F4 power amplifier

[Rewind]

Excellent questions.

 

[Zen Mod]

F4 is pretty close to "wire with no gain", bringing not so much of own character in game

pretty much, everything you hear is character of your preamp

and , that applies from 0V of input to F4 and all the way to clipping point which is , say, in range of 40Vpp

of course that THD of F4 is rising with output level, but I can bet that THD of your preamp is rising more rapidly ......

 

[soldersmoker]

Excellent reply!

 

[Zen Mod]

let's be realistic, all mine are .......

 

[soldersmoker]

And you were so matter-of-fact for that one answer--then, back to abnormal!

 

[Giovanni1968]

Hello everybody, read many posts in the thread but it got too long for an absolute non-tech like me... started the process of building an F4, a skilled tech will build it for me, got the boards with the mosfets and the 4U case (in black flavour), next step caps and transformers but still bit nervous about my choice of heading the F4 as many suggested since preamp being an old AR SP9 with 20+ volts gain which should match with the F4, at the bottom mere (...) 90db/w speakers but with a small listening environment 25W should do it (using a KT88 push-pull now which sounds pretty nice to my nerd's ears).

Would it go all well (which I have no doubt on the builder's end) are the SP9 and 90db/W JBL L220 good match with the F4?

In a future occurrence how about a BA-3, is it easy as a DIY for a beginner and a good match with the F4? I would move the SP9 and the tubes to another room (along with turntable) and wondering about the BA-3 to remain in the Pass environment.

Grazie

 

[6L6]

The SP-9 is going to be fantastic with F4!! It has the ability to output more voltage than the F4 needs to hit the rails, I.E., it’s going to unlock 100% of the potential of the amplifier.

 

[Giovanni1968]

The SP-9 is going to be fantastic with F4!! It has the ability to output more voltage than the F4 needs to hit the rails, I.E., it’s going to unlock 100% of the potential of the amplifier.

Hopefully the L220 will be a good match too

 

[CoGT3]

Since we are on the subject about preamps….

Any chance a XP-22 would have enough voltage gain for the F4? I can’t find any listed maximum voltage output, only gain of 9.3dB balanced. Speakers are efficient 93dB with impedance mostly around 10 ohms dropping to 8 at the worst.

I have a bad itch to try this diy FW amp thing out and can’t decide which to try first. Main amp is a XA30.8, not looking to replace it. Actually looking for some of those unique presentations Papa puts into his FW designs to give a different presentation than the XA30.8. I’ve read enough F4 threads I can’t remember who said it (think is was 6L6) but someone made a comment for those of us who live off vinyl with a good phono pre the F4 is the bomb if you preamp can drive it. They also commented that compared to the F5 is was maybe a kinder and maybe richer presentation which is up my alley. Phono pre is a XP-25 which does kick out plenty of voltage and everything is run balanced.

If the XP-22 will not have enough drive for the F4 I understand you can add the BA3 input gain stage which basically gets you to the BA3 amp with complimentary output stages. My naive understanding is that the BA3 gain stage shares characteristics with the .5 and .8 Pass amps which I think would push the presentation toward my XA30.8.

Thanks for any input, I love the simple elegance of the F4 just not convinced my preamp will match up well enough to reach the amps potential.

 

[6L6]

You could tweak the gain on the XP-22... You'll probably void the warranty, but everybody knows that awesome lives on the side of tweaking and optimizing things to make your system perfect.

The BA-3 gain stage is a mini F5.

F4 is awesome.

 

[CoGT3]

Everyone says the same about the F4 and if it worked out it I would build a second to run mono balanced. Without voiding the warranty, any chance there is enough gain/voltage? Or do I look elsewhere like M2x, AJ, F6? I have some Tokin SITs set aside for Papas Tokin “VFET” amp so haven’t looked at the SIT amp versions.

 

[Acoustic Emotion]

Dear Audio Friends,

I am thinking about exchanging the F6 channel boards in my First Watt DIY amp with F4 boards.

My preamp (DAC) is a Gustard X20 Pro connected directly to the FW amp with RCA interconnects. The DAC gives the following output voltages.

