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Stax CP-X Electrostatic Direct Pickup System

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Stax CP-X Electrostatic Direct Pickup System

Postby tubes4life » 15 Dec 2009 04:00

People talk about the Weathers FM pickup often. It's was a legend in its own time in terms of sonic quality. According to the late J. Gordon Holt, this pickup was one of the best phono cartridges he had ever heard. Unfortunately few, if any, have survived the ravage of time. Aside from a picture of it in the VE's cartridge database, I've never had the good fortunate of ever using one nor have I ever seen one in person.

The Stax CP-X with the POD-XE Oscillator/Demodulator, is a stereo unit said to be similar to the Weathers in operating principle. According to the info available from the Stax website, the first Stax designed electrostatic cartridge, CP-20, a mono unit, appeared in 1952. A stereo unit, CPS-40 came out in 1962 and the CP-X came out in 1970. The CP-X was upgraded to CP-X/Type 2 in 1975. In 1977, Stax introduced a new model CP-Y using a permanent electret element with an IC built into the body of the cartridge. Shortly afterwards, the CP-X was discontinued.

I acquired my CP-X System in 1976. It had to be imported directly from Japan as there were no distributors in the US at the time. I limped along with the skimpy instructions in Japanese that came with the unit. Eventually I wrote to Stax for help. They were nice enough to send me a copy of a hand-written instrcuction manual in Janpanese with English translations. Only then was I able to set up the unit half way decently.

CP-X stayed in my system for quite a while. I liked it so much I actually made a tubed 10db voltage amp to replace the fet/bipolar one in the POD-XE, as I had and probably still have a fixation in wanting to keep my audio chain as "solid-state free" as possible. :)

The CP-X was out of storage the other day for photos. Afterwards, I actually set it up in the Panasonic SP10 and listened to it through the ARC SP8/Cary 805C/Quad 63. I was amazed at how wonderful it still sounded. A few hours later I found myself still playing records. I had to pratically force myself to quit. Nothing had generated quite so much excitement for me in audio recently.

The Stax CP-X sounds drastically different in character from magnetic cartridges. Sonically the difference between the CP-X and the magnetic cartridges is closely parallel to the difference you hear from the electrostatic headphones/loudspeakers and their dynamic counterparts.


Published specifications of the cartridge:

Freq. Response: 10-40kHz; 10-50kHz for Type2
Channel Separation: 22db(1kz); 24db for Type2
Effective Tip Mass: 0.3mg
Compliance: vetical - 10
horizontal - 20; 15 for Type2
Optimum Tracking Pressure: 1 g.
Stylus shape: 0.2 x 0.8 mil; 0.3 x 0.8 for Type2
Vertical Tracking Angle: 15 degrees
Weight: 9.5g; 8.5g for Type2


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Postby Blue Angel » 15 Dec 2009 12:00

Hello toobs

Thanks for the really marvellous pics and text. On another board, there's an associate going by the nick "mafioso" and I am sorely tempted to ask him to czech out the security arrangements around your locality :D

Very nice toys you have.

ba
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Postby hypertone » 16 Dec 2009 05:22

Wow! That is amazing, I never knew they made such a thing. 8)
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Postby Whitneyville » 19 Dec 2009 10:29

I had forgotten about the Stax carts. Anyone remember the "break-though" catridge that used light beams and photocells to "read" the grooves? It had a stylus but it was supposed to be "teamed" with the "linear tracking" quasi-disc lathes of the time. Help this old brain out on this one. I remember seeing one on a B&O turntable those many years ago.
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Postby tubes4life » 20 Dec 2009 04:22

Hi Whitneyville -

If not the ELP Laser Turntable, are you by any chance thinking about the cartridge that Toshiba had put on the market many years ago? You can find the specifics of it in the VE Cartridge Database.

https://vinylengine.com/library/aurex-toshiba/c-100p.shtml

Indeed, photocells were used in a few linear tracking arms including Goldmund and B&O as the sensor for control of the catridge carrying arm.

