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.
 
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.
 
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
 
SY;2350227 You can accomplish the same thing more easily (and said:
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

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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.)
 
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.
 
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.
 
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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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Loading Included

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.

I'll have a standard 47k at the buffer's input and while I'm not sure what optimum capacitance is for the AT92ECD, I'll probably go somewhere between 270 and 470pf, also at the buffer's input. I think the mfgr's allow for 100-200pf cable capacitance and I'll take that into account, too. I'm watching this thread and still comtemplating how to go about this project. I'll be building on a less than 1" X 1" perf board, so the board shouldn't weigh more than a few grams, hopefully allowing total cartridge/buffer weight to still be within limits for the Akai AP-D2's tonearm. Best Regards To All, Edgar
 
I'll have a standard 47k at the buffer's input and while I'm not sure what optimum capacitance is for the AT92ECD, I'll probably go somewhere between 270 and 470pf, also at the buffer's input. I think the mfgr's allow for 100-200pf cable capacitance and I'll take that into account, too. I'm watching this thread and still comtemplating how to go about this project. I'll be building on a less than 1" X 1" perf board, so the board shouldn't weigh more than a few grams, hopefully allowing total cartridge/buffer weight to still be within limits for the Akai AP-D2's tonearm. Best Regards To All, Edgar

Hi, fair enough, but I cannot see any real advantage over the usual arrangement, rgds, sreten.

(And I can see some noise issues with a less than ideal buffer op-amp.)
 
Noise Consideration

Hi, fair enough, but I cannot see any real advantage over the usual arrangement, rgds, sreten.

(And I can see some noise issues with a less than ideal buffer op-amp.)

I'm a bit wary of the potential for noise, myself, and I think noise should be minimal, as the buffer stage will run at unity gain. There is, of course, some noise added to the system by a unity gain stage and one objective of my experiment is to determine if the added noise can be heard, and if audible, do any sonic benefits outweigh the added noise. Advantages are only theoretical at this point, but I expect less hum pick up from tone arm wiring and greater immunity to electrostatic hum pick up by less than perfectly shielded interconnect cable between turntable and amp because of the LM833 buffer's less than 1k output impedance. I'll also be able to "cut n try" various loading capacitance values. Best Regards, Edgar
 
You are simply adding noise, weight and complication. The cartridge itself has a low impedance at hum frequencies, so a unity gain buffer won't help there. The cable will already be at least pseudo-balanced so pickup should already be largely cancelled. Capacitive loading is not a problem as the cartridge is designed to require it. All the normal reasons for a local 'head preamp' don't apply. If the problems you are trying to 'solve' really existed then you would not want unity gain anyway, but 10-20dB. People don't even do this for MC cartridges.
 
advantages of a headshell amp

Well, there are a lot of reasons why a headshell amp shouldn't be done, perhaps, but turntable to preamp cable length is pretty well fixed maximum 6' or 2m. If I could get 8' or 2.5m, I wouldn't have to walk around three sides of the piano from the record shelf to the turntable, then back around two sides of the piano to adjust the volume after I start the LP. Perhaps there is a better IC and gain than IMfree707 has suggested? A particular FET? rewiring of the turntable arm? Putting a preamp PCBlet in the base of the turntable? I could eliminate the whole disco mixer and use a real $125 PA mixer with tone controls to switch from LP to CD to radio, if the turntable put out 1V PP 600 ohm source. The PAS2 preamp is not at all ideal for 2010, it shorts non-selected inputs, and if you turn the CD player on while the selector knob is on "phono", then it could damage the output stage of the CD player or radio. And a $1000 state of the art Preamp is definitely not happening at my income level. The better half is already griping about the $400 Hammond Organ, the $600 speaker pair, the $200 Peavey amp, the mixer, the graphic equalizer , the digitech . . .
 
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If you can't avoid a very long cable, then put an ordinary RIAA preamp near the turntable and run a long cable to the line stage.

Any well-designed source should include a series resistor in its output so it can be shorted without harm. A few hundred ohms is sufficient for protection, and won't seriously affect cable driving for reasonable domestic lengths.
 
If you can't avoid a very long cable, then put an ordinary RIAA preamp near the turntable and run a long cable to the line stage.

Any well-designed source should include a series resistor in its output so it can be shorted without harm. A few hundred ohms is sufficient for protection, and won't seriously affect cable driving for reasonable domestic lengths.

I agree totally, and having modded a fair number of CD players over the years I have yet to see one that did not have one or more resistors in series with the outputs - besides all modern op-amps are short circuit protected. (This incidentally is also true of all solid state tuners wrt output resistors that I've tinkered with as well.)

FWIW I would take a slightly refreshed PAS-2 over any disco mixer I've encountered any day. (UREI, Peavey, etc.) How about a diy 12B4 line stage or an Aikido - will easily outperform anything you've got on hand.
 
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Some Headshell!

I will soon have a turntable and cartridge...snipped text...

Hi Guys, my Akai AP-D2 turntable and AT92ECD Cartridge have been here a few days. The non-removable headshell, while small and having little mass, has "thong" coverage and obviously no space for even my very compact interface board. I didn't even consider using my interface. I built it beforehand, so I may test it, external to the headshell, at a later time. I did design and build my VinyLiberator pre-amp board, as demonstrated in the You Tube Video.

I had absolutely no problem in empirically determining the proper component values for proper RIAA Playback response by simply using my Audio Generator and 'Scope. The 90pf trimmer is quite noisy while being adjusted, but gives a sweet +/- 4db or so "fine tuning" range at 20khz to bring overall response to optimum. I didn't use a cartridge load capacitor because recommended load capacitance was 100pf and is approximately the value provided by tonearm wiring and output interconnect cable. The pre-amp seems only a little noisy and sounds really close to RIAA, as far as I can hear. Thanks, Guys

VinyLiberator You Tube Link
 

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