DB25 summing PCB - MK2??

[mig27]

Hey everyone,

Can you please help with two quick questions re. the shunt resistors.

For a 32 channel project with 4 PCBs, do I install the shunt resistors on all PCBs or just one?

I'm also planning to hook this up to proper summing amps (V475, V275 etc.).
Will I then have to uninstall the shunt resistors or are they not getting in the way?

Thanks!


Michael

 

[ruffrecords]

Hey everyone,

Can you please help with two quick questions re. the shunt resistors.

For a 32 channel project with 4 PCBs, do I install the shunt resistors on all PCBs or just one?

You only need to add the summing resistors to one board.

I'm also planning to hook this up to proper summing amps (V475, V275 etc.).
Will I then have to uninstall the shunt resistors or are they not getting in the way?

Thanks!


Michael

The shunt resistors are only needed for passive mixing. For virtual earth mixing e.g. Neumann V475 you do not need shunt resistors.

Cheers

ian

 

[currentstatus]

Question... I've used 5k11 resistors (no shunt) on a 32ch active project (4 boards) using a Neumann v475c amp.. are the value of the resistors correct?

I'm getting overdrive at +4db input.....

 

[ruffrecords]

Question... I've used 5k11 resistors (no shunt) on a 32ch active project (4 boards) using a Neumann v475c amp.. are the value of the resistors correct?

I'm getting overdrive at +4db input.....

Can you explain what you mean by overdrive? If you  send+4dBu to a single input, what do you get at the output?

Cheers

Ian

 

[tommypiper]

Just discovered these boards and thread, thanks to Ian.

Ian, how do we wire an insert return with your pcbs? 

In order to have it switchable, insert in or out,  I'd either have to cut the traces to break the connection and get the signal at J7 - OR I could take the signal from the resistor hole and go to my switch, include the resistor along the way, and return to the other resistor hole.  If that's the way to do it, how do you recommend best soldering wires to the small resistor hole - use a resistor lead and solder the wire to the lead?  I'm sure I'm missing an obvious solution!  ;D

Also, is J7 a standard size -- are there standard terminal block things that will solder onto J7 to make it easy to connect wires to J7? 

 

[ruffrecords]

Just discovered these boards and thread, thanks to Ian.

Ian, how do we wire an insert return with your pcbs? 

In order to have it switchable, insert in or out,  I'd either have to cut the traces to break the connection and get the signal at J7 - OR I could take the signal from the resistor hole and go to my switch, include the resistor along the way, and return to the other resistor hole.  If that's the way to do it, how do you recommend best soldering wires to the small resistor hole - use a resistor lead and solder the wire to the lead?  I'm sure I'm missing an obvious solution!  ;D

The DB25 board was designed to be as simple as possible. There were lots of requests for extras features but if I have added them all it would probably have cost twice as much. I incorporated J7 as a means of making all the signals available so you could easily wire to the DB25.  Since it was introduced there have been several questions about incorporating extras like pan switches which were solved by using J7. However, in all cases, the mix bus resistors needed to be mounted vertically with no connection to the input in order to isolate them. The outputs of the additional circuits are then wired to the free ends of these resistors. This is a little bit of a kludge but for a small number of inputs it is doable.

So, if you want inserts on just a couple of inputs, you can use this method but if you want them on all eight it is likely to be unwieldy and unreliable. In other words more trouble than it is worth. If I wanted to do this I would be tempted to simply route the inputs via a patch bay. I know the insert is then not easily switched in or out but patching in any outboard is very easy- I am not sure how you do that round the back of a summer.

Anyway, I guess it is just a reflection of modern work flow. As there have been quite a number of question about options that require disconnecting one end of the resistors I am toying with the idea of including another connector like J7 right next to it that breaks the input connection unless you short it out. This would solve your problem and simplify many other types of mods.
[quote
Also, is J7 a standard size -- are there standard terminal block things that will solder onto J7 to make it easy to connect wires to J7?
[/quote]

J7 takes a standard 0.1 inch pitch header or you can wire directly to it.

Cheers

ian

 

[tommypiper]

...adding a pan control to a mono channel.

