Playing With Panasonic Strain Gauge Cartridges (And A Dedicated Phono Stage)

Administrator
Joined 2004
Paid Member
So a friend has asked me to design a strain gauge phono pre-amp based on the Panasonic strain gauge cartridges.

I've got an EPC-460C and an EPC-465C on hand along with two NOS replacement styli.

(Even better I get to keep one of the cartridges when the project is done.)

I have spent several days reading VE and AK threads about these cartridges and how to amplify them.

Many people seem to like them without any sort of EQ, but based on measurements posted on VE and elsewhere this doesn't seem correct to me. Reviews back in the day show FR plots that by today's standards look pretty bad.

I might mention I have heard several modern strain gauge cartridges and thought they sounded absolutely terrible. I also heard a Panasonic with some limited EQ that didn't.

To be fair my frame of reference are a couple of Ortofon Per Windfelds. I have several others including a couple of SPUs, but I feel the PW is the most accurate cartridge I own.

From extensive study it seems like the following might be true..


  • HPF at 50Hz

  • Shelving HPF filter between 500Hz - 2.21kHz

  • As yet undetermined rolloff between 2.21kHz and 10kHz..
All subject to change as I figure this out.
 
Administrator
Joined 2004
Paid Member
Well both seem to be electrically good..

460C 900 ohms DCR

465C 869 ohms DCR

In both cases the channel to channel match is within an ohm or so using my Keithley 2002 bench meter.

I am planning on about 3mA of current through the elements as this is a value I have seen frequently recommended. Higher currents have been advocated, but self heating may be an issue so I won't go there.

The two channels are inverted relative to one another, Panasonic accommodated for this in their CD-4 demodulators by adding an inverting stage on I think the left channel. (I need to check)

Since my line stage and that of my friend can invert polarity independently on each channel I may just not bother to address the polarity issue at this point in this design. Given my design proclivities this might be best addressed in a phono stage with output transformers.
 
I have spent several days reading VE and AK threads about these cartridges and how to amplify them.
Hi Kevin, have you searched DIYAudio? There is some good info in this thread.

http://www.diyaudio.com/forums/analogue-source/211081-strain-gauge-jfet-pre-amp.html

  • HPF at 50Hz

  • Shelving HPF filter between 500Hz - 2.21kHz

  • As yet undetermined rolloff between 2.21kHz and 10kHz..
.
My latest design includes HPF at 50Hz. As you correctly state, theory says that there should be a shelf between 500 and 2.1kHz (with 12dB gain), in practice my WIN strain gauge sounds fine without this, but my EPC460 sounds shy on treble. I have recorded an LP played with EPC460 via EMU sound card, and playing around in Sound Forge, I find that a boost of 4 to 6 dB gives a pleasant response. Although theory states that it should be 12dB, applying this much gain results in a very shrill response. So clearly, neither the WIN nor Panasonic are perfectly linear strain gauges, and it is not possible to derive one standard response curve that will will suit all. Unfortunately it seems that we have to tailor the eq to suit individual cartridges :(

My audio friend has a EPC451, we recorded his test record (white noise) and a spectrogram did indeed show a drop in response from 500 to 2.1kHz. However once again, equalising in sound forge showed that about 6dB lift provided the preferred presentation.

Why do you want a roll-off between 2.1 and 10k? There are no poles or zeroes in the RIAA standard to correct for, in fact the natural response of the Pana (based on whit noise analysis) seems to drop off after 10k so if anything a boost may be needed.

At the moment I am playing with a Lyra Skala so I haven't listened to either strain gauge in a short while. But they are very rewarding if you get them running properly, with my clear preference being for the WIN.

Regards,
 
There was an Electro Research EK-1 strain gauge / preamp back in the early 80,s and each strain gauge pick up required specific frequency compensation. No plug and play unless you wanted wild frequency response swings...,

I believe Gordon Holt reviewed in Stereophile and loved it back then and it was over 3k+ for the system
Alot of money in the early 80,s
 
There was an Electro Research EK-1 strain gauge / preamp back in the early 80,s and each strain gauge pick up required specific frequency compensation. No plug and play unless you wanted wild frequency response swings...,

I believe Gordon Holt reviewed in Stereophile and loved it back then and it was over 3k+ for the system
Alot of money in the early 80,s
That's interesting, I have not heard of the Electro Research before but it certainly confirms my own observations ie each cartridge needs its own eq.

