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 1st January 2004, 01:30 AM   #1
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
Default Another LM1875 question

Hi everybody,

I went to a local surplus store, called American Science and Surplus (www.sciplus.com i think) and couldn't pass up a $12 subwoofer I saw there. It is an 8-inch 50W sub made my LA Sound. I then built a nice bandpass box for it, and connected it to my sub amp I built with an LM3886. It sounded great, deep, tight and no distortion. Yes, it distorted when the amp did, but other than that, it was great sounding. Now, I want to build a new amp for it. I want to use 2 LM1875 in bridged mode, to get around 50W or so. Can the LM1875 drive a 4-ohm speaker in bridged mode? If so, about how much wattage can it put out?

Since the heatsink would be mounted to the aluminum backplate, the chips must be insulated. I have these blue rubber like things from a heatsink from a computer ps with both a 7905 and a 7912 on it. This thing is for insulating it, right? I would prefer though, to just get the insulated version of the chip. Is there a power output reduction when using the insulated version of the chip? I would prefer using the insulated version also because I wouldn't have to buy special screws that don't conduct.

Finally, can somebody direct me to a good bridged circuit?

Thank you, Mike
  Reply With Quote
Old 1st January 2004, 02:02 AM   #2
diyAudio Member
 
Join Date: Feb 2002
Location: Charleston, SC
I'm just finishing up an 1875 amp using non-insulated chips. I just mounted them similar to this using a bar across the chip and a silpad behind it for insulation. Then you don't have to mess with nylon bolts and such. My amp isn't bridged so can't help ya with the first question.
  Reply With Quote
Old 1st January 2004, 04:06 AM   #3
Banned
 
Join Date: Nov 2002
Location: Adelaide, Australia
Send a message via MSN to SkinnyBoy
the chips won't run bridged into 4 ohms... it means they would be running 2 ohms each, and they aren't designed for it...
  Reply With Quote
Old 1st January 2004, 01:22 PM   #4
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
I looked at the datasheet. These chips can put out 4A of current. Wouldn't they power a 2-ohm speaker fine? It's sort of like the 3886. It has no problems with a 2-ohm load. I have ran 2 4-ohm sub speakers in parallel off of a 3886 and it had no problems with it. Would it work, just putting out more heat?

Does the insulated version of the chip lose power like the insulated 3886 does?

This is a low powered chip. I can't see it needing fancy mounting brakets. All it should need is a screw- it works on my 3886 amp which puts out much more heat.

Has anybody experimented with bridged 1875s and could tell me how they work into a 4-ohm speaker? A schematic would also be appreciated. If not, what is a good, simple chip to use for this that can put out about 50W.

I tried this speaker with a 100W amp and it started to burn and smell with the volume up so I want to keep the power output not too much above 50W. What size fuse would I need to protect the speaker? And where would I place it? Inline with the speaker wire?

Thanks, Mike
  Reply With Quote
Old 1st January 2004, 02:44 PM   #5
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
Quote:
Originally posted by soundNERD
I looked at the datasheet. These chips can put out 4A of current. Wouldn't they power a 2-ohm speaker fine? It's sort of like the 3886. It has no problems with a 2-ohm load. I have ran 2 4-ohm sub speakers in parallel off of a 3886 and it had no problems with it. Would it work, just putting out more heat?
Look at the power dissipation curves on the datasheet. You'll find that the chip will dissipate a lot of power when driving a low impedance with a fairly high power supply. Neither the 3886 nor the 1875 are rated for driving a 2-ohm load. If you want more power, use the proper amp.
But go ahead and try it, maybe we'll hear another spectacular tale of how the gainclone blew, taking a speaker with it.
  Reply With Quote
Old 2nd January 2004, 01:02 AM   #6
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
How could the chip blowing ruin the speaker? DC on the output? I would run it from +-24V supplies. What would be a good chip to use? I like National because they give free samples of their chips.

-Mike
  Reply With Quote
Old 2nd January 2004, 01:10 AM   #7
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
There have been a few stories posted here about chip amps blowing and taking out the speaker, yes by putting DC on the output.
Look closely at pg. 4 of the datasheet at http://cache.national.com/ds/LM/LM1875.pdf .
There are other chips out there. How about this one (consider wiring two in parallel instead of briding them into a 4-ohm load):
http://cache.national.com/ds/LM/LM4780.pdf
  Reply With Quote
Old 2nd January 2004, 09:14 AM   #8
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
One LM3886 driving a pair of MJ15003/04 in the supply pins will have no problems driving 200W/2R with 40V supplies.

Use a heatsink big enough for a 200W amp.
  Reply With Quote
Old 2nd January 2004, 12:32 PM   #9
diyAudio Member
 
Join Date: Sep 2003
Location: Illinois
djk,

How would I connect the 3886 to get 200W into a 2ohm load? How much power could it put out with +-24V? I really want under 100W, since that is the max rating of the speaker.

If I were to use a single 3886 but also use the MJ15003/4, would I get more power into 4-ohm speaker?

I have decided on a 3886, and last night ordered 5 samples of the insulated version. Where can I find a good, simple circuit for it? If the schematic comes with a PCB design that would be good also.

Thanks, Mike
  Reply With Quote
Old 3rd January 2004, 12:12 PM   #10
djk is offline djk
diyAudio Member
 
djk's Avatar
 
Join Date: Feb 2001
Location: USA
"If I were to use a single 3886 but also use the MJ15003/4, would I get more power into 4-ohm speaker?"

Boosted LM3886/MJ15003/04 on 40V is 200W/2R, or 100W/4R

Boosted LM1875/MJ15003/04 on 28V is 100W/2R, or 50W/4R

Basic scheme in bridged mode:

http://www.mif.pg.gda.pl/homepages/t...s/simp200W.gif
  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
BrianGT LM1875 feedback capacitor question Sparky OR Chip Amps 13 31st December 2008 05:54 PM
LM1875 power supply question (easy) madisonears Chip Amps 12 24th February 2008 10:41 AM
2 general gainclone questions, and an lm1875 question Adam M. Chip Amps 11 4th March 2005 11:53 AM
LM1875 battery question Fragen Chip Amps 17 14th October 2004 09:58 AM


New To Site? Need Help?

All times are GMT. The time now is 01:57 PM.


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