NewYorkDave

Fully balanced summing box, 16 channels, with level and pan
« Reply #40 on: December 21, 2005, 04:05:39 PM »
Brad: No, you'd leave it at 4.7K.
Svart: No. Reverse log would only be down 1dB at center.


Svart

Fully balanced summing box, 16 channels, with level and pan
« Reply #41 on: December 21, 2005, 04:09:06 PM »
I see what you mean, I was only noticing the approx taper, not really considering the strict numbers.

 :thumb:
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

NewYorkDave

Fully balanced summing box, 16 channels, with level and pan
« Reply #42 on: December 21, 2005, 04:13:47 PM »
...but I should mention that some console mfrs. approximate that curve with a rev-log pot, slugged with suitable loading on the output. Either way, off-the-shelf pots have to be slugged somehow to give the correct taper.

Svart

Fully balanced summing box, 16 channels, with level and pan
« Reply #43 on: December 21, 2005, 04:20:23 PM »
I like how there are so many different tapers that are almost the same.. commercial log, true log, audio taper.. etc.. they make life fun!
Welcome to the GroupDIY leper colony! when something falls off, we just replace it with a tube!
occupation: General Electron Mayhem

Alesis X2 information repository:
http://www.theopiumdenproductions.

babyhead

Fully balanced summing box, 16 channels, with level and pan
« Reply #44 on: December 24, 2005, 09:51:05 PM »
Mr Dave:

All I want for Xmas the 11 pole pan resistor string!!!

I'm going to pimp this out...14 channels with PEC pots and Forssell makeup gain. 8 with pan, 4 mono...

You'll be reborn a king!

  :guinness:  :guinness:  :guinness:

bradb

Fully balanced summing box, 16 channels, with level and pan
« Reply #45 on: December 27, 2005, 04:04:31 PM »
My futurelec order arrived, 10 days later.. USPS shipping from NY NY to new jersey.  that is slow, but its the holiday season.

i'll be gone next weekend but soon i'll have a 4 channel that i can report on.  i'll be using edcors for now.

bb

NewYorkDave

Fully balanced summing box, 16 channels, with level and pan
« Reply #46 on: December 29, 2005, 01:22:56 PM »
babyhead--

I may have misunderstood your question earlier. If you're talking about simply replacing the continuous-track 20K dual linear pot with a 20K stepped dual linear pot, made up of strings of 2K resistors, then that would work fine. You'll still need to keep R1, R2 and R3 as shown on the schematic.

babyhead

Fully balanced summing box, 16 channels, with level and pan
« Reply #47 on: January 01, 2006, 08:14:50 PM »
Quote
If you're talking about simply replacing the continuous-track 20K dual linear pot with a 20K stepped dual linear pot, made up of strings of 2K resistors, then that would work fine. You'll still need to keep R1, R2 and R3 as shown on the schematic.


That's what I'm talking about! I was scratching my little head on that one. So R2 and R3 slug the pot into the correct tapers I gather...

Another solution might be:

http://www.futurlec.com/PotRot.shtml

Dual 2k lins.

Thanks Dave! :guinness:

FrankSL

Fully balanced summing box, 16 channels, with level and pan
« Reply #48 on: July 04, 2006, 06:11:23 AM »
I asked omeg and they make only 22k linear, can the circuit be modified easily to use 22k?
Thanks,
Frank

Fully balanced summing box, 16 channels, with level and pan
« Reply #49 on: March 29, 2007, 06:49:22 PM »
Quote from: "NewYorkDave"
That's the balanced implementation of "Orban panpot" I was talking about earlier. Here's an example:

http://groupdiy.twin-x.com/albums/userpics/10031/balpanpot.jpg

EDIT: The "cw" and "ccw" markings on the pots are transposed. The clockwise end (cw) should be at the top. I'll fix the drawing.

OK, it's fixed now.


Hey Dave, do you still have this around?  I thought I had saved it once long ago, but I can't find it anywhere on my hard drives.  i'm kind of thinking along these lines for something i wanna try and bread up with an api 325 ish circuit, basically just a signal into a fader into a Discrete op-amp into an output transformer into a balanced orban style pan pot.  i can't for the life of me google up any actual details on it however, such as value of series R's as ratio to the pan pot value, etc etc....  i'd like to work on tailoring it to fit my app, but don't quite know where to start.

just in case...  or maybe someone has the technical details about the orban style panpot?  sure wish i was an aes member right about now....


mrclunk

Fully balanced summing box, 16 channels, with level and pan
« Reply #50 on: August 16, 2007, 04:17:31 PM »
for reasons given  above bummmp....

would like to find this orban pan circuit too, possible option for my summing box.

ta
paul

mrclunk

Fully balanced summing box, 16 channels, with level and pan
« Reply #51 on: August 17, 2007, 06:26:11 AM »
So i believe i found the Orban active pan circuit.
http://www.geofex.com/Article_Folders/panner.pdf

letterbeacon

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #52 on: July 10, 2018, 06:18:41 PM »
I'm planning to build a 16 channel summing mixer as designed by NYD in this thread (schematic attached).

