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Old 1st November 2010, 06:58 PM   #11
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Default Loading Included

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Originally Posted by sreten View Post
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
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Old 1st November 2010, 07:12 PM   #12
sreten is online now sreten  United Kingdom
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Quote:
Originally Posted by imfree707 View Post
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.)
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Old 1st November 2010, 08:12 PM   #13
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Default Noise Consideration

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Originally Posted by sreten View Post
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
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Old 1st November 2010, 09:28 PM   #14
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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.
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Old 1st November 2010, 11:50 PM   #15
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Default 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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Old 2nd November 2010, 11:05 AM   #16
DF96 is offline DF96  England
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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.
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Old 2nd November 2010, 02:54 PM   #17
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Quote:
Originally Posted by DF96 View Post
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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Last edited by kevinkr; 2nd November 2010 at 02:57 PM.
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Old 15th November 2010, 06:02 PM   #18
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Default Some Headshell!

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Originally Posted by imfree707 View Post
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
Attached Images
File Type: jpg VinyLiberator U-1, jax, HF adj.jpg (106.4 KB, 35 views)
File Type: jpg RIAA by edgar wb lg.jpg (63.1 KB, 36 views)
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