EMI TG 12434 "Spreader" Schematic

[clintrubber]

Maybe something like this will work: http://sound.westhost.com/project21.htm

Related stuff, but not frequency selective, like the discussed spreader is.  But could be made to do so tough.

 

[abbey road d enfer]

Nothing in the article justifies the all-pass phase-compensation networks.
The low-pass filter introduces only 6° phase-lag at LF.
By virtue of their different characteristic pulsation, the two APF's transform this in +/-3° (3° lead at LF, 3° lag at HF), at the expense of extreme temporal distortion (APF's introduce 180° phase-shift of HF content).
APF's are not minimum-phase networks, so they are impossible to compensate when de-matrixing.
The audibility of phase is an ongoing debate, but I would think that compensating a 6° difference between channels by imposing a global 180° shift on both channels is a rather crude method.
It looks to me as a typically British approach, not too dissimilar to the networks used in Vox's AC30 vibrato and Surrey Electronics' Spectrum Shifter. I believe Michael Gerzon and Alan Blumlein had a predilection for phase-shifters, which probably explains why these circuits were used in audio applications, when in other parts of the world, they were strictly for scientific applications.

 

[Strawtles]

Right now I had confirm that the TG12434 is the same circuit of the TG12416 with less components
It seems that this circuit is really complex (two boards with a lot of components)

 

[noulou]

I've got one unit here. Tg12416 V.A.L. module. When i get the time, i'd love to help.

 

[clintrubber]

I believe Michael Gerzon and Alan Blumlein had a predilection for phase-shifters, which probably explains why these circuits were used in audio applications, when in other parts of the world, they were strictly for scientific applications.

Interesting observation.

Unless I miss something here, same goes for the Sum & Diff approach. The required freq-selective crosstalk between L & R could be done without going to that format.

 

[clintrubber]

Right now I had confirm that the TG12434 is the same circuit of the TG12416 with less components
It seems that this circuit is really complex (two boards with a lot of components)

Any pics or schematics you could link to ?

Thanks!

 

[ed rees]

I've got one unit here. Tg12416 V.A.L. module. When i get the time, i'd love to help.

This would be great, thank you!

 

[Strawtles]

Hi noulou, can you post some some photos of the circuit? It may be useful!

 

[noulou]

Front

 

[Strawtles]

Can you add some photos of the electronic circuit?

 

[tv]

Hrm, hrm ....

"Don't judge a book by its cover....
IT'S THE INSIDE THAT COUNTS! ...."

 

[noulou]

pcb1

 

[noulou]

pcb2

 

[Strawtles]

Thank you Noulou 
How it sounds?

 

[noulou]

Thank you Noulou 
How it sounds?

No idea. I'll let you know once i hook it up.  :-[

 

[Lee_M]

I'll explain how the passive spreader control in the REDDs worked. Maybe someone can design a spreader from this info?

The left and right mics go through a pair of S&D transformers.
Then the sum and difference lines each has it's own bridged T fader.
This is followed by the spreader, which is just another set of bridged T attenuators connected to work inversely.
After this is the shuffler, which would be desirable and was probably included in the TG unit.
Then the signals go through another set of S&D transformers to make them back into Left and Right signals.

For the shuffler, this looks like a 600 ohm version, no idea what it's from but it has the same topology as the Redd.17's 200 ohm shuffler.
http://www.richardbrice.net/EMI_shuffler_circuit.GIF

I know this thread is pretty darn old, But I read this post and realised the REDD passive spreader sounds like a very similar design to the passive "width" control in the BBC MX1/8 mixer.

The schematic and circuit description are in the attached PDF.

I'm in the middle of building the BBC width control to use as an external unit, But had been struggling to make sense of the "differential fader" mentioned in the PDF.
Inverted Bridged-T attenuators sound like a good plan for this. Thanks!  ;D

I'm not sure what alternative transformers you could use for DIYing it, The only reason I discovered the deisgn was because I had some of those BBC transformers on hand and was trying to find their intended use.
All I can tell you is that they have 1 primary and 4 secondaries, Although that's obvious from the schematic. I guess I should have done some basic measurements on them before I wired them up...

 

[abbey road d enfer]

I'll explain how the passive spreader control in the REDDs worked. Maybe someone can design a spreader from this info?

