Lou Dorren: A new CD-4 Demodulator!!! [ARCHIVE]

[loudorren]

Hello CD-4 Quadraphures,

rolvkarsten,

The semiconductor phono pickup is a variable resistance device and not a generator like MM or MC pickups. The natural response therefore is a 6 dB per octave roll off. The RIAA curve is actually a 6dB per octave slope broken up slightly at the middle.

In the theoretical Ideal RIAA curve, the first 6dB per octave section starts at 50Hz and stops at 500Hz. This is followed by a flat section that starts at 500Hz and ends at 2122Hz. The 1KHz 0dB reference level is in this flat area and is used as the overall curve reference. The second 6dB per octave section starts at 2122Hz and continues to 15KHz in the CD-4 system.

The absolute realization of the ideal curve in the cutter pre-emphasis and playback de-emphasis networks is impractical. The compromise is to insure that the networks are at the 0dB point when crossing 1KHz.

To insure that the semiconductor pickup follows the curve a slight tweak is added to the pickup elements to provide the 0dB point crossing. Euphonics and Technics used this method to simplify the pre-amplifier. Soundsmith puts their tweak in the pre-amp. Either method will yield proper results.

The semiconductor pickup is biased and therefore requires one of the two channels to have a 180 degree phase inversion.

Someone asked me awhile back about the Technics 3480 CD-4 demodulator which was the first one that I modified with limiting and Phase Lock Loop. It was the unit that I used to convince RCA to adopt CD-4 Quadraphonic record technology. I have included a picture of one.

For all of you, the last image is the circuit board parts placement legend for the new demodulator.

Lou Dorren

 

[rolvkarsten]

Lou.

If you extend the first 6 dB/octave section of the RIAA curve further from 500 Hz you will at 2122 Hz have a level approximately 12.5 dB below the RIAA level. In my view - considering the strain gauge cartridge having a 6 dB/octave rolloff -this gives the strain gauge an error of 12.5 dB at 2122 Hz.

Now, you say that the absolute RIAA realization at the encoding stage is impractical/not used. I also understand from your post that the strain gauge cartridge response is slightly tweaked, in other words not straight 6 dB/octave. So this makes it all more understandable for me. :sun

I understand the strain gauge has to be biased, but I don't understand the need for one channel to have a 180 degrees phase inversion.

Thank you for your reply!

BTW I was looking after a Panasonic at ebay, but the last one went for over USD260. I am in great doubt if such an old cartridge can be worth this much. On the other hand, maybe it is, because maybe the old amplifiers did not at all made justice to the concept.

All the best
Rolv-Karsten

 

[loudorren]

Hello 4 channel Buffs,

rolvkarsten

The semiconductor pickup has a left and a right section. The anodes of both are biased from a plus supply voltage through a load resistor for each side. If the stylus tracks a lateral grove (mono or L+R), one channel of the pickup will output mono while the other channel will output -mono. This also happens with MM or MC pickups, but because they do not require bias, the manufacturer can simply reverse the leads of one channel internally. To correct the problem in the semiconductor pickup a 180 degree phase inversion is required in one channel of the preamp.

Lou Dorren

 

[rolvkarsten]

Thank you, Lou!
RK

 

[rolvkarsten]

While at the topic; Here is the Win SDT-10 taken from an article in Swedish "Radio & Television" November 1975 (issue 11/1975)

Rolv-karsten

 

[Doug G.]

Ah, so yet another strain gauge cartridge! I wonder how many different models there actually have been?

Do you know if the SDT-10 came with a Shibata or similar stylus Rolv?

Doug

 

[ChristopherLees]

Hello 4 channel Buffs,

rolvkarsten

The semiconductor pickup has a left and a right section. The anodes of both are biased from a plus supply voltage through a load resistor for each side. If the stylus tracks a lateral grove (mono or L+R), one channel of the pickup will output mono while the other channel will output -mono. . To correct the problem in the semiconductor pickup a 180 degree phase inversion is required in one channel of the preamp.

Lou Dorren

yes but it's soooo much better when you don't do that and just reverse the polarity of one of your speakers instead..

 

[rolvkarsten]

yes but it's soooo much better when don't do that and just reverse the polarity of one of your speakers instead..

I thought a polarity reversal was a polarity reversal? And that the end result was the same.

RK

 

[rolvkarsten]

Ah, so yet another strain gauge cartridge! I wonder how many different models there actually have been?

Do you know if the SDT-10 came with a Shibata or similar stylus Rolv?

Doug

I do not know, Lou. All text in the article was the one underneath that picture and that does not mention the type of stylus. However, I googled "Win SDT-10" and one text indicated the SDT-10 was from the mid-sixties, so probably not Shibata. But it could have been changed to Shibata later of course.
BTW I remember that Jon Iverson made the Electro Research EK-1 preamp to go with the Panasonic cartridges. (EK = Electro Kinetics.) I think that preamp is as rare as an unicorn!
Seems all these strain gauges are higly regarded by the "insiders". Maybe I should buy that USD600 one at ebay after all...
Anyway, I'll do some more research on the Win and its stylus!

