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imfree707 1st November 2010 02:27 AM

Magnetic Cartridge Intimacy
 
I will soon have a turntable and cartridge that I'll be able to install my proposed Cartridge Interface Buffer in. Does anyone have experience with placing resistor/capacitor loading with a unity-gain LM833 buffer intimately, in a head shell, immediately behind the cartridge. My theory is that such an arrangement will provide optimum, stable cartridge loading, while converting the cartridge output to low impedance, sharply reducing tonearm wiring and interconnecting cable hum pick up. It should, at least, sound good, so I should be able to post results on You Tube in a couple weeks. What do you think, Guys. Thanks.

SY 1st November 2010 05:17 AM

Back in the '70s, I worked with Murray Zeligman on a design that put an MC headamp right into the headshell (we used CM860 FETs and remote powering). He sold it to Dynavector. Novel, but I'm not convinced it got us anything. You can accomplish the same thing more easily (and, I think, better) by simply running the cartridge/interconnect/preamp input as balanced.

andyr 1st November 2010 10:51 AM

Quote:

Originally Posted by imfree707 (Post 2350101)

My theory is that such an arrangement will provide optimum, stable cartridge loading, while converting the cartridge output to low impedance, sharply reducing tonearm wiring and interconnecting cable hum pick up.

What do you think, Guys. Thanks.

Methinks you are trying to solve a problem you shouldn't have? :confused:

I don't have any hum emanating from the tonearm wiring and IC to my phono stage. And my cart is a LOMC (0.28mV).

But then I use a shielded phono cable.

I would've thought you might put your device at the base of the tonearm but certainly not in the headshell, as you will increase mass there.

Regards,

Andy

indianajo 1st November 2010 02:26 PM

[QUOTE=SY;2350227 You can accomplish the same thing more easily (and, I think, better) by simply running the cartridge/interconnect/preamp input as balanced.[/QUOTE]
Once again, I ask the question: When you convert a single ended mag phono input (op amp with negative to preamp common, RCA input ring to preamp common) to balanced (op amp minus input to cartridge RCA ring), do you add the RIAA resistors and caps that are the positive input, also on the negative input? Does replacing the RCA jack ring -cable shield with 2 wire cable, XLR input, and shield to headshell only, provide hum reduction benefit?

kevinkr 1st November 2010 03:07 PM

Quote:

Originally Posted by indianajo (Post 2350606)
Once again, I ask the question: When you convert a single ended mag phono input (op amp with negative to preamp common, RCA input ring to preamp common) to balanced (op amp minus input to cartridge RCA ring), do you add the RIAA resistors and caps that are the positive input, also on the negative input? Does replacing the RCA jack ring -cable shield with 2 wire cable, XLR input, and shield to headshell only, provide hum reduction benefit?

Answer to first question would generally be no, in your specific case without any schematic it is hard to know. In a fully differential phono stage the eq network would likely be passive and be placed across the phases after the first differential stage and before the second - in a feedback based or passively equalized design you'll never get two networks to be identical and by so doing you seriously degrade cmrr and you will also likely have increased frequency response and phase errors due to the mismatch between phases.

Your best bet would be transformers, but if you are using a MM cartridge I can't imagine that there would be any benefit at all, typically differential input stages are 3dB noisier than a comparable single ended input stage - of course if the first stage in your phono stage is an op-amp you already have this issue, although the best op-amps are quiet enough in practice that it does not matter with any typical MM cartridge.

I am running a ZU/Denon DL-103 moving coil with 0.35mV output at 5cm/sec into step up transformers and have no hum from the wiring or transformers even with unbalanced input and output. (My speaker system and amps are -3dB at 35Hz so significant levels of hum would be audible.)

SY 1st November 2010 05:07 PM

Quote:

Originally Posted by indianajo (Post 2350606)
Once again, I ask the question: When you convert a single ended mag phono input (op amp with negative to preamp common, RCA input ring to preamp common) to balanced (op amp minus input to cartridge RCA ring), do you add the RIAA resistors and caps that are the positive input, also on the negative input? Does replacing the RCA jack ring -cable shield with 2 wire cable, XLR input, and shield to headshell only, provide hum reduction benefit?

