Phono Preamp for Strain Gauge Cartridge

[xcortes]

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.

[Paul Joppa]

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.

[keto]

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!

[xcortes]

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.

[keto]

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.

[xcortes]

I'm using 6v SLAs and 1k resistors to bias the cartridge. My DC blocking resistors are 0.1uF and respone is indeed heavy on bass so I'm pretty sure the 0.68 is more adequate.

[keto]

Were your Kalsons responsible for that recent seismic activity???
I've got an extra set of orange-drop 0.068uF caps, if you want to try 'em.

[xcortes]

Ja! No, I was at an elevator at the office, not having fun with the Pany.

I'm indeed counting on using those orange drops.

[keto]

I'll "drop" off those 0.068uF's on my next trip in from the city. The more I listen to the LP of Sergiu Luca playing Bartok on his Carlo Bergonzi (Cremona 1733) violin, the more I think the full 12dB treble boost might be a good thing. Or is that the Nonesuch Digital curse? In terms of gain and output Z the 6EM7 would be an interesting option for this application, or maybe that super-linear 6CM7? http://frank.pocnet.net/sheets/049/6/6CM7.pdf

[keto]

Woohoo! The strain-gauge cartridge is now biased up/down via a C4S current "load" (R) and a C4S current "sink" (L). It doesn't really show in the picture, but all the LED's are lit. The sound is fabulous. I don't know if that's because two 3K resistors have been replaced with C4Ss, or because I'm just amazed that the idea worked. SLA batteries are set up for +/- 12vdc, R2 is 6780 and R1 is 224.

[xcortes]

Amazing! I have to implement the C4S in mine, I guess.

The SG with my unRIAAed Eros is a beast. I would put it against Doc's $5k jap low DCR MC anyday (just kiddin' since i've not heard the latter).

[keto]

I'll be curious to see what you think of the C4S version. To me the difference is not subtle. I doubt this combo is anywhere near Doc's new MC cartridge. Just a hunch. But hey, imagine a Bottlehead strain-gauge cartridge, with the SR and C4S in the cartridge itself, and the LED's lighting the grooves :-)

[xcortes]

I finally purchased and received a "RIAA" tests lp and did my measurements with the uneq-ed Eros. And a couple of days ago the guys from "Emia" released their own preamp and with the release they published their measurements of the cart:

The no-EQ curve is identical to what I got and his chart is nicer than mine so I'm pasting his.

In any case. There's definitely some compensation within the cart as it doesn't behave completely like a velocity device.

Leaving aside the extremes peaks that have to be dealt with (the bottom one with the input capacitor) there's only 4 dB to boost -although the frequencies are the same-.

Saludos

[keto]

Looks like that input cap could also be tuned lower than 50hz. Maybe a 0.1uF (33.8hz @ 47K)?

[xcortes]

The red phono repro fell on my lap so now I have an Eros that I can fully devote to the project!

Stay tuned