The left and right mics go through a pair of S&D transformers.
Then the sum and difference lines each has it's own bridged T fader.
This is followed by the spreader, which is just another set of bridged T attenuators connected to work inversely.
After this is the shuffler, which would be desirable and was probably included in the TG unit.
Then the signals go through another set of S&D transformers to make them back into Left and Right signals.

For the shuffler, this looks like a 600 ohm version, no idea what it's from but it has the same topology as the Redd.17's 200 ohm shuffler.
http://www.richardbrice.net/EMI_shuffler_circuit.GIF

I know this thread is pretty darn old, But I read this post and realised the REDD passive spreader sounds like a very similar design to the passive "width" control in the BBC MX1/8 mixer.

The schematic and circuit description are in the attached PDF.

I'm in the middle of building the BBC width control to use as an external unit, But had been struggling to make sense of the "differential fader" mentioned in the PDF.
Inverted Bridged-T attenuators sound like a good plan for this. Thanks!  ;D

I'm not sure what alternative transformers you could use for DIYing it, The only reason I discovered the deisgn was because I had some of those BBC transformers on hand and was trying to find their intended use.
All I can tell you is that they have 1 primary and 4 secondaries, Although that's obvious from the schematic. I guess I should have done some basic measurements on them before I wired them up...

I believe it's time to take some distance with the guts and consider the usefulness of these gizmos.
IMO, there is no doubt about the usefulness of the "spreader", which is basically a width control.
That can be implemented in M/S mode, with the added flexibility of inserting an EQ in the S path, which would allow controlling what frequencies are narrowed or spread. A much simpler implementation uses two opamps and a bunch of resistors and caps, doing the essential bits of the concept: narrowing bass and spreading treble.
But regarding the shuffler, I don't see any evident justification; indeed, in the preliminary, this has been designed to emulate the x-talk behaviour of phono cartridges, which apparently had some specific euphony.
I believe the main point is that it correlates surface noise, which makes it less apparent by about 3dB.
It turned out that the shuffler had also the virtue of centering the noticeable fact that many Beatles (and others) had e.g. drums and guitars on one channel , and vocals and guitar solo on the other , which was at some time quite disturbing.
I never figured that out when I was a kid, since I had a mono set, but it came eerily apparent when I was in a restaurant in Elkhart, where they had only one speaker per room.
Today, I don't see any serious justification for the use of such a device. There are pan-pots on every channel, and any DAW offers the possibility of duplicating tracks, inverting polarity and adding EQ.

 

[Lee_M]

I believe it's time to take some distance with the guts and consider the usefulness of these gizmos.
IMO, there is no doubt about the usefulness of the "spreader", which is basically a width control.
That can be implemented in M/S mode, with the added flexibility of inserting an EQ in the S path, which would allow controlling what frequencies are narrowed or spread. A much simpler implementation uses two opamps and a bunch of resistors and caps, doing the essential bits of the concept: narrowing bass and spreading treble.
But regarding the shuffler, I don't see any evident justification; indeed, in the preliminary, this has been designed to emulate the x-talk behaviour of phono cartridges, which apparently had some specific euphony.
I believe the main point is that it correlates surface noise, which makes it less apparent by about 3dB.
It turned out that the shuffler had also the virtue of centering the noticeable fact that many Beatles (and others) had e.g. drums and guitars on one channel , and vocals and guitar solo on the other , which was at some time quite disturbing.
I never figured that out when I was a kid, since I had a mono set, but it came eerily apparent when I was in a restaurant in Elkhart, where they had only one speaker per room.
Today, I don't see any serious justification for the use of such a device. There are pan-pots on every channel, and any DAW offers the possibility of duplicating tracks, inverting polarity and adding EQ.

Thanks for the concise reply, Those are some excellent points.

On the one hand, I can definitely agree with it potentially being useless...This isn't something that I'd ever really felt like I needed or had ever considered building until I inadvertently discovered the schematic, whilst searching for info on these transformers.
But on the other hand, It's a fun and easy project that costs me very little as I already have most of the parts for it.
Plus it seems a bit more interesting than simply building another preamp or EQ.

If it turns out to be useless, It can be easily disassembled into a couple of passive M/S encoders.

I don't have any computers in the studio, I'm using an iZ RADAR for recording which is fairly minimal when it comes to modern digital editing features (especially compared to DAW's)  so having external M/S converter boxes and other odd gizmos can be extremely handy.

What you said about phono crosstalk having euphonic properties sounds very interesting, Can you direct me to any links where I can read more on this?
I had a look on google for more info but pretty much every result I found is about reducing or eliminating phono cartridge crosstalk.

