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Old 19th April 2012, 01:36 AM   #1
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Default Strain Gauge jFET Pre-amp

I have recently discovered strain gauge cartridges after worrying about MM vs MC cartridges for the last 30 years. Happy to say that my newly acquired Win strain gauge is the best thing I've ever heard, even though its about 40 years old. Sorry to say that I've wasted a long time listening to sub-standard cartridges. Oh well.
Anyway, the Win came without a line stage so I decided to build my own. The lack of equalisation is a bonus, though requirement for balanced output is a complication. Anyway my circuit is attached, with a few notes.
1. This is battery powered. I'm too lazy to build shunt regulators, and too stupid to understand Akido-style circuitry. Batteries take no time to implement and are quiet.
2. Idss of my matched jFETs is 12.5mA, I have gone with standing current of 10mA. CCS is 20mA, adjust on test.
3. Upper cascode resistor goes to output rather than B+. Learnt this trick from John Broskie (Nelson Pass also uses this in Zen V8 or 9). This will reduce distortion and reduce gain, luckily i still have plenty of gain to drive my power amp in this "ultra-linear cascode" configuration.
4.. Originally I used output capacitors to block DC, I am now using a 1:1 transformer (Lundahl 1527XL) which is much more transparent, I prefer it to the caps I had
5. The variable resistor across the output is actually a 24position switch with resistors ranging from 100k down to 50R (salvaged from an old switched attenuator, resistor values have not been calculated to give precise steps in this application). This provides variable gain, switches are necessary to ensure close channel matching. Therefore I have no preamp any more, just a source selector. Also allows me to set recording level for needle drops (into my EMU 1212M sound card).

That's about it. Sounds brilliant, as do all our own creations.
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File Type: pdf strain gauge fet amplifier.pdf (5.0 KB, 528 views)
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Old 19th April 2012, 04:49 AM   #2
SY is offline SY  United States
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The Win cartridge was brilliant, one of my all-time favorites. But... you still need EQ. Not RIAA, since the output is displacement sensitive, but you have to EQ for the midrange where RIAA has a zero and a pole.
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Old 19th April 2012, 08:06 AM   #3
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Quote:
Originally Posted by hazard500 View Post
3. Upper cascode resistor goes to output rather than B+. Learnt this trick from John Broskie (Nelson Pass also uses this in Zen V8 or 9). This will reduce distortion and reduce gain, luckily i still have plenty of gain to drive my power amp in this "ultra-linear cascode" configuration.
Who in turn learnt it from William Z Johnson (Audio Research).

It's a partial cascode, first used by WZJ in US 4647872 in 1985. This is a form of local feedback.

If you design carefully, you can incorporate the EQ mentioned by SY in this part of the circuit.
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Old 19th April 2012, 11:14 PM   #4
AVWERK is offline AVWERK  United States
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Have you measured the frequency response yet? Wasn,t this one of the issues with the strain gauge concept along with poor seperation figures?

I remember the EK-1 from.., Robertson? Every time you bought a new Panasonic cart, you had to send the control center back for recalibration

The idea of extremely low moving mass should have favored the strain gauge over all the moving coils we see today

So what is the achillies heel of this promising idea that never quite took off?

BTW I have had 4 MC,s made by SAo Win and found them exceptional but never tried his strain model- alas..

Regards
David
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Old 19th April 2012, 11:20 PM   #5
SY is offline SY  United States
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Originally Posted by AVWERK View Post
So what is the achillies heel of this promising idea that never quite took off?
The installed base of noncompliant preamps. Audiophiles hate the idea of being tied to one preamp, that's much less fun. Technically, it's a great approach.
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Old 19th April 2012, 11:33 PM   #6
AVWERK is offline AVWERK  United States
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The Riaa preamp lobyists would surely pull together and kill off any new strain that might slowly infest the audio world
Can we start a moving coil disease of some kind to pull things back into proper balance?
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Old 20th April 2012, 06:04 AM   #7
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Originally Posted by SY View Post
The Win cartridge was brilliant, one of my all-time favorites. But... you still need EQ. Not RIAA, since the output is displacement sensitive, but you have to EQ for the midrange where RIAA has a zero and a pole.
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Originally Posted by Mark Kelly View Post
It's a partial cascode, first used by WZJ in US 4647872 in 1985. This is a form of local feedback. If you design carefully, you can incorporate the EQ mentioned by SY in this part of the circuit.
Firstly I will say that I have been using this cartridge with no equalisation for a few months, and there are no obvious response errors - but maybe I have cloth ears!! I have looked looked at bode plot of RIAA curve vs strain gauge reponse and it appears to me that at a theoretical level, assuming that strain gauge matches RIAA between 50 and 500Hz:
- Strain gauge will suffer from lack of hf, and needs a shelf lift of over 10dB above 2,122 kHz. Increasing gain is required above 50kHz to match the enhanced RIAA curve
- Strain gauge will have too much bass below 50Hz and needs a LF roll off.

Have I got this right? In reality, the actual RIAA response is much flatter than a bode plot and response errors are not noticeable (well i would expect to hear 10dB).. I'll have to think how I can engineer something into the partial cascode feedback loop - anyone got any ideas on how to boost hf? Maybe a variable adjustment, so I can set it for best sound, and increase each year as my hearing fades.
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Last edited by hazard500; 20th April 2012 at 06:16 AM.
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Old 17th December 2012, 11:55 AM   #8
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No replies?
I just wanted to say that I think you are right. I have an old Technics SG I would like to try, but my Technics CD-4 decoder did not work to well, so I am planning to use a preamplfier PCB I have laying around, just biasing the cartridge. That should work. But as you, I realized the frequency response could not be ideally flat.

Best regards
RK
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Old 18th December 2012, 01:19 AM   #9
andyr is offline andyr  Australia
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Quote:
Originally Posted by hazard500 View Post
4. Originally I used output capacitors to block DC, I am now using a 1:1 transformer (Lundahl 1527XL) which is much more transparent, I prefer it to the caps I had.
Yes, caps introduce their own sonic signature (and can be fearfully expensive! ) but they have a far wider bandwidth than a transformer. The only spec I could find for the Lundahl 1527XL said that its upper rolloff point was 150KHz - which IMO is not high enough.

Plus they cause a signal loss.

Regards,

Andy

Last edited by andyr; 18th December 2012 at 01:33 AM.
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Old 18th December 2012, 04:20 AM   #10
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Originally Posted by andyr View Post
Yes, caps introduce their own sonic signature (and can be fearfully expensive! ) but they have a far wider bandwidth than a transformer. The only spec I could find for the Lundahl 1527XL said that its upper rolloff point was 150KHz - which IMO is not high enough.

Plus they cause a signal loss.

Regards,

Andy
150kHz is not high enough? What source are you listening to, even if its 192kHz then there's nothing above 96kHz.

Oh, I just checked the spec sheet - pass band is 10 hz to 150 kHz +/- 0.2kB. You can really hear 0.2dB drop at 150kHz?
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