Back in the thread it was asked if you could use a different sized pot for the pan pot and I was also wondering if I could use a 10k dual linear pot, seeing as CAPI do one with a centre detent fairly cheaply.

NYD said no because...

Well, here's the problem: every part of the circuit is highly interactive.

The input impedance of the mixer should be around 5K at a minimum in order to work with a wide variety of equipment. Lower than that is no problem for most true professional equipment, but most definitely a problem for consumer and "prosumer" stuff. And the output of the mixer needs to have a level and source impedance compatible with the input of a mic preamp.

The input impedance of the panpot circuit is fairly constant at about 50% of the value of the pot--and this is much better than the "usual" dual-lin panpot circuit, which has an input Z that's all over the place. The panpot needs to be fed from a source impedance of not more than 20% of the pot value. That, along with the minimum input impedance requirement, sets the limit on the value of the level pot.

The panpot needs to be loaded by a resistance equal to the pot value; this consists of the mixing resistor and the buss impedance in series. A 200-Ohm buss impedance is negligible compared to the value of the mixing resistor in the case of this design, so we can ignore it and use a 10K mixing resistor for a 10K panpot, 20K for a 20K pot and so on.

The impedance of the buss is all channels (mixing resistor in series with the panpot source impedance, which varies from zero to about 15% of the pot value) in parallel, which is forced down to the desired value by Rshunt.

If we scale everything up to allow a 50K dual for the pan--which means ~50K mixing resistors-- the output impedance will be too high. If we adjust the shunt on the output to compensate, the total attenuation will be ridiculous, more than twice the already-substantial figure of the circuit as I drew it. You'd be lucky to achieve anything approaching an acceptable signal-to-noise ratio. If we scale it down to use a 10K panpot, the input impedance will be too low to be acceptable for "general purpose" use.

Say if I halved the values of all the resistors and pots allowing 10k pot to be used for pan and a 4.7K pot for the input attenuator. Will this mean that the input impedance will be 2.5k? If so, I know that's low and as NYD says:

Quote
The input impedance of the mixer should be around 5K at a minimum in order to work with a wide variety of equipment. Lower than that is no problem for most true professional equipment, but most definitely a problem for consumer and "prosumer" stuff. And the output of the mixer needs to have a level and source impedance compatible with the input of a mic preamp.

But seeing as I'm building this for my small project studio, and will be mainly connecting a Fireface 400 and Behringer ADA8200 to it, an impedance of 2.5k should be ok? I.e. I'm not going to be connecting a wide variety of consumer equipment to it.

It's bad design if I was making a mixer to be used by anyone anywhere, but would it suit me? The Fireface has an output impedance of 75r and the Behringer is 1k, I think.

Would this also halve the insertion loss from 46dB to 23dB?

Thanks for your help.

JohnRoberts

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #53 on: July 11, 2018, 01:17:46 PM »
I'm planning to build a 16 channel summing mixer as designed by NYD in this thread (schematic attached).

Back in the thread it was asked if you could use a different sized pot for the pan pot and I was also wondering if I could use a 10k dual linear pot, seeing as CAPI do one with a centre detent fairly cheaply.

NYD said no because...

Say if I halved the values of all the resistors and pots allowing 10k pot to be used for pan and a 4.7K pot for the input attenuator. Will this mean that the input impedance will be 2.5k? If so, I know that's low and as NYD says:
if you scale everything down a factor of 1/2... the input Z will be the 4.7K pot in parallel with the wiper load (2.4k in series with perhaps another 4k).  So not quite 2.5k, but perhaps low for consumer gear.
Quote
But seeing as I'm building this for my small project studio, and will be mainly connecting a Fireface 400 and Behringer ADA8200 to it, an impedance of 2.5k should be ok? I.e. I'm not going to be connecting a wide variety of consumer equipment to it.

It's bad design if I was making a mixer to be used by anyone anywhere, but would it suit me? The Fireface has an output impedance of 75r and the Behringer is 1k, I think.

