G-Pultec with Gain Attenuation- (16 Channel G-Pultec Mixer)

[stuartpre]

Hi, this is my first post so please be patient with me.

I have previously built 5 G-Pultecs for a studio I worked in, they worked.

I am no longer a pro engineer, i'm back in the playing and writing.

I am thinking of building my own simple G -Pultec based console.

The idea: 16 Pultecs in a frame with outputs wired to a 'New York Dave Passive summing mixer', including pan and fader for each channel.

So far so good.

Now I figure i need a gain stage that I can control as the output, so of course the G-Pultec SRPP buffer,...

Q1. What is the gain of Gyraf's SRPP buffer?

Q2. What is a simple and effective method to attenuate the SRPP buffer? (ie master fader-there are off course L and R SRPP Buffers).

Q3. How do I change the actual gain of the SRPP buffer?/Can it be increased/decreased and how much without compromising the Sound Quality?

A further bonus question;

Given each "Channel" has a complete G-Pultec acting as Channel; input, EQ and makeup Gain...

Q4. Can Channel attenuation be designed into the actual G-Pultec and therefore removed from the 'New York Dave passive summing
mixer'? Pro/Cons, suggestions? (Hint: If a suitable solution is applied to the SRPP buffers in the makeup gain amp on the output of the summing mixer, can't the same design be incorporated into each Channel (Each Channel being a G-Pultec)?

Thanks.

 

[ruffrecords]

NYD's passive balanced mixer is designed to be used with a mic pre and will typically need 30 to 40dB of make up gain. This is probably rather more than the Gyraf SRPP can provide.

If we take a step back for a moment and look at what you are trying to achieve you quickly realise that there is no need for the output transformers in each Pultec EQ and no need for a balanced mix bus. So, there is no reason not to use unbalanced channel and pan controls as would be done in a regular mixer. This means you do not need the output transformer on each EQ output. If you keep this as a simple but unbalanced bus then adding a master control is very easy. You just add is across the the bus before the gain make up amp. Another advantage is the the output from each EQ is much higher because there is no step down from the the output transformer so you need less gain make up and the Gyraf SRPP will now probably have enough gain to fulfil this function.

Interesting project.

Cheers

Ian

 

[stuartpre]

Thanks guys. I will take that advice, neat.

Now I have started to look at the overall cost and I have begun to rationalize the metal work and PSU's

Firstly, I have decided to build in 4 modules of 4 to achieve my 16 channels. So 2 modules on either side of the master module.

Questions:

As the SRPP PCB has; supply, transformers (that I wont fit, except for master L R) and the tube stage...It makes sense to install only 1 SRPP PCB per 4 channel module and, then construct the further 3 tube stages on an adjacent board of the same size, and they will be sharing the PSU on the SRPP board.

So the current for heaters  is 4 times ECC88, =  1.2A as I understand it. The LM317 is rated at 1.5 A.

Q1. Will the LM317 be happy or should I upgrade to LM350 3A?

Q2. Are there any significant issues related to oscillation, hum or cross-talk that may be generated by my PSU sharing scheme?


On the subject of my master module.

The Master Module will:

-be the summing bass
-have L AND R SRPP with output transformers
-have control room monitor switching, volume and multiple 2-tk select (both +4 and -10).

Questions

Q3. How do I 'tap' the master L R output (for control/monitor CCT) without degrading the signal going to print? Is there a simple neat trick to isolate my Control room/monitor stage from the Master L R ouptut? Line Transformer?
Or should I be looking at an IC solution for the Control Room/Monitor CCT?

Q4. Is there any issue in my L R SRPP's sharing a PSU (independent of my 4 modules)?

Q5. Would 2 further SRPP's be suitable for buffering my Control Room/monitor stage? (May they share the L R Master SRPP PSU?)
      Or should I be looking at an IC solution for the Control Room/Monitor CCT?

 

[ruffrecords]

Firstly, I have decided to build in 4 modules of 4 to achieve my 16 channels. So 2 modules on either side of the master module.

Questions:

As the SRPP PCB has; supply, transformers (that I wont fit, except for master L R) and the tube stage...It makes sense to install only 1 SRPP PCB per 4 channel module and, then construct the further 3 tube stages on an adjacent board of the same size, and they will be sharing the PSU on the SRPP board.

So the current for heaters  is 4 times ECC88, =  1.2A as I understand it. The LM317 is rated at 1.5 A.

Q1. Will the LM317 be happy or should I upgrade to LM350 3A?

The rating is probably less important than the heat sink. With twice the current there will be twice the dissipation . You need to get this heat away without significantly increasing the temperature of the regulator which basically means a bigger heatsink.

Q2. Are there any significant issues related to oscillation, hum or cross-talk that may be generated by my PSU sharing scheme?

Yes, lots of them. Most mixers employ a central power supply and take great care in distributing power. There is no reason you cannot have distributed HT and heater supplies but you still need to take great care with the overall grounding scheme.

The Master Module will:

-be the summing bass
-have L AND R SRPP with output transformers
-have control room monitor switching, volume and multiple 2-tk select (both +4 and -10).

Questions

Q3. How do I 'tap' the master L R output (for control/monitor CCT) without degrading the signal going to print? Is there a simple neat trick to isolate my Control room/monitor stage from the Master L R ouptut? Line Transformer?
Or should I be looking at an IC solution for the Control Room/Monitor CCT?

There are two principal ways to does this. The first is to tap off unbalanced before the output transformer and feed via a buffer. This is the way it is done in most project mixers. The other way is the way we used to do it at Neve which is to do it after the output transformer by using a 10K:10K bridging transformer.

Q4. Is there any issue in my L R SRPP's sharing a PSU (independent of my 4 modules)?

None other than those already mentioned.

Q5. Would 2 further SRPP's be suitable for buffering my Control Room/monitor stage? (May they share the L R Master SRPP PSU?)
      Or should I be looking at an IC solution for the Control Room/Monitor CCT?

That would work. As I mentioned earlier, most mixers have a single supply shared by all circuits so a couple of extra SRPPs sharing the supply of the bus amps should be no problem.

Note that small Neve mixers had an entirely passive monitor section. Often this would begin with a 10K:600 transformer feeding the monitor level and mute pots. The 12dB loss in the transformer brings the +4dBu typical level down to the normal sensitivity of most active speakers so it works quite well even today.

Cheers

Ian