F4 power amplifier

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#3,484
Last year I built a direct box for bass guitar using the F4 circuit but scaled way down to just a couple of watts. The output section used 5 pairs of irf610/9510 mosfets driving a Jensen JT-11-DMPC. The 9510's were the good old Intersil ones by the way. I used 18V rails and 50ma of bias on each transistor so I didn't use any heat sinks.

I needed to drive long lines (100-200 meters) in a number of venues where I play and this box really works well. With a gain of one and lots of output current, I don't have the 20-25 dB of loss you get with DI boxes like the JDI.

The unit uses unregulated supplies just like the big F4 and the biggest issue was hum. It took a couple of transformer swaps and a lot of tinkering with parts placement, but in the end everything turned out well.

Bottom line - the F4 circuit works REALLY well with transformer output.

I have been meaning to start a thread on this DI box project but..well...you all know how that goes.

Graeme
 
#3,487
buzzforb said:
Hey I got one of those xformer thingies. Guess i could hook it up to F4:devilr: Toaster time!
Click to expand...

Buzz

If/when you do this I would like to hear what you think of this relative to your BA3/F4.

Now that I have done it a few times (with help on wiring transformers correctly) I really like the sound and simplicity. As I mentioned above I would consider doing a full blown F4 with interstage transformers to see how they drive my SEAS woofers.

Thanks

Bob
 
#3,488
If you got the gain, they got the grunt. That is biased at normal levels. If you are quazy, just bias them higher and raise rail voltage and you definitely have no issues. it is current amp, so it laughs at tough load. As for combinations, I have all boards in quesiton populated, its just matter of taking time to throw it together. I have always wanted tro try Lovoltech JFet SRPP FE for F4. Taylor it to specific Zin of jfet free F4 and you got an amp that can swing big V and shoot big I
 
#3,489
First Watt F4 Amplifier

Hello everyone, I am a new member on this forum and I write from Italy, I need your help to make an amplifier First Watt F4, draft Nelson Pass. Who can give me tips on buying the necessary equipment? It 'an existing kit of this amplifier? Thank you all for the help I will be given later, I look forward to hearing from you
Pierluigi
 
#3,490
There are no complete kits available AFAIK,

The PCBs can be purchased from the diyAudio store, there is a link at the top of the page.

P-F4-2V20 - Pair of F-4 boards (Makes 2 channels; Rev 2.0) - Boards

A semiconductor kit can be purchased from a forum member h_a

http://www.diyaudio.com/forums/swap-meet/178558-matched-fets-your-f1-f4-f5-aleph-j.html

Amplifier chassis with pre drilled and tapped heatsinks will be available soon from the store.

http://www.diyaudio.com/forums/site...n-stock-chassis-requesting-your-feedback.html

Large value PSU capacitors can be purchased from forum member umut1001

http://www.diyaudio.com/forums/vendors-bazaar/163913-50v-68000uf-kendeil-caps-sale.html

I have purchased from all of the above without any problems.

Welcome to the forum and happy building :smash:
 
#3,491
buzzforb said:
If you got the gain, they got the grunt. That is biased at normal levels. If you are quazy, just bias them higher and raise rail voltage and you definitely have no issues. it is current amp, so it laughs at tough load. As for combinations, I have all boards in quesiton populated, its just matter of taking time to throw it together. I have always wanted tro try Lovoltech JFet SRPP FE for F4. Taylor it to specific Zin of jfet free F4 and you got an amp that can swing big V and shoot big I
Click to expand...

Buzz - thanks for the input
You have me re-reading this thread and the BA3 thread
I would like to build a "bass" amp (driving SEAS w26s) to replace my Crown K1s.
Having done two headphone F4s with interstage transformers makes me lean to a full F4 with the same - love the simplicity - but there is the appeal of something new - the BA3
I also have been following the F6 thread and that is one intriguing amp. What would we do w/o DIY
Best
Bob
 
#3,492
Hooked up my Jensen F4 to the Ravens in my system a couple of days ago
Kept the F3 driving the PHL midranges
Moved the Aleph 30s from Ravens to drive the SEAS W26 bass driver

First of all I must say that I forgot that the Alephs can put out in the lower range.

Overall I find that I have a wider soundstage, better localization of instruments. nice tight bass and a system that I can listen to without fatigue.

The transformer F4 continues to intrigue/fascinate me. Once and if I can get my head around it I would like to build a F6 and try it to drive the PHLs moving the F3 to the Ravens. Maybe the Ravens favor a current amp. Don't know but it could be an interesting experiment.I do know that the Crown K1s may be headed to CraigsList

Maybe a crazy question but could the F3 be used to drive a stock F4 (no interstage transformer)? Same question about the Aleph30 - can it be used to drive stock F4?

Best


Bob
 
#3,494
I have a quick question for the experts --

Are the transistors that buzzforb sells, the Fairchild FQA19N20C(FQA16n25C) and FQA12P20 , direct substitutes for the IRFP240 / IRFP9240 when building a F4?

Are any changes to the circuit required or suggested?

Thanks,

Jim
 
#3,498
Here's another question - how much input voltage would it take to drive a F4 from the XLR input to get 5 watt into 50ohm?

I know there is a way to compute that, but I haven't a clue to the formula.

Also, does the drive voltage requirement increase or decrease for a given output if you use the balanced vs. the RCA input?
 
#3,499
let's go backwards :clown:

say that F4 will squeeze +/- 20V sine at output (round number because of clarity ; real life - it's in ballpark )

that's 40Vp-p

that's 40V/2 peak

that's (40V/2)/1.41 RMS

as P is U^2 / R

that's (for 8R) : 14V14^2 / 8 = 25W

now go backwards

:rofl:
 
Last edited:
#3,500
So if you have that same input signal of 40v p-p (14vRMS)

into 50R

14^2 / 50 = 8w ?

Is that correct?

Next question - with a 2v source (computer, Dac, CD player, etc...) how much signal would you get from 30db gain? (such as a BA-3 front end...) I'm not super clear on how db works...

And does using it (the BA-3) balanced change the gain or is it the same?
 
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