I just got a J2 so cannot comment about the F4, but I am extremely happy with the amp. I am amazed at how nice a 25 wpc amp sounds and the build quality is what you would expect but from Mr. Pass but at a bargain basement price which makes it a low risk experiment, which is watt this hobby is all about and that is what makes it fun and expensive.
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You're not getting deluged with replies, so I thought I'd chime in.
I also own a J2, in my case driving my horns. It's everything Photonman said. If you buy from Mark at Reno Hi-Fi (as I did), as far as I understand things they will let you try out the F4 for a certain period, and return it if you're unhappy.
I wanted to make sure you understood that the F4 is a buffer, and has no voltage gain, so it requires a fairly hefty voltage drive to reach full power.
I owned a Rawson F4 clone. Sound quality was outstanding, but it just didn't work very well with my Zu Druids. With the F4, it's difficult to separate power issues from gain issues. My preamp had 20dB gain and I suspect that wasn't enough, but it's also possible the F4 didn't have enough power. If your preamp's gain is higher than 20 dB and you have relatively high efficiency speakers, it may work great for you.
My pre-amp has 20 db of gain. How much output voltage are you talking about?
What pre amps would have enough gain and output voltage to make an F-4 sound good?
I know about Marks offer at Reno Hi Fi, How ever I would like to here more about the amp from all the experts in this audience before I take the plunge and get one.
I love mine. Ths is the best, and most flexible, First Watt amp I've had--to include the F3, F4, F5, J2, and an Aleph J clone. A hint of warmth and great clarity without becoming brittle.
The often unmentioned thing about this amp is the size of the soundstage. Surprising for a ss amp.
I would not recommend using it with speakers under 90db. If you do, you wll need two for a monoblock configuration. Interestngly, that sound is just a little warmer and more tubelike.
If you have a statement or near statement preamp, tthe F4 is an HUGE sleeper.
The often unmentioned thing about this amp is the size of the soundstage. Surprising for a ss amp.Could you be more specific, Writer2? Is the F4's soundstage truly large in all three dimensions? And does it actually extend beyond the width of the speakers, and even behind them? Even the J2 couldn't do that, said Larry, at least not compared to his parallel fed SET amp
With your really great recordings how precisely are images rendered by the F4? Do vocals and instrumentals each seem to be oriented in a defined space? Lastly, how is the F4 when playing orchestral recordings? Do you hear any distortion towards the louder and/or more complex parts? If the sound does then get a bit harsh or distorted, does it get less so if complex and/or orchestral material doesn't have a lot of deep bass? Or if you happen to be using a pair of powered subs that take over from the F4 at around 70Hz?