Summing Amps - what do they do?

a summing amp is used to bring multiple sources together

it may seem simple but it has some special requirements that a simple line level unity gain amp doesn't



a simple situation of two line outputs joined to gether to make a simple mixer.

each amp sees the other as a load

simple solution is to feed the two sources through a resistor such that each output/amp is not overloaded (loaded DOWN)



as this gets larger and more and more are brought together ... that summing point seems to get near zero

virtual earth summing junction

the signals seem to get smaller

so

the following amp .. the summing amp ... needs to be low noise and high gain ... and headroom to burn

and a high group time delay ( don't ask )

and a phase shift under 90deg at the crossover point ( err definately don't ask)



they are just better amps than your average line level, EQ, buffer ... general purpose amp



some Mic-pre amps do find themselves used in this situation.

The Neve BA283 and API2520 and cousins are obvious old time popular units



A modern use for these things might be a PASSIVE summing unit

for analog mixdown from Digital Interfaces



might also be used to bring two stereo programs to one

....

stereo mix with a parallel compressed mix

mastering engineer trick



add the tube feel of the Tube Tech and you might have some of that MASTERING magic





all a bit too short and incomplete as an explanation

but I hope it gets the ball rolling

Kev, awesome explanation!



Would you use a summing amp for sub mixes, bus out certain groups like bass, kick, snare... and maybe even the lead vocals?



Example, to give all the center panned tracks better imaging?



Or... to use it for the sweet tracks in a song... kind of like adding a hook from a sound designers POV ... ear candy...



Or... are they more for the final mix ?

all of the above



current trend is to do sub mixes in both stereo and mono from the DAW and then sum these together in analog for a final mix



yes imaging is one of those qualities people are looking for



I guess the more inputs to the passive mixer there are

the more people tend to take out of the sub groubs



some units don't have a pan ... as these are hard to get trimmed equally across all channels

same goes for faders

...

here is why top end mixing desks are SO MUCH money ... quality parts



back to the point

some units have a switch for left, right and centre

and have a fader as a rotary switch with matched resistors so each channel tracks the same



The DAW can make a stereo synth or guitars or drums and the imagining is ok

then these are brought into a larger mix using the passive unit and then the centres like lead vocal

... kick and snare perhaps.



As for the sweet tracks

well people use whatever trick they can to make a musical part sit inside the mix but easily heard... without pushing it on top of the mix



some musical parts get transistors and some get tubes

at the time of recording

or later at the mixdown

:roll:

or a bit of both



it's all about colour

and then about imaging and openess and ... glue

all at the same time



difficult to put into words but when you hear it you will be chasing that feel for the rest of your recording life

Hey Kev...



"when you hear it you will be chasing that feel for the rest of your recording life"



beautifully put...



now thats a signature definitely worth using!