Best Firstwatt amp for backloaded horns? - diyAudio
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Old 20th July 2010, 06:53 PM   #1
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Default Best Firstwatt amp for backloaded horns?

I know this is a very subjective question that I need to answer by listening to them myself and seeing what I like, but I only have enough money to build one firstwatt amp and I don't know anybody else who has one that I can listen to. I've read reviews of all of these amps and cruised the forums, but I still can't narrow it down.

This is my current setup: Speakers are home made Austin A166 back loaded horns with Fostex FE166e drivers. I put phase plugs and coated them with modpodge like everybody suggests. They're very efficient and can fill the room with a watt or so per channel. For the preamp I've got a Firstwatt B1 as well as a Transcendent Audio Grounded Grid preamp I built. The source is a Rega P3 turntable, Denon DL-103 needle, and a Cambridge Audio 640p phono stage.

My current amplifier is a TubelabSE, a 45 based SET. This is the only "high end" amplifier I've ever owned (previously used old Marantz integrated and a T-amp). I'm very happy with the Tubelab, but naturally I'm curious about hearing what the a high end solid state amp has to offer.

I like the silky smooth, natural sound of the Tubelab, but I don't want a solid state amp that just simulates the tube sound. I just want the Firstwatt that's best suited for my horns, and to try something different.

Here's what my research has come up with so far:

F1j - Pros: Cool Transconductance amplifier, ideal for fullrangers without crossovers. Has an "analytical" sound that should offer a different flavor than my 45 SET.
Cons: push pull rather than single ended. I can't use it with a conventional speaker if I choose to in the future.

F2j: Pros: Same as the F1j except single ended with a more tubey flavor.
Cons: Might sound too similar to what I already have, both F1 and F2 seem to have been superceded by newer F series amps, can't find anybody saying these sound better than an F3 or F4. Can't use with conventional speaker if I don't like it with my horns.

F3:Pros: Reviews say it sounds alot like a 45 SET. Really low distortion. I can use it with "normal" speakers if I don't like it with my horns.
Cons: I don't want something that sounds identical to the 45 SET I already have. Uses negative feedback so I'm afraid it'll overdamp my fullrange drivers.

F4: Pros: Current buffer. Versatile, I can use it as a proper amp, or I can use it with my tube amp to drive a subwoofer. Or I can use it as a headphone amp.
Cons: Seems to reflect the quality of the preamp hooked up to it, so I don't know how it'll sound with my Grounded Grid pre. Might have to build an Aikido or something.

F5: Honestly this one doesn't seem like a good fit for high efficiency horns.

J2: Pros: 6Moons' review makes it sound like this amp supercedes the F3.
Cons: Have to wait until the schematics are published.

With all that in mind, I've narrowed my choices down to the F2, the F3, the F4 or the J2. The J2 and F4 are in the lead now, but the F4 only because I can easily repurpose it if I don't like it as an Amp. Does anybody have any experience with these amps and backloaded horn speakers that might point me in the right direction?
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Old 20th July 2010, 06:59 PM   #2
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Probably of more impotance is the driver in the back loaded horn. The Fostex likes to have a relatively high output impedance. F1J is probably too high, F2J is probably more appropriate.

But almost any of them will be good...

All my Fs are still at the parts gathering stage (F1, F2, F4)

community sites,, ........ commercial site planet10-HiFi
p10-hifi forum here at diyA
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Old 20th July 2010, 07:36 PM   #3
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The F2 was my first impulse too, in fact I already have the PCB's for it. The more research I did, though, the less clear my decision became.
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Old 20th July 2010, 09:21 PM   #4
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F3 - cheaper than J2 or M2 and sounds great with the

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Old 20th July 2010, 11:58 PM   #5
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The F3 does look good...Is it still possible to find LU1014's or do I have other JFET options?
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Old 21st July 2010, 12:32 AM   #6
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Deep Surplus sells them. Just use the search tool here.
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Old 21st July 2010, 02:47 PM   #7
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Thanks for pointing me in the right direction. One more stupid question: Where are the F3 schematics? I know the service manuals used to be in the downloads section on the old site, but I don't see them anymore. Couldn't retrieve them through either
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Old 21st July 2010, 04:10 PM   #8
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I dont think you could go wrong with the F5. It may not suit your taste.
I enjoy it more than an F3 clone with lowther PM5a's. I agree with Nelson however, I have some small FF85K fostex's that sound golden honey sweet with the F3 clone.

The F2 is an easy project to build point to point. The F5 not much more complicated, and there are still options for PCB's for F5 around.
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Old 21st July 2010, 05:28 PM   #9
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I picked up boards for the f5 about a year ago but I decided against building it. If 1 watts can rupture my eardrums, I doubt I'd ever take the f5 out of first gear. Seems like one more gain stage than I need.

The fostex's I'm using are 6 inches and they're responsible for the entire frequency range (no woofer or tweeter), is there a firstwatt that might squeeze an extra db or two at the top and bottom? Especially the top, the bass is actually pretty good as it is if I use an amp without negative feedback.
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Old 21st July 2010, 08:27 PM   #10
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I've used the F5 with my sachiko horns for a long time and it sounds really good. Its not that high a gain (good for your sensitivity) and is very versatile if you choose other speakers. Another option may be the DeLite?

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