Simple Neve Summing "Mixer" Design

[XAXAU]

EDIT: I changed the brand in the title from "API" to "NEVE"!

Hi guys!

I need some help with designing a simple API summing "mixer". I say "mixer" cause I don´t need inserts, pan- or gainpots for the input channels.
Basically I just want the benefits of analog summing and the ability to impart some great tone onto my mixes. I´ll probably sum channels into stems then sum the stems to a stereo track.


So what are my requirements?

8x Inputs
2x Vu Meters
1x Stereo Stepped Gain
1x Attenuator for monitoring
2x Outputs (AD & monitors)

I´d prefer transformer balanced inputs, mainly for tone but my wiring is always changing and it ain´t pretty.
I would be injecting my Moog Voyager as well which is switchable to balanced @ 600ohm.


Stuff I already have:

JLM Kraftwerk Kit
JLM VU2 Stereo VU Buffer Kit


Stuff I plan on buying:

2x Sifam AL20SQ-Retro
2x Elma Attenuators (CAPI)
1x Elma Attenuator THT


Stuff I need help with:

* Find suitable high quality input transformers
* Find  high quality components like chassis, cables, connectors, caps etc
* Signal path
* Encouragement 

Signal path from talking to Jeff Steiger:

DA - (8x) Input trafo (floating -) > 47R > ACA (DOA 2520 & Trafo EA 2623-L) (floating -) > Stepped Attenuator >BO (DOA 2520 & Trafo EA2503-L) > AD


That´s all for now!  8)

 

[ruffrecords]

I am not clear if this is 8 inputs split 4 left and 4 right or 8 stereo stems split 8 left and 8 right.

Either way, have you considered a passive mixer? It can still be transformer balanced but you only need two transformers, one for left and one for right.

Cheers

Ian

 

[XAXAU]

I am not clear if this is 8 inputs split 4 left and 4 right or 8 stereo stems split 8 left and 8 right.

Either way, have you considered a passive mixer? It can still be transformer balanced but you only need two transformers, one for left and one for right.

Cheers

Ian

Hi Ian!

It´s 8x mono (4 left & 4 right) inputs I had in mind.

Yes I´ve looked at passive summing and then using Neve 1272´s, your TLA or any other spicy preamp. Do you mean that the passive summing can be transformer balanced if the passive summing network goes into 2x input transformers or?

I´ve also looked at Neumann V475-2C and CAPI 2-ACA-BO.

I´d prefer not to do passive summing because I´ll probably do more than one trip through the summer, summing together the stems then summing the stems to get one step closer to the console vibe.

So that leaves the Neumann as well as the 2-ACA-BO which I was originally planning to do. The Neumann is $200 recapped straight down my mailbox so that´s the cheapest alternative and from what I hear it´s a great sounding summer.

If the colour isn´t enough I can always add on a line driver after whatever summing unit I end up using.

 

[ruffrecords]

Hi Ian!

It´s 8x mono (4 left & 4 right) inputs I had in mind.

Yes I´ve looked at passive summing and then using Neve 1272´s, your TLA or any other spicy preamp. Do you mean that the passive summing can be transformer balanced if the passive summing network goes into 2x input transformers or?

Basically yes. That is in essence what happens in the Neumann 475-2C although it is closer to virtual earth mixing than passive mixing but the principle is the same - you mix balanced signals together then connect them all to one transformer followed by an amp. In the DB25 thread there is a discussion between me and a DIYer who did exactly this with the Neumann. He used my DB25 board  for the interface and summing resistors and just wired it to the Neumann.

Cheers

Ian

 

[XAXAU]

Basically yes. That is in essence what happens in the Neumann 475-2C although it is closer to virtual earth mixing than passive mixing but the principle is the same - you mix balanced signals together then connect them all to one transformer followed by an amp. In the DB25 thread there is a discussion between me and a DIYer who did exactly this with the Neumann. He used my DB25 board  for the interface and summing resistors and just wired it to the Neumann.