Actually just done a bit more research and here is an interesting article

Audio Investigations: Equalizing the strain gauge

This blog references the EK1 phono system, and also mentions that he prefers 6dB of eq between 500 and 2.12k, which is very close to my own experience.
 
Administrator
Joined 2004
Paid Member
Interestingly I installed the 465C on one of my Southers and added about 5gms of mass to the too light arm wand so I could use a 6gm counterweight to apply a tracking force of 3.5gms.

I modified my Muscovite Mini and added 6dB of boost between 500Hz - 2.1kHz, did nothing above that, but it strikes me it can be a bit bright on some material. VTA definitely plays a role.

Surrounds surprising good. My line stage allows me to invert outputs independently of each other.
 

Attachments

  • 20170515_221653.jpg
    20170515_221653.jpg
    705.8 KB · Views: 1,972
Administrator
Joined 2004
Paid Member
Hi Kevin, have you searched DIYAudio? There is some good info in this thread.

http://www.diyaudio.com/forums/analogue-source/211081-strain-gauge-jfet-pre-amp.html


My latest design includes HPF at 50Hz. As you correctly state, theory says that there should be a shelf between 500 and 2.1kHz (with 12dB gain), in practice my WIN strain gauge sounds fine without this, but my EPC460 sounds shy on treble. I have recorded an LP played with EPC460 via EMU sound card, and playing around in Sound Forge, I find that a boost of 4 to 6 dB gives a pleasant response. Although theory states that it should be 12dB, applying this much gain results in a very shrill response. So clearly, neither the WIN nor Panasonic are perfectly linear strain gauges, and it is not possible to derive one standard response curve that will will suit all. Unfortunately it seems that we have to tailor the eq to suit individual cartridges :(

My audio friend has a EPC451, we recorded his test record (white noise) and a spectrogram did indeed show a drop in response from 500 to 2.1kHz. However once again, equalising in sound forge showed that about 6dB lift provided the preferred presentation.

Why do you want a roll-off between 2.1 and 10k? There are no poles or zeroes in the RIAA standard to correct for, in fact the natural response of the Pana (based on whit noise analysis) seems to drop off after 10k so if anything a boost may be needed.

At the moment I am playing with a Lyra Skala so I haven't listened to either strain gauge in a short while. But they are very rewarding if you get them running properly, with my clear preference being for the WIN.

Regards,

I did a search on google but the diyA thread did not come up immediately. I'll have a look.. (Edit: I take that back, having just taken a look I did indeed read your thread.. :) )

Using cascode 6DJ8 in front end, cartridge is currently providing cathode bias to the amplifier circuit. Current is ~ 2.5mA which is a bit low. Eventually I design something specific with about 4mA of bias current for the cartridge.

The EQ reduces gain in the pre-amp by about 10dB overall giving me about 48dB of gain which given my system gain budget is quite a bit shy of what I need. (I design for redbook CD levels to +3dB or so)
 
Administrator
Joined 2004
Paid Member
Seems like partially correcting the region between 500Hz and 2122Hz works better than the more aggressive approach I initially took because the highs above 10kHz are not boosted as much.

From 6.5dB shelving HPF I went to 3.5dB and it seems to be better overall.

So far I have not established a need for cut below 50Hz..

I still think the response may need to be knocked down a bit above 10kHz.

Debating on whether a new Jico stylus would provided superior performance to the NOS shibata. It has improved considerably with use but after an indeterminate number of years I find it hard to believe that the rubber components have not hardened significantly.

One thing that is evident is that there is a lot of detail, and they track HF well. Quite sensitive to VTA (SRA). Sometimes it sounds a bit artificial and HIFI, have not decided whether this is a cartridge artifact or the material. Bass response is quite good as well.

Tonight I will share graphs of the EQ tried so far.

They are sensitive to the quality of the arm and effective mass needs to align properly with the cartridge's rather low compliance. Tracking the 465C at 3.5gms currently.

A phono stage design will ultimately come out of this which I will share here. I have not yet decided on any of the details other than it will utilize vacuum tubes for gain.

