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plgsekip
diyAudio Member

Join Date: Jun 2001
Location: Krakatoa
Cherry LF Com NFB and LM1875, possible?

Hi AndrewT and all other members,
Here is the schematic for my LM1875 as well as extracted materials from Dr. Cherry's publication on LF comp.
Please help on finding the "correct" value. :-) Has anyone done a simulation on this LM1875 with Cherry's LF comp?
Attached Images
 My LM1875 and Cherry LF Comp NFB.png (110.9 KB, 509 views)
Attached Files
 My LM1875 and Cherry LF Comp.pdf (197.9 KB, 98 views)
__________________

-luq-

 14th February 2015, 08:43 AM #502 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders starting with R3=1k and C2=220uF. Gain is 23times. The added resistor is 44k (use two 22k in series) = RF3 The added capacitor CF2 = R3*C2/RF2 = 1k*220uF/22k = 10uF Change R2 from 22k to 66k (use 68k and add a 1M or 2M parallel trimmer to set the output offset to zero mVdc, then replace trimmer with the nearest value fixed resistor). C1 = C2 *R3 / R2new / sqrt(2) = 220uF*1k/66k/1.4 <= 2u38F (use 2u2F) Your LF bandwidth has increased from 3.3Hz to 1.1Hz You could reduce all the capacitors by a factor of 2.2 to C1 = 1uF, C2 = 100uF & CF2 = 4u5F, for F-3dB=2.4Hz or reduce by a factor of 3 to C1 = 0u68F, C2 = 72u6F, CF2 = 3u3F, for F-3dB=3.5Hz In these latter two cases you are allowed to use capacitors substantially smaller than the original and still have adequate LF bandwidth. BTW, I have a discrete Power Amplifier with R1=2M2, R2=66k, R3=1k, NFBupper=RF2=22k, RF3= 44k, (two smd 805 in series in the copper trace), C1=1uF, C2=CF1=120uF (measures 120u8F), CF2=5u5F (two 10uF electro in series) __________________ regards Andrew T. Last edited by AndrewT; 14th February 2015 at 09:00 AM.
 14th February 2015, 10:00 AM #503 plgsekip   diyAudio Member   Join Date: Jun 2001 Location: Krakatoa Thanks. Please allow me some time to build this. :-) I am interested to try it, especially on the trick of making zero dc offset. And I suppose all the capacitors are of the electrolytic ones, except for the C1 (input cap). BTW, the 10k pot works better than 100k or 20k. __________________ -luq-
danielwritesbac
diyAudio Member

Join Date: Nov 2007
Quote:
 Originally Posted by plgsekip Hi Daniel, I have read the post but could not find the reason why you changed 22k to 15k. Could you elaborate more on this?
Certainly. We don't know whether your source will perform better with a lighter or heavier load. So, you'll need to try, to find out.

If your feedback resistor is 22k and feedback-shunt resistor is 1k, then it is possible to use a range of input load from 4.4k to 21k.
__________________
DIY Models & AppsClipNipperLM1875TDA7293Powerful Parallel TDA7293 kit ♦ My post has opinion.

 15th February 2015, 08:38 AM #505 danielwritesbac   diyAudio Member     Join Date: Nov 2007 AndrewT's post#502 is utterly fascinating. The LM1875 is a standard operational amplifier eagerly willing to obey any orders you give it. However, those particular orders are for really rocking bass and higher durability, simultaneously. Therefore, fascination! __________________ ♦ DIY Models & Apps ♦ ClipNipper ♦ LM1875 ♦ TDA7293 ♦ Powerful Parallel TDA7293 kit ♦ My post has opinion.
 15th February 2015, 08:45 AM #506 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders Many blame different amplifiers for no bass, or spitty treble, or muddled midrange, or any number of excuses for not liking the "PERFORMANCE" of the amplifier. It is usually NOT the amplifier that is at fault. It is the BUILDER'S IMPLEMENTATION. Build the amplifier correctly, test it thoroughly to ensure you have built it correctly, then listen to the amplifier PERFORMING correctly. __________________ regards Andrew T.
 15th February 2015, 10:26 AM #507 plgsekip   diyAudio Member   Join Date: Jun 2001 Location: Krakatoa I totally agree with post 506. I have learned quite a lot recently through discussions in this forum. I can also see the mistakes I made previously and how each of the components affect the overall system performance. Especially for an amp that I have left about a year since my first built of this LM1875. Additional report, I have just implemented a log-law faking resistor, a parallel of 1k2 resistor to the 10k pot. It causes my amp a bit warmer than without it. Do you think these log-law faking resistors have something to do with this temperature increase? (Uh, still asking question... Didn't I say I have learned?) An one more thing, as this amp improves, so do my ears... :-D The russian caps I use as the input cap really works well. Many many thanks. :-) __________________ -luq-
 15th February 2015, 10:30 AM #508 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders The 1k2 both loads the Source and loads the Receiver. I can't see why the extra loading, on the Receiver input, leads to an increase in temperature. Unless somehow that resistor which is connected to +IN, very indirectly, causes the Receiver to oscillate. Surely that symptom would be obvious. __________________ regards Andrew T.
 15th February 2015, 10:34 AM #509 plgsekip   diyAudio Member   Join Date: Jun 2001 Location: Krakatoa Thanks AndrewT for the prompt reply. I'll check it again and remove the resistor to see whether it affects the temperature. It is a slight/minor increase that needs no worry, but I enjoyed a cool heatsink as it was before. :-D __________________ -luq-
plgsekip
diyAudio Member

Join Date: Jun 2001
Location: Krakatoa
LM1875 + Dr. Cherry's LF Compensation

As promised, this is the schematic. Please help checking the component value especially the polarity of capacitor (C7) in the LF compensation feedback. It is a 10uF and I guess it should be of electrolytic type.

As the total feedback resistor is now 66k, isn't the gain of my amplifier now 67 times?

I assume that the input impedance Rin should be equal to total gain feedback capacitor. 22k Rf and 22k Rin, now 66k Rf and 66k Rin, and if one manages to equalize them (Rf = Rin) then zero mV of dc at te output. Is it correct?

BTW, I have removed the 1k2 resistor from the vol pot, the temperature is lower than with the resistor installed, actually I soldered them right at the pot's legs.

Another thought, I think an amplifier board should have a ground bus, I noticed Dr. Cherry established a bold straight line for ground bus in the middle of his NDFL PCB as in ETI May 1983. With similar ground bus, created by connecting the pixels of veroboard using soldering tin, my LM1875 does not make thump sound when turned off. Heatsink is cool even without 100nF bypass cap (I just experimented by removing them to observe what happens). So, thinking that unusual heat is a symptom of oscillation, I think it's not oscillating, right?

Thanks
Attached Images
 lm1875_cherry_lfcomp.png (30.3 KB, 476 views)
__________________

-luq-

Last edited by plgsekip; 15th February 2015 at 01:52 PM.

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