Help with a Paul Klipsch Tetraphonic Circuit

[chrisaudio]

Hi Klipsch Korner! I have a fun project that I'm hoping someone can help with — many thanks in advance!

I'm trying to Recreate the "Tetraphonic Pseudo 4-Channel Stereo Array", as published by Paul Klipsch in his notes from June 1971, and reprinted in the amazing Dope From Hope book. I've attached an annotated plan of the circuit in question.

I have a medium-level understanding of electronics and circuit diagrams, but I haven't undertaken any solo projects like this for over 20 years and then I was always under the supervision of a professor. I also realize this plan is from pretty dated, so symbols are different, etc. (So please assume grade-school level compounded with a couple decades of forgetfulness!)

However, I'm pretty competent at soldering and assembly so I'm confident that I can connect the components securely and safely and in the right configuration. I'm going to be building this on a Breadboard first, before making an enclosure for it and hard-wiring the components.

But before I start, there are some areas I'm unsure about — which I've notated on the image under A, B, C, D and E. They are:

A: Grounding I'm assuming the four Earth points that connect to one side of the potentiometers are a common ground which is different from the audio ground — is this correct? If so, should I be grounding this device using a ground lug, similar to what's found on a turntable?

B: Double gang Potentiometer? The symbol in the bottom-right appears to show a double-gang potentiometer, is this correct? Could I use two individual pots here or is that asking for trouble? What about two fixed resistors? (See "E")

C: Do these amplifiers have three inputs? Each amplifier has three signals 'entering' it — but I assume the top and bottom are the common "-" of an unbalanced audio signal, while the centre line is the 'hot' signal from each signal derived from the circuitry in the "Bud Box". Can you confirm this?

D: Resistor values So the note on the diagram says "All fixed resistors are 25kΩ 1/4 watt". This is very helpful for the four fixed resistors, but what about the potentiometers? I have successfully sourced some NOS 25kΩ resistors, and also some 25kΩ pots but I don't know if that's the right move. Am I missing something?

E: "16Ω L Pad" I read this to be a 16Ω attenuator for the left side of the signal, which kinda makes sense since there is some serial connectivity going on in the L+R and R-L sides of the circuit. Do you understand this note to be simply labelling the circuit, or is Mr. Klipsch saying that the double-gang pot (see "B") should have an impedance of 16Ω? And if this is the case, why is it not a fixed pair of 16Ω resistors?! I have a feeling this is a rookie question, or I'm interpreting it wrongly, so I'd love to be enlightened!

I appreciate any help or pointers you can give me, and also please don't hesitate to call out any misleading observations I have made. Thank you!

 

[ClaudeJ1]

I built and USED the little PWK resistor box for 30 years. If PWK modified this box further for home in his later years, it had to be in the 90's, since I was at his home in 1985 and he only had 3 front channels back then, which he demonstrated for me.

After he passed in 2002, Valerie Klipsch moved into a smaller home, where in 2007, roughly 18 of us went there to look at Paul's Jubilee mono speaker. She had the same Khorns and Single belle in another room (1985's version), with the Addition of 2 more Klipshorns (Rosewood finish vs. the original Walnut ones).

Unlike the evolution of 5.1 THX Home Theater Methods of the 90's, his had no time delays. Just attenuation of the center and rears with some further modifications.

In my 3 channel box (still have it, no longer used), I thought a pre-amp balance control would make the potentiometers in the R and L channels in the box redundant/unnecessary, so I only used the Fixed resistors for R and L, and only made the Mono Middle variable.

You don't need potentiometers for the R and L channels. Just the resistors will do.
The only thing I put a potentiometer on was the Center channel. I ended up, by ear, tweaking down a 5-6 db Attenuation of the center channel to make it sound "right" and it worked for Rock, Jazz, Pop, Symphonies, etc. very well without further calibration for the ROOM at the time.

My "Calibration Recording" was the song "Mediterranean Sundance" from Al DiMeola's "Elegant Gypsy" album (later on CD version).
Paco DeLucia (Flamenco Guitarist) in the Left channel and Al DiMeola in the Right channel.

It's what they used to call a ping-pong recording, with each musician having his own channel. So, starting from from least output position I turned up the Center Potentiometer until the R and L guitars sound "moved" towards each other 1/4 of the way in from each side. Then I never had to touch the potentiometer after that, until I moved to a different room.

If I were to do this today, adding the 2 Side/Rear channels, I would use a Dual Potentiometer to do it. Then the controls would be, Full R and L, attenuated Center MONO, and attenuated. I can't help you any further, since I use full Dolby Atmos for my system.

I suspect that you would have similar attenuation of the rear R and L channels. The Dual Ganged Potentiometer would allow easier attenuation of the rears, IF your room was perfectly symmetrical. If not, then having separated Potentiometers for each would add another element of Asymmetrical balance control. In other words, more knobs to twiddle!

