Which preamp for Amp Camp?

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#181
xrk971 said:
That is clever and elegant but I wonder what the effects of the feedback of the audio output current is going to be on the filament current, which controls the tube’s supply of vacuum path electrons? Perhaps the thermal time constant is too slow to matter.
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Don't worry X, Pete is a seasoned engineer, his designs are mature prior to release. The 18FX6 heater works with 18V AC and DC which means 25.4V peak, well within spec on reasonable use of the SSII. 🙂
 
#185
xrk971 said:
That is clever and elegant but I wonder what the effects of the feedback of the audio output current is going to be on the filament current, which controls the tube’s supply of vacuum path electrons? Perhaps the thermal time constant is too slow to matter.
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Much too slow to matter indeed. The trick is not new, it was already used by the Melos SHA-1 in the early 1990's (it was a more complicated amp, with a fet+darlington output and a dc servo).
 
#190
xrk971 said:
That would have been the Pocket Class A headphone amp that Twocents brought over. That HPA/preamp is a SE Class A MOSFET source follower with a BF862 n channel JFET input stage with local feedback. It produces a lovely sound that is H2 dominant and descending higher orders of harmonic distortion as is typical of SE Class A. THD is in range 0.05% driving loads as low as 250ohms.

As I have not been able to find a schematic of the SSII yet, but based on 6L6’s statement that 1-2% is wrong, I will retract my estimation until I can see the circuit. If it has a CCS, probably will be much lower in similar 0.05% THD and as low as 0.002%. But depends on how the tube is handled as that is the H2 and THD generator.

But it is true that adding a sweet sounding pre with higher H2 distortion to a SS amp can make it sound very sweet.
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I just saw the posted specs:
Starving Student II Headphone Amplifier – diyAudio Store

It seems my estimates were not too far off as specified THD is 0.47% to 1.8% at 150ohm load depending on CCS or resistor plate load. Not 0.002% to 0.05% though.

All H2 dominant though so should sound very sweet. Typical reasons why tubes sound great.
 
#191
xrk971 said:
I just saw the posted specs:
Starving Student II Headphone Amplifier – diyAudio Store

It seems my estimates were not too far off as specified THD is 0.47% to 1.8% at 150ohm load depending on CCS or resistor plate load. Not 0.002% to 0.05% though.

All H2 dominant though so should sound very sweet. Typical reasons why tubes sound great.
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I have the original starving student works great as a preamp I also have Pete Millit’s NuTube headphone amp used as a preamp 👍
 
#195
xrk971 said:
It seems my estimates were not too far off as specified THD is 0.47% to 1.8% at 150ohm load depending on CCS or resistor plate load. Not 0.002% to 0.05% though.
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If you want lower distortion, this mix of Millet and SOHA II will do, while preserving the dominant H2. It wouldn't be much more expensive to build and use about the same power. It might be a bit trickier to adjust.
 

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#196
Realizing that this is an old thread, but ...

What would be the difference between B1 Buffer and Schitt SYS being used as volume control for ACA (other than the joy of building it myself)?

My input would be ipod Classic, primarily, but I may end up adding vinyl to this setup and then I'd need an input switch.
 
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