Passive Summing Amp with several busses calculation help needed

[mohausler]

Hi there.

I was wondering whether anyone here could help me with a good formula for a passive summing box with several busses. I always wanted to design a box like that as the idea of analog summing is more about analog parallel processing for me rather than the effect of analog summing itself. Most analog summers on the market just have one bus, I would like 4 subgroups. There is only one widely common product out there, the Heritage Audio MCN32. That one is active, huge and has all these controls I don't really need, in fact rather not have. I also just came across the Undertone Pyra-Sum which is very close to what I wanted to build, but has a slightly different signal flow of what I would like and, quite frankly, considering that it is nothing but a few resistors, switches and connectors a bit overpriced imo. I also think that mono-ing channels can be solved slightly better as you loose the right input completely in their design.

I worked out input impedance, output impedance, resistor values. How would I work out the required make up gain?

Thank you!

 

[ruffrecords]

If you do not slug the bus with a resistor then the make up gain is equal to the number of inputs fed to the bus. So if you have 16 inputs you need a gain of 16 times or 24dB.

Cheers

Ian

 

[mohausler]

If you do not slug the bus with a resistor then the make up gain is equal to the number of inputs fed to the bus. So if you have 16 inputs you need a gain of 16 times or 24dB.

Cheers

Ian

Thank you very much!

 

[abbey road d enfer]

This is assuming the input impedance of the make-up amplifier is very high compared to the bus impedance, which is 1/N the value of the injection resistors, N being the number of feeds.
In most cases it is close enough, but when the number of feeds is low, and a mic preamp is used for make-up, the bus impedance is no longer << than the preamp's input impedance.
Not a big issue, but it is just to show that adjustment may be necessary.

 

[ruffrecords]

This is assuming the input impedance of the make-up amplifier is very high compared to the bus impedance, which is 1/N the value of the injection resistors, N being the number of feeds.
In most cases it is close enough, but when the number of feeds is low, and a mic preamp is used for make-up, the bus impedance is no longer << than the preamp's input impedance.
Not a big issue, but it is just to show that adjustment may be necessary.

And it also assumes the source impedance of the inputs feeding it are very low compared to the bus feed resistor - so don't leave any unused inputs open circuit.

Cheers

ian

 

[scott2000]

If I wanted to sometimes use a summing box like this as a master effects insert only, can you just add an input to the summing bus output...after the resistors? Is there anything to consider or must be added or taken in account?

Also, if no external amplifier is in the chain like in the picture, (just summing bus to master inserts) couldn't a first master insert send/return be the summing amplifier?

Hope this makes sense.

 

[ruffrecords]

Probably the main issue with what you propose is that master insert send/return will likely not have sufficient gain to bring the bus signal up to line level unless it is one of the old 600 ohm input tube based types that has about 40dB gain at the front (in other words is effectively a mic pre plus a compressor).

Cheers

ian

 

[scott2000]

Probably the main issue with what you propose is that master insert send/return will likely not have sufficient gain to bring the bus signal up to line level unless it is one of the old 600 ohm input tube based types that has about 40dB gain at the front (in other words is effectively a mic pre plus a compressor).

I think I follow.

What about the other thing where I would like to bypass the summing completely and only use the master insert effects to treat a stereo signal? Can I put/add a stereo input after the summing bus with no worries? Or am I not following the above correctly?

 

[ruffrecords]

I think I follow.

What about the other thing where I would like to bypass the summing completely and only use the master insert effects to treat a stereo signal? Can I put/add a stereo input after the summing bus with no worries? Or am I not following the above correctly?

Not directly after the summing bus because bus level is so low. Easier just to add an insert point before the master insert effects and patch the input to it.

Cheers

Ian

 

[Matt Syson]

Agreeing with ian of course but should point out that if you are using resistors to create a 'mini, sum bus' (for example parallel processing where you have the direct feed from a previous summing stage)
and want to blend it with a return from a processor of some sort. This can be done with a pair (mono unbalanced) or quad (balanced) of resistors but this will incur about 6dB level drop for UNCORRELATED signals)(and intermediate levels for elements in/out of phase at various frequencies so is actually quite complex AND) most likely variable if the source impedances are dissimilar. In almost all cases it will simply work and you turn up the level by up to 6db somewhere else in the system so you can almost ignore it. The (summing)resistor values must be high enough so that the drive current capability of the 'source' is never compromised (distortion) do the resistors would probably need to be around 3K Ohms (an output stage from gear using TL072 chips is starting to get a bit more distorted at around 2K ohms of load) and other gear 'semi pro' could be anything.
The equations you need are essentially Ohms law once you have worked out the scheme you want to use and take into account all impedances around the whole circuit. in real life use some 3K resistors and just turn something up until you get what you want is probably close enough (unless the driving amplifier has a particularly high output impedance and poor drive current capability.
The definition of 'Best' is whatever works as YOU expect and like. ALL other 'designs' have their own parameters so not necessarily 'best' for you.

 

[JohnRoberts]

I never understood the attraction of passive combining but to answer your question about the post makeup gain stage mic preamps are typically used.

Mic preamps are not optimized for the relatively low gain, and include extra front end circuitry for phantom power that is unnecessary.

It would not be difficult to roll your own high quality dedicated 25dB gain stage, it could even be DC coupled to avoid those evil capacitors.

JR

PS; do not read this as me advocating passive combining, yadda yadda.

 

[scott2000]

It would not be difficult to roll your own high quality dedicated 25dB gain stage, it could even be DC coupled to avoid those evil capacitors.

I'm not too worried about getting enough gain...
even have this buffer built that would be neat to try since I never use it but it's signal is inverted....Need a transformer I guess?..
It's from a book so I removed values jik.... Has 26dB of gain.....

 

[JohnRoberts]

I am not a tube guy so I have nothing constructive to offer.

JR

 

[ruffrecords]

Passive summers are really quite simple. They are just a bunch of resistors and connectors. They typically accept line level inputs and create a relatively low level output. In the process of summing the signals together some attenuation is involved. The minimum attenuation is exactly equal to the number of inputs to a bus. So if there are 8 inputs to a bus the minimum attenuation is 8 times (18dB). For 16 inputs it is 16 times (24dB) and for 32 inputs it is 32 times (30dB).

The input impedance of each input is close to the value of the sum of the two summing resistors connected to that input. So for a 10K input impedance you would typically use a pair of 5K1 resistors. The bus impedance is simply the channel input impedance divided by the number of inputs connected to the bus. So for a 10K input impedance, 8 channels connected to the bus gives a bus impedance of 1.25K, 16 channels gives 625 ohms and 32 channels gives a 312 ohm bus impedance. The bus impedance is essentially the output impedance of the passive summer.

Since the passive summer attenuates the inputs, you need to follow it with an amplifier to restore the signal level to line level. From the above you can work out how much gain it needs. Its input impedance should be at least ten times the bus impedance.

That is all there is to it really. Some people make unsubstantiated claims about how passive mixing can improve or alter sound and charge large sums of money for their products . But a bunch of resistors in a box is not likely to do this nor does it cost very much. Attached is a picture of one I made in an old computer switch box I got on eBay for about £10.

For more info on passive summing check out the DB25 folder on the DIY page of my website:

Custom Tube Consoles - DIY

Cheers

Ian