DB25 summing PCB - MK2??

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I recently replaced psu lytics in my friends SSA-2 tubetech. it's the first edition without the mono/stereo toggles.
Had to turn it round quick so i couldn't spend too much time on it.

Each input xlr legs has 24k9 res to sum bus - (hence quoted 50k input z)
Sum bus to input tx - input buffered by 2 stages of E88C and feedback to tx secondary (zero field)
Then through attenuator switch onto classic pultec make up amp - see MP1A schem on gyraf site. ECC83 driving ECC82 to output tx (tertiary winding feedback)
stabilised HT supply, mosfet heater supply, standard 7815/7915 meter drive supply.
fantastic layout/build quality - nice one tubetech!
 
mikeyB said:
I recently replaced psu lytics in my friends SSA-2 tubetech. it's the first edition without the mono/stereo toggles.
Had to turn it round quick so i couldn't spend too much time on it.

Each input xlr legs has 24k9 res to sum bus - (hence quoted 50k input z)
Sum bus to input tx - input buffered by 2 stages of E88C and feedback to tx secondary (zero field)
Then through attenuator switch onto classic pultec make up amp - see MP1A schem on gyraf site. ECC83 driving ECC82 to output tx (tertiary winding feedback)
stabilised HT supply, mosfet heater supply, standard 7815/7915 meter drive supply.
fantastic layout/build quality - nice one tubetech!
Click to expand...

Thanks for doing that. So it looks like they have just two input transformers with the zero field thing going on. Don't suppose you took any pics of the internals?

Cheers

Ian
 
Rshunt = ((Rseries / N) * Z) / ((Rseries / N) - Z)

where
Rseries = R1+R2 or R3+R4 or R5+R6... and
Rseries is a lot (>= *10) greater than source impedance (most often <100ohm from line level sources)
N = number of assigned inputs
Z = wanted output impedance to feed the input of a makeup stage (maybe 200ohm for a mic preamp makeup stage)
Insertion loss = Z / (Rseries + Z) or in a dB readout LOG(Z / (Rseries + Z))*20
Gain of summing PCB(s) following makeup stage probably at least enough to make up the insertion loss number
 
I am back from holiday in Sherwood Forest. Several people have asked about calculating the resistor values to use with this PCB so I have put together a short document showing how to do this. I have also had another batch of PCBs delivered so if anyone needs any I have plenty.

Cheers

Ian
 

Attachments

  • db25config.pdf
    236.2 KB · Views: 170
A couple of reminders.

1. The second batch of db25 boards has arrived so anyone who missed the first time round can get in now.

2. Any ideas of what we would want to see in a standard case for a groupdiy passive summer. DanDeurloo of collectives cases has said he would be willing to do one if there is enough interest. So what do we want in it? Obviously some db25 cutouts at the back and a couple of XLRs for the outputs - that's about it for a truly passive summer and it would not need to be very deep either. If you want to go internal tube gain make up then we need mountings for the TwinLineAmp (TLA), its power supply and mounting for a couple of VTB2291 output transformers. We'd need an IEC connector cut out on the rear panel. Again the front panel could be completely blank or perhaps some people would want a pair of VUs and maybe a stepped output level control. Perhaps there's enough variability here that threre is no standard front panel. How about a blank aluminium one that could be cut/engraved to taste by Frank Rollen?

Anyway, those are my initial thoughts.

Cheers

Ian
 
I would like to see a 2 space 500 module, with 2 Dsub25 boards mounted on the faceplate.

Those will fit with JLM audio INX5 500 series dingo cards, which can be pretty easily configured for virtual earth summing, with a second stage to re-invert (with or without transformers) and adjustable gain.

Then a spot for 2 Gain knobs or a concentric one for left and right or gain and balance.

The boards fit, that is what I am building now, but would love to have faceplates.

My plan is to make it 16 channel summing, and have the gain knobs have switched positions based upon the number of active channels (gain changes with different numbers of channels).

I figured 18 channels (I was planning to wire in the back two inputs manually).  The outputs are on the back.

But I realize this is a little "500 specific".

I have measured, and your cards will fit in front of the INX5 cards.

Prototyping now, will post pics when done... behind a few projects.

b
 
If I can say my bit... it would have:
a tube gain make up stage
VU meters
Stepped output level control
Something like switch channels 1-4 to mono

Almost too bad it looks completely like the SSA 2B like that... but I think that is the feature set to go with!
 
Trench Recordings said:
how about a left/right mono switch ?
Click to expand...

The db25 PCB is simple because it does not have provision for faders, AUX sends or left/right/mono switches. You could add it to a couple of channels using flying leads but if you want to do it on all eight then it would rather defeat the object of the PCB. However, to make the board more flexible I included a header so you could run a cable from it to front panel mounted controls but you would have to also have the bus mixing resistors there as well.

Cheers

Ian
 
Can someone check these maths.

I'm summing 8 channels - none of them are mono.

If I use 5k resistors for each RxH and RxC and I'm trying to match to 300ohms. I get 357ohms for the shunt resistor?
 
Humner said:
Can someone check these maths.

I'm summing 8 channels - none of them are mono.

If I use 5k resistors for each RxH and RxC and I'm trying to match to 300ohms. I get 357ohms for the shunt resistor?
Click to expand...

None of you channels is mono so you must have 4 on each bus. You have 5K resistors so Rbus = 10K/4 = 2500.

You want a Z of 300 so

Rsh = (2500 x 300)/(2500 - 300) = 341 ohms.

Cheers

Ian
 
Ian,
great work - timing is perfect for me too.
I would like to get 4 PCB for the summing. This is for my active summing project using Neumann v475.
should I PM you?
Ship to the US?
Also if you have other items available - please send me a link.
thanks,
chris
 
whiskeynipple0088 said:
Ian,
great work - timing is perfect for me too.
I would like to get 4 PCB for the summing. This is for my active summing project using Neumann v475.
should I PM you?
Ship to the US?
Also if you have other items available - please send me a link.
thanks,
chris
Click to expand...

As Pierre said just email me from the link on my web site:

http://www.ianbell.ukfsn.org/EzTubeMixer/

where you will find prices and shipping charges or just PM me if you prefer.

Cheers

Ian
 
Hi Ian,

Thanks for your help - boards arrived a few days ago, I'm waiting on some panel's to be made up and then start piecing everything together. Can't wait

Rob
 

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