Valve Itch phono

diyAudio Chief Moderator
Joined 2002
Paid Member
Salas - did you test by ear as well when changing the resistor?

That will do soon in HMC Jfet old build for a friend. He got an MC cart now that needs interfacing. Will check FFT first and then real audio test. The generator's source impedance is 50 Ohm BNC standard, nearer to a Denon 103, but I am inclined that the damping for square waves is a solely secondary winding issue. There are RC snubber options also of course.
 
diyAudio Chief Moderator
Joined 2002
Paid Member
Further reporting on secondary loading, 22K kept the slight HF rise measured on oscope when applied in the 43dB original JFET phono (non cascode). The RIAA curve gained beyond 13KHz. So I paralleled 100K to go 18K that gave me the smoothest square wave before. RIAA HF part came back to normal. 977 gain with that load is ~15dB. If not able to do it technically by calibrated FFT driven with noise signal, I suggest a 50K trimmer instead of input resistor on the Itch and listen. When best perceived with given SUT and RIAA tolerance, take the trimmer out, measure it, and fix a near value good resistor. Different SUT will do different thing, and the Itch ain't same input capacitance as the one phono I tested on. SUT under damping (47K) may well be needed since it combined OK with Fran's curve subjectively and maybe explains it wasn't dull with his HF RIAA capacitor value as I was afraid it could.
 
diyAudio Chief Moderator
Joined 2002
Paid Member
I tested 22K after the Partridge 977 & sounds so good with the Puresound P10 MM, after I used 10K, what's better 22K or 18K?

Incidentally there was cart setting afternoon today for a second hand old TT in very good condition (after little care) with the original HMC simplistic that carries the SUTs now, and 18K was very nicely balanced. Could clearly hear bias settings through test disc, minute VTF changes, record mat etc. Good tonality on music.
 

Attachments

  • DSCF2853.JPG
    DSCF2853.JPG
    157.2 KB · Views: 373
  • DSCF2859.JPG
    DSCF2859.JPG
    243.5 KB · Views: 370
  • DSCF2860.JPG
    DSCF2860.JPG
    224.3 KB · Views: 370
Felipe: looks like after christmas maybe?


***************************************
I intend doing the resistor on mine later tonight - butI need to have a good detailed listen before I do the swap.

Just in case I'm confused here, or for others coming along after, you do mean changing R1 (47k) for something in the range of 18-22K?

Might be worth updating the sch or maybe the parts list to show the change....


Fran
 
diyAudio Chief Moderator
Joined 2002
Paid Member
It did bring up HF for 47K VS 18-22K on scope and FFT, but it could be damping better on Itch's input and makes small difference. Still a little more cleaner is enough to keep it there. Is the new resistor quality the same as when it was 47K terminated?