Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

Beginner's Gainclone, HiFi LM1875, The Amplifier Board
Beginner's Gainclone, HiFi LM1875, The Amplifier Board
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 14th February 2015, 02:48 AM   #501
plgsekip is offline plgsekip  Indonesia
diyAudio Member
 
Join Date: Jun 2001
Location: Krakatoa
Default 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?
Thanks in advance.
Attached Images
File Type: png My LM1875 and Cherry LF Comp NFB.png (110.9 KB, 483 views)
Attached Files
File Type: pdf My LM1875 and Cherry LF Comp.pdf (197.9 KB, 91 views)
__________________

-luq-
  Reply With Quote
Old 14th February 2015, 09:43 AM   #502
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
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 10:00 AM.
  Reply With Quote
Old 14th February 2015, 11:00 AM   #503
plgsekip is offline plgsekip  Indonesia
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-
  Reply With Quote
Old 14th February 2015, 11:15 AM   #504
danielwritesbac is offline danielwritesbac  United States
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by plgsekip View Post
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.
  Reply With Quote
Old 15th February 2015, 09:38 AM   #505
danielwritesbac is offline danielwritesbac  United States
diyAudio Member
 
danielwritesbac's Avatar
 
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 & AppsClipNipperLM1875TDA7293Powerful Parallel TDA7293 kit ♦ My post has opinion.
  Reply With Quote
Old 15th February 2015, 09:45 AM   #506
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
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.
  Reply With Quote
Old 15th February 2015, 11:26 AM   #507
plgsekip is offline plgsekip  Indonesia
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-
  Reply With Quote
Old 15th February 2015, 11:30 AM   #508
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
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.
  Reply With Quote
Old 15th February 2015, 11:34 AM   #509
plgsekip is offline plgsekip  Indonesia
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-
  Reply With Quote
Old 15th February 2015, 02:48 PM   #510
plgsekip is offline plgsekip  Indonesia
diyAudio Member
 
Join Date: Jun 2001
Location: Krakatoa
Default 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
File Type: png lm1875_cherry_lfcomp.png (30.3 KB, 451 views)
__________________

-luq-

Last edited by plgsekip; 15th February 2015 at 02:52 PM.
  Reply With Quote

Reply


Beginner's Gainclone, HiFi LM1875, The Amplifier BoardHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Beginner's Gainclone, Safety and, The Power Supply Board (please contribute) danielwritesbac Chip Amps 118 18th June 2008 07:12 PM
Beginner's Gainclone, LM3875, The Amplifier Board danielwritesbac Chip Amps 37 22nd May 2008 07:43 PM


New To Site? Need Help?

All times are GMT. The time now is 10:07 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.79%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki