It’s time to scratch that itch again and I’m trying to figure out which low-gain preamp to build for a stereo F6. I plan to convert it to balanced operation later, so a pre that can work with both would be great. A pre that could be converted to higher gain later would also be good, because I’d also like to experiment with the F4 and BA-3 follower stage, but I’m not opposed to swapping out for a completely different pre if necessary.
Here's the list of candidates I've cobbled together from what I've seen discussed in previous forum posts:
And here’s where I’m stuck:
I'm also open to recommendations on the attenuator and switch choice. I've looked at the goldpoint switched attenuators and the AMB one (briefly) and I don't know enough about how they work to know if one is inherently better than the other. I’m going to put the attenuator and switch in a passive chassis separate from any active gain stage.
Here's the list of candidates I've cobbled together from what I've seen discussed in previous forum posts:
- BA-3: ~15x gain
- BA-1: ~6x gain, configurable up to ~20x, down to ??
- Korg NuTube [+B1]: ~6x gain, and what’s the story around phase inversion?
- Bottlehead BeePre: $too $expensive
- Bottlehead S.E.X.: ~6x gain
- WHAMMY: ~5x gain, gain can be lowered, relies on op-amps (not a bad thing, just different), wants batteries
- Balanced Zen line stage: 10-20dB gain
- AMB alpha 24: configurable gain, but I don’t know enough about it and I’m not sure how much experience people have with it
- ImPasse: high gain at 20x
- ZM's Iron Pumpkin or Iron Pre: I don't know enough about these
- Starving Student: 7x-10x gain
- ACP+: 2.8x gain
- F3 as pre: 4.3x gain
And here’s where I’m stuck:
- Are there any obvious contenders I’ve overlooked?
- Are any of these contenders currently constrained due to unobtainium parts? (e.g. hard to find tubes, jfets, etc)
- Do any of these have known drawbacks to their sound?
- Which ones have PCBs in a store I can buy or known-good gerbers I can send off for one-off PCB ordering?
- For those that run on batteries, is there a known good solution for running them off wall power with a regulated power supply instead?
- Are there subsets of pre-amps that share a common power supply? If I can build a chassis and power supply and swap out boards like in other FW amp designs, that would be ideal for trying different flavors.
- Are there any that wouldn’t pair well with an F6?
I'm also open to recommendations on the attenuator and switch choice. I've looked at the goldpoint switched attenuators and the AMB one (briefly) and I don't know enough about how they work to know if one is inherently better than the other. I’m going to put the attenuator and switch in a passive chassis separate from any active gain stage.
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CD level sources directly into the BA-3 preamp could cause clipping at its output.
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Sure. I was going to avoid unity-gain buffers. Check these back-of-the-napkin working-backwards calculations, though:
The F6 will push 25W/8R or 50W/4R (and I'm using 4R speakers) which translates to ~14V. It has 14dB of gain which is ~5x. From what I understand, that means Vin of 2.828V is enough to clip the F6's output. I don't listen nearly that loud.
I'm looking at DACs that output 1V or 2V max and I suspect I could get away with a purely passive solution except for source material that happens to be super quiet. I've seen numbers like -20dB thrown around in reference to the levels that CDs are sometimes recorded at. To amp that up 10x to bring it back to 0dB would take the F6's 5x and a pre-amp's 2x.
So basically I'm looking at any pre with a gain between 2x and 6x as highly likely to work for me, and with some DACs I could probably get away with a purely passive solution (especially balanced DACs that output 4V). 6x gives me quite a lot of headroom. A BA-3 solution, with it's 30x gain, means I'd be attenuating the signal down by quite a lot, bringing it much closer to the noise floor than is desirable or necessary.
As always, if my formulas or understanding is wrong, please correct me.
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I have used my F6 with just a passive 10k stepped attenuator for quite a while.
No problems, sound was very good, but,
even with my Jericho horns volume needed to be turned up nearly full. This was for folk and acoustic music.
Recently I added an F3 pre, this gives much more headroom to alter the volume, also I feel that the sound is slightly warmer and richer. (This may just be playing a bit louder.)
So setup now is,
Marantz network server -- 10k stepped attenuator -- F3pre -- F6 -- speakers.
No problems, sound was very good, but,
even with my Jericho horns volume needed to be turned up nearly full. This was for folk and acoustic music.
Recently I added an F3 pre, this gives much more headroom to alter the volume, also I feel that the sound is slightly warmer and richer. (This may just be playing a bit louder.)
So setup now is,
Marantz network server -- 10k stepped attenuator -- F3pre -- F6 -- speakers.
That's fascinating because the Jericho sensitivities are so high. What's the Vout on your Marantz?
Im using BA3 Pre with my F6 on a pair of pensils with alpair 10p (so 89-ish db sensitivity, 6 ohm). This is a small living room, less than 30 sq m probably (maybe even less than 20). I wouldn't say i listen very loud but my volume control tends to hover between 10 and 13 o'clock most of the time. I rarely crank it beyond 14 (that starts to be really loud). No noise felt or heard from the pre (though there is a slight bit of hum from the F6 itself with ears right against driver). Anecdotal, but hopefully useful nonetheless.
That's very useful, because I was thinking of the BA-3 pre for the longest time, and those specs match another set of speakers I have. That gives me a very good reference point.
What's your source's Vout?
I'm curious about this one as well. The PDF says it's optimized for 32R loads for both headphones and RCA and I'm curious about the implications that has for using it to drive the F6 which has an input impedance of 100k. I don't know enough about this stuff yet to know things like that off the top of my head. My intuition says the 100k load would be easier on the ACP and that it should have no problem, but I don't know if there are other factors I'm unaware of.
Separately, my understanding of the B1 is that it converts one downstream input impedance into another, and that could be useful here to down-rate the 100k to 32 to keep the ACP happy. Would a buffer be helpful, or even necessary?
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