• RCA output level：3Vrms @ GAIN=+0dB
• XLR output level： 6Vrms @ GAIN=+0dB

The gain of the preamp can be changed to +6db and +12db (not sure how much voltage gain this gives).

My speakers are open baffle, with the Lii Audio Silver 10 driver on top complimented by a 15 inch helper woofer (15PR400) driven by a SAM300D plate amp (taking high level input from the full range driver terminals).

Lii Audio Silver 10 Sensitivity: 98.6 dB（2.83V/1m）

My room is 4 x 4 meters.

My questions:

• Will the DAC/Preamp + F4 combo be able to drive the Silver 10 driver to normal listening levels?
• Is it correct that +12db volume gain will translate into 4 extra voltage gain (resulting in 3 volt (at 0db) + 4 volt (at 12db) = maximum 7 volt output over RCA?

Thanks for any input!

Regards,

Koos

 

[Zen Mod]

do you have data about preamp - what is maximum output voltage ?

gain is one thing ( 6db is 2V/V, 12db is 4V/V) of coin, while max output voltage is saying which max input voltage can be amplified without clipping on output

that one, output voltage, is what is feeding your amp

 

[Acoustic Emotion]

Dear Zen Mod,

Thank you for your quick response, much appreciated (as well as all your other shared information, very helpful!)

Unfortunately, I could not find the maximum output voltage, the only information regarding output of the X20 Pro what's in the manual is this:

Frequency response： 20-20kHz /-0.08dB
SNR： >126dB
Channel crosstalk：-138dB @ 1kHz
THD+N： <0.0006% @ 1kHz
IMD： <0.0006% @ 1kHz
RCA output level: 3Vrms @ GAIN=+0dB
XLR output level: 6Vrms @ GAIN=+0dB

I searched the web and found several other threads about this DAC and people "complaining" about the abnormal high output voltages. However, no real numbers were given for the maximum output voltage.

Is this something I can just measure myself, for example by playing a test tone and measuring the RCA connections?

Regards,

Koos

 

[Zen Mod]

aha, in that case let's assume that they're logical

F4 having RCA input, so taking RCA in account

X20 - 3Vrms at 0db gain

means 6Vrms for 6db of gain

means 12Vrms for 12db of gain

12Vrms on input of F4 will give roughly 12Vrms on F4 output

12Vrms@8R is (P=Urms^2/R) 18W

which is almost 25W@8R , as F4 is declared (25W@8R being 14V14rms)

so, just try it .......

if you want to be sure , play 1KHz/0db sine with your DAC/pre , put DMM in RCA output, set to Vac, and write down measured values for 0db, 6db and 12db gain
black probe to RCA shield, red probe to RCA hot

 

[Acoustic Emotion]

Dear Zen Mod,


Thanks for your clear explanation! This is not only helpful for me for this specific topic, but will help me to calculate these thngs myself in the future

I will measure it accordingly for the ease of mind and order the PCB thereafter. I think it will be a wonderful combination, as the output stage of the DAC is a superb sounding one to my feeling.

Will post the outcome once the F4 boards are being fed by the X20, can't wait!

Happy 2022

Regards,

Koos

 

[Acoustic Emotion]

While I am ordering parts at this moment for the F4, the 5K ohm 25 turn Bourns 3296W is not available on short notice.

May I replace it with a 2K or 10K ohm Bourns 3296W ?

 

[6L6]

Where are you ordering from?

It doesn’t need to be a 25-turn, doesn’t need to be a Bourns, and there are many different versions that vary only in physical layout...

That said, order a 10K if you cant find a 5K.

 

[Acoustic Emotion]

I am ordering from Conrad.nl as I am living in The Netherlands. For Digikey and Mouser I need to pay import tax + expensive shpping compared to Conrad.

Thanks for the explanantion, I will check if I can find a similar alternative.

Regards,

Koos

 

[Acoustic Emotion]

Looks like I found an exact copy of the Bourns...

https://www.conrad.nl/p/weltron-wel...5-slagen-lineair-05-w-5-k-9000-1-stuks-447100