F4 power amplifier
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EUVL
diyAudio Member

Join Date: Oct 2003
Follower with Gain

I know it is a bad habbit, but I could not resist the temptation again.

I also have not simulated or built. It is the basic circuit without (much) protection, etc. It should have a gain of about 20dB, same output capability as the F4, and no global feedback. I think bandwidth should not be too bad. Back-of-the-paper calculation says over 200kHz.

Please feel free to comment / criticise.

Patrick
Attached Files
 follower with gain.pdf (63.1 KB, 791 views)

RoboMan
diyAudio Member

Join Date: Jul 2006
Re: Follower with Gain

Quote:
 Originally posted by EUVL Please feel free to comment / criticise.

Nice try! I think an OCL amp should at least have temperature compensation to keep bias and DC offset. Don't you think so?

Babowana
Formerly "jh6you". R.I.P.

Join Date: Jul 2006
Re: Follower with Gain

Quote:
 Originally posted by EUVL Please feel free to comment / criticise.

+42V is a misprint of -42V at minus rail?

Is this procedure of gain estimation right?
2x(20*log(2x((2.2/0.68)x(39/82))))~20dB

Very interesting.

 26th March 2007, 03:08 PM #144 EUVL   diyAudio Member   Join Date: Oct 2003 > +42V is a misprint of -42V at minus rail? Thank you for noting. I thought I should put in some bugs to test attentions (joke). Gain is calculated by : 1st stage = 2200/(680+1/Yfs of ZVN3310A at 5mA) 2nd stage = (2x100=39)/(82+1/Yfs of IRF9610 at 100mA) 3rd stage = Rloadxn/(Rloadxn+0.47+1/Yfs of IRFP240 at bias) where n=no of devices in parallel. The 100 ohm 5W resistor is shared by both the upper and the lower half, so they get the 2x when calculating gain. One should of course calculate also the lower half and take the average, to be precise. As transconductances do vary from device to device, one should measure AC gain and adjust the value of the 100ohm 5W resistor. My calculations showed that it might want to be between 120 to 135 ohm if you want exactly 20dB gain, at working temperature. With the values shown, the IRFP240s are biased close to 1A each. Going further up on voltage for more power is difficult, so in that case better to make it a fully differential amplifier (a la Aleph-X) by converting the first stage MOSFETs to 2 diff pairs. It should then delivery 100W Class A. I think the circuit is simple enough, even compared to the F4. It will not have the best distortion figures around, but I expect its harmonic spectrum to be mainly second and third. The sex appeal (to me) is that it has no global feedback, something I have yet to try in power amps. (The F4 also has no global feedback.) As to thermal compensation, I don't think it has been an issue with Aleph type circuits, so I do not expect runaway here as well. The IRFPs has a slight increase in bias with temperature, but they do settle to a steady state value. If you are realy worried, you may also use 2SK1529/2SJ200, which is a bit more stable in that respect. One last point I want to add. The circuit is nothing new or intelligent. You can find all elements in the first 4 chapters of Hill and Horowitz. But that does not mean that it has to sound bad. Patrick Steen, I guess you are building a F4. Fancy having a modular frontend to try this out ? All standard PASS DIY components you have in your drawer.
Babowana
Formerly "jh6you". R.I.P.

Join Date: Jul 2006
Quote:
 Originally posted by EUVL The 100 ohm 5W resistor is shared by both the upper and the lower half, so they get the 2x when calculating gain.

I am still scratching my head on the first 4 chapters of Hill and Horowitz ^^~
Never ending story for me . . .

I do not understand well how the 100 ohms 5W contributes to the gain.

 26th March 2007, 05:05 PM #146 Babowana   Formerly "jh6you". R.I.P.     Join Date: Jul 2006 Ayo . . . babo . . . The 100 ohms 5W changes the zero potential of the middle point between the two 39 ohms to above or below zero. I'm sure I should read the first 4 chapters more #^^#
 27th March 2007, 03:17 PM #147 cviller   diyAudio Member     Join Date: Dec 2005 Location: Toronto It plays.... I have assembled my f4 clone, and it plays quite nicely, but it doesn't get hot It seems to be running in class AB with only 3x60mA idle current. I have P1 all bottomed out, but I can only adjust from 2mA to ~60mA. Is it simply because 4.7k is not enough? Should I increase R9 or have I just placed some components wrongly? Still, I'm impressed with the details, although it lacks some in the bottom. This will most likely be sorted with some more current and hot transistors. If I don't fix it it will be a nice summer amp!!!! It's so strange to have a Pass circuit running so cold, I'm getting used to the warm radiation from my modified babbelfish. I had no problem adjusting the dc-offset. A picture of the setup: http://viller.org/audio/2007mar_f4/a...y/P1040594.JPG Few more pictures of populated boards: http://viller.org/audio/2007mar_f4/assembly
 27th March 2007, 03:54 PM #148 EUVL   diyAudio Member   Join Date: Oct 2003 If you measure voltage across TL431 (from D1 to D2), you should be getting something between 7 and 8V. Anything less means you have too little voltage to bias. Then try to change R9 to 5k and P1 to 10k. > Is it simply because 4.7k is not enough? I didn't see any 4.7k on the circuit. Which one -- Rxx ?? Patrick
cviller
diyAudio Member

Join Date: Dec 2005
Location: Toronto
Quote:
 Originally posted by EUVL If you measure voltage across TL431 (from D1 to D2), you should be getting something between 7 and 8V. Anything less means you have too little voltage to bias. Then try to change R9 to 5k and P1 to 10k. I didn't see any 4.7k on the circuit. Which one -- Rxx ?? Patrick
Thanks, I'll try that tonight. The 4.7k is P1 which was supposed to be 5k.

 27th March 2007, 06:09 PM #150 cviller   diyAudio Member     Join Date: Dec 2005 Location: Toronto The voltage diff between gates is 8.03V, but I use transistors with vgs 4.42 So is this logic correct: Required voltage for 1A through pair: 4.42V+4.42V+2x0.47V = 9.78 I only need approx 0.5A, but still, 8V is way too little. I just need to increase R9 a few kOhm and I should have the beast running.

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