Phono Preamp for Strain Gauge Cartridge

[xcortes]

I heard a Strain Gauge cartridge. And I liked it. Until I knew "it only works with a special preamplifier". I mean, a hardcore BH doesn't buy blackboxes, does he? (well, maybe a DAC je, je, je).

But I kept studying. And I learned that:

1. There's "vintage" strain gauge cartridges. Not only the expensive modern one with the black box attached.
2. SGs are semiconductors that behave resistively and the resistance varies with pressure.
3. Because of 2 above the cartridge needs a bias current.
4. The nature of being pressure sensitive, vs speed sensitive (as  MM or MC cartridges are) implies that their response is close to that of the RIAA curve. ie  no equalization is required (there's arguments on both sides here – no need to get into those -).

So I designed a phono preamp for a Panasonic cartridge. The cartridge puts out 3mV when the current is 4mA and has an internal resistance of 1k ohm.

The preamp is based on a 437A preamp. It uses the highly praised Peerless R-080 preamp. Has no less than three EXO 01 chokes in the PS. No electrolitics in the PS. A shunt regulated PS with two C4S. So it’s a single stage with a gain of 40 without eq with a very sturdy PS and a nice tube.

So far I’m thinking batteries for the SG bias although a C4S should work great. I also need to design the C4S and probably DC heaters for the filaments.

But before that I thought I’d present my idea here for any comments or suggestions.

Thanks,

Ps. I DO have all the parts as well as chassises to build them quickly.
PPS. The schematic and PS are for one channel.

[2wo]

Hi Xcortes,

My first cart was a Stanton 681eee. After a few of the other MM's of the day, I got a Ortofon MC 20, then a Denon 103d.

Then I heard the Roland Research, SG and was hooked. I still have mine. There was a small Op amp based board and the full discreet FET amp.
 You will need filters. After the RIAA curve is applied, at cutting to the groves on the LP. The amplitude, still is not linear.
At least I think you will need some sort of HF shelving filter, or maybe some sort or "shadow RIAA". don't know, just saying.

I will put you in touch with a friend of mine who has been working on an Roland, SS, based solution for some time...John       
   

[Paul Joppa]

Yes, a displacement signal would need shelving treble boost from 500Hz to 2122Hz - about 12.6dB boost.

[xcortes]

My original design will not work since the 437A at the designed parameters would have too high plate resistance.

On the needed eq. I studied the cart's response curve and, in line with Paul's comments, would need adjustments. The good thing is that those adjustments are completely in line with my system crossover points. I can just increase the volume in the mids and I'd be pretty close.

Let me count my losses, reasses and I'll be back.

Thanks

[xcortes]

Yes, a displacement signal would need shelving treble boost from 500Hz to 2122Hz - about 12.6dB boost.

I know Paul is on vacation but still...

First sonic tests are very positive so I want to proceed to modify a preamp for the needed shelving.

My calculations tell me that if I replace R1 in a Seduction with a 75k resistor, R2 with a 23k one and the 33n cap with a 3,300pF one I get time constants of .323 uS and .076 uS corresponding to 492 and 2,097 hz and to a -12.6dB reduction. I would take out the 10nF and 1.2nF resistors.

In an Eros phono I would replace the 10.5k resistor with a 23k one, the .03uF cap with a 3,300pF one and remove the 0.010uF cap.

I'll give this a bump when Paul returns.
 

[mellowgroovy]

Yes, a displacement signal would need shelving treble boost from 500Hz to 2122Hz - about 12.6dB boost.

I know Paul is on vacation but still...

First sonic tests are very positive so I want to proceed to modify a preamp for the needed shelving.

My calculations tell me that if I replace R1 in a Seduction with a 75k resistor, R2 with a 23k one and the 33n cap with a 3,300pF one I get time constants of .323 uS and .076 uS corresponding to 492 and 2,097 hz and to a -12.6dB reduction. I would take out the 10nF and 1.2nF resistors.

In an Eros phono I would replace the 10.5k resistor with a 23k one, the .03uF cap with a 3,300pF one and remove the 0.010uF cap.

I'll give this a bump when Paul returns.
 

How about your preamp frequency response after insert this equalization?

Thanks,

[keto]

Hey Xavier,

After crunching those numbers, its good to see you came out with something similar.

"....My calculations tell me that if I replace R1 in a Seduction with a 75k resistor, R2 with a 23k one and the 33n cap with a 3,300pF one I get time constants of .323 uS and .076 uS corresponding to 492 and 2,097 hz and to a -12.6dB reduction...."

but I would take out the 10nF and 1.2nF resistors. caps!

Here are the numbers I came up with for the Seduction circuit:

A           12dB           7dB          5dB

fz          2000          1500         1333

fp           500            666          750

R1         79577         66433       61873

R2         26525         53051       79597

C2          .003          .002         .015
 

I just have to approximate those R values and then its off to Mouser...

[keto]

Okay, now the real-world values, for altering the Seduction circuit for the Panasonic strain-gauge cartridges...

75K, 23K2, 33nF (12.6dB)
60K4, 48K7, 22nF (7dB)
61K9, 80K6, 15nF (5dB)

and a .068uF cap in the input, to block DC current and make the 50hz high-pass with the 47K5 grid-bias resistor.

R's are 1% 1/4W and C's are 5% 600V orange-drops.

[xcortes]

My listening tests tell me it's more like 5dB than 12dB. We'll definitely need to get a RIAA test lp.

The sound through the Seduction is sooo good I'm tempted to mod the Eros already.

[keto]

That Eros option should be awesome.
That's a direct-coupled circuit, right?
Will the Seduction-based C and R values also work there?

[xcortes]

That's a direct-coupled circuit, right?

yep

Will the Seduction-based C and R values also work there?

yep!!!!

[Paul Joppa]

Yes, a displacement signal would need shelving treble boost from 500Hz to 2122Hz - about 12.6dB boost.

I know Paul is on vacation but still...

First sonic tests are very positive so I want to proceed to modify a preamp for the needed shelving.

My calculations tell me that if I replace R1 in a Seduction with a 75k resistor, R2 with a 23k one and the 33n cap with a 3,300pF one I get time constants of .323 uS and .076 uS corresponding to 492 and 2,097 hz and to a -12.6dB reduction. I would take out the 10nF and 1.2nF resistors.

In an Eros phono I would replace the 10.5k resistor with a 23k one, the .03uF cap with a 3,300pF one and remove the 0.010uF cap.

I'll give this a bump when Paul returns.
 

I'll look at this more closely later - but at first glance, I think you are implementing a 12dB treble cut, not a boost. It may be more difficult to implement with the Eros.

[xcortes]

Shakes. When I read “shelving” treble boost I interpreted it as “compensating” treble boost. I guess I’m not that fluent in English as I should!

[keto]

Oooops. We can probably do something with the parts on hand. I'll look at the numbers when I get home tonight. Thanks, Paul!

[keto]

How about removing the corresponding RIAA filter and doing the treble-boost after the Seduction/Eros circuit, using 50K:500 mic transformers? Would -20dB overall be too much loss? + the -12dB below 1Khz?