Another LM1875 question

soundNERD · 1st January 2004, 02:30 AM

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

Dustin Haug · 1st January 2004, 03:02 AM

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.

SkinnyBoy · 1st January 2004, 05:06 AM

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...

soundNERD · 1st January 2004, 02:22 PM

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

paulb · 1st January 2004, 03:44 PM

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.

soundNERD · 2nd January 2004, 02:02 AM

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

paulb · 2nd January 2004, 02:10 AM

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

djk · 2nd January 2004, 10:14 AM

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.

soundNERD · 2nd January 2004, 01:32 PM

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

djk · 3rd January 2004, 01:12 PM

"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

1st January 2004, 02:30 AM	#1
soundNERD diyAudio Member Join Date: Sep 2003 Location: Illinois	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
BrianGT LM1875 feedback capacitor question	Sparky OR	Chip Amps	13	31st December 2008 06:54 PM
LM1875 power supply question (easy)	madisonears	Chip Amps	12	24th February 2008 11:41 AM
2 general gainclone questions, and an lm1875 question	Adam M.	Chip Amps	11	4th March 2005 12:53 PM
LM1875 battery question	Fragen	Chip Amps	17	14th October 2004 10:58 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	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

1st January 2004, 03:02 AM	#2
Dustin Haug 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.

1st January 2004, 05:06 AM	#3
SkinnyBoy Banned Join Date: Nov 2002 Location: Adelaide, Australia	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...

1st January 2004, 02:22 PM	#4
soundNERD 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

2nd January 2004, 02:02 AM	#6
soundNERD 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

2nd January 2004, 02:10 AM	#7
paulb 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

2nd January 2004, 10:14 AM	#8
djk diyAudio Member 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.

2nd January 2004, 01:32 PM	#9
soundNERD 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

3rd January 2004, 01:12 PM	#10
djk diyAudio Member 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

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)