Ok, the Sowters 1990 transformers arrived, replaced the partridge ones at 10X, changed the incoming capacitor and the grounding a bit and got a bit better results on fft. Gain now is 62dB, fine for my AT-ART9.
Resolution has improved and is even quieter. Even happier with the sound!
Resolution has improved and is even quieter. Even happier with the sound!
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Very nice build. Good THD at such strong 1.25V RMS output. Except the noise floor level isn't optimized when without input signal padding. Because one mV region synthesized signal is generally noisy. Thus the THD+N figure shown is invalid. But OK for straight out interface connection. Possibly your real THD+N is around 0.1% unweighted. Published phono specs are many times A weighted though.
Hello everyone, I'm in the process of ordering all components for the Itch. PCBs and 6N2PEV are underway. I'm not confortable about the tube heat within a closed case, like almost everyone seems to build. My plan is to have the passive components on the down side, and the tubes going "through the roof" of the preamp box. Of course not only because of the heat, but as well for its look.
I might have a few spare PCB as it seems the minimal JLPCB order was 5, so if you are located in Switzerland and want a board set let me know by pm. I have boatloads of 6SN7 as well, more that I will ever use, just let me know. I'm not in a hurry, this is only a part order phase, the build is likely to happen later this year - or even next winter. Meanwhile I will continue to pre-amp my Technics 1210 through a cheap Pro-Ject Phono Box S2. Too much sand in there but does the job until better.
As for the PSU, I will scourge my transformer box, but most likely I will order a custom-made Toroidy with all needed windings. Unless someone here think it's a bad idea
So far thanks to all the authors, especially Salas and Jphabc54 for their development work and all others for their contributions !
I might have a few spare PCB as it seems the minimal JLPCB order was 5, so if you are located in Switzerland and want a board set let me know by pm. I have boatloads of 6SN7 as well, more that I will ever use, just let me know. I'm not in a hurry, this is only a part order phase, the build is likely to happen later this year - or even next winter. Meanwhile I will continue to pre-amp my Technics 1210 through a cheap Pro-Ject Phono Box S2. Too much sand in there but does the job until better.
As for the PSU, I will scourge my transformer box, but most likely I will order a custom-made Toroidy with all needed windings. Unless someone here think it's a bad idea
So far thanks to all the authors, especially Salas and Jphabc54 for their development work and all others for their contributions !
Any SUT designed for 47k output side loading will work fine with the Valve Itch. The T-20 is such a unit.
But keep in mind its a 32dB gain (times 40) SUT optimized for very low output MC cartridges. 0.1-0.15mV. Those usually have <5Ω internal coils resistance spec too. T-20's usual pair was the original Ortofon MC20 cart. If you will not aim at such a cartridges category you will not exploit the T-20 correctly.
But keep in mind its a 32dB gain (times 40) SUT optimized for very low output MC cartridges. 0.1-0.15mV. Those usually have <5Ω internal coils resistance spec too. T-20's usual pair was the original Ortofon MC20 cart. If you will not aim at such a cartridges category you will not exploit the T-20 correctly.
It also abruptly lowers the reflected load to the cart. Because input Ω is 47k/primary to secondary x ratio squared.
In my experience, 10x/20dB SUT is good for 0.4-0.5mV class carts, 20x/26dB SUT for 0.2-0.25mV class carts.
40x/32dB SUT is good for 0.1-0.15mV low impedance exotic class.
In my experience, 10x/20dB SUT is good for 0.4-0.5mV class carts, 20x/26dB SUT for 0.2-0.25mV class carts.
40x/32dB SUT is good for 0.1-0.15mV low impedance exotic class.
I used Sowter recommended values 330pF and 10kohm after doing some measuring. Also I use a 470 ohm loading resistor on the input of the transformer, which I found I liked more. Total loading as seeing from the cart - AT-ART9 - is about 230 ohm.
Transformer is wired as 10X, so with 47k input, initial loading was 470 ohm.
Transformer is wired as 10X, so with 47k input, initial loading was 470 ohm.
