I have what is probably a dumb question, but I'm stumped, so I'll ask anyway: Does the LTA MZ 2 preamp have reasonably enough voltage swing to make the F4 sound good on something like Hornshoppe horns (nominally 94db/1m/1w)? LTA says the MZ 2 has about 11 dB of gain, depending on impedance (10 db at 4 ohms and 12 at 14 ohms).
That's what I am seeing. With your 94 dB speaker, the loudest it would play would be about 67 dB which isn't very loud in a quiet room. Since that's the loudest part of the music, the softest notes might be 18 dB lower or very hard to hear.
I'm surprised they claim adequate volume for a 95 kB speaker.
Cool design though. Maybe you should take a look at the Impasse pre-amp. It was specifically designed for the F4 with 26 dB gain and can input and output either balanced or single ended. Of course, balanced output takes 2 F4s.
That's a fascinating possibility. I'll definitely keep the Impasse in mind. I'm not all-in on the Hornshoppe horns yet (with the Heil tweeters), as I have a friend who had an F5 driving a pair of Altec 605s that sounded fantastic. I'd like to buy a pair of GPA 604s, but I'd need somebody local to build me a set of 9 cubic foot speaker cabinets. Not too keen on being Mr. General Contractor for that project!
If you are still deciding on speakers, maybe you should build a pair of open baffles using the Scandinavian Audio Labs SAL 08C08 from the diyAudio Store. They seem to be in the neighborhood of the kinds of speakers you are looking at.
Whatever you decide, it's bound to be a fun project.
I had a pair of Wild Burro Betsys a few years ago, so I'm familiar with the OB magic those were able to pull off (even though they aren't the best drivers ever). Again, thanks for the great suggestion!
Just wanted to give a heads up that Hugh Dean and I have developed a nice preamp with 40v p-p output with low distortion and SE Class A harmonic profile. It's been tested in prototype stage already. JPS64 will be releasing Gerbers for easy construction soon. It's a 5 transistor affair, very simple but clean output. Sound is superb and can drive any amp to clip without adding hardly any distortion.
I decided to convert my F4 to the crippled F4 using Zen's schematic and installing a BA-3 pre board inside the case. It has made the F4 more in line with the BA-2 with BA-3 driver and made the amplifier much more versatile. Not hard to do and I have a set of spare j-fets for another project being that I already had the stuffed BA-3 board.