I think you may have just inadvertently added fuel to the DIY fire with your comment about adding EQ to the "S" path, That sounds like a very interesting feature to add to this box!  ;D
I've got a few odd BBC inductors lying around, So I guess some passive EQ bands could work nicely in this.

 

[abbey road d enfer]

What you said about phono crosstalk having euphonic properties sounds very interesting, Can you direct me to any links where I can read more on this?

Honestly no, because I had never heard or read about this before. So at the moment, the only evidence I have is that given on the phaedrus.com site.
The idea was that the apparent sound stage appeared narrower at low frequencies (<600Hz) they would leave it untouched at LF and narrow it at higher frequencies.
To me it seems counterproductive with the ubiquitous elliptic equalizer used in vinyl mastering, necessitated for reducing the vertical amplitude of the stylus.
Indeed, this is moot in view of music being made for digital delivery today.
These brilliant engineers at EMI probably got carried away, working out solutions for a non-existing problem.

I had a look on google for more info but pretty much every result I found is about reducing or eliminating phono cartridge crosstalk.

Indeed, narrowing the stereo image at HF reduces the audibility of surface noise. That may be the only objectively demonstrable benefit.

I think you may have just inadvertently added fuel to the DIY fire with your comment about adding EQ to the "S" path,

Not new, though. M/S #101 being manipulating the relative levels of S and M, adding EQ is M/S#102.

I've got a few odd BBC inductors lying around, So I guess some passive EQ bands could work nicely in this.

Beware that a passive MS matrix may be sensitive to input/output impedances.

 

[Lee_M]

Thanks for taking the time to help, It's greatly appreciated. 

It seems that phaedrus.com site is no longer a registered domain and the wayback machine isn't getting me to any of the older pages.

What you describe sounds like a very novel idea, It seems quite funny to me that while most of the hi-fi world is trying to reduce phono crosstalk the techs at EMI were seemingly coming up with ways to amplify its effect...Was this after the "white lab coats in the studio" period?

narrowing the stereo image at HF reduces the audibility of surface noise. That may be the only objectively demonstrable benefit.

I've got a very minimal understanding of phono technology, Despite my love of it, But I guess this is the same principle as why the mono/stereo switch on my old sony TA-70 helps drastically reduce surface noise when set to "mono" for mono vinyl.

I was just getting around to puzzling over the impedances, I'm still trying to understand how they are worked out so please correct me if I'm wrong!
Being a BBC unit, I'd assume its specced for 600R source and load...But I'm guessing this could be different as it's designed to be used within a mixer.
The 1k2 resistors are making me question this too, Is there some kind of parallel relationship happening with them (and/or other components) which brings it down to 600R or is it simply working at a different impedance altogether?

I'm intending to mainly use this as a 2-bus processor, On the main inserts on my mixer.
The inserts are fully balanced (electronically) with a 20R output impedance and 20k input impedance.
Should I be thinking about termination on the i/o connections of this unit or is that not a concern?

Thinking about it a bit more, I'll definitely need to work out the ratio that these transformers are wired for so that I can get my T-pad values correct.
I'm guessing there is a very slight increase in impedance from the first stage of transformers (Going by the 1k8 resistors at the M/S connections, either side of the fader) Which is then cancelled out by the second stage of reverse-connected transformers after the differential fader.
Am I on the right track?

Regarding the EQ, Maybe completely passive isn't the easiest way to go....Although I'd definitely like to stick with LC based filters.
Is installing a buffer circuit in the "S" channel likely to cause undesirable phase issues with the M/S to L/R decoding?
I'm thinking a way around any potential issues or discrepancies produced by adding circuitry to just one channel could be to add the same active circuit to both channels but have the "M" channel with a resistive pad equivalent to the losses of the LC circuit (with an amplifier for makeup on each channel) and only actually include the LC components in the "S" channel. Does that sound feasible?
Kinda like running a stereo Pultec with the filters (but not amp) bypassed in one channel.

The more this project puzzles me the more I'm intrigued by it. 

Plus it seems a bit more interesting than simply building another EQ.

Looks like I'll be eating those words!
I'm fairly set for "standard" EQ's, but it seems like this could be a very different tool altogether.

M/S #101 being manipulating the relative levels of S and M, adding EQ is M/S#102.

Where can I study the full course?  ;D