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Old 20th April 2013, 02:37 AM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Recommended RIAA Values for this Circuit

Hi, I have a circuit I came up with that uses a grounded grid feedback configuration. I am simulating it in Tina. Attached are the circuits and simulation. What configuration/component values do you all recommend for such a system?
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Old 20th April 2013, 03:11 AM   #2
SY is offline SY  United States
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Just out of curiosity, how did you come to this circuit? There's some features that seem... odd. Your cartridge will not be happy. The gain is likely to be extremely low and conformance to RIAA will be problematic.
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Old 20th April 2013, 03:15 AM   #3
BRSHiFi is offline BRSHiFi  United States
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http://www.tubecad.com/2007/08/blog0116.htm

Best for MC carts as the input impedance of this unit is insanely low. The schematic is incomplete as I will be adding an input transformer for MC carts. I will post the MM version in a few.

Last edited by BRSHiFi; 20th April 2013 at 03:35 AM.
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Old 20th April 2013, 03:50 AM   #4
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Interesting circuit... If the cartridge needs to be loaded anyways, why not use the cathode impedance to do it?

I particularly like JB's thought to use a transformer as the cathode resistor to ditch the huge coupling cap.

dave
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Old 20th April 2013, 03:51 AM   #5
BRSHiFi is offline BRSHiFi  United States
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Thats the whole idea. Are there transformers that can match MM carts to the low impedance, or will I need to custom here?
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Old 20th April 2013, 04:32 AM   #6
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Assuming you have enough gain / low enough noise to do MC direct (Tina will tell you that) i see no reason why you couldn't use a 500:50K mic transformer in reverse. It is sort of a fun idea using a MC stepup in reverse.

dave
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Old 20th April 2013, 04:41 AM   #7
BRSHiFi is offline BRSHiFi  United States
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Will try that. MM's should be OK on the noise. MC direct will be too noisy unless a stepup transformer is used.

I like the idea of balanced phono stages but moist turntables use RCA and not XLR unless you modify them. That will void the warranty totally on my new unit.
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Old 20th April 2013, 05:14 AM   #8
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You have to understand that this stage will provide about a 450-500R load to the source so any step up device will take that value lower. A 1:4 ratio quickly brings you to 30 ohms so apply your SUT's carefully. Using a transformer in the other direction to bring load up to MM levels (47K) will require that the stage be quiet at MC levels. Essentially I don't think you can win both battles with this design unless it is quiet enough for MC without a SUT.

Maybe you need to look for a pair of Telefunken ECC 803S's :-)

dave

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Old 20th April 2013, 05:58 AM   #9
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You really can't make an RIAA curve unless you can change gain by 40dB from low to high frequencies. Non-inverting feedback can't reduce gain below 0dB, so you have to start with quite a lot. There're reasons phono equalizers are made with several gain stages.

All good fortune,
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Old 20th April 2013, 06:16 AM   #10
rjm is offline rjm  Japan
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Oh dear.

Look, MJ is cool and all, but don't go copying circuits wholesale if you can't read Japanese.

JB was wrong about the circuit. The red "X" he marked to indicate what he thought was an incorrect connection is itself wrong. The circuit is correct as originally drawn, the connection does not short to ground, it appears to set the gain, connected as it does to the "MC gain switch".

The left and right halves of the circuit are maked "MC head amp" and "Phono EQ amp" respectively. The RIAA network is right there in the circuit, bottom right, where is says "NF型EQネットワーク". The key point here is the MC/MM switch only engages the low impedance "grounded grid" input stage when MC cartridge is selected, MM carts are routed directly to the second, high impedance second stage.

Don't use the "MC head amp" circuit with a MM cart, you will be completely screwed...
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Last edited by rjm; 20th April 2013 at 06:23 AM.
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