Panasonic Strain Gauge pre-amp

[AudioReflex]

Hi all ...



Microphone pre-amp ..


After much reading .. about Strain Gauge Carts .. Could someone simplify this for me,


With the Strain gauge installed to a Standard turntable ...
Plugging the inputs of a Turntable into a Microphone pre-amp will work?


Cheers
Mike

 

[rayma]

It still needs substantial EQ, but different from RIAA.
Playing With Panasonic Strain Gauge Cartridges (And A Dedicated Phono Stage)

 

[AudioReflex]

It still needs substantial EQ, but different from RIAA.
Playing With Panasonic Strain Gauge Cartridges (And A Dedicated Phono Stage)

So that would be a Yes to Microphone pre-amp?


At this stage ... i would just like to plug it into a pre-amp to be able to hear the Cart.




Ive got enough HiFi gear after the preamp to trim the sound.



I'm aware about EQ's and Load settings for the Strain gauge cart ... and from what i can figure out ... to put in simple terms ... is that it's more of a microphone.


Cheers
Mike

 

[kevinkr]

No, it won't work.. Panasonic strain gauge cartridges require approximately 4mA of DC for normal operation.

And the channels are 180° out of phase with each other.

You need a proper strain gauge pre-amplifier. Also they badly need EQ.

See the attached picture below. I have actually measured a couple of my cartridges and they closely match the frequency response shown in the upper graph from a long ago review in Audio (August 1974) - unfortunately I have misplaced those response measurements.

The graph below it is the response of my most recent EQ which I am finally happy with. (It only took 8 tries lol)

It is nothing like a microphone, if you want to compare it to anything at all a variable resistor would be more appropriate.

I write this as I listen to a 465CR on a 12 inch prototype tonearm into my strain gauge pre-amplifier.

 

[AudioReflex]

No, it won't work..
You need a proper strain gauge pre-amplifier. Also they badly need EQ.

It is nothing like a microphone, if you want to compare it to anything at all a variable resistor would be more appropriate.
.

Cheers for that ... will hunt for a proper strain gauge pre-amplifier




Mike

 

[kevinkr]

Good luck! I hope you can find something suitable. Might want to read through my thread as there is quite a lot of information about the cartridges as well.

 

[scott wurcer]

Good luck! I hope you can find something suitable. Might want to read through my thread as there is quite a lot of information about the cartridges as well.

Welcome to the no one pays any attention to anything posted club.

 

[kevinkr]

Have a look at that thread, and laugh!

For the past 2 years I have put up with a constant barrage of requests for another board run. I finally offer to do something about it and it's been almost a week now and no takers.

Lot of progress on EQ very lately, I think I finally figured it out, and not surprisingly the solution that works is simpler than what it replaced.

Working at home and have been very, very busy. Just about to see up a small EE lab in the home office. Not returning to Needham office until September..

On other side of all of this we'll have to get together..

 

[Mark Tillotson]

Cheers for that ... will hunt for a proper strain gauge pre-amplifier
Mike

The key phrases useful here are "instrumentation amp" - or "high CMRR". Many traditional instrumentation amps are very limited bandwidth however, mainly due to having excessive gain - you typically want an instrumentation amp with lowish gain which then feeds another modest gain block - this makes the most of the gain-bandwidth products. Another thing to look out for is voltage noise spec.

 

[kevinkr]

Hi Mark,
We're talking about an inexpensive phono cartridge, and one that isn't usually configured for balanced operation, and requires an integral source of clean DC ideally from a CCS.

There are very limited choices and most are more than 40 years old, the best solutions today are all DIY, there are no modern off the shelf solutions.

 

[scott wurcer]

Hi Mark,
We're talking about an inexpensive phono cartridge, and one that isn't usually configured for balanced operation, and requires an integral source of clean DC ideally from a CCS.

I was under the impression these carts are configured as a half bridge so they can't be used deferentially. There are BTW instrumentation amplifiers with more than enough BW at G =100 if it was possible to use one here.

 

[kevinkr]

From the outside it really looks pretty much just like a ~1K rheostat, the resistance being a function of the displacement of the stylus from its resting position.

I wasn't clear, balanced operation isn't possible with one..

 

[AudioReflex]

Hi all ... i would like to thank you for your input.


I just secured a EPC-451C .... and have been reading as much as possible about a suitable pre-amp to drive the cart ... which led me to this forum.

It still needs substantial EQ, but different from RIAA.
Playing With Panasonic Strain Gauge Cartridges (And A Dedicated Phono Stage)

much appreciated ... part of my reading

Good luck! I hope you can find something suitable. Might want to read through my thread as there is quite a lot of information about the cartridges as well.

much appreciated ... part of my reading

Welcome to the no one pays any attention to anything posted club.

i try to ...

The key phrases useful here are "instrumentation amp" - or "high CMRR". Many traditional instrumentation amps are very limited bandwidth however, mainly due to having excessive gain - you typically want an instrumentation amp with lowish gain which then feeds another modest gain block - this makes the most of the gain-bandwidth products. Another thing to look out for is voltage noise spec.

Hence .. asking about micrphone preamp ...

Hi Mark,
We're talking about an inexpensive phono cartridge, and one that isn't usually configured for balanced operation, and requires an integral source of clean DC ideally from a CCS.

There are very limited choices and most are more than 40 years old, the best solutions today are all DIY, there are no modern off the shelf solutions.

