and a bit of noise, and that is useful for me.

So, any opinions on the matter? 


I'm also trying to decide if the noise penalty of leaving the inputs open all the time will be problematic, or if I should switch the line amp inputs on and off.  The sends to the effects will of course be switched, and most of the effects are pretty quiet, so if the op-amps are reasonably quiet it shouldn't be a problem, right?  I will only really know once I've tried it, but if anyone has any past experience I'd like to hear it.


Gabriel

[bibi]

Since no one else is replying here you go:  http://sound.westhost.com/project94.htm
You shouldn't have a problem adding more inputs to make this an 8 channel stereo mixer.
Check the ESP site further and you will find even more mixing amps and the theory behind them.
What you want to look for is an active mixing stage so you don't need to worry about plugging or muting inputs.

It's really what opamps do best and they certainly aren't expensive!

Good luck.

[PRR]

> how many input stages I can comfortably sum into a single summing amp

As many as you want, though better with only what you need.

96 inputs to an opamp is not unknown. Tube can do this too.... look at Hammond organ, how do they get all those tonewheels to come out one place? Gotta be a 44++input mix in there somewhere.

Open/closed inputs is ambiguous, and effect depends on active or passive mixing. If you desire zero interaction between knobs, without buffers, active is simpler, and muted inputs should be disconnected. With passive there is interaction, but less as you INcrease the number of inputs, but disconnecting muted inputs raises gain on other inputs (which is actually musically useful).

Just do it. An opamp and pots and resistors on a hunk of board. Diddle and fiddle. I think you are over-thinking.

[38Super]

Another thing you may want to consider is using linear pots on each channel to create a left and right out - a pan function.  I'd suggest using dual or quad opamps in DIP packages.  Check audio projects on the net for recommendations.  Analog devices makes some good stuff, as well as many other suppliers.  Suggest using bipolar transistor opamps: they are typically, not always, quieter.  Easier if you create a bipolar supply (+/- 15V):  that way you don't have to DC offset your opamp inputs and capacitor couple inputs.

Suggest buffering each input and feeding low impedance opamp output to a pot and/or pan pot and then summing using another opamp.  You could gain up each buffered input channel to something resonable like 10 and use pot to attenuate to desired level.  As long as you're building it, why not have multiple independently buffered outputs for use on multiple amps, unbalanced out, etc.  You could add a high cut cut if you want.

See if you can find schems for simple 4 or 8 channel mixers to see how they implement circuit functions.


cheers,

rob

[G._Hoffman]

Another thing you may want to consider is using linear pots on each channel to create a left and right out - a pan function.  I'd suggest using dual or quad opamps in DIP packages.  Check audio projects on the net for recommendations.  Analog devices makes some good stuff, as well as many other suppliers.  Suggest using bipolar transistor opamps: they are typically, not always, quieter.  Easier if you create a bipolar supply (+/- 15V):  that way you don't have to DC offset your opamp inputs and capacitor couple inputs.

Suggest buffering each input and feeding low impedance opamp output to a pot and/or pan pot and then summing using another opamp.  You could gain up each buffered input channel to something resonable like 10 and use pot to attenuate to desired level.  As long as you're building it, why not have multiple independently buffered outputs for use on multiple amps, unbalanced out, etc.  You could add a high cut cut if you want.

See if you can find schems for simple 4 or 8 channel mixers to see how they implement circuit functions.


cheers,

rob

I don't really need a balance or pan pot, just because of the nature of how I'm using it, but that is otherwise pretty much what I'm doing.  ±15V PS, buffered inputs with the level control after the buffer, and a simple summing amp.  If I was going to do any kind of flitering it would very likely be a high pass filter, as I tend to like quite bright delays and such (as a recording engineer, I can get a lot of nice brightness without making things sound shrill by putting a low pass filter on the feedback.  One of my favorite tricks.)  But for this, that probably won't be happening.


Gabriel