Regards,
Andy
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Stax CP-X Electrostatic Direct Pickup System

Postby Biotron » 31 Dec 2009 22:15

Hello Toobs

I just noticed your post and had to join. I have had a Stax POD-XE since 1973. I have by-past the internal 10X solid state amp and go direct into the AUX input of my C22 preamp. The power supply smoked and built a new one. Other than that it has played (and chirped) to many a tune. :lol:
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Postby tubes4life » 01 Jan 2010 00:01

Hi Biotron -

You are the 3rd person I know of who has the cartridge since 1976. It's just unbelievable that not many people have heard about this unique cartridge. Ever since Dec. 15 when I posted this thread, I have been using it on a daily basis. I must have listened to at least 50-60 albums since then and maybe more. I have come to the conclusion that it is by far the most beautiful sounding cartridge I've ever had.

I also tried bypassing the 10X fet/bipolar voltage amp, but the gain was not sufficient for my system. Years ago, I built a 10X tubed version using two 12AX7s. I took out the factory printed circuit board and put the one I built with the tubes in. The power supply had to be attached outboard. I was happy with it until I moved to my current address where it developed serious radio interference. In desperation, I removed it and put the original board back in and it has been working fine ever since.

I think the internal 10X voltage amp imparts just a wee little bit more brightness than I'd like in the sound. Bypassing it seems to cure it, if you have enough gain downstream. I've tried inserting another line stage preamp between the "POD Out" and my system preamp, thus bypassing the 10X amp, but have not been happy with the resulting sound.

For the past few days, I've been experimenting with different interconnects and have been quite pleased with what I heard. I think It'll stay connected to my system for the foreseeable future.

Regards,
Andy
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Postby Biotron » 01 Jan 2010 21:48

Hello Andy

I understand less than 50 total Stax POD units made it into the United State and were brought in by members of the Stax family. I got mine through John Iverson (Electro Research). I was "lucky" enough to be in the high end HiFi business from 1971 - 1980 and had a chance to do allot of "comparing" of various components. One of the best systems I have heard was the Stax POD thru my C22 and Iverson’s A75 amps (2) and force field speakers (no moving parts) - just incredible ! The Stax is slightly thin but really likes the tubes third harmonic and the balance is most excellent. Over the years I have compared it against many carts. The sales reps would always wind up at my house for some odd reason and they would bring their latest and best sounding stuff. Drove my wife nuts as we rarely played a song all the way thru…
Is the interference you were hearing from a local broadcast station or cell tower? Could be you can add some high value caps to the input and run the RF to ground without altering the audio. The gain is OK on my C22 but I have to watch it (turn it down) when I change my audio source
I just finished putting my computer together and at first chance I will post some of the pictures I have taken over the years

Regards
Galen
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Postby tubes4life » 02 Jan 2010 03:15

Hi Galen -

Yes, John Iverson must have been the one who talked highly about the Stax. I remember reading some very favorable comments on the Stax by someone in the industry in one of those underground magazines in the 70s. He must have been the one who got me interested in the cartridge. If my memory still serves, he also said the only way to make the cartridge better was by making a better oscillator/demodulator. I was never able to get someone to design a new oscillator for me.

One of the owners of Stax told me that he was able to adapt the cartridge to the Rabco SL8E. He said the performance was out of this world. The only thing was that you have to make sure the capacity of internal wires of the new arm to match perfectly with that of the Stax UA-7. Otherwise, it will oscillate. I was never able to figure out a way to put the cartridge on any other arm.

Your A-75 must be the Electro Research which has since become a classic. I drooled over it so bad at the time. I wonder why John Iverson dropped out of the hardware business. What a shame! Well, he's a good writer. I always enjoy hearing what he has to say.

