[Xenophon]

  Not from me but from Nelson:

F4 Power Buffer

Like everything else from First Watt the F4 is a unique amplifier. It has no feedback and no voltage gain, only current gain. This design is sometimes called an “impedance converting amplifier”, but more commonly referred to as a buffer – in this case, a power buffer.

The F4 is Class A impedance converting amplifier, having no voltage gain or feedback. Its input impedance is 47,000 ohms, and its output impedance is about 0.2 ohms. It is suitable for driving a high sensitivity loudspeaker with the output voltage of a preamp or other line-level audio source. It is also useful with a less sensitive loudspeaker in a bi-amped configuration where it takes input from the output of a conventional amplifier.

As a stereo amplifier with single-ended inputs and outputs, it will deliver up to 25 watts into 8 ohms with a damping factor of 40. It will do 50 watts into 4 ohms, and as a mono-block amplifier with parallel inputs and outputs, it will do about 100 watts into 2 ohms. As a mono-block amplifier with balanced inputs and outputs the power output rating is 100 watts into 8 ohms at 1%.

The amplifier operates Class A to 25 watts (50 peak), and the distortion is 2^nd and 3^rd harmonic in character, rising or declining in proportion to the output power. The amplifier has a direct coupled input and output, with a -0.5 dB rolloff around .1 Hz and 200 KHz. It does a clean square wave at 100 Khz.

The simplified circuit of the F4:

The combination of a simple Class A circuit operated without feedback and the good objective performance gives us a superb sounding amplifier. The low distortion, bandwidth extension, and high damping results in midrange clarity, treble detail, and control on the bottom end. While these are available from most good solid state amplifiers, the F4 also brings depth, imaging, midrange warmth and top-end sweetness.

Overall, it is one of the best sounding amplifiers, and if you can live with unity voltage gain in your amplifier, it is possibly your best choice.

Yeah, I was also thinking about this.  Basically if I understand correctly the F4 is a pass-through window for the pre-amp so any gain would have to come from there.
--> If the voltage swing theory is correct then it might not be a good idea for the HE-6.
 
Sounds like my F3 might not be the optimal pairing then...hmmm...will sleep on it.  Guess I could still pull the plug on the deal without too much costs.  Hard to judge what the impact of the voltage limitation is in real terms, especially because I don't listen to heavy bass slamming music.  But the description about ultimate clarity and detail is alluring, as is its harmonics profile which gives a teeny bit of sweetness according to Nelson Pass.
 
If anyone has any additional insights, do share 'm.

 

[ALRAINBOW]

I did a post a few pages back I would read it
My observations was not taken very well
But it is the truth. It can be summed up as
Being thin. It has extreme detail but lacks proper impact
And what I felt was the worst was not that it clipped
It to me was the tonal balance being bright
It makes the classical horns off there sound
The whole presentation seems much closer and had very good staging
But as I said the insrtumints are just wronge in sound.
Al

 

[potterma]

   
What's the math on on both a single F4 (SE) and dual F4s in balanced mode?

Well, the maximum unclipped output voltage is +/- 20 V, which will give you 8 W into a 50 ohm load.  The nominal power rating is 25 W into an 8 ohm load.  The F4 manual describes bridged mono blocks that will do 100 W into 2 ohms.  Bridging gets you more current for lower impedance loads, but won't increase the maximum output voltage.
 
Hope that was helpful.

 

[potterma]

  Yeah, I was also thinking about this.  Basically if I understand correctly the F4 is a pass-through window for the pre-amp so any gain would have to come from there.
--> If the voltage swing theory is correct then it might not be a good idea for the HE-6.
 
Sounds like my F3 might not be the optimal pairing then...hmmm...will sleep on it.  Guess I could still pull the plug on the deal without too much costs.  Hard to judge what the impact of the voltage limitation is in real terms, especially because I don't listen to heavy bass slamming music.  But the description about ultimate clarity and detail is alluring, as is its harmonics profile which gives a teeny bit of sweetness according to Nelson Pass.
 
If anyone has any additional insights, do share 'm.

Its not a theory.  Its Ohms Law.  Works every time....

 

[Xenophon]

Another probably stupid question:  the F3 provides +12.5 dB gain.  Is that an absolute limitation for the output or is this the extra gain provided by the amp only.  What I mean is this:  if you feed it via a DAC that provides -say- 12 dB gain, does the gain provided by the F3 come on top or will the output, whatever you do, be limited to +12.5 dB vs baseline.  I hope this makes some sense...  

 

[potterma]

  Another probably stupid question:  the F3 provides +12.5 dB gain.  Is that an absolute limitation for the output or is this the extra gain provided by the amp only.  What I mean is this:  if you feed it via a DAC that provides -say- 12 dB gain, does the gain provided by the F3 come on top or will the output, whatever you do, be limited to +12.5 dB vs baseline.  I hope this makes some sense...  

