• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Recommended RIAA Values for this Circuit

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

dave
 

rjm

Member
Paid Member
2004-05-02 2:41 pm
Kyoto
phonoclone.com
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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Oh dear.
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".

I am a Japanese. rjm is correct.

The correct NF equalizing CR values are indicated in the following Web
(sorry written in Japanese but you can read the schematic).
It is noted that the first stage of 12AX7 is for MC head amp which
you can eliminate for MM cartridge.

Tube Pre
 
kingneb said:
Hi, I have a circuit I came up with that uses a grounded grid feedback configuration.
I can't quite work out what this circuit is for. It clearly is not for MM as the input impedance is far far too low. It clearly is not for MC as the input noise is far too high. It clearly is not intended to have accurate RIAA equalisation as the open loop gain is insufficient and depends too much on valve parameters.

Was someone running a competition for weirdest use of a 12AX7, but nobody told me?
 

BRSHiFi

Previously known as kingden
2008-08-28 4:27 am
Evanston, IL
It seems to me that the ultimate best way to accomplish low noise MC amplification is a fully balanced phono preamp. My turntable is a Clear Audio Concept unit. It has permanently attached RCA cables so modifying the unit will void the warranty.

As far as tube parameters go for RIAA, it seems that a cathode follower is the best way to combat that problem. Other designs I am experimenting with are hybrid op-amp/12AX7.
 
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The circuit as shown will also have an input impedance which varies with frequency, due to the RIAA network. The original Japanese circuit shown by Broskie does not have this problem. Even with that, I find the idea of a grounded-grid 12AX7 rather unusual for MC - I would have thought that the noise and impedance would be too high.

I have never used MC but I suspect that for that you either need BJTs or a good transformer.
 
OK. That would imply that 1/f is generally small for those valves. Of course, the 2.5 in 2.5/gm is only an average figure as it depends on the details of the triode geometry.

I once read somewhere (can't remember where) that European cathodes were generally better made than US cathodes so had lower 1/f. Not sure if this is true or just the usual transatlantic bragging.