Cheers

Ian

Cheers! Will read up on that in your DB25 thread then.

Which of these summing solutions will give the least noise if I was doing 2 roundtrips? Summing to stems then summing the stems down to a master stereo track? Will the API summer have the dB drop like traditional passive summing as well if it had 2x input trafos post summing network?

I guess I need to be educated on which summing network does what. I know that a passive summing network needs resistors in series so that one channel does not load the other channels and this also sets the gain. Not connecting a channel will screw up the balance of the summing network. That´s basically all I know! Will active- or zero ohm summing be different?

I read the DB25 thread and the Forsselltech paper and some things became a bit clearer but I can´t say I fully comprehend these concepts! 

http://www.forsselltech.com/media/attachments/summing_buss.pdf

 

[ruffrecords]

Cheers! Will read up on that in your DB25 thread then.

Which of these summing solutions will give the least noise if I was doing 2 roundtrips? Summing to stems then summing the stems down to a master stereo track?

Properly designed, there is almost zero noise difference between passive and virtual earth summing.

Will the API summer have the dB drop like traditional passive summing as well if it had 2x input trafos
post summing network?

As far as I  know, the API is a straightforward virtual earth mixer so you cannot add a transformer post the summing network. You either have to unbalance each input first, or sum hot and cold separately for it to be balanced

I guess I need to be educated on which summing network does what. I know that a passive summing network needs resistors in series so that one channel does not load the other channels and this also sets the gain. Not connecting a channel will screw up the balance of the summing network. That´s basically all I know! Will active- or zero ohm summing be different?

There are basically three variations:

1. Passive summing into a transformer (Neve mixers in the 70s used this)
2. Virtual earth summing (API) (sometimes called active or zero ohm)
3. Virtual earth summing via a transformer (Neumann)

All three use series resistors in each input.
The number of channels in  a passive mixer sets the loss and hence the gain make up required to restore the level and hence the noise level.
The number of channels in a virtual earth mixer sets the noise gain and hence the noise level
Not connecting a channel has no effect on a virtual earth mixer except make is susceptible to noise pick up.
Not connecting a channel on a passive mixer will reduce the bus loss and raise the signal level (by how much depends on how many channels).

Bottom line for your application is that the Nuemann is a good choice.

Cheers

Ian

 

[XAXAU]

Properly designed, there is almost zero noise difference between passive and virtual earth summing.As far as I  know, the API is a straightforward virtual earth mixer so you cannot add a transformer post the summing network. You either have to unbalance each input first, or sum hot and cold separately for it to be balancedThere are basically three variations:

1. Passive summing into a transformer (Neve mixers in the 70s used this)
2. Virtual earth summing (API) (sometimes called active or zero ohm)
3. Virtual earth summing via a transformer (Neumann)

All three use series resistors in each input.
The number of channels in  a passive mixer sets the loss and hence the gain make up required to restore the level and hence the noise level.
The number of channels in a virtual earth mixer sets the noise gain and hence the noise level
Not connecting a channel has no effect on a virtual earth mixer except make is susceptible to noise pick up.
Not connecting a channel on a passive mixer will reduce the bus loss and raise the signal level (by how much depends on how many channels).

Bottom line for your application is that the Nuemann is a good choice.

Cheers

Ian

Thanks for educating me Ian!

If I´d go for the API (building yourself is way more fun, don´t you think?) would it be possible to use input transformers on every input channel and unbalance them, send the signals through 47K bus resistors then into the ACA? As I previously said it´s mainly for tone but my wiring is all over the place plus I might move to London or Malaysia where all sorts of weirdness is going on in the wires!  ;D

Also if I do a passive summing mixer is it possible to have input transformers? I have EZ1290 in mind.

If yes then what impedance would I look for? 10K in, low Z out?

 

[ruffrecords]

Thanks for educating me Ian!

