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In Reply to: RE: Is that what you want? posted by Triode_Kingdom on February 07, 2016 at 15:46:54
Hello TK,Whit the help of the blogspot poster (alexander IRC) and yourself, I went the HFAC route and don't look back since then.
I agree with you, HFAC benefits are say, "less apparent" with drivers, but hey, it's an easy all-around heater solution once you know the basics of those cheap transformers' "hacking".
I am about to try HFAC for heaters on my RIAA preamp. More on this later on!
It's the input of dedicated men like you and Alexander that keep the tube DIY scene alive. Thanks again for your implication.
Cheers,
Joris
Edits: 02/07/16Follow Ups:
Thanks for your comments. The reason I don't generally recommend ultrasonic heating for drivers and preamps has to do with bleed-through. It's a concern that the HF energy might enter the signal path and be amplified in those low level stages. Even if overload isn't an issue, the possibility of IM or spurious responses is much greater than when this energy is used only to heat the output stage.
Keep us posted on your results with the phono preamp. Should be interesting!
--------------------------
Buy Chinese. Bury freedom.
Hello,
For what it's worth, so far I have breadboarded an input stage using HFAC on an indirectly heated pentode in triode mode. From our previous posts I was aware of the risk for oscillation from spurious injection of HF so I scoped it and there was none. That is why I feel confident applying this to my RIAA preamp.
Regards,
Joris
Just a further precision on what I meant by "unsuitability" through my one and only experience. I was more referring to the sound they produce since in my test there was no other disadvantadge to HFAC. I seem to recall you tried it once as input? I might be mistaken as well - Sorry about that. It's been a while since I messed with these.
However I do I recall you pointing spurious HF emissions to be watched, because that made me push the analysis on the scope more precisely focused on the input stage and overall stability. I must add that the power stage (2 stage amp) was a russian DHT tetrode trioded, also fed from an HFAC module.
The sound is almost unreal because the background just disappears - One must hear it to believe it!
When used on the input stage on breadboard, I found that *somehow* soundwise I still preferred the raw AC approach when hum is no issue as with IDHTs, but not by much.
RIAA preamp is one place where the benefits of HFAC are prominent and justify the *very light* sound trade-off wrt to direct AC, in fact it is not much to pay for silent black background, LF AC almost inevitably causing hum on the high-gain input stage of the preamp.
Just some random thoughts of the moment.
Curious to read your thoughts about the sound as well
Best regards,
Joris
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