Inspired by the Fokin, LuFo, and SuSyLu designs I want to build a symmetric follower amp based on a balanced choke for some SuSy magic. Also, I'd like to have some tube glow for voltage gain in front. See attached circuit (very much simplified).
The LuFo/SuSyLu is a finished design of a (balanced) follower stage using LU1014 JFETs with cascodes to increase the voltage capability of the LU. However, one could also use some power SITs (like a THF51 or similar) to build the whole thing with less parts. The SITs may also perform differently in this application, but I don't really know.
I have most of the parts (THF51 SITs, LU1014s, various tubes, heatsinks) to start messing around with this, but I don't know if I should go with the SITs or the LUs. Yes, I could try both and then compare the simple THF51 vs the cascode LU1014, but I'd like to hear your thoughts on the SIT vs LU question. Which way would you go, and why?
The LuFo/SuSyLu is a finished design of a (balanced) follower stage using LU1014 JFETs with cascodes to increase the voltage capability of the LU. However, one could also use some power SITs (like a THF51 or similar) to build the whole thing with less parts. The SITs may also perform differently in this application, but I don't really know.
I have most of the parts (THF51 SITs, LU1014s, various tubes, heatsinks) to start messing around with this, but I don't know if I should go with the SITs or the LUs. Yes, I could try both and then compare the simple THF51 vs the cascode LU1014, but I'd like to hear your thoughts on the SIT vs LU question. Which way would you go, and why?
Attachments
If you use the LU’s, they will need a cascode in order to handle voltages larger than 20v (that’s max rating). So some N channel MOSFETs above them like LuFo/SuSyLu.
In practice, it’s very hard to get perfect matching of all things in both legs of the balanced amp. So the dominant third may not be an issue. That was predicted to be the case on SuSyLu, but in practice, THD was a bit higher and amp was dominant 2nd harmonic even though running in balanced mode.
In practice, it’s very hard to get perfect matching of all things in both legs of the balanced amp. So the dominant third may not be an issue. That was predicted to be the case on SuSyLu, but in practice, THD was a bit higher and amp was dominant 2nd harmonic even though running in balanced mode.
Yep. I guess that's what I wanted to ask.
I believe these two questions are related. In theory, the two symmetrical halves will cancel the even order harmonics. In practice, they will still tend to do this, but not perfectly. Despite it's circuit symmetry, the SuSyLu is still 2nd order dominant.
That said, I am not in any of the 2nd or 3rd order camps. I have tried both, heard the differences, and can't say that I generally prefer one over the other.
What I do like is low distortion, and no cap at the speaker output.
Yes, sure, no DC blocking cap at the output means the output DC must be low and stable. The SuSyLu has shown that this can be done in a good way. In addition, I am running all my amps with a DC protection module anyway (I almost burnt down things with a misbehaving F5 a while ago, I am not going there anymore). But that's not the question here.
What are your thoughts on SIT vs. LU+cascode? Which way would you go, and why?
What are your thoughts on SIT vs. LU+cascode? Which way would you go, and why?
I have more than enough amps. Making one more is overkill (but fun), making two is just just wrong. That's why I am thinking about going with SITs or LUs, not both.
So far there is only one point on the pro/contra list: LU wins on low distortion (due to higher transconductance).
Any other thoughts on SIT vs. LU?
So far there is only one point on the pro/contra list: LU wins on low distortion (due to higher transconductance).
Any other thoughts on SIT vs. LU?