If I´d go for the API (building yourself is way more fun, don´t you think?) would it be possible to use input transformers on every input channel and unbalance them, send the signals through 47K bus resistors then into the ACA? As I previously said it´s mainly for tone but my wiring is all over the place plus I might move to London or Malaysia where all sorts of weirdness is going on in the wires!  ;D

I think that should work. The transformers would unbalance the signal so you could then follow it with a straightforward virtual earth mixer like the API. I don't know the details of the API so you would have to check their docs to see what resistor value to use but 47K should be OK.

Also if I do a passive summing mixer is it possible to have input transformers? I have EZ1290 in mind.

If yes then what impedance would I look for? 10K in, low Z out?

It is possible to do passive mixing with input transformers - we did it  at Neve in the 70s all the time. The EZ1290 is probably overkill for a small number of inputs but what you want is tone so that is not really the issue. I would use 10K:600 input transformer, either original Neve 31267 or the current Carnhill equivalent. If you are feeding into the EZ1290 then you can do the mix either balanced (using pairs of 300 ohm resistors or unbalanced (using 600 ohm resistors). You have four signals per bus so the bus loss will be 12 dB and the bus impedance will be 150 ohms. The 10K:600 transformer loses 12dB as well so the total loss is 24dB and that is easily made up by the EZ1290.

Cheers

Ian

 

[XAXAU]

I think that should work. The transformers would unbalance the signal so you could then follow it with a straightforward virtual earth mixer like the API. I don't know the details of the API so you would have to check their docs to see what resistor value to use but 47K should be OK.It is possible to do passive mixing with input transformers - we did it  at Neve in the 70s all the time. The EZ1290 is probably overkill for a small number of inputs but what you want is tone so that is not really the issue. I would use 10K:600 input transformer, either original Neve 31267 or the current Carnhill equivalent. If you are feeding into the EZ1290 then you can do the mix either balanced (using pairs of 300 ohm resistors or unbalanced (using 600 ohm resistors). You have four signals per bus so the bus loss will be 12 dB and the bus impedance will be 150 ohms. The 10K:600 transformer loses 12dB as well so the total loss is 24dB and that is easily made up by the EZ1290.

Cheers

Ian

Cool thanks! Are you referring to Carnhill VTB9046 10K:600K -13dB? Is the 600ohm R´s to match the input trafo or? How did you come up with that number?

I´m leaning towards doing a passive Neve mixbus and if so I´ll probably put JLM GoBetweens on the front together with mic & hi-z jacks and toggle between summing and micpre with a 4pdt changeover. Would be such a waste NOT to do it, right?  8)

Is the mu-metal version of the VTB9046 the same trafo? Don´t see people using it here and there´s talk of placement and oscillation issues and such?

 

[ruffrecords]

Cool thanks! Are you referring to Carnhill VTB9046 10K:600K -13dB? Is the 600ohm R´s to match the input trafo or? How did you come up with that number?

Yes, that's the one. It's Carnhill's version of the 31267 we used at Neve back in the 70s.

The 600 ohm was an off the top of the head stab at the right value to load the transformer. You have 4 inputs per bus so the load on the transformer secondary will be the resistor plus the other three resistors in parallel. With 600 ohm resistors the load is 600 + 600/3 = 800; a bit on the high side. If you use 470 ohm resistors that drops to 470+470/3 = 627 ohms which is close enough. The bus impedance will be 470/4 = 118 ohms which is a good source impedacne for the 1290.

I´m leaning towards doing a passive Neve mixbus and if so I´ll probably put JLM GoBetweens on the front together with mic & hi-z jacks and toggle between summing and micpre with a 4pdt changeover. Would be such a waste NOT to do it, right?  8)

You can certainly switch between summer and mic pre but I don't know how you would do the HI-Z input.

Is the mu-metal version of the VTB9046 the same trafo? Don´t see people using it here and there´s talk of placement and oscillation issues and such?

