F4 power amplifier

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#2,681
Sorry if this has been answered elsewhere -- it's a long thread.

1. Is there any reason that using a single pair of output fets as opposed to 3 would be a problem? I am used to 300b amps, so 8W is fine and a slightly worse damping factor is also fine. Any other concerns in doing this?

2. The 47K input resistor, can this be increased? I'd like to add a tube driver, and 100K (or higher really) would be beneficial in terms of rp and coupling cap size.
 
#2,682
dsavitsk said:
Sorry if this has been answered elsewhere -- it's a long thread.

1. Is there any reason that using a single pair of output fets as opposed to 3 would be a problem? I am used to 300b amps, so 8W is fine and a slightly worse damping factor is also fine. Any other concerns in doing this?

2. The 47K input resistor, can this be increased? I'd like to add a tube driver, and 100K (or higher really) would be beneficial in terms of rp and coupling cap size.
Click to expand...


1. just go for it

2.same

3. 😀
 
#2,683
dsavitsk said:
... Is there any reason that using a single pair of output fets as opposed to 3 would be a problem? ...
Click to expand...
That depends on if you keep the 1.5A bias through 1 pair of devices or you only use .5A. There will be a distorion penalty in reducing the number of output devices. More so when driving lower load Zs (4 ohm). And, along with lower max output, even more so when using only .5A bias.
😀
 
#2,684
Hi all,

perdone my stupid question ( I understand nada of amps) is it possible
to employ more pair of mosfet like 5 or 6 for example with 2,5-3A? I need more power, and does the rest of the schematics remain identical? :cannotbe:
I like very mutch Pass amplifiers, they sound so well 🙂

Cheers,
Paolo
 
#2,685
inertial said:
Hi all,

perdone my stupid question ( I understand nada of amps) is it possible
to employ more pair of mosfet like 5 or 6 for example with 2,5-3A? I need more power, and does the rest of the schematics remain identical? :cannotbe:
I like very mutch Pass amplifiers, they sound so well 🙂

Cheers,
Paolo
Click to expand...


You can do that, but what you really need to do is to ask yourself what you need: voltage swing, current or gain ?

If you have <4 Ohm speakers, more devices and more current might help you, but the stock F4 is quite good at driving small ohmic loads.

More voltage swing is what you need if you have driven the F4 into clipping and still need more juice. You can raise the voltage slightly, but the jfets don't like it too much - if you get close to their rating, you'll have to cascode them. If you up the voltage, you'll have to increase the current as well, but you would be better of bridging two F4's with a nice balanced input.

If you want gain, you should look for a different amp...
 
#2,686
Oh thanks,

Really I need about 40 W on 8 ohm ( my speaker are 83dB/w but 8 ohm near resistive :cannotbe: ) I think 25w of the stock F4 is a bit little. I am dreaming of build this jewel in the future and I want planning and understand best way I can .
Two paralleled F4 is an option but I strongly prefer only one maybe
a bit "boosted" , if possible :devilr:

Cheers,
Paolo
 
#2,689
I assume you have two stereo F4's.

What you do is you take your balanced output (do you have that) on your preamp and wire +/- to r and l on the F4, speaker should be connected between the two "hot" connectors - then you have a balanced monoblock. Now do the same on the other channel and you have two.

20 times gain is not that much...
 
#2,690
Thanks cviller,

Oh poor me, my preamp has unbalanced output (SE triode) so I have to take another road , I presume, am I wrong?
I was souspicious of this than I have read "balanced mode", for this
issue I have ask for a "boosted" version of the single F4 , stereo.
If I raise the voltage , what power can I expect? maybe 30-35 watt?😕

cheers,
Paolo
 
#2,694
Christian,

I hooked the F4s up to my 95dB speakers at home with just the output of my Cd player and they play loud enough for normal listening, but not enough for intense listening. Pretty much what you said. I think that the speakers we were trying at Burning Amp were less than 84 dB efficient. A poor choice for the purpose.

Hope to see you next year. We are making BIG changes to BAF for next year should be waaay bigger and more organized. Lots better auditioning.

Mark
 
#2,695
Now you know what ~2V sounds like. 😀 The problem with F4 and no preamp is that it sounds so good that you want to listen to it louder, but you cannot. :cannotbe:

If the plans hold, I'll be at the BAF next year for sure. And if you need help organizing, I'll be in san fran from july/august.
 
#2,697
F4 without preamp.

I do not feel the deficiency of the volume, an equivalent in my configuration in all cases without a pre-amplifier.
We look at this for example:
Source: Mechlabor STM-610 stúdio tape-recorder unbalanced.
Amplifier: 2x2 that is four Nelson Pass F4 unbalanced.
Loudspeaker: MJK MLTL with Fostex FE-206
I plan the construction of something however.
What your opinion we say it about this:

http://www.borbelyaudio.com/adobe/balline.pdf

The normal volume enclosed a breast on a picture you may see it, what I listen to.

Gyuri

PS:
Would it not be possible to do something in order for it to be possible to put up pictures with a more normal resolution?
I hope for it I was not it once now OT.
 

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#2,700
inertial said:
Hi guys,

perdone my stupid question: is it the F4 25WRMS on 8 ohm and (how much) ??WRMS on 4 0hm ?😕 Am I wrong?

Cheers,
Paolo
Click to expand...

F4 service manual says:

"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 100 watts into 2 ohms."

You could read full details in http://www.firstwatt.com

Cheers,

🙂
 
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