Nevey Sum Bus

[alexc]

Bus resistor values vary and for more channels, the value is lower.

If I was doing 8 channels of balanced I'd probably go somewhere between 4K7 and 10K.
Probably more towards 10K.

Bus terminating or shunt resistance then is calculated according to how much gain you decide to run.
So that depends on the turns ratio of your input transformer (if you have one) and/or the gain available in your makeup amp.

For a balanced bus into a neve style make up amp (unbalanced) you need a transformer.

Basically you 'knock down' the bus levels as far as you have make up available.

ie. the impedance of the bus, which is the parallel of the unterminated bus and the termination resistance should be low, for various reasons, not least of which is that is what makes switching channels in/out not jump up and down in level too much.

Anyway, there's a lot of opinion, some calculations etc as to what is the way to go.

8 channels balanced into a 1:2 transformer (neve stuff apparently used 1:2)  with 35dB gain might be 300R or so.

Best to calculate it. I'm feeling a little lazy right now, so I'll leave that to you.    I'll post more if you remain stuck.

 

[alexc]

Individual level pots and pans pots changes the game significantly in terms of impedances and what not.

I haven't done that so I can't give you the right answers about bus resistors and shunt resistance. 

And for API issues, ClassicAPI is definately the goto place for info.

Cheers

 

[Scrappersa]

Bought the BA283 boards from Igor. My plan is 16 mono channels with 1k volume pots, 10k panning pots, 2.2k resistors slugged to the panning pots, 6.8k summing resistors into the BA283 boards with Carnhill VTB1847 as the output transformers. Are the input transformers necessary since my signal will be unbalanced when it hits the BA283's?

 

[alexc]

Not for unbalancing, no.

Good luck with it and let us know how it goes for you.

Cheers

 

[lotus]

Hi,if the signals entering the ba283 are unbalanced like the previous poster,there would still be a good need for the line input transformers if you want to ad a line insert ,right?
Also thanks Alex for the help so far.Eric

 

[Scrappersa]

If I want to have multiple outputs, can I just split them off the output transformer or will that cause impedance problems? I want a mix out, and one or two monitor outs. Maybe a headphone out but probably not.

 

[alexc]

if by 'line insert' you mean a send output and return input, then :

Adding line inserts certainly would not be a good idea at the summing junction.

That is a very low level and impedance sensitive place so the less you mess with it there, the better!

Even with a transformer, the levels on the secondary are very low so I doubt that would be a good idea either.

I can't think of any simple way to do this. Really, it would require a dedicated buffered send output.
I would think it would be better  to do inline after the sum unit 

Sorry - without redesigning the make up amp to provide a buffered out and switched return,  I don't think it would fly.

 

[alexc]

You can mult the outputs fine provided you only connect balanced devices to the mult and your total load doesn't go lower than 600R. 

The neve output stage is quite powerful so will have not trouble driving 2 balanced devices.

In my next summing bus, I am mult'ing the main outputs to feed an onboard 'stereo pico compressor'.
This is a transformer unbalanced solid state input, so probably some K input Z.
Multing that is no problem at all.

If you connect a mult to an unbalanced device you will unbalance everything hanging off the outputs.

Headphones?  I assume you mean a headphone amp device.

Again, as long as balanced input and load > 600 no probs.

As with all these things, the more you futz with it, the more likely to have problems.
1 mult OK, 2 prolly OK. I would just stop at that point and rethink things a little


So "don't do dat" 

 

[Scrappersa]

Ooooh, that might be an issue then. My power amp driving my monitors is an Adcom GFA 545 which happens to have unbalanced RCA outputs. I'll have to figure out a solution to this. Any ideas?

 

[alexc]

Unbalanced inputs (at the amp) you mean?

It's not a problem for the summing box - it's just that it will be running unbalanced, because one of it's output pins will be grounded at the amp's unbalanced input.

When you ground one leg of a transformer balanced output, the level rises 6dB. That can be an issue but usually OK.

Of course, you lose the galvanic isolation that balanced transformer coupling provides.

The other thing is that hifi amps are usually set up to accept quite low levels - they are high sensitivity, needing a 1Vpp to 4vpp or so for full output. 'cd level' or -10dB style inputs.

A nevey styled summing amp puts out more like 10vpp typical and peaks out at maybe 24vpp or so.

So it is high level '+4dB' style output and is best matched to a low sensitivity amp.

Otherwise you need to turn it down. You may as well just buy some tascam stuff

I would 'fix' the situation by building a balanced, low sensitivity tube amp  8) 8)
http://www.groupdiy.com/index.php?topic=49119.0

Or you can just make an attenuator box or even make a transformer isolation box with cheap ebay jensen 10K:10K transformers and an attenuation network.

IF you are trying to feed a high quality, high level balanced signal chain AND you are mult'ing to an unbalanced, low level amp chain, then  ..  not so good.

iso transformer for unbalancing and an attenuation network best.

Otherwise you are forcing things to be what they are not wanting to be. It works but .. not as satisfying.

Like recording without beer.

 

[Scrappersa]

Hey Alex, I'm loving the summing mixer that I made. I ended up going with all of the values I posted here plus using the BA283's for makeup gain. I don't have an input transformer, but I'm using Ed Anderson's Neve transformer for the outputs. The only problem I've been running into is that my gain is a bit too hot. Looks like I might need an attenuator. What switch/board/values did you end up using for yours, and how has it worked out for you?

 

[alexc]

Hi

I used 4K7 bus resistors and a 620R bus shunt. And 17 channels max connected.
Bus loss was 28dB approx

Input traffo was 1:1. Gain make up + output traffo was 18dB (min level control) to 36dB(max level control).
BA amp ended up with 100R resistor + 2K pot (linear I think)

I generally use it with the level knob at around 3 o'clock or so. Typical usage is a stereo DAW at strong studio level out, maybe 4 synth/fx modules playing and 2 or so analog channels playing.

No changes since then and I am using it all the time - noise is very low, distortion extremely low and I have no problems setting a gain depending on how much stuff I have playing out thru the unit.

Worst thing is I get a little crosstalk, somewhere in the region of -50dB to -45dB or so for the louder set channels. This is not a problem for me at all.

----

If your gain is too hot, decrease the value of the shunt resistor to increase the total bus loss.

Or, vary your BA amp gain arrangements. You can easily get 20dB or so variable gain so that should take care of your needs.

Cheers

 

[dirty1_1garry]

Great project!
any schematics or structure of your summer?

 

[alexc]

I didn't draw up any - the 'voltage summing ' or 'passive summing' network is virtually straight ahead 'NY Dave' implementation - balanced with L/R assign switches only.

The makeup amp is the neve BA283 'line amp' section  (3 transistors and a gain trim) driving an output transformer single-ended style. You can find that pretty easy around here.

The only bits to work out are i) balanced sum resistors  ii) bus 'slugging' resistor  - I mentioned my values in here somewhere.

PSU is a 24V regulated external plug-pack

There's nothing else. You can find 'NYD summing' network schematic somewhere in this 'Mix' section.

---

It still works great every time as the 'monitor mixer' for my ITB mix + all my DIY outboard boxes (tube and discrete stuff)

I'm doing another summing unit soon for expansion, based on 'current summing' with a Lawo card.

Just do it!