BAF 2013 folded cascode LSK Pre line amplifier adapted for strain gauge cartridge - diyAudio
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Old 16th September 2015, 09:59 PM   #1
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Default BAF 2013 folded cascode LSK Pre line amplifier adapted for strain gauge cartridge

I thought the LSK line amp from BAF2013 could be used with a Panasonic EPC-450C strain gauge cartridge.
4 mA through the cartridge's strain gauge elements. Opposite polarities on bias to avoid an inverting stage, or swapping cartridge leads.
Any comments?

/RK
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Last edited by R-K Rønningstad; 16th September 2015 at 10:00 PM. Reason: typo
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Old 16th September 2015, 10:06 PM   #2
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What excitation voltage are you planning on using?

Edit: I just realised you are talking about audio. Haha
I thought you were talking about taking strain measurements during fatigue testing.
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Old 16th September 2015, 10:32 PM   #3
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It is a device like shows in the picture attached. I think ca 4mA of bias current. Could probably make some regulator for 5VDC from a 7805 or something. I just made a simulation to look at positive bias for one channel and negative for the other.
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Old 17th September 2015, 12:46 AM   #4
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Optimum excitation voltage has a significant effect on strain gauge performance.
Basically you want the highest voltage without overheating the strain gauge. I'll have to refer to my notes. My brain doesn't remember that level of detail anymore.
A Salas shunt reg should make a noticeable improvent for this application.

I'll think about the amp side of things once I remember my strain gauge stuff. I'm no expert about amps but folded cascode definitely gives you high performance.
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Old 17th September 2015, 08:11 AM   #5
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I'm trying to find whether the cartridge uses 120 Ohm 350 Ohm or 1000 Ohm strain gauges. Would be nice if they use 1000Ohm strain gauges.
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Old 17th September 2015, 08:16 AM   #6
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Would be nice to know. I do not know this.
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Old 17th September 2015, 07:34 PM   #7
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I found this at Vinyl Engine - The Home of the Turntable, thanks to one Flavio.




Postby flavio81 » 13 Jun 2010 05:00

Ok, here's the information i've found:

Here's how you get sound from it, directly from Technics patents:

ImaThe cartridge needs, per channel, 4mA or 5mA of current through it.
The internal resistance of the cartridge is 1000 ohm per channel (= per semiconductor element). When deflecting the needle with my finger, i get around 100 ohm.

Panasonic SE-405 demodulator specs say the input impedance (of that preamplifier) is 4700ohm, which means that for 4mA, they are using around 18 to 22V of bias voltage.

For me, if i want to use 9V of bias tension and 5mA of current for the semiconductor cartridge, the "R" (resistor) value on that diagram would have to be 800 ohm, so the total resistance (1800 ohm) together with the voltage (9V) gives 5mA.

The capacitor is there to filter out the bias voltage (DC). I guess any high enough value will work...

The output in theory would go to the AUX input, no RIAA compensation is required, like if it was a ceramic cartridge!!!

Another additional consideration i've found is that one of the two channels must be reversed in phase to give a correct stereo output. Or you can invert the leads of one of the speaker channels...
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Old 17th September 2015, 07:37 PM   #8
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Quote:
Originally Posted by 2 picoDumbs View Post
I'm trying to find whether the cartridge uses 120 Ohm 350 Ohm or 1000 Ohm strain gauges. Would be nice if they use 1000Ohm strain gauges.
So Ref to my other post, it seems the elements are 1000 Ohms.
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Old 17th September 2015, 07:42 PM   #9
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Quote:
Originally Posted by R-K Rønningstad View Post
It is a device like shows in the picture attached. I think ca 4mA of bias current. Could probably make some regulator for 5VDC from a 7805 or something. I just made a simulation to look at positive bias for one channel and negative for the other.
Elements tweaked for RIAA curve match looks suspicious. A SG is a constant displacement transducer, I can't see the mechanical elements varying their properties at low frequencies. I would think you still need a preamp with a different de-emphasis curve. A ceramic cartridge simply "plows" through the RIAA curve there are considerable errors in doing this.
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Old 17th September 2015, 07:59 PM   #10
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Yes. I had a discussion with the late Lou Dorren at the QuadraphonicQuad forum some years ago regarding this. I think I found quite some deviation, but I think Lou argued it was not that important (due to other errors if I remember).
I made a simple LTSpice simulation, and I believe I found quite some dBs error. I can probably dig out this again from some harddisk.
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