Rolv-Karsten

 

[Doug G.]

yes but it's soooo much better when don't do that and just reverse the polarity of one of your speakers instead..

I would also like to know why it is soooo much better to reverse the polarity of the leads to one speaker.

Then one would have to remember to change it back when using a regliar cartridge.

The phase reversal of one channel is done in the dedicated circuitry for the strain gauge cartridge and doesn't affect the input from a magnetic.

Doug

 

[Doug G.]

Doh! Color me dorked up! I should have remembered that the WIN SDT-10 is the cartridge designed by MR. Sao Win!

And then he licensed his design to Panasonic for their own cartridges which became the EPC-450C, 450C II, 451C and 460C.

I don't think I would buy the 600 dollar cartridge, Rolv. That is definitely a gouge! Maybe there will be others in auctions that only get to 200+ dollars :^)

Doug

 

[rolvkarsten]

Thank you for the advice, Doug. That's a good advice.
I was prepared to pay 150 when the broken one went for 108. But above 100 is enough, I believe. But the market rules, you know!

 

[ChristopherLees]

I would also like to know why it is soooo much better to reverse the polarity of the leads to one speaker.

Then one would have to remember to change it back when using a regliar cartridge.

The phase reversal of one channel is done in the dedicated circuitry for the strain gauge cartridge and doesn't affect the input from a magnetic.

Doug

as one channel has extra circuitry to correct a phase issue, you end up with different electronics in each channel...urrrghhh..

compare that with just a polarity switch on one speaker wire, and the answer is a no brainer.......and it doesn't take long to change the speaker wire polarity on one channel....miles quicker than changing anything else...and why would you ever need to change turntables/cartridges /preamps anyway...if you have the strain guage working properly into a good sounding dedicated box..
In the mid 1970's I had a Sao win cartridge into my own valve box , which my brother had made/designed, it had enough gain to go straight into an audio research d76a poweramp...it was fantastic..just reverse the negative /positive at one end of one speaker wire..which at the time were a set of magneplanar tympani 1D's...

The Box that Sao Win provided with his cartridge was not an exciting piece of
electronics to listen to...sonically it sucked.....and that's putting it mildly..some of my friends used less kinder words like
"heap of s%^t"....and there were some issues with the cartridges..from my memory of thirty years ago they varied a bit, you could get a good sounding one and an awesome sounding one..he also had some hand made ones that were better than the production models..

 

[ChristopherLees]

I thought a polarity reversal was a polarity reversal? And that the end result was the same.

RK

not when electronics are used, in only one channel of the preamp to correct a 180 degree phase inversion , vs no extra electronics and a simple speaker wire phase reversal

 

[rolvkarsten]

as one channel has extra circuitry to correct a phase issue, you end up with different electronics in each channel...urrrghhh..

compare that with just a polarity switch on one speaker wire, and the answer is a no brainer.......and it doesn't take long to change the speaker wire polarity on one channel....miles quicker than changing anything else...and why would you ever need to change turntables/cartridges /preamps anyway...if you have the strain guage working properly into a good sounding dedicated box..
In the mid 1970's I had a Sao win cartridge into my own valve box , which my brother had made/designed, it had enough gain to go straight into an audio research d76a poweramp...it was fantastic..just reverse the negative /positive at one end of one speaker wire..which at the time were a set of magneplanar tympani 1D's...

The Box that Sao Win provided with his cartridge was not an exciting piece of
electronics to listen to...sonically it sucked.....and that's putting it mildly..some of my friends used less kinder words like
"heap of s%^t"....and there were some issues with the cartridges..from my memory of thirty years ago they varied a bit, you could get a good sounding one and an awesome sounding one..he also had some hand made ones that were better than the production models..

Seems Lou (around TP17 phono schematics at pg 24 of this thread) in one channel has an inverting opamp in the signal path, while in the other a non-inverting. And as I understand this the output signals from the analogue switch (or whatever it is) are summed and sent to the output buffer and further on. The summing is really not a summing. The mux selects either MM/MC or SG, but the MUX outputs goes to this summing eventually.
So different electronics for left and right with strain gauge. I doubt very much one is able to hear this!
Please correct me if I am wrong, I just quickly looked at the schematics.

BTW I am curious about the boxes provided with the old SGs, Technics and Panasonic CD-4 demodulators from the seventies... Probably no match for even better MC and MM amplifiers of the day. Then you mention Win's own not so good box. Probably the EK-1 was good (but still 1980 tech), and Rowland Research had a box. So... Requiring dedicated - maybe not that good sounding - interface amplifiers did not make it possible for the SGs to shine through. But can a 1975 SG from Panasonic aspire to some almost state of the art title in 2009 even with state og the art interface electronics?

