Apologies if already mentioned....
First Watt SIT-3 is reviewed in Stereophile Jan 22, 2019
First Watt SIT-3 power amplifier | Stereophile.com
"The SIT-3 is the third, and possibly the last, in Nelson Pass's series of First Watt designs using a run of special transistors that Pass personally commissioned from SemiSouth Laboratories, now defunct."
First Watt SIT-3 is reviewed in Stereophile Jan 22, 2019
First Watt SIT-3 power amplifier | Stereophile.com
"The SIT-3 is the third, and possibly the last, in Nelson Pass's series of First Watt designs using a run of special transistors that Pass personally commissioned from SemiSouth Laboratories, now defunct."
Dunno, but using a directly heated triode preamp to any of the VFET variants seems to add more magic, for whatever reason, so you could consider it an opportunity.
The lower the output power, the less gain is required for full output from a typical source.
The rated 18W is 12Vrms into 8R, so the 12dB gain (about 4x) gives full output for 3Vrms input.
All the voltage gain in the amplifier is provided by an audio line transformer connected
as a 1:4 step-up (4x = +12dB). That's all the voltage gain available without adding more
active circuitry, which would have been in conflict with the design goals for the amplifier.
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XSIT
Dear Guérilla,
I don’t own an SIT-3, it would be great if we may have an answer from Mr Pass.
I guess that’s all we have had, due to his intellectual property and company’s policy of how much information that can be revealed.
Fortunately from his kindness, we are able to have a circuit diagram of the DEFISIT and photos from many reviewers.
Few of us having successfully built it and enjoying the heavenly sonic signature of this unique topology.
Based on my “cloning” I may have an answer that would satisfy your question, but it is related only to the THF-51S.
The little gain is due to the pinch off voltages of the SIT’s gate and the P channel mosfet complementary. Once the gate voltage reach the pinch off threshold, the amp ceases functioning due to Ids is dropping to zero.
The little gain becomes more and more tiny due to non linear response near the pinch off area, in my case another half a volt from pinch off to an acceptable distortion level.
In conclusion, my SIT-3’s clone can only produced an excellent linearity within 1volt of gate swinging from a selected bias voltage.
Hope that would help, and please bear in mind, mine is not an SIT-3.
Cheers.
.
Dear Guérilla,
I don’t own an SIT-3, it would be great if we may have an answer from Mr Pass.
I guess that’s all we have had, due to his intellectual property and company’s policy of how much information that can be revealed.
Fortunately from his kindness, we are able to have a circuit diagram of the DEFISIT and photos from many reviewers.
Few of us having successfully built it and enjoying the heavenly sonic signature of this unique topology.
Based on my “cloning” I may have an answer that would satisfy your question, but it is related only to the THF-51S.
The little gain is due to the pinch off voltages of the SIT’s gate and the P channel mosfet complementary. Once the gate voltage reach the pinch off threshold, the amp ceases functioning due to Ids is dropping to zero.
The little gain becomes more and more tiny due to non linear response near the pinch off area, in my case another half a volt from pinch off to an acceptable distortion level.
In conclusion, my SIT-3’s clone can only produced an excellent linearity within 1volt of gate swinging from a selected bias voltage.
Hope that would help, and please bear in mind, mine is not an SIT-3.
Cheers.
.
Attachments
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As pointed out earlier, the gain figure came from the transformer, and I
didn't feel like using an active voltage gain stage.
3 volt rms into the high impedance input will clip the amp at 18 watts into 8.
Of course the output stage is simply a follower - you can drive it with some other
circuit without problems.
didn't feel like using an active voltage gain stage.
3 volt rms into the high impedance input will clip the amp at 18 watts into 8.
Of course the output stage is simply a follower - you can drive it with some other
circuit without problems.
The group that even will get to hear this amp is rarified.
It would be nice to hear one to round out my VFET experience. I have a Sony TAN 8550, Yamaha B1, Sony TA 4650, and Pass Clone VFET. I have heard SIT models at show, and the enormous VFET beast. I am listening to Pass Clone VFET with Stax headphones and transformer coupler as I write this, very special sound.
I was hoping Sean at Zu would have one in operation at the last CAS 2018 but SIT3 was on static display only.
It would be nice to hear one to round out my VFET experience. I have a Sony TAN 8550, Yamaha B1, Sony TA 4650, and Pass Clone VFET. I have heard SIT models at show, and the enormous VFET beast. I am listening to Pass Clone VFET with Stax headphones and transformer coupler as I write this, very special sound.
I was hoping Sean at Zu would have one in operation at the last CAS 2018 but SIT3 was on static display only.
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