Adding something other than a simple L-C-R pan switch al la New York Dave is non-trivial. So I have come up with a simple scheme that should work. It uses the 'Neve Trick' we used to make simple balanced monitor level controls with a single pot but extended to turn it into a pan pot. It relies on the bus being connected to a transformer rather than an electronically balanced input. The schematic is attached. The guy who first asked for it has built one and it works which is why I am now posting it to the group.

To implement it you need to connect to the hot and cold inputs of the channel from J7 to the pan pot circuit. R3 and R5 connect the bus directly to the channel cold input so you can fit them as normal to the PCB (R1C and M1C for example). R4 and R6 can have their bus end connected to the DB25 board (R1H and M1H for example) and flying lead taken to the pan pot sliders.

Cheers

Ian

http://groupdiy.com/index.php?action=dlattach;topic=52068.0;attach=27365  (See page 8 for original link if this doesn't work - I don't know how else to link...)

Ian, thanks so much.

Couple questions about this pan control.  (Looks very useful!)
Why does it require a transformer to work?  (A transformer input on the summed stereo output, not on the channel?)
(If my amp doesn't have a transformer input, may I put in a simple 1:1 transformer ahead of it to make the pan work?)

What is the function of the 4k7 R?

What is the approx impedance of the channel with this pan control, when centered, when fully panned?  (Do we still need to add resistors to the channel on the DB25 pcbs?)

Thanks!

 

[ruffrecords]

http://groupdiy.com/index.php?action=dlattach;topic=52068.0;attach=27365  (See page 8 for original link if this doesn't work - I don't know how else to link...)

Ian, thanks so much.

Couple questions about this pan control.  (Looks very useful!)
Why does it require a transformer to work?  (A transformer input on the summed stereo output, not on the channel?)
(If my amp doesn't have a transformer input, may I put in a simple 1:1 transformer ahead of it to make the pan work?)

When the pan is hard panned one way, the channel that is off has both outputs connected to the input cold line. This is therefore a large common mode signal and so how much of it appears in the output depends on the common mode rejection ratio (CMMR) of the input it is fed to. Transformers are exceptionally good at rejecting this type of common mode signal; electronically balanced inputs tend not to be so good which is why I recommend a transformer. I have not tried it without a transformer so you may well be able to get it to work satisfactorily.

What is the function of the 4k7 R?

The 4K7 sets the law of the pan pot. Linear pots are used because they are more consistent in pairs. However, on their own the give too much attenuation at the centre pan position (over 6dB). The 4K7 reduces the attenuation at the centre pan position to around 3 to 4dB.

What is the approx impedance of the channel with this pan control, when centered, when fully panned?  (Do we still need to add resistors to the channel on the DB25 pcbs?)

Thanks!

In the centre, the input impedance is approximately 3.5K. When fully panned it is about  2.5K
The 10K resistors R3, R4, R5 and R6 are the bus feed resistors and could be fitted to the DB25 board sitting upright so as to be able to connect them to the pan pot.

Cheers

ian

 

[tommypiper]

Thanks, Ian!

BTW, I've seen mention that unconnected inputs into a passive summer need to be terminated.  On unconnected inputs, do the hot and cold inputs (pin 2 and pin 3, into a balanced summer) need to be shorted to each other, or connected with a resistor, or what type of termination?  I guess this is to preserve the overall impedance scheme, or are there other reasons? 

One may end up doing a mix needing only half the channels, so what's the most effective method to solve this.  For example, you have a 16 channel summer, but only an 8 channel D-to-A output on your computer/recorder/device, and you're doing an 8 channel mix down into the 16 channel summer.  How do you terminate the unused 8 channels?  I'm thinking I could design a switch on each channel that terminates the unused inputs.  Or I could solder a jumper or resistor across  empty male XLR connectors and plug them into the summer input channels (or into the XLR inputs on the DB25 snakes)... 

 

[ruffrecords]

Thanks, Ian!

BTW, I've seen mention that unconnected inputs into a passive summer need to be terminated.  On unconnected inputs, do the hot and cold inputs (pin 2 and pin 3, into a balanced summer) need to be shorted to each other, or connected with a resistor, or what type of termination?  I guess this is to preserve the overall impedance scheme, or are there other reasons? 