Rolv-Karsten

 

[ChristopherLees]

So different electronics for left and right with strain gauge. I doubt very much one is able to hear this!
Please correct me if I am wrong, I just quickly looked at the schematics.
Rolv-Karsten

extra electronics vs a straight wire??

I forgot to mention a side issue that stereo amps without dual mono power supplies tended to sound a little better when an out of phase stereo signal runs through them, perhaps because when heavy dynamics occur it is arriving out of phase (not quite together) and gives the amps power supply a bit more headroom as a result...either way it's an easy enough experiment to do with a normal cartridge...wire one channel out of phase and reverse it at the speaker end..you still hear everything in phase..

BTW I am curious about the boxes provided with the old SGs, Technics and Panasonic CD-4 demodulators from the seventies... Probably no match for even better MC and MM amplifiers of the day. Then you mention Win's own not so good box. Probably the EK-1 was good (but still 1980 tech), and Rowland Research had a box. So... Requiring dedicated - maybe not that good sounding - interface amplifiers did not make it possible for the SGs to shine through. But can a 1975 SG from Panasonic aspire to some almost state of the art title in 2009 even with state og the art interface electronics?

Rolv-Karsten

I was into stereo in 1975 not quad...but if the technics cartridge had been better than the win in stereo I would have used it....but I didn't...the win was bl##dy fantastic(...you could tweak the hand made ones, sliding the stylus on the rubber yolk) ....the technics cartridge sold for around $150 in the mid 1970's
(it wasn't really even on the same page as the win)

as for their phono boxes...well a mid 1970's mass production unit from a japanese manufacturer vs a four 12ax7 valve unit full of brand new English mullards, that can be plugged directly into a valve poweramp...with matched electronics in each channel...and only listening in stereo.....mmmm


the only strange thing was recording a cassette tape out of phase...the vu meters went against each other the whole time....I remember distinctly recording "little feat" "The last record album" USA pressing and John Lennon "Roots" Adam XXII lp and boy did those meters seem to bounce everywhere...

 

[rolvkarsten]

Well, extra electronics is one thing. I can agree on that. But Lou's demodulator seems to differ only in configuration, not in the number of opamps in the signal chain.

That power amp tip of your's is a good one, BTW!

Seems the Panasonic cartridges either could be used with those mass marked demodulators, or expensive Electro Research and the like amplifiers. Nothing in between. While for MM and MC cartridges you can get preamplifiers for 50, 100, 150, 200 and so on up to 20.000........... The whole spectra.

RK

 

[rolvkarsten]

And here is the Win FET-10:

RK

 

[loudorren]

Hello CD-4 quadralifers,

ChristopherLees, Doug G., rolvkarsten,

I did not realize that phase inversion would strike up quite a discussion. In fact each of you is correct in part of the explanation. The point that Chris made has validity which must be taken into account when designing a universal phono pre-amp. Just switching in a 180 phase inversion amplifier creates a problem. This comes from the signal delay created by each amplifier stage. If you have one extra amplifier in the Right Channel then you have in the Left, a mono signal input on both channels will have a slight phase difference a the output due to the differential delay difference caused by the extra stage. This delay becomes worse as the frequency increases.

As rolve observed, in the new demodulator I made sure the the Left and Right pre-amplifier chains have the same number of amplifiers in all cartridge modes.
Also all the quad amplifier chains also have the same number of amplifiers.

One more point here. The amplifiers for the left and right channels should be the same type. This is because different amplifier designs have different delay.

Switching the speaker phase will work for 2 channel reproduction but becomes cumbersome and easily forgotten during playback. By the way the switch matrix to use in Quad is quite extensive. Good design always prevails!

With regards to amplifiers sounding better with independent power supplies, this is true when comparing to a poorly designed stereo power supply. I have seen many stereo amplifier designs where it was quite obvious that the power supply was designed to test one channel, not two. Low frequency energy can be directional, but in a great number of recordings it is simply placed in the middle of the stereo sound field which makes it L+R or mono. This makes both channels pull high current from the power supply. If the supply can not handle this, the supply rail voltages will drupe reducing the amplifier output capability. Remember that an amplifier is nothing more than a control system that doles out the energy from the power supply to the load based on the audio input control signal.

A quick QQ CD-4 demodulator update. Time is getting short for the Jon Urban delivery and evaluation. Stay Tuned.

Lou Dorren

 

[jrborg]

It is possible to use the same op amp to have +gain (non inverting, 0 degrees) and -gain (inverting, 180 degrees). I'm working on a restoration preamp that uses just that to switch between lateral and vertical records.