Best solution is to not use coax, but a twisted pair with a shield. XLR or DIN connector. Yes, ground the shield at the TT end. Balanced input on the phono amp, preferably using a circuit with high CMR. EQ can, as Kevin said, be run between the two phases just as if it were a single-ended circuit- you don't need high CMR after the first stage. This arrangement is very resistant to externally induced hum.

indianajo 1st November 2010 05:57 PM

thanks
 
Thanks for the answer. I see on review that the PAS2 has the RIAA caps on the second stage of the 12AX7 input tube, not the first. I think this RA88a disco mixer has the RIAA caps (ceramic) before the ST33078 op amp, but I'll put on my reading glasses and take another look at the traces. The disco mixer 33078 for 4558 swap pulled the hiss down equivalent to the PAS2 with new film plate resistors, but the hum is still a little higher. The hiss is equivalent to the gas heater pilot and less than the ST120 fans , so not a problem, I'm not that much a purist. New electrolytic PS caps in the disco mixer didn't help the hum. Forward, as Ken Keysey's bus said.The box has room to fly a little op amp board above the main one for differential buffering, and 33078s are 3 for a dollar, so I'm going to play with that. Tonepad's op amp PCB is $13 with shipping, so I guess I'll try that. Input transformers are not the sort of thing I'm going to find on the junkshelf at salvation army, where most of my parts come from. The cartridge is a Shure M97 Era IV, nothing really exotic.
I complimented Imfree707 privately for his boldness in making a little SMC PCB at our age for the headshell (I'm 60, he's 55) but he's going to try perfboard and DIP packages first, I believe. SMC's are scary to the repairman, particulary multilayer PCB's, but maybe they aren't as hard to assemble the first time as all that. I had a **** of a time finding tonepad's single dip op amp PCB's, much less mythical SMC single op amp PCB's. Making PCB's is a whole nother adventure, I see on construction that you can do it with a laser printer, clay paper, hair peroxide and HCL toilet cleaner, but I'm going to delay that adventure.

SY 1st November 2010 06:11 PM

Quote:

Originally Posted by indianajo (Post 2350858)
TI think this RA88a disco mixer has the RIAA caps (ceramic) before the ST33078 op amp

If that's true, the design is badly flawed from a noise and RIAA accuracy standpoint.

sreten 1st November 2010 06:22 PM

Hi,

The massive flaw with this approach is that MM's are designed to work
with typical cable capacitance and input loading, in nearly all cases the
response will be far worse than if the buffer is not there, so pointless.

rgds, sreten.

indianajo 1st November 2010 06:34 PM

Thanks Srefen, Sy. I think this disco mixer can be improved for not much work or $. I Like the price ($15) power consumption and footprint (6"x10" on top of the Steinway) If you think RIAA filter before op amp is funny(first is the only op amp in the chain, right to the slider pot mixers with other mag phono in, CD player, mike and out the back to the power amp) another guy bought a newish disco mixer in Romania that connected the two mag phono inputs to the 1-2 fader pot, both ends, before amplifying. Saves one dip op amp and a couple of disc capacitors. I thought that was a riot. He built a little flying op amp just to get the mike talkover to work right. Will try buffer amp single ended before RIAA network before I try balanced input and go to the trouble of changing the connectors on the turntable to XLR and making cables with $1 a foot 1 T.P. shielded cable. (I have 16 feet left).
The 1 T.P. shilded teflon cable is about .2"x.15" cross section. I've got 30' of belden twinax out in the garage, .25" dia, two 22 ga parallel inside the coax shield not very close together and not twisted. If I do decide to go balanced, would that be lower in capacitance per foot than the 1 T.P. teflon and more like a RCA plug coax phono cable?


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