Now that I put the original 10X amp back in, I have only occasional chirps. With different cables and other associated equipment Stax's "thinness" can be fleshed out without diminishing its transient response which in my opinion has no peers. However, I have to add I've never heard a strain gauge cartridge. The transient response makes reproduced music uncannily realistic. It's ideal for chamber music, jazz and vocals.

Will love to see your pictures.

Regards,
Andy
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Postby Biotron » 03 Jan 2010 00:35

Hi Andy

I have updated a few pics to my Gallery. Sorry for the poor quality as they are a few years old. The pics of the force field speakers were taken in late 1974 at Iverson’s home / shop in L.A. The "speaker" was mounted on a Plexiglas frame without an enclosure and is standing in front of an old sand filled Wald corner folded horn for bass. With no enclosure and no moving parts you were basically listening to the electronics. One of the more sonically interesting pre-amps was a home brew using the input driver board out of an A75 amp. It has very high gain (all class A) with variable DC offsets on the input and out output. This got a little interesting as the preamp as well as the A75 amp can pass DC so if the record was warped it would pass it along to the speakers. The FF speaker would also displace air continuously like a fan if you used a battery on the input. The black foam was to keep fingers (and prying eyes) out as was a high voltage device. Very in-efficient but boy was it clear. I know this is not the right forum for this but it’s still interesting for those of us on the quest. I’m in the middle of putting my HiFi room back together and will post more / current pics when I can.

Iverson worked on an FET Strain Gage set up but I never got to listen to it. The CP-X as you know will track just about anything and has a most excellent transit response. For my more casual use I use a Demon MC in either a Grace 940 arm or another Stax UA7 mounted on a separate mounting plate combined with a PS2 preamp.

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Postby Whitneyville » 22 Jan 2010 07:21

Tubes, I think it was the ELP turntable/catridge I was remembering. Gonna revolutionize the audio world. But I guess they forgot about the transparent albums and the "picture discs" (that sucked anyway). I do recall vagely the Toshiba cart after looking at the VE library. Man, that was a loooong time ago! Remember the B&O electrostatic cartridges on their "linear tracking" turntables you slid the album in like a side-loading CD player? The whole system was bugged with tech-problems. Too far ahead of their time I guess. I also remember the amps of that era that you slid you finger across to change the volume and on the tuners, change frequences. I wonder if any of them still work?
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Stax CP-X Electrostatic Direct Pickup System

Postby decca4 » 23 Apr 2010 21:47

Hi

Unusual thread on an unusual product. I have a CPX-2 that I used for a short period in the late 70th. Absolutely extraordinary is what I recall, super fast and delicate upper with a firm bottom end. The reason for giving up was that it was very difficult to make the decoding unit stable. Currently this is disassembled and lying in a box. Over the years I have often thought of trying it again and today I took it out and just to have a look at it. My idea is to build a new decoding unit with a crystal oscillator. The pictures posted by "tubes4life" are excellent! In particular the picture of the needle is of interest as it shows probably the main reason for this cartridge to sounds so good. In the picture you can see two metal elements in the body of the cartridge which are at 45° to the cantilever just above the needle. This is where the signal is picked up in much the same manner as is done by the Decca cartridges and the IKEDA. Although representing different systems they sound wise have that in common that they are ultra fast and very dynamic. They all have that in common that the signal is picked up over the scanning needle and not at the end of an imperfect and resonant cantilever.

Has any of you had any trouble with instability of the POD-X?

Is there any more info about how to adapt the STAX to a RABCO?

Very nice knowing that there people out there using this fantastic cartridge.
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Postby tubes4life » 13 May 2010 21:42

Hi, Decca4 -

The oscillator in the POD-X(E) drifts as it warms up. User needs to make minor adjustments from time to time during the course of extended listening sessions. The drift, however, always follows the same pattern through time and is rather predictable. I consider this a minor annoyance and am not particularly troubled by it. But you do have to make it a practice to check the meters on the POD every time you have lowered the cartridge onto the record to see whether adjustment is needed. It's like brushing and zapping the record each time before it's played. After a while, it becomes a second nature.

