I’ve been devouring every THF-51S thread I can find—µ-Follower, DEF, Sissy-style buffers, you name it. Rather than ask for drawings, I’d love to pick your brains on how you coaxed the best out of this beast:
- Bias voicing – I see followers running anywhere from 1.6 A @ 28 V (BAF2015 rebuilds) up to 3 A @ 62–70 V in the 45 W/70 W µ-Follower monsters. How did you settle on your Iq/Vds pair, and what happened to the 2nd-harmonic line as you crept hotter or cooler?
- Thermal chores – 200 W dissipation per monoblock turns these into winter space-heaters. Any success stories with quiet fans or clever chimney cases that didn’t blunt the sonics?
- Front-end flavor – Pure iron for voltage gain vs. the 12-JFET buffer route: what differences did you hear in mid-band texture or sound-stage “air”?
For low power driving efficient loads, you can make Common Source operation do the job, but when you get above that you will likely want to use them as Common Drain followers, and for that you can get good results class A without feedback.
I personally prefer a neutral front end for SITs, letting them dominate the "personality" and
mostly you want to pay attention to the load line so as to balance the character you want.
One caution, the high wattage ratings of some of these parts demands some serious cooling to remain reliable, so you want to be conservative in your expectations.
I personally prefer a neutral front end for SITs, letting them dominate the "personality" and
mostly you want to pay attention to the load line so as to balance the character you want.
One caution, the high wattage ratings of some of these parts demands some serious cooling to remain reliable, so you want to be conservative in your expectations.
I'll be back in few years, when I'm (hopefully) able to grasp some understanding of written bellow ......
Reading your note on the SIT-4 again, Tokin in Common-Source with only a 2SK170 in front—I’m reminded how that topology seems to maximize the ‘triode-like’ bloom at modest bias (≈1 A, 40 V). The SIT-5 drives the same THF-51S in Common-Drain and lets a beefy partner share the current, trading a touch of warmth for lower Rout and wider bandwidth.
Kind of like: if THF-51S was a person. The SIT-4 let it do the talking (but it might ramble off-topic a bit but we like it). The SIT-5 pairs it with a co-host so that the whole speech is a bit more structured.
Now... I’m wondering about a compromise: a follower load-line around 2A @ 50 V—below SIT-5’s current but above SIT-4’s—paired with a single J-FET buffer and modest step-up iron.
Would the plots suggest 2ᵑᵈ/3ʳᵈ balance stays constant while absolute THD slides down 🤔🤔 keeping dissipation under ~90 W per channel. Would that still preserve the “character” you aim for when you talk about working the device along the right part of the curve?
Maybe I am too greedy.