A lot of design iterations are shared in this thread, please look for the very last design for the one you should build if you are considering building anything at all. Not all of these iterations sound particularly good, you have been warned. :D
 
Administrator
Joined 2004
Paid Member
Hi Shooter, You aren't part of the Dallas crew I know perchance? :)

I haven't figured out how to do a 3dB/octave filter, but some EQ appears to be needed.

Currently I have a very simplistic EQ with about 3.7dB of boost between 500Hz - 2122Hz which means the area above 10kHz is not excessively boosted, but doesn't fully compensate for the trough.
 

Attachments

  • current_eq.PNG
    current_eq.PNG
    80 KB · Views: 892
Administrator
Joined 2004
Paid Member
I wasn't sure if it was you or not. I hope you will find what I have come up with interesting. There will be more gain, mid band, and LF correction (optional) and the topology itself deals with the HF peak above 10kHz. Output impedance will be much lower making for good compatibility with your TVCs.
 
Administrator
Joined 2004
Paid Member
Version 1 Schematic Design

As I've remarked before I design phono stages with deliberately high output levels in order to integrate well with systems with digital sources with maximum output levels of 2.0Vrms @ 0dBFS.

With approximately 57dB of gain @ 1kHz the maximum output level of this combination of EPC-465C and strain gauge phono stage should result in approximately 2.1Vrms @ 5cms/sec at 1kHz.

Detailed description:

The first stage is nothing more than a 6DJ8 mu follower although the battery bias scheme may make that less than obvious. The batteries last many years and allow me to establish an operating point that is more than usually independent of the tube's slope of transconductance. In this case I have chosen 4mA as the target current as this is the value Panasonic mentions in the specification for these cartridges.. This yields a gain of approximately 30dB in the first stage and the follower provides an output impedance to the network of < 1K ohm.

Something less obvious is the input stage is non-inverting and provides bias to the cartridge.

The second stage is a hybrid mu-follower using a DN2540 as the upper element, again battery bias is applied, this reduces dependence to operating current on fet threshold voltage, it also allows for fairly large source resistance which improves the performance of the follower. The follower will drive transformers effectively output impedance is < 200 ohms.

A Salas SSHV set for approximately 200 - 220V would make a good power source for this design. Local decoupling is a must.

You will note that there are a lot of 47/100 ohm resistors, unless you want a VHF oscillator you will not omit these. These particular resistors need to be placed right at the socket.

Yes, I like batteries, they're cheap and quiet sources of bias. There are two A23 12V batteries and a single AA per channel.

Note that at your discretion I might recommend the use of a 5.1V low capacitance TVS diode on the cathode of each channel to protect the cartridge in the event of a tube or other component failure.

This version is currently in use here at home with an EPC-465C.

**Note** C1 may be increased to 0.1uF if LF roll off is not desired. (This is what I am currently using)

No provisions have been made to invert one channel relative to the other, you will need to invert at the speaker or if using transformers on the outputs invert one relative to the other. My line stage provides the ability to invert either or both channels as desired right from the front panel.

The design is shared with the explicit understanding it is for personal use only.

Edit: Please see POST # 73 for the updated design. (Major improvement)
 

Attachments

  • Strain_Gauge_Phono_V1.PNG
    Strain_Gauge_Phono_V1.PNG
    57.2 KB · Views: 898
  • Gain and phase Strain Gauge Phono V1.PNG
    Gain and phase Strain Gauge Phono V1.PNG
    72.3 KB · Views: 903
Administrator
Joined 2004
Paid Member
GTHICM was here today, but did not get to hear the newest iteration of the front end design.

The improvement over what he heard is substantial; it is much more dynamic and quicker. Gain is 10dB higher, which results in better integration with the rest of the system.

It is also very quiet with a low subjective noise floor which I have not yet measured.
 
Yes, I did hear the previous iteration this morning and it did sound promising. But as observed, the dynamics were not quite there to my liking. The sound was what could be described as polite, but interesting with good imaging, nonetheless. Unfortunately, I will not be able to hear this latest iteration for several months because of overseas travel plans. This however confirms that this rather unique cartridge holds quite a bit of promise. Not bad at all for a relatively meager cost, with the benefit of replaceable styli.