Preamp for LM1875 - diyAudio
Go Back   Home > Forums > Amplifiers > Chip Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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
Thread Tools Search this Thread
Old 25th July 2009, 04:45 PM   #1
tom91 is offline tom91  United Kingdom
diyAudio Member
Join Date: Jul 2009
Default Preamp for LM1875

I've built an LM1875 power amp, using the circuit diagram from a kit ( I've not included a volume control yet.

It sounds great when I plug it into the headphone jack on a small radio, and I can get a decent amount of volume without distortion or the chip getting too hot. However, when I use it with my MP3 player, I don't seem to get enough volume out the amp even with I have the MP3 volume on full. It works fine with other amps.

Could I build some sort of preamp to boost the input audio level? Any ideas on circuits? 741 or LM386 might be suitable? (I know the LM386 is technically a power amp, but the radio is probably putting a similar level out it's headphone jack and that seems to be fine.
  Reply With Quote
Old 25th July 2009, 06:29 PM   #2
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
what is the sensitivity of your speakers and what is the gain of your power amp?

Have you measured the output voltages of your sources?
Have you checked that all your source outputs are free from DC offset?
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 26th July 2009, 07:53 AM   #3
diyAudio Member
Join Date: May 2008
If you need more gain, you could increase R5. E. g. replace 180k with 1M8. That could lead to higher noise however. Considering the really low cost it should be worth a try.

Another alternative would be to decrease R4 and increase C3 by the same factor. E. g. replace 10k with 1k and 22 with 220. In that case you will need enough space for the bigger capacitor.

In both cases your amplifier will be louder, so be careful, when you connect other sources without volume control. If it still is not enough or if it is too much, experiment with different values. The gain for your amplifier is (R4+R5)/R4. C3 and R4 are a filter, and C3xR4 should remain the same, before and after any modifications for a start.

Follow the link within your link and download the datasheet. Everything you need to know is explained in it.
If you've always done it like that, then it's probably wrong. (Henry Ford)
  Reply With Quote
Old 26th July 2009, 03:08 PM   #4
tom91 is offline tom91  United Kingdom
diyAudio Member
Join Date: Jul 2009
I tried a 1M8 resistor at R5, and the gain is much better. I can now get a decent sound even with the volume on my MP3 only halfway up. Problem solved.

Thanks for your help!
  Reply With Quote


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 PCB, Which To Use Paswa Chip Amps 264 18th January 2013 09:38 PM
LM1875 as preamp digi01 Chip Amps 6 19th September 2008 01:43 PM
lm1875 siemensc65 Chip Amps 4 23rd May 2006 03:28 AM
Lm1875 soundNERD Chip Amps 14 12th October 2003 02:26 AM

New To Site? Need Help?

All times are GMT. The time now is 09:17 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2