That is right, it ensures the bus impedance does not rise which would alter the level and possibly affect the performance of the gain make up amplifier's input transformer (if there is one). I many cases it will not matter too much. Suppose you have 16 inputs with 5K resistors in each leg (hot and cold). The bus impedance is 10K/16 = 625 ohms. You would normally slug this with a resistor to drop the impedance to around 200 ohms . To do this you would slug the bus with a 300 ohm resistor which reduces the bus impedance to 205 ohms. If you only used 8 of the inputs, the bus impedance would be 10000/8 = 1250 ohms. This in parallel with the 300 ohm resistor is 242 ohms. I think any have decent gain make up amplifier would be able to cope with a source impedance of anywhere between 150 and 300 ohms so 205 to 242 should not worry it.

On the other hand, if you had 32 inputs, the slugging resistor would need to be 555 ohms. If you only used 8 inputs, the bus impedance would rise to 384 ohms (555//1250) which might be a problem for some transformers.

The other issue is noise/interference pick up. The unterminated inputs can  and will pick up any interference in the air. Much of this ought to be common mode and should be rejected by the balanced mix bus and in any case the level should be well below normal signal levels.

Bottom line is that for an 8/16 input summer you can probably get away with leaving the unused inputs unterminated. If you want to play safe, just wire up a 25 way D-type connector with hot/cold pair shorted and plug it into the unused DB25 input.

Cheers

Ian

 

[ruffrecords]

It has come to my attention that someone has made some counterfeits of my DB25 board and has attempted to sell them here on the Black Market. The moderators have kindly deleted his post but you can imagine how angry this makes me. If you discover anyone trying to sell copies of this, or any other of my PCBs, please let me know.

Cheers

Ian

 

[[silent:arts]]

Not again. I thought we are over and done with things like that.
Sorry to hear this, and - somehow -, welcome to the club.

 

[Deepdark]

Not again. I thought we are over and done with things like that.
Sorry to hear this, and - somehow -, welcome to the club.

Someone copied your work too? Damn it's a plague here

 

[[silent:arts]]

Someone copied your work too? Damn it's a plague here

Not mine, but a friends, years ago.

 

[Deepdark]

Not mine, but a friends, years ago.

Hope it wasn't the mic and mod guy lol.

 

[axl]

Hi guys!
Do you still sell these?

 

[Rocinante]

Ian (Ruff Records) has a page/store in the white market where you can pick up the db25 pcb's and bunch of other cool pro audio related pcb's.

 

[ruffrecords]

Hi guys!
Do you still sell these?

Yes, I do. Check out my web site:

http://www.customtubeconsoles.com/

and click on the Ruffrecords PCB Emporium tab.

Cheers

ian

 

[ruffrecords]

I am now down to the last five boards. at the moment I do not plan to make any more but I was wondering if it was worth making a MK2. Lots of people seem to want to break out signals between the 25 way and the summing bus to add pots, pans and so on so I was thinking of extending the board a bit and adding another connector to allow people to do this without cutting tracks.

Thoughts?

Cheers

ian

 

[Audioman]

Hi Ian,
I think the new version of the db25 needs :

- Pan switch (L C R)
- balanced insert (send/return) option in every channel
-  another good improvement is the possibility to have a balanced inserts on the master of the summer

if you change the side of the board (more bigger) Theres no problem for me.

You can raise also the number of channels for pcb
( eg 16 instead of 8 ) a good summing mixer needs at least 16 channels

This is my Thoughts for my little experience.

P.S. during reading of the thread i know tha the shunt resistors are needed only on the last pcb (eg i have mount the shunt resistor on every four DB25 pcb on my summer) i understand ?

i also read that if you use the neumann V475 you don't need the shunt resisistor is it right ?

I am now down to the last five boards. at the moment I do not plan to make any more but I was wondering if it was worth making a MK2. Lots of people seem to want to break out signals between the 25 way and the summing bus to add pots, pans and so on so I was thinking of extending the board a bit and adding another connector to allow people to do this without cutting tracks.

Thoughts?

Cheers

ian