Finally made a LM1875 amp - Page 15 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps

Chip Amps Amplifiers based on integrated circuits

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 16th December 2011, 07:39 PM   #141
jemraid is offline jemraid  United Kingdom
diyAudio Member
 
Join Date: Aug 2004
Location: NE Derbyshire
Good for you Steve!

You won't be disappointed :-)

Jim
__________________
FH3 build using very simple tools and a point to point Gain Clone with largish power supply. http://www.jasread.com/fh3/index.html
  Reply With Quote
Old 18th January 2012, 05:47 PM   #142
diyAudio Member
 
formula 22's Avatar
 
Join Date: Sep 2011
Location: El Triunfo
Hello all!
Is there a difference between the feedback in this amplifier made ​​with resistors: [(200k/10k) +1, and [(20k/1k) +1], if there is in what?
If measurements can?
thank you
  Reply With Quote
Old 18th January 2012, 06:26 PM   #143
diyAudio Member
 
Join Date: May 2008
The higher values will lead to more Johnson noise. Whether that becomes audible depends on the rest of your system, especially on the sensitivity of your speakers.

You will notice a difference in DC offset.

Depending on the parasitic capacitance of the PCB, higher values resistors can lead to different amp behaviour at high frequencies and can make it necessary to use a compensation capacitor, when you wouldn't need one with lower resistor values for the same gain.
__________________
If you've always done it like that, then it's probably wrong. (Henry Ford)
  Reply With Quote
Old 19th January 2012, 06:11 AM   #144
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
You wouldn't want the feedback resistor higher than 115k. I doubt if you'd actually want it that high, but 200k is not especially valid nor desirable for LM1875 non-inverting amplifier.
  Reply With Quote
Old 19th January 2012, 10:30 AM   #145
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
High values of NFB resistors allow lower values of DC blocking capacitors.
This can be a very good compromise if you want to adopt AC coupled amplifier with wide audio bandwidth and do not want to use any polarised nor electrolytic capacitors.

If the Power amp gain is relatively low then 200k for both Rin and NFB upper leg resistor will not create much additional noise.
If you use Cherry's suggested NFB, the Rin resistor can be 3 times the NFB upper leg resistor and still maintain DC balance of LTP currents/voltages.
__________________
regards Andrew T.
  Reply With Quote
Old 19th January 2012, 10:29 PM   #146
diyAudio Member
 
Join Date: Jan 2012
From BrianGT in 11th post -"Wire one of the GND connections on the PSU board to the chassis ground."
Would any of the 4 holes marked pg+ or pg- be the ones? And share it with a wire that goes to the amp board? There is no chassis yet. Where would the chassis gound be made?
  Reply With Quote
Old 21st January 2012, 05:35 AM   #147
diyAudio Member
 
danielwritesbac's Avatar
 
Join Date: Nov 2007
Quote:
Originally Posted by AndrewT View Post
High values of NFB resistors allow lower values of DC blocking capacitors.
This can be a very good compromise if you want to adopt AC coupled amplifier with wide audio bandwidth and do not want to use any polarised nor electrolytic capacitors.

If the Power amp gain is relatively low then 200k for both Rin and NFB upper leg resistor will not create much additional noise.
If you use Cherry's suggested NFB, the Rin resistor can be 3 times the NFB upper leg resistor and still maintain DC balance of LTP currents/voltages.
I can't get that to work right. The soundfield collapses down to a tiny size and the output is somewhat raspy/analytical. This sound happens when its run out of speed. The highest figures I can get you is 115k with 3.9k divider for gain. That works okay. You could go higher if lower gain, but a computer source or mp3 player might start internal clipping if amp gain is too low.

But the good news is that this 3.9k is within range to use a 100uF cap for NFB cap, and the great news is that it is very easy to find excellent quality 100uF (and smaller) caps. This can be paralleled with a really tiny value polyester dip cap for excellent results since the frequency response will stay level. At this point, NFB cap selection gets extremely easy.

Do you mind posting a sketch involving the Rin resistor value relating to your comment about Dr. Cherry's NFB tapping? If I remember correctly, the speaker zobel resistor has to be a 3w or better model so as to avoid accidentally releasing the feedback loop upon resistor failure. Probably a less expensive resistor could be used if speaker zobel cap is 22nF. Its still enough to block some RF, but the smaller value cap removes some risk, and nobody would ever have an excuse to remove the zobel for audio purposes since that is so very far above the audio band.

Last edited by danielwritesbac; 21st January 2012 at 05:46 AM.
  Reply With Quote
Old 23rd January 2012, 03:43 AM   #148
diyAudio Member
 
Join Date: Jan 2004
Location: Livermore, CA
I've sent BrianGT this question via email along with some other kit-related questions, but this should be super-simple to answer here. For the kits purchased from chipamp.com -- are the LM1875T chips meant to be included in the kit, or not? The kit I received included everything but these chips and I was trying to figure out if the omission was intentional or not.
  Reply With Quote
Old 23rd January 2012, 06:00 AM   #149
diyAudio Member
 
Join Date: Jan 2004
Location: Livermore, CA
Quote:
Originally Posted by FromageTheDog View Post
I've sent BrianGT this question via email along with some other kit-related questions, but this should be super-simple to answer here. For the kits purchased from chipamp.com -- are the LM1875T chips meant to be included in the kit, or not? The kit I received included everything but these chips and I was trying to figure out if the omission was intentional or not.
Holy smokes, talk about customer service. Posted here just wanting a quick answer before I sent in my DigiKey order, but seems even that was unwarranted; just got tracking info via email informing me that the missing bits are on their way! Thanks Brian! Can't wait to start building this thing.
  Reply With Quote
Old 7th March 2012, 12:12 AM   #150
diyAudio Member
 
Join Date: Nov 2007
Would it be possible to replace the 22k resistors on BrianGT's amp boards with 1/4 watt instead of 1/2 watt. Having a hard time sourcing 1/2 watts that small in Canada. Thanks for your help.. JohnnyP
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
LM1875...how low can you go? Gcollier Chip Amps 7 6th May 2010 04:43 AM
Home Made Tubes?? has anyone made one, is it possible?? John Biles Tubes / Valves 7 11th December 2005 02:29 AM
Finally made it in the PassDIY pages! Blues Pass Labs 23 28th July 2004 05:05 AM
Lm1875 soundNERD Chip Amps 14 12th October 2003 03:26 AM
Finally made a new webpage! Duo Everything Else 24 17th January 2003 01:29 PM


New To Site? Need Help?

All times are GMT. The time now is 09:14 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2