Phono Preamp for Strain Gauge Cartridge

xcortes

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

Attachments

The Eros has a resistor to ground as part of the eq Network. Maybe we can use that resistor as part of the new network and just put the series resistor/cap before?

We could use larger resistor values and compensate with a smaller cap?

We would need to adjust the C4S on top to account for the change in current draw through the shunt resistor.
 
keto's should work but 20dB is a lot to loose.

Here's mine, between the triodes of the Seduction. It is not amenable to use with a pentode stage, since it relies on a low source impedance (much less than the 75K resistor; in Seduction it's about 4-5K). The parts marked * are already part of the Seduction circuit.
 

Attachments

  • strain guage eq.GIF
    strain guage eq.GIF
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Thanks Paul! Using some parts on hand, I've installed a 4.86dB treble boost, with poles at 800hz and 1400hz. At least I think that's what I've done: 3300pF and 60K4 in parallel and 80K6 to ground. We'll have to get a tone sweep LP into the picture before proceeding further. It was sounding a little dull with no treble boost and now its sounding a little bright. But then I've been playing Mahler 5 all week so take that with a grain of salt!
 
Thanks Paul!

As keto says the absolute truth will be revealed once we get the test tones lp. I'm working on this.

In the meantime I installed input caps and a resistor to bias the cartridge with batteries on the Eros. Removed the eq leaving only the 75k resistor. Even after padding the input on my FP3 by 20dB I have headroom to spare. In spite of the excessive gain the circuit is completely silent and superb sounding.

 
I've been using a 1.3Ah 12v SLA battery on each channel, to bias the strain gauge cartridge. The multi-turn 5K Vishay resistors (set at 3K) bias each side at + and - 2.6vdc and 3.4mA. A 0.068uF input cap both blocks dc current from the first triode stage and creates a 50hz high-pass with the 47K5 input grid bias resistor. The jury's still out on the treble boost. I have a 4.8dB one installed at the moment, and will try the 12dB one calculated by Paul Joppa as soon as those parts arrive. In the meantime, I'll install the C4S upgrades to the Seduction circuit.
 

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