I have never used either of these so I cannot comment but the mu-metal can version would be a lot easier to use on a PCB.

Cheers

Ian

 

[XAXAU]

The 600 ohm was an off the top of the head stab at the right value to load the transformer. You have 4 inputs per bus so the load on the transformer secondary will be the resistor plus the other three resistors in parallel. With 600 ohm resistors the load is 600 + 600/3 = 800; a bit on the high side. If you use 470 ohm resistors that drops to 470+470/3 = 627 ohms which is close enough. The bus impedance will be 470/4 = 118 ohms which is a good source impedance for the 1290.

Cool so if I do 16 input channels it´s 523+523/7=598 ohms and bus impedance 523/8=65 ohms?

And what if I skip the VTB9046 input trafos? What impedances should I be aiming for then?

You can certainly switch between summer and mic pre but I don't know how you would do the HI-Z input.I have never used either of these so I cannot comment but the mu-metal can version would be a lot easier to use on a PCB.

The JLM Go Between kit has an IDC connector where you can jack in his FET DI kit (https://www.jlmaudio.com/shop/fet-di.html) and the mic input signal will be overridden without popping thanks to relay switching.

I just figured the mu-metal version would be better if I´m doing 8x VTB9046 ins plus 2x VTB9045 ins and 2x VTB9049 outs while having the power supply in the same case. From pictures of the trafos it looks like both the regular and mu-metal has holes for screws?

 

[ruffrecords]

Cool so if I do 16 input channels it´s 523+523/7=598 ohms and bus impedance 523/8=65 ohms?

Correct. But don;t forget the bus loss is now 8 times which is 18dB so with the 12dB loss in the input transformer you now need 30dB of make up gain.

And what if I skip the VTB9046 input trafos? What impedances should I be aiming for then?

You have to load the input transformer with close to 600 ohms so there is no need to change the resistors so the bus impedance is the same. You lose the 6dB gain of the input transformer so you need 6dB more gain and will possibly have 6dB more noise.

The JLM Go Between kit has an IDC connector where you can jack in his FET DI kit (https://www.jlmaudio.com/shop/fet-di.html) and the mic input signal will be overridden without popping thanks to relay switching.

Sounds like a plan.

I just figured the mu-metal version would be better if I´m doing 8x VTB9046 ins plus 2x VTB9045 ins and 2x VTB9049 outs while having the power supply in the same case. From pictures of the trafos it looks like both the regular and mu-metal has holes for screws?

Pass; as I said I am not familiar with these transformers.

Cheers

ian

 

[XAXAU]

Pass; as I said I am not familiar with these transformers.

I have emailed Colin @ Audiomaintenance asking about the mu metal versions of the Carnhill input transformers. I also asked if he ever did produce a toroid with better flux shielding together with Carnhill (saw that discussion in an old gdiy thread > https://groupdiy.com/index.php?topic=42198.msg525165#msg525165).

Waiting for a reply.

Meanwhile I´m slowly gathering bits and pieces for the project. I sent you an email regarding 2x DB25 pcb´s.

I emailed Carnhill as well.

 

[XAXAU]

Well it seems like everyone I emailed is on vacation or their noob alarm rang?  ;D

Anyway I've decided to do an external PSU instead! I'm looking for a suitable box, any suggestions? I´m thinking a 5-pin neutrik for the PSU (Ground, +24v, 0v, +48v, 0v). What cable should I use?

Also I'm wondering where to connect the VU meters? I'm thinking on the inputs of the vtb9045's and outputs of the vtb9049's. Would that work if I'm using JLM VU2 buffers?

What case should I get if I want something nice that will last a lifetime? Was planning on doing front and rear panels at http://www.frontpanels.de/.

 

[ruffrecords]

I sent you an email regarding 2x DB25 pcb´s.

I emailed Carnhill as well.

I posted them on Monday.