Whatever the input signal is, the F3 will amplify it by 12.5 dB, up until it either runs out of current capability (for low impedance loads) or it will run into its voltage limit of +/-13 V.  That is to say, you can't feed it a large input signal and get it to put out higher voltage.  You'll just drive it into clipping...
 

Thanks. This is really helpful and addresses some of the confusion that I had. Can I ask about the j2? The manual does not have the maximum output voltage in the way that it is listed for the. F amps. Thanks

Sorry, I don't know about the J2.  If someone with a J2 can do some measurements, we could find out.

 

[musicinmymind]

   
As long as you watch the volume knob you should be fine, but your Krell might have some hiss like mine so I recommend getting some resistors to attenuate that. Because my amp has a lot of gain, I have a lot of attenuation using 100ohm and 3ohm resistors (-32dB). If you buy wirewound, you will not get coloration or reduction in SQ unlike the HE-adapter.
 
BTG does indeed make some nice stuff. The queue might be a bit long these days. If you have absolutely no need for the TRS adapter that Hifiman provides, you good cut that up to use an adapter for the time being.

 
Wow...100ohm, that should lot of resistance...are you using 100ohm in series and 3ohm in parallel?

 

[ALRAINBOW]

I do not know the specs but the sound was about the same
As the F3. But the F3 was sounding a bit better

Al

 

[Xenophon]

@potterma: ok, thanks for that clarification. Crap, why is this so bloody hard to get right!? Will confer with Renohifi but the way things are starting to look it might be better if I just kill the deal and take my losses. Little point in spending upwarss of 3k usd with duties, taxes and transport in if one knows beforehand that the result won't be optimal.

 

[preproman]

  Well, the maximum unclipped output voltage is +/- 20 V, which will give you 8 W into a 50 ohm load.  The nominal power rating is 25 W into an 8 ohm load.  The F4 manual describes bridged mono blocks that will do 100 W into 2 ohms.  Bridging gets you more current for lower impedance loads, but won't increase the maximum output voltage.
 
Hope that was helpful.

So would a pair of F4s have any advantage over the F1/F1J besides more watts?

 

[brunk]

  @brunk  and others who care:  the difference is the amp.  F2, F3 and some others do not have the voltage output swing that the F5 and the F1/F1J do.  Simple math, really.  Its a lower voltage limit on some of the First Watt amps.  They've the current ability, but not the voltage swing...  With the First Watt amps, its about voltage swing, not rated power into 8 ohms!!!

 
Lol I never looked at the voltage swing closely, but made the assumption it was similar to other First Watt offerings. It clearly is not. Thanks for clarifying that.

  Yeah, I was also thinking about this.  Basically if I understand correctly the F4 is a pass-through window for the pre-amp so any gain would have to come from there.
--> If the voltage swing theory is correct then it might not be a good idea for the HE-6.
 
Sounds like my F3 might not be the optimal pairing then...hmmm...will sleep on it.  Guess I could still pull the plug on the deal without too much costs.  Hard to judge what the impact of the voltage limitation is in real terms, especially because I don't listen to heavy bass slamming music.  But the description about ultimate clarity and detail is alluring, as is its harmonics profile which gives a teeny bit of sweetness according to Nelson Pass.
 
If anyone has any additional insights, do share 'm.

The F4 is a current source amp, meaning you need a voltage buffer like a First Watt B1, or a serious preamp to give what it needs, the yin to the yang If you want a top quality buffer on the cheap, look into a Salas Hot Rodded DCB1 on DiyAudio. Incredible price/performance.

 

[potterma]

  So would a pair of F4s have any advantage over the F1/F1J besides more watts?

For the F4(s) to be viable, you need a pre-amp with substantial gain.  The F4 gives you the ability to drive low impedance loads with its higher current output.  For the HE-6, it may not be the best option, unless you have a way to amplify the signal first.

 

[Steve Eddy]

@Steve Eddy


Yes, but one thing you don't realize, Steve, is that the F5 will push 8 Watts into 50 ohms.

But the person I was replying to said specifically "60 ohms." So that's the number I was working off in my response.

se

 

[brunk]

  For the F4(s) to be viable, you need a pre-amp with substantial gain.  The F4 gives you the ability to drive low impedance loads with its higher current output.  For the HE-6, it may not be the best option, unless you have a way to amplify the signal first.

Master-1 preamp can do 19v from XLR That's what I'll be trying out when I get around to building the F4. It won't be balanced amplification, but who cares if it sounds awesome, which i hope it does.

 

[preproman]

  Master-1 preamp can do 19v from XLR That's what I'll be trying out when I get around to building the F4. It won't be balanced amplification, but who cares if it sounds awesome, which i hope it does.

 
I was thinking the Master 1 as well.  I'll be looking forward to your impressions..