Your idea of building a better decoder with a more stable oscillator is very interesting. I often wonder whether it has ever been done before. It seems to be the only performance bottleneck of this unique phono cartridge design.

I had a friend who adapted the Stax to a Rabco SL-8 years ago and was very happy with the results. However, the total capacitance of the Rabco tone arm wires and the interconnect cables needs to stay within the range of the original dedicated Stax UA-7 tonearm cable in order to make it work properly.

Not too long ago I adapted the CPX to the Souther SLA-3. I didn't want to destroy the original Souther wand so I had to find a suitable steel rod with the same outer diameter of 3/32". Checked with every local hardware store and distributor but nothing usable could be found. I nearly gave up until finally stumbled on a piece of arc welding rod at a metal fabricator's workshop. After the welding compound was stripped off the rod, the outer diameter was precisely the same as the original Souther wand. The collar holding the cartridge was a section cut from a ball-point pen body. I had to look through quite a few ball point pens to find one with an inside diameter exactly the same as the original Stax arm. The collar and the rod were then glued together using epoxy.

I experimented with different interconnects and found none of my "audiophile" cables produced any output from the cartridge. I eventually found a pair of old Audio Technica cables that worked. I had only to make slight adjustments on the detectors on the POD-XE.

The CP-X tracked perfectly on the Souther. It sounded excellent but not any better than on the UA-7. In operation, the Stax/Souther combination leaves something to be desired. Static noise is heard from the speakers whenever the cartridge is raised or moved, or when the carrier is swung from the play position to the upright position for record change. Touching any metal part of the arm produced the same annoying statics. I think there are simply too many loose fittings on the arm that can cause interference. In contrast, CP-X/UA-7 combination is vertually silent in operation. I suspect CPX/Rabco will also be prone to extraneous noises as the Souther.


Cheers.
Andy


13530
Home-made wand for the CPX with Souther

13529
Souther wand, donor ball-point pen, and wand for the CPX

13527
CPX on Souther SLA-3

13528
CPX and SLA-3 on Thorens TD-125 in SME plinth
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Stax CPX

Postby Smooty » 24 Sep 2010 12:50

Just thought I'd chime in here if I may. I too have a CPX, mine is a Type 2 version and I have had no issues with the DOPE box as far as stability or smoking itself. I do find that it takes better than a hour for the settings to zero in, every 20 minutes I just adjust each channel to bring it back to snuff. Oh, is that the stability problem you're talking about or actual circuit design stability as in high frequency oscillation? Whatever, all I can say is this thing is lightening fast with a perceived response from DC to Channel 20, perhaps not as beefy sounding as a Koetsu Onyx, definitely, or others of that ilk, a little thinner or perhaps more even sounding than the typical MC, more like Dynavector 23R/17D cartridge sound, but they never achieve the quickness of this thing. Bought mine in the mid-80s to match up with my electrostatic speakers, on the advice of Walt Bender. Definitely a very cool piece, showed to a buddy of mine who is way deep into Western gear and he had not even heard of it let alone listened to one. The number of units stateside sounds about right, when I bought mine the guy said something very similar, I am very thankful he threw-in 2 additional styluses, because I'm sure they either super expensive or near impossible to locate.
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Re: Stax CPX

Postby tubes4life » 13 Oct 2010 14:35

Smooty wrote: I do find that it takes better than a hour for the settings to zero in, every 20 minutes I just adjust each channel to bring it back to snuff. Oh, is that the stability problem you're talking about or actual circuit design stability as in high frequency oscillation?


Sorry I overlooked your question and comment. The stability problem I was referring to is its operating point has a tendency to drift, requiring periodical adjustments during extended listening sessions especially in high ambient temperature. As you mentioned, the circuit can oscillate when drifted off course although I never find this unduly annoying. In normal comfortable room temperature I think the circuit is actually quite stable.
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