Cheers

Ian

 

[ruffrecords]

Well it seems like everyone I emailed is on vacation or their noob alarm rang?  ;D

Anyway I've decided to do an external PSU instead! I'm looking for a suitable box, any suggestions? I´m thinking a 5-pin neutrik for the PSU (Ground, +24v, 0v, +48v, 0v). What cable should I use?

A 5 pin XLR is often used for mixer power supplies. The cable should be screened and connected as described in my grounding101.pdf  document. which you can find in the power folder here:

http://www.customtubeconsoles.com/diy

Cable should be rated for the expected current draw with a good safety margin.

Also I'm wondering where to connect the VU meters? I'm thinking on the inputs of the vtb9045's and outputs of the vtb9049's. Would that work if I'm using JLM VU2 buffers?

No point connecting to the 9045 inputs, the signal level is too low to show. You should attach them where you think it is important to know the signal level and where you also know what signal level to expect. You should also think about where you want to be able to measure levels for set up and for monitoring outputs.

What case should I get if I want something nice that will last a lifetime? Was planning on doing front and rear panels at http://www.frontpanels.de/.

Mechanical design is a whole different ball game. Do you want a rack mounting unit or a desktop unit? Do you have slider faders? How many channels do you want? How wide will each one be? How do you plan to wire to/from and bwtween channels?

One way to avoid a lot of mechanical deign is to buy a standard 19 inch sub-rack. This gives you a basic frame onto which you can mount your own front and rear panels. Just add top and bottom covers if it is rack mounted of for a desktop unit make your own wooden cosmetic cheeks.

Cheers

Ian

 

[XAXAU]

A 5 pin XLR is often used for mixer power supplies. The cable should be screened and connected as described in my grounding101.pdf  document. which you can find in the power folder here:

http://www.customtubeconsoles.com/diy

I read that document and it was very insightful. Will have to refer back to it once I start wiring the unit.

Cable should be rated for the expected current draw with a good safety margin.No point connecting to the 9045 inputs, the signal level is too low to show. You should attach them where you think it is important to know the signal level and where you also know what signal level to expect. You should also think about where you want to be able to measure levels for set up and for monitoring outputs.

Well I obviously didn´t think that through!  What do you mean by measure levels for setup? I think I´ll just send the outputs of the transformers to the VU meters.

Mechanical design is a whole different ball game. Do you want a rack mounting unit or a desktop unit? Do you have slider faders? How many channels do you want? How wide will each one be? How do you plan to wire to/from and between channels?

One way to avoid a lot of mechanical deign is to buy a standard 19 inch sub-rack. This gives you a basic frame onto which you can mount your own front and rear panels. Just add top and bottom covers if it is rack mounted of for a desktop unit make your own wooden cosmetic cheeks.

I will be mixing level and pan ITB so there´s no need for that kind of control, it´ll be a summing unit with 2x mic pres, 2x drive rotaries, 2x trim rotaries, 2x VU meters. I might build a monitor section and switch between the output trannies and external in (DAW).

I plan on using a 2-3U 19" rack but I don´t know which ones are really well made? I´d rather spend some extra cash on one that lasts.

 

[ruffrecords]

My apologies, the word 'mixer' in the heading confused me for a while. There are several threads on similar topics and I confused your thread with another one.

If you are just summing then all you need is meters on the outputs. There are lots of rack mounting boxes available. Local ones tend to be cheaper so probably a good idea to look there first.

Cheers

Ian

 

[XAXAU]

My apologies, the word 'mixer' in the heading confused me for a while. There are several threads on similar topics and I confused your thread with another one.

If you are just summing then all you need is meters on the outputs. There are lots of rack mounting boxes available. Local ones tend to be cheaper so probably a good idea to look there first.

Cheers

Ian

Cheers! Any suggestions for power cable?

 

[ruffrecords]

Cheers! Any suggestions for power cable?

I use Farnell part number 1891258 but I need the heavy gauge for heaters. You could probably get something in the same product range but with a lighter gauge.

Cheers

Ian