..... but luckily ... there are a couple of choices .... with DIY being one of them

I was under the impression these carts are configured as a half bridge so they can't be used deferentially. There are BTW instrumentation amplifiers with more than enough BW at G =100 if it was possible to use one here.

Cheers for the info ....




I have a variety of T/T's ... phono stages and cartridges ... and have compared modern MM carts ... to MM carts from the 70's ... along with a couple of shootout matches of MC's. There has been some surprising results.


Googling has led me to this Cart >> Strain Gauge .... and look forward to hearing what all the fuss is about.

for the past 2 years I have put up with a constant barrage of requests for another board run. I finally offer to do something about it and it's been almost a week now and no takers.

.... Still taking in (reading) your Thread ..







Cheers
Mike

 

[billshurv]

Attached is a simple way of doing the basic cartridge interfacing. No EQ on this. Done by a member on VinylEngine. Nice as its in the 'junk box' territory for being able to knock up for an experiment.

 

[kevinkr]

Uses a similar biasing arrangement to what I plan in my next design. My latest EQ will work with this if you add a second opamp.

 

[AudioReflex]

Attached is a simple way of doing the basic cartridge interfacing. No EQ on this. Done by a member on VinylEngine. Nice as its in the 'junk box' territory for being able to knock up for an experiment.

Uses a similar biasing arrangement to what I plan in my next design. My latest EQ will work with this if you add a second opamp.

The info is much appreciated


Cheers
Mike

 

[AudioReflex]

Finally ... some updates

I received the Panasonic EPC 451c and have built the pre-amp


As this would be the 1st time i have ever soldered up a complete board design ...
( i would be in the beginner, novice category (Usually do minor repairs only))

I chose to go with the pre-amp design as per the diagram by Chris.
Like to thank / acknowledge CD for the easy to follow design.
I built it exactly as per the schematic ... and this should reflect in the position of parts and wires on the board.

I made some changes to suit being ....
Dual 9v batteries (Dual 6x AA's spec'd)
1K resistors for the LEDS (220K was spec'd)
Deleted the DPM ... in lue of exposed, removable battery's
Standard Caps ... 1uF non polar ... (No PIO caps available)
Using OpAmp KA4558 at the moment << (it appears to be fine) ... Waiting for LME49720NA ... 6 weeks delivery time from the USA.


Attached are some pictures of the setup ...

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I have some questions

STRAIN GAUGE CART
Panasonic EPC 451c is described as 1000 Ohm's.
I measured the resistance on both sides ... and got the following results
Poles >> GREEN - RED = 691 Ohm's
Poles >> BLUE - WHITE = 764 Ohm's

Is this correct? ... could someone measure their EPC 451c to confirm please.
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

OPAMP>> OpAmp KA4558 ... I have attached a picture of it's schematic ... Connections OK?

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

PRE-AMP
I have not connected it to anything just yet .. and am testing the AMP and EPC connections with a multi-meter to confirm that all is fine.

(in one of the pictures i have marked up the results)

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

AMP side .. (Refer to picture) .. comes up with correct polarity .. along with 5 volts & falling ... Also .. 1.6mA showed up.
It seems good to connect to a main amp ... however .. Is there another method of test i can do?


-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

EPC side .. (Refer to picture) ... RHS has correct polarity .. all looks fine <<>> However ...
>>> the LHS ... polarity is reversed. (Following the schematic >>> LEFT STRAIN GAUGE)
Is this the way it was intended? <<>> or i should correct this via >> just reverse the LHS connection by putting Earth to the Centre Pole and Negitive to the Outer Pole.

Voltage and mA wise ... both Left and Right show up 9.4volts and 2.5mA ... which is basically the Battery Volts / mA's

How can i test ? ... to ensure that the Voltage is good to go for the EPC 451c ? ... and NOT burn out the Cart.
(what am i missing?)


Any added advise please.


Cheers
Mike

 

[kevinkr]

The cartridge resistance varies significantly from sample to sample. I've seen resistance values in the low 600 ohm range to 1K depending on the specific cartridge.

To test the pre-amp you can either plug the cartridge in or connect a 1K resistor across the inputs - you should see somewhere around 3V. (It's not critical)

The cartridge is not very electrically fragile so as long as you are reasonably careful you should not have a problem.

If you want to implement EQ, my latest design will work with this pre-amp, but you will need to add a second op-amp on each channel to boost the gain back up.

 

[AudioReflex]

The cartridge resistance varies significantly from sample to sample. I've seen resistance values in the low 600 ohm range to 1K depending on the specific cartridge.

Thats a wide variation from Spec ...

To test the pre-amp you can either plug the cartridge in or connect a 1K resistor across the inputs - you should see somewhere around 3V. (It's not critical)

I added the 1K resistor .. and it still maintained 9v ...

Either something not right ... or the Opamp is not suitable.


as per the method principal of the picture >> I did a Test of reducing a 9v battery by connecting 3x 2.2k resistors and achieved 3v .. unsure of the problem at this stage.

If you want to implement EQ, my latest design will work with this pre-amp, but you will need to add a second op-amp on each channel to boost the gain back up.

I thank for your knowledge on this subject .. and for your design ... will need to sort out the above first

 

[kevinkr]

To your question about the voltage polarity to the cartridge, these cartridges have opposing polarity outputs so inverting one relative to the other is necessary, generally it is done with another stage or as is done here the supply voltage polarity is inverted.

Check to make sure your RCA shells are actually connected to the circuit ground, it sounds like something is open.

The op-amp should be fine, not my first choice but adequate for experimentation. (May or may not be a bit noisy)