Pultec SP-3 - Documentation anyone?

[amplexus]

This odd beast has come across my bench. Haven't been able to find much more than a brochure about it. Fully passive 'mixing' section, with a pair of 12ax7 doing gain makeup duties driving a pair of 6AQ5 pushing the outputs.

Don't suppose anyone around these parts has a schematic or any documentation kicking around in their boneyards? It's not an overly complicated thing, but a map would save me some time! Will be bypassing the 'panning' matrix and would like to make the gain variable.

 

[TwentyTrees]

I've nothing helpful to add, but I absolutely LOVE the fact that it has a knob just labelled "spare"! Might start doing that on some of my builds...

Hope you're able to find some docs.

 

[JMan]

Did you ever find documents for this? I've seen these around here and there and always looked at them with envy, even though they seem fairly impractical in a modern studio. (Not that practicality has much place in my wonts). It's easy enough to find the "simplified diagram," but I've never been able to dredge up a schematic when trying to satisfy my curiosity.

 

[amplexus]

Nope. Never did. It was pretty straightforward to trace on the amp side though.

Realistically the simplest unmodded use case is as a stem or buss summing mixer. It has like 7 inputs? And 3 of them have both level and pan control - the other three just panning. You could send your drums out to this thing for panning and summing and they'd sound HUGE.

As it is I modded it to have passive individual L R input attenuation, and then a single stereo output attenuator post transformer for trim/gain staging. Sounds like god just as a 'processor'

 

[EugeneKei]

Hey gents. Any schematic or turret layout? would be nice to have and try to build this one magic device)

 

[amplexus]

I did actually sketch out the amp section. Not the panning and summing network though.

It looked like the amp section has about 20ish dB of gain if I recall during the initial testing. Will post it up later.

 

[EugeneKei]

Already have dual mono EQP-1A clone, and I believe it would be great to build this beast also for panning and summing.
Appreciate any sharings

 

[amplexus]

I can't promise I made no errors- it was quick and dirty tracing just so I could note what went where.

 

[EugeneKei]

I can't promise I made no errors- it was quick and dirty tracing just so I could note what went where.


View attachment 103705

Looks interesting. Does it have just output transformer? Are there two output transformers?
How many inputs and outputs does it have? Audio In_A and Audio In_B mean 2 inputs? the same question regarding outputs. Will it be possible later with no rush add the rest elements of the scheme? just to have rotary switches, potentiometers, etc
Thank you

 

[amplexus]

Looks interesting. Does it have just output transformer? Are there two output transformers?

Yes. The original unit was unbalanced in, stereo balanced out. One transformer per channel.

How many inputs and outputs does it have?

There are 3 main inputs and one "spare" input with no level control, but continuously variable pan. There are then 3 "Echo" inputs that had attenuation and could either feed the Pan pot of the corresponding main input, or be hard switched to the L or R bus. There are two balanced outputs.

Audio In_A and Audio In_B mean 2 inputs?

The amp stage has only two 'inputs' fed from the summing busses for L and R. I simply labelled them A and B, but they can be thought of as left and right.

Will it be possible later with no rush add the rest elements of the scheme? just to have rotary switches, potentiometers, etc

No. The unit has been returned to the client and we actually (reversibly) removed the entire summing/panning network from the circuit as it was not needed for the application they wanted to use it in. As such I had no need to document that side of things. Attached is a block diagram that may help.

 

[EugeneKei]

Got it, thanks a lot. Hope somewhere to find full schematic or layout, as it would be really interesting to try to build this beast)

 

[amplexus]

I wish you the best of luck! I did a fairly thorough search- but ultimately not exhaustive as at the end of the day i was able to do what i needed without one, and I could pretty quickly trace the amp side. If you have luck I'd love to know.

 

[EugeneKei]

Yes. The original unit was unbalanced in, stereo balanced out. One transformer per channel

May be you know what transformers are there? on a picture it says just "triad 45496"

 

[amplexus]

To the best of my recollection that is the only external marking on the output trafos. You can see the rest of the can has nothing else on it. I _think_ they are the same output iron as used on the MEQ and EQP1. The PT is better marked.

 

[emrr]

Output cap on the ground side of the transformer, yeah!

 

[amplexus]

Output cap on the ground side of the transformer, yeah!

Yea it's an odd arrangement. Is there an advantage to it that you're aware of?

 

[emrr]

It's a pre-1940 thing. There's a well reasoned purpose I've read in old books, but it escapes me at the moment. It mostly went away post-war.

 

[soapfoot]

Don’t MEQ-5 and EQH-2 have a similar arrangement?

 

[Matt Syson]

Well since the capacitor will have a large electrical 'surface area', having it grounded will mean that it won't 'capacitively couple to high impedance nodes of the surrounding circuit, especially if the outer foil is grounded. Having it in the 'hot' side would lead to either positive or negative feedback depending which other components/wires were near it. Large electrolytics used on power rails in equipment can be a handy 'screening plate' if you place them carefully.

 

[amplexus]

Well since the capacitor will have a large electrical 'surface area', having it grounded will mean that it won't 'capacitively couple to high impedance nodes of the surrounding circuit, especially if the outer foil is grounded. Having it in the 'hot' side would lead to either positive or negative feedback depending which other components/wires were near it. Large electrolytics used on power rails in equipment can be a handy 'screening plate' if you place them carefully.

Makes sense. In this _specific_ unit those caps are big chassis mounted, hermetically sealed PIO caps that aren't particularly near anything, and of course are in a grounded steel can... But if the practice was more widespread I suppose it wouldn't make sense to change established design protocols.