Valve Itch phono

[merlin el mago]

Was funny do the measurements and very interesting to know the result values.

 

[merlin el mago]

Vinyl Engine calculator Cartridge/Amp Loading/Resonance

Phono Stage Load:
47 KΩ
Cart Inductance:
500 mH
Total capacitance:
480 pF

Resonance:
10.3 kHz

Optimum Load:
32.3 KΩ

If I change the 47K input resistor load of Itch for 32.3KΩ I will load better my cartridge or not, because Shure recommended 47K 400-500pF for loading?

 

[Salas]

Attached scope measurements, SG set to 100kHz 100mV, 1st pic scope set 0.1V/cm 2us shows the measurement 100kHz 100mV only attached SG, 2nd pic without the 100K input resistor scope set 5V/cm 5us, 3rd pic with 100K input resistor scope set 0.5V/cm 5us.

2nd measurement result 8Vpp

3rd measurement result 0.5Vpp

0.5/8 = 0.0625 ratio log = -1.20411 x 20 = -24.0823dB = aprox. 260pF

1st looks like 200mVpp

2nd looks like 7.2Vpp if set to 2V/cm or 17Vpp if set to 5V/cm

3rd looks like 0.9Vpp

 

[merlin el mago]

2nd pic is 5V/cm

 

[merlin el mago]

0.9/17 = 0.05294 log = -1.27620 x 20 = -25.52412 dB = 300pF?

 

[merlin el mago]

300pF (Itch + internal wire) + 220pF (Linn Ekos) = 520pF surpassed the maximum recommended by Shure for V15 type III

 

[Salas]

That would be -25.5dB i.e. 300pF in Kevin's table including internal wiring pF from RCA to grid and external wiring pF from generator to RCA. 11% error included even if the voltage measurements are correctly done.

 

[Salas]

I would also try the 100k at the grid for third measurement after all wiring. 2nd meas=ref only wires from gen to grid. 3rd meas=drop for +100k at grid. I would also verify ref and drop -dB vs DMM that has spec for 100kHz VAC if available. Also vs 6N2PEV to verify measurements can show some differences i.e. not too low resolution.

 

[kevinkr]

Merlin, where did you place the 100K resistor? The best place is right at the RCA jack end of the cable connected to the generator, that way you are only measuring the capacitance in the pre-amplifier.

I was very surprised at the high levels of capacitance in some of my designs! (Which confirmed the somewhat disappointing results I was getting with some cartridges.)

In terms of cartridge loading I would recommend sticking as close to the manufacturer's recommended loading as possible, this is because in addition to electrical resonances in the cartridge there are mechanical ones as well, the cartridge is designed to provide the correct loading only into the specified load resistance and capacitance.

 

[merlin el mago]

I placed the 100K soldered at RCA input.

 

[kevinkr]

That's good IMO, you do want to measure all of the capacitance in the pre-amp right to the jack.

 

[merlin el mago]

That sounds good and was very kind from you to help me. Also I enjoyed a lot of doing the measurements.

Cheers
Felipe

 

[Salas]

With 100kΩ at grid could focus only to layout+base+tube Miller so to know better about RCA and input wiring contribution. To can improve those if needed because wires can be bad pF guys http://www.diyaudio.com/forums/parts/285128-measuring-capacitance-some-rca-cables.html

Also the valve base local wiring can add substantial pF between grid and anode on top of the base's own that Miller multiplies along Cga. Can be redone and remeasured if needed.

 

[merlin el mago]

Good advice Salas, now watching futbol Barça.

 

[merlin el mago]

I'm here Salas.

 

[kevinkr]

Merlin, if you can take some photographs of how the itch is put together Salas or I may be able to advise on how to get the capacitance down a bit.

 

[merlin el mago]

With 100kΩ at grid could focus only to layout+base+tube Miller so to know better about RCA and input wiring contribution. To can improve those if needed because wires can be bad pF guys http://www.diyaudio.com/forums/parts/285128-measuring-capacitance-some-rca-cables.html

Also the valve base local wiring can add substantial pF between grid and anode on top of the base's own that Miller multiplies along Cga. Can be redone and remeasured if needed.

Attached pics.

1st pic scope 50mV/cm direct to signal generator
3rd pic scope 5V/cm signal generator probe direct to grid

 

[merlin el mago]

Attached more pics.

1st pic scope 0.5mV/cm signal generator probe to 100K (100K soldered to grid)

 

[merlin el mago]

Merlin, if you can take some photographs of how the itch is put together Salas or I may be able to advise on how to get the capacitance down a bit.

Attached pics.

 

[merlin el mago]

Salas what about the latest measurements?