Help! Parallel 2x 3875 per channel design - please advise. - 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 March 2004, 06:48 PM   #1
loong is offline loong  United Kingdom
diyAudio Member
 
Join Date: Feb 2004
Location: Earth
Question Help! Parallel 2x 3875 per channel design - please advise.

I have most of stuff on hand, and want to have a small intergearted amp (for my 4x5m room). Being a newbie with very little electronic knowledge, it' better to seek for comments from experts here.

Note:
- LM3875TF x 2 per channel ( parallel ), no servo
- Power +/- 35V dc 250 VA per channel
- Cap 2x 1,000UF /per chip
- All R 1/4W 1% except Rout 0.1Ohm 3W 5%

1). Will this design work, particularly for 88dB speakers?
2). Is the input impedence is too low (output from CD go directly to the Amp via 50K pot).

3) More ideas for improvement are always welcome.

Many thankssss in advance!
Attached Images
File Type: jpg 3875x2.jpg (44.3 KB, 753 views)
  Reply With Quote
Old 16th March 2004, 06:54 PM   #2
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
No.

Read this link to understand theory/issues:

Bridgeclone
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 16th March 2004, 07:48 PM   #3
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
1) Yes it will. You should be able to get around 110 W into 4 ohms according to the Overture Design Guide spreadsheet. As always, YMMV.
2) The input impedance should be OK.

The only suggestion that I would make is to use a zobel network per chip, before the 0.1 ohm output resistor, rather than just one zobel after the resistors.

When constructing, do not connect the outputs (do not install the 0.1 resistors) until you have done some basic but important testing. You are basically connecting the outputs of two amps together with nothing but 0.2 ohms between them. If something goes wrong, it will go wrong in a most spectacular way.

Each amp should function perfectly on it's own, with no oscillations, no excessive heat, and no other signs of trouble. You should measure the offset voltage of each amp, and ensure that it is not too different. Remember that the difference in voltage here will create a current flowing from one amp to the other through just 0.2 ohms of resisitance! Also measure the AC output of each amp with a steady test tone. It should be very close to the same. If it is too different, then the gain of one amp is higher than the other. This also creates a current from one amp to the other which will cause extra wasted heat and unnecessary drain on your power supply. Find matched pairs of gain setting resistors using an ohmeter to ensure that the gains are equal.
  Reply With Quote
Old 16th March 2004, 08:18 PM   #4
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
I disagree with macboy. At the very least, you need 0.1% tolerance resistors setting the gain to make sure each chip is sharing the load. This is discussed in National's App Note AN-1192 and in the thread I mentioned.
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 16th March 2004, 10:28 PM   #5
GregGC is offline GregGC  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Toronto
Default Re: Help! Parallel 2x 3875 per channel design - please advise.

Quote:
Originally posted by loong
I have most of stuff on hand, and want to have a small intergearted amp (for my 4x5m room). Being a newbie with very little electronic knowledge, it' better to seek for comments from experts here.

Note:
- LM3875TF x 2 per channel ( parallel ), no servo
- Power +/- 35V dc 250 VA per channel
- Cap 2x 1,000UF /per chip
- All R 1/4W 1% except Rout 0.1Ohm 3W 5%

1). Will this design work, particularly for 88dB speakers?
2). Is the input impedence is too low (output from CD go directly to the Amp via 50K pot).

3) More ideas for improvement are always welcome.

Many thankssss in advance!

I'd say if this is your first project why use 2xLM3875? For 88dBm speakers and the room you specified single IC LM3875 should be just fine for 6-8 Ohm speakers.

I'd keep it simple as a first attempt. You may find you don't need more any way.

/Greg
  Reply With Quote
Old 17th March 2004, 02:17 AM   #6
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
Quote:
I disagree with macboy. At the very least, you need 0.1% tolerance resistors setting the gain to make sure each chip is sharing the load.
Did you not read my comment or do you just enjoy disagreeing with others? I said...
Quote:
Find matched pairs of gain setting resistors using an ohmeter to ensure that the gains are equal.
We do not need to use high precision resistors because we are not mass-manufacturing this thing. Hand matching will suffice quite well enough, and in fact can give better than 0.1% results.
  Reply With Quote
Old 17th March 2004, 02:19 AM   #7
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
Quote:
For 88dBm speakers and the room you specified single IC LM3875 should be just fine for 6-8 Ohm speakers.
Judging by the load in the schematic loong posted, I would assume that the speakers are 4 ohm. In that case, two parallel 3875's will perform much better than one.
  Reply With Quote
Old 17th March 2004, 05:22 AM   #8
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
Quote:
Originally posted by macboy
We do not need to use high precision resistors because we are not mass-manufacturing this thing. Hand matching will suffice quite well enough, and in fact can give better than 0.1% results.
Well then you had better describe the procedure to loong who described himself as a newbie on how to get better than 0.1% match with what probably is his 3-1/2 digit multimeter...
__________________
Aerodynamics are for people who can't build engines. Enzo Ferrari
  Reply With Quote
Old 17th March 2004, 12:23 PM   #9
diyAudio Member
 
bigparsnip's Avatar
 
Join Date: Apr 2003
Location: Cambridge
One thing that I would sujest which may be of some use here, is to use seperate input caps for each chip (rather than the single, common one currently being used). This will allow slightly diferent input bias curents to flow into each chip (which won't be identical) without creating a potential difference between the two. But, you would still need to use very close tolerance resistors to set teh feedback for the circuit.
  Reply With Quote
Old 17th March 2004, 01:57 PM   #10
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
Quote:
Well then you had better describe the procedure to loong who described himself as a newbie on how to get better than 0.1% match with what probably is his 3-1/2 digit multimeter
Point taken. I made the assumption that he had access to a decent multimeter, but heh, you made the assumption that he had 0.1% resistors in his parts drawer... or at the local supply house. I think that more people have access to a 4.5 digit or better DMM than to .1% resistors. It doesn't matter though. We agree that the gain setting resistors must be very close in value so that the gains are equal and don't cause problems. That's really the point. Whether they are expensive high precision devices or just hand-matches cheapies is not.
  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
need advise: 3-channel tripath with 12V power supply smoerk Class D 7 17th August 2007 04:06 PM
Is a bridge / parallel configuration of 3875 is better for my speaker ? 100db Chip Amps 7 8th March 2006 11:42 AM
I need advise for three channel Parallel LM4780 project. Padel Chip Amps 8 22nd November 2005 12:27 AM
Parallel 3875 or single 3886 for low impedance speakers? Groucho2004 Chip Amps 2 17th July 2004 09:31 PM
==>> Parallel 4x LM3875 for 2 ohm speaker - pls advise.. metha Chip Amps 24 18th April 2004 09:29 PM


New To Site? Need Help?

All times are GMT. The time now is 02:12 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