Bottlehead Forum
General Category => Technical topics => Topic started by: xcortes on January 18, 2012, 01:24:50 PM
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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
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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
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Yes, a displacement signal would need shelving treble boost from 500Hz to 2122Hz - about 12.6dB boost.
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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
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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.
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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,
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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...
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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.
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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.
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That Eros option should be awesome.
That's a direct-coupled circuit, right?
Will the Seduction-based C and R values also work there?
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That's a direct-coupled circuit, right?
yep
Will the Seduction-based C and R values also work there?
yep!!!!
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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.
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Shakes. When I read
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Oooops. We can probably do something with the parts on hand. I'll look at the numbers when I get home tonight. Thanks, Paul!
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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?
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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.
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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.
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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!
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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.
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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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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.
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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.
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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.
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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
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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.
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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).
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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 :-)
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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
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Looks like that input cap could also be tuned lower than 50hz. Maybe a 0.1uF (33.8hz @ 47K)?
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The red phono repro fell on my lap so now I have an Eros that I can fully devote to the project!
Stay tuned
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Xavier,
How did I miss this thread when it came out???
An old audio buddy of mine used a Panasonic strain gauge cartridge in the mid 70s. It was amazing but I could never find one. This was way, WAY before ebay.
I'm staying tuned!
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I used the Panasonic 451 with the Roland Research stage back in the 80's and 90's. It was outstanding, I still have it. Unfortunately the supply of cartridge's/stylus have pretty much dried up. there are a few options but they are expensive. I friend of mine who is actively working on this, ordered a stylus off the bay, which turned out to be a piece of crap knock off...John