Would this also halve the insertion loss from 46dB to 23dB?

Thanks for your help.
I am not a fan of passive mixers so good luck.

JR
John Roberts
http://circularscience.com
Tune it, or don't play it...

bluebird

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #54 on: July 11, 2018, 07:04:37 PM »
I would go for it if you have some time to waste...and a little money to waste as well. 2K input is fine for most things. I think even a TL072 can drive 2K. Just know there is going to be a lot of cross talk between channels if you plan on making up the gain with a traditional mic pre. Theres not a lot of options and its overall a messy way to make a mixer but I made one a long time ago and it was pretty fun for a while. It got parted out eventually ;D

If you really want to mix out of the box for cheap just get a little Mackie 16 channel board.  But a cool tube line amp or a set of 1:1 transformers in the main buss insert and have all the mojo you need!

letterbeacon

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #55 on: July 11, 2018, 07:12:37 PM »
Thanks for your replies.

Say if I built it with the values NYD specified -with 20k bus resistors, therefore keeping cross talk down- and then built a 1272 type circuit as a line amp rather than mic pre, would that be better?

I assume that if I built the 1272 -which is a line amp- I wouldn’t need to shunt the summing bus impedance down to 200r as if it was feeding a mic pre and therefore there wouldn’t be as much insertion loss.

bluebird

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #56 on: July 11, 2018, 08:45:10 PM »
Its the impedance of the summing amp input thats going to allow cross talk as well as the size of the summing resistors. In a perfect world you would have the summing resistors feeding into a virtual earth. An op amp's inverting input with the non inverting input grounded. The feed back resistor would be scaled to your summing resistors parallel resistance. Its a bottomless pit for voltage at that inverting input node so no signal can slip into another channels summing resistor. Any type of amp that has an input transformer will allow some signal to slip by. And the larger you make the summing resistors the less signal your getting to the summing amp and the more noise you'll be amplifying to get your level back to unity.
Its been done on large format consoles. I believe the old 80 series Neve desk's have a summing amp with a transformer input. Probably something like the 1272.

So go ahead and build it and just be prepared to change components until you get a good balance of noise vs. crosstalk. You can use IC sockets for the summing resistors so you can change them out.

« Last Edit: July 11, 2018, 08:51:30 PM by bluebird »

ruffrecords

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #57 on: July 12, 2018, 03:20:19 AM »
Its the impedance of the summing amp input thats going to allow cross talk as well as the size of the summing resistors.
Crosstalk in a passive mixer depends on two factors - bus loss and return loss.

With 10K bus resistors and a 200 ohm bus impedance, the bus loss is 40dB

The return loss depends on any common impedance between left and right buses which is basically determined bythe driving source impedance. Imagine a centred pan pot. Signal on the left bus can leak  via the pan pot, back to the input and from there down the other leg of the pan pot to the right bus. The bus resistors, pan pot and input source resistance form a pot divider with the source resistance at the bottom. So the smaller the source resistance the lower the return loss. In a simple passive mixer, the source resistance will be about one quarter of the gain pot resistance. If you use a 2K pot and 10K pan pot and bus resistors, the return loss will be about 36dB.

Crosstalk is the sum of bus loss and return loss which in the example given  is 76dB.

Quote
Its been done on large format consoles. I believe the old 80 series Neve desk's have a summing amp with a transformer input. Probably something like the 1272.

Neve used the BA283 unbalanced output for driving pan pots and buses. It has a source impedance of less than 20 ohms so return loss was very small (usually better than -60dB) . Neve achieved crosstalk figures in the =90dB region. The bus amp was indeed a 1272.

Virtual earth mixing has its ow problems. Both systems are capable of near identical performance when implemented properly.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

letterbeacon

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #58 on: July 12, 2018, 07:48:59 AM »
Thanks Ian and Bluebird - this is really helping me get my head around the subject. I've also been reading the documents for the  EZ Tube Mixer, which is also really useful.

What are the formulas to work out the insertion loss and then the return loss? I would like to work out what the cross talk amount would be for NYD's schematic that I posted earlier.

JohnRoberts

Re: Fully balanced summing box, 16 channels, with level and pan
« Reply #59 on: July 12, 2018, 10:20:55 AM »
For a simple stereo mix I wouldn't expect crosstalk to be a huge issue (more of a concern for broadcast mixing).

Since most passive boxes use variable gain mic preamps for make up gain knowing the precise loss is not a big deal either.

The loss will be a function of how many stems are being summed, more stems more loss.

JR 
John Roberts
http://circularscience.com
Tune it, or don't play it...


 

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