If you want to do it the same way as the schematic, good luck finding a 16 ohm L pad. The schematic says "pseudo," meaning "fake or not genuine." I'm not even sure if PWK actually implemented this in the 90's, since he thought of it in 1971? Maybe Roy Delgado would know?

There are so many economical 5.1 choices today, in Pre Pro and fully amplified mutli channel receivers that I would not bother with this implementation. That's just me.

Aside from that, when my room dimensions were less than the original 15 feet, adding the Center Channel box became unnecessary.

Remember that PWK would ONLY listen to his own full Symphony Orchestra Recordings (he hated commercial recordings, calling them "dilute stereo").
He was all about application of all the Principles learned from the 1933 bell Labs technology, as stubborn as one can get, audio wise. LOL.

He used only 2 widely spaced Omni Microphones for all his recordings on the best Reel to Reel recorders of the day, which was roughly the time of my birth.

'Nuff said.

 

[chrisaudio]

Thank you ClaudeJ1, this is so extremely helpful. I realize this approach is fairly redundant these days, so this is more of an experiment of curiosity than striving for a perfect setup!

A couple of follow ups...

You don't need potentiometers for the R and L channels. Just the resistors will do.

This is really helpful; but to confirm, do I need to replace the variable resistors on the L and R channels in the circuit with a fixed resistor, or can I remove them completely and wire from the existing fixed resistor to the amp input?

If I were to do this today, adding the 2 Side/Rear channels, I would use a Dual Potentiometer to do it.

That was my thinking too... Do you suggest using a 25kΩ potentiometer, as specced by PWK for the fixed components? This is where my component knowledge fails me.

Thank you again!

 

[ClaudeJ1]

Yes to your first follow up.
Source impedance of pre amps is typically much lower, so 25 K is a great middle value to work with, since front end impedance on power amps is 50-100K.

 

[babadono]

If you have gain controls on the main LCR power amps I don't see why any pots are needed on them. If the rear speakers are 4 ohm nominal an 8 ohm L pad could be used on them. L pads are not just standard dual pots. The L pad is the volume control for the rear difference signal speakers Dynaco back in the day ( Bob Carver era ) had a difference signal back channel generator called Quadaptor. Last time I googled it info was available. PWKs diagrams assumed unbalanced equipment so the ground symbol is ground, analog signal ground and power ground. 3 inputs on amps? I think that was just PWKs way of showing shielded cables and the connections to the "bud" box.
Like all unbalanced equipment the shortest cables are the best and with the lowest possible resistance shield, because chassis and signal ground currents are being carried on the same shields.

 

[THD+N]

That circuit diagram does not look correct for deriving rear L-R and R-L signals. Unless your amps are modified inside the chassis, the only way to get true difference signals is with active circuitry (op-amps/discrete transistors).

 

[babadono]

@chrisaudio Oops sorry I mis spoke not Carver but David Hafler came up with the pseudo quad circuit. I can't download the files associated they're too big. But if I found them so can you. Google "Quadaptor Dynaco". Perhaps the best (easiest) way to come up with parts to try this out would to just buy an old Quadaptor.on FleaBay or where ever. Curiously it looks like PWK and Mr. Hafler hooked the rear speakers up differently with plus and minuses swapped. And yes @THD+N it is probably not the 100% accurate way to derive L-R and R-L but it is relatively cheap

 

[ClaudeJ1]

@chrisaudio Oops sorry I mis spoke not Carver but David Hafler came up with the pseudo quad circuit. I can't download the files associated they're too big. But if I found them so can you. Google "Quadaptor Dynaco". Perhaps the best (easiest) way to come up with parts to try this out would to just buy an old Quadaptor.on FleaBay or where ever. Curiously it looks like PWK and Mr. Hafler hooked the rear speakers up differently with plus and minuses swapped. And yes @THD+N it is probably not the 100% accurate way to derive L-R and R-L but it is relatively cheap

I have a Dynaco Quadraptor. I never liked Quadrophonic recordings, and celebrated it's demise.

I'll ship it to anyone here that wants it. You know how to get a hold of me if that's your next experiment.

I even had a bucket brigade time delay box with a pair of speakers I sold it all off.

Modern HT matrixing works pretty well and I can emulate the Ambience of many different spaces in my Yamaha CX-A5200 Pre Pro. Even with all that cool stuff, I still like plain 2 channels for music, where there is only one sound stage presented as a "wall of sound" with various ambiences in the recordings themselves without further processing.

 

[babadono]

There ya go @chrisaudio free Quadaptor to experiment with! Very cool offer @ClaudeJ1 And yes I like straight 2 channel too. But sometimes a little "ambience" mixed in is called for. I am experimenting getting my Yamaha DSP-1 set back up after years of non use. It's a gimmick like all effects but hey this is a hobby, right? It is first gen DSP and needs noisegating as no signal noise is intolerable otherwise.