F4 Preamp Question

[Metropolis]

Hi All,

I have built an F5, an Aleph J (my favorite) and an F6 (twice).

I love the AJ's smooth, relaxed presentation, but sometimes could use a little more headroom.

So, I was thinking about building an F4 and pairing it with a bottlehead BeePre - a 300B tube based preamp. Apparently it can swing 18v before clipping. This should give me plenty of headroom while also giving the lushness of a 300B (kinda like the AJ)... at least that is my uninformed theory.

Does anyone have any experience with this preamp and the F4? If yes, please let me know what you think.

FYI - I am using Zu Omen mkII speakers with a Peachtree novaPre driving either the F6 or AJ (depending on mood).

Thanks!
Matt

 

[Zen Mod]

"Input impedance is 56K ohms with balance control centered. Output impedance is 500 ohms single ended and 1000 ohms balanced. Gain of our prototype at 1kHz measures 8.3dB. Frequency response is within 1dB from 17Hz to 30kHz. -3dB points are 7Hz and 70kHz. Noise level will depend upon the brand of 300B you choose, but our prototype’s S/N measures -91 dB at +4dBu output. THD measures 0.065% at 1kHz at 2Vrms output. Maximum output before the onset of clipping (which presents itself as a very gradual rounding of the sine wave trough) is 18Vrms. "

say that you have standard 2Vrms source

that is 5.65Vpp

8.3db means gain of 2.6V/V

so , you end with 14.7Vpp

that is pretty much lower than 40Vpp , as F4 input demand/swing capacity

though ....... it could be enough for your needs ........ who knows , without further info - room size , your SPL habits etc.

 

[Metropolis]

Spl habits are probably 80 to 90 db (most listening is done at night when my wife is asleep). Speakers are rated at 97db. I sit about 12 ft from the speakers and have a powered sub for low end support.

 

[Zen Mod]

forgot to write - 14V7pp is around 3W4/8R

that's soooooo tight .......

can you try with some eeeny weeeny amp in same power category ?

you can always connect F4 on amp's out , as described in manual , and save some greenies not buying preamp

 

[Metropolis]

Thank you so much! (I am a complete novice with this - when I build the amps, it's pretty much paint-by-number)

This info helps a lot! So, even though the BeePree is 18v output, the gain structure is too low to comfortably work with the F4. How much gain would I need then?

Thanks!

 

[Metropolis]

@ZenMod - I just thought of another question - would using the beepre instead of the NovaPre help with my headroom problem with the Aleph J?

The only spec I can get on the NovaPre are this:
Output: 7V rms/11.2 peak
output impedance: 50 ohms (that's supper low)

 

[Zen Mod]

7Vrms - as max swing preamp is capable of ?

and - is that typo > how's Aleph J here , suddenly ?

 

[Metropolis]

The user manual only list 7V rms as the output, 11.2 volts as the peak... they don't provide any specs beyond that.

Aleph J was always here in the thread 🙂

It is the need for additional headroom in my system with the Aleph J that started me looking into the BeePre with the F4 - because, to my ears at least, the Aleph J is a very sweet, relaxed sounding amp that still provides lots of detail. I love that about the Aleph J, but sometimes it runs out of power - I hear compression/congestion in the mid-range when this happens.

My goal with investigating the BeePre/F4 combo is to try and get similar beautiful sound with a little more power.

Based upon the information you have already provided me with, it looks like the Beepre/F4 combo will not accomplish this.

Thanks!

 

[Zen Mod]

F4 will give you some , comparing to Aleph J ......... but how much , only proper test can reveal

if you don't mind some additional heat and opening Zu's , you can divide driver connections and biamp them ..... Aleph J to tweet , and feed F4 from Aleph J output then F4 to 10" ER driver ... F4 giving grunt which J isn't capable of

if that works , search for proper preamp , with intention to put F4 solo ....... or build bigger Aleph J

BA3 (bal?), Impasse ,

(you know that I'm biased ...... )

 

[Metropolis]

Cool! Thank you very much! Based upon the information you gave me, I may still build the F4, but I may feed it with a 2a3 power amp (bottlehead stereomour II)... I need to do some research into it.

Thank you!

 

[Zen Mod]

swing (amount of) with 2A3 is still going to be problematic , even if buffered with F4 , presuming that you're going to use 8 ohms tap on OPT

different thing if you have , say 600R , non-attenuated , tap

edit: just searched ; with 3.5W out , 16R tap on Stereomour ,closed with appropriate resistor (100R ?) will give you something in vicinity of 22Vpp or even few more

that will be , buffered via F4 , delivered as ~8W at Zu's 8R

 

[Metropolis]

That may actually work. The Zu's are 12ohm nominal (I don't think they drop below 10 ohms).

I think I read somewhere that people got good results with 20R or 22R.

I probably will need to get a bunch of resistors and see what works best - 3 or maybe 5 watt resistors?

 

[Zen Mod]

ooops

if Zu's are 12R nominal , said swing will give even less watts

you really need preamp capable of 40Vpp of healthy swing

that's 14Vrms

 

[Metropolis]

I think I am going to try the Stereomour 2a3 amp. I always wanted to try an SET amp.
If it doesn’t have enough volume and I love the sound, I can convert my Aleph J to an F4. If that still doesn’t have enough volume, I can build a second F4 and make them a pair of parallel monoblocks- that should be plenty loud.