LM3875 Chipamp - 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 25th March 2015, 01:22 PM   #1
Philfr is offline Philfr  France
diyAudio Member
 
Join Date: Dec 2012
Default LM3875 Chipamp

Hi all,
Planning to build LM3875 amp with pcb from Chipamp..
Non-Inverting LM3875 Stereo Kit | Chipamp Electronics
To drive small "monitor" speakers (6 Ohm), I dont need much power, maybe 5/10 watts.
First time building LM3875, I will use toroid 80Va 12 vac, and snubberized PSU :

Click the image to open in full size.

Amp schematic :

Click the image to open in full size.

About Cs (1500 uF), not sure it's the good value for my use ?
The goal is to have a very smooth response (500 to 4000 hz), sounstage is not a problem, just tone amp.
Your advice ?
Phil.
  Reply With Quote
Old 25th March 2015, 01:32 PM   #2
diyAudio Member
 
Mark Whitney's Avatar
 
Join Date: Feb 2013
Location: Netherlands
Have a go at designing/building a P2P version. It is more fun and you learn a lot more.
__________________
Regards Mark.
  Reply With Quote
Old 25th March 2015, 03:22 PM   #3
johnr66 is offline johnr66  United States
diyAudio Member
 
Join Date: Mar 2009
Only 5 to 10 watts? I'd use the LM1875. With 12.6-0-12.6 2A transformer you'd get around 12 watts of unclipped power both channels driven continuous into 6 ohms.

Either way, use single bridge rectifier. Why incur four diode drops?

Decouple IC with film caps near to the IC. If main filter caps are more than 10cm from the IC use ~330uf 'lytics near the IC. Audio doesn't need DC coupling. Use caps as shown on datasheet or risk other complications.

Add RF lowpass on input and don't forget proper ground layout.
  Reply With Quote
Old 25th March 2015, 03:58 PM   #4
diyAudio Member
 
Join Date: Jul 2014
.
I have to agree with johnr66. The LM1875 seems much better suited to your requirements.

At the same time I wonder. Is there some specific design goal that calls for the changes you made to the "Typical Application" circuit in the data sheet?

The chip and the circuit it works in are designed side-by-side by a team at least one senior, and several junior, engineers. They work in a lab equipped with tens of thousands of dollars worth of test equipment. The chip they finally design will be manufactured in the millions, there's no room for error.

All of which is boiled down for we DIYers into the "Typical Applications" circuit. A simple little thing with hundreds of thousands of dollars in research and development costs behind it.

If you just want to fool with a chip, do this or that and see what happens, well then I say go ahead and power to you. That's what these forums are for.

But if you just want to get the job done and go on to something else, then I can see no reason to change what the factory--which created the chip in the first place--recommends. Doing so is just throwing away a very large amount of engineering expertise, which seems pointless unless it's required by some specific design goal.
.

Last edited by bentsnake; 25th March 2015 at 04:01 PM.
  Reply With Quote
Old 25th March 2015, 04:03 PM   #5
Philfr is offline Philfr  France
diyAudio Member
 
Join Date: Dec 2012
Hi Mark,
I had thought P2P, but I need a huge magnifying glass

Hi John,
Why LM3875 and this psu, cause I have.
I will add a safety loop breaker and a "proper" ground.
"If main filter caps are more than 10cm from the IC use ~330uf 'lytics near the IC"
I keep your advice.
Thank's.
Phil.
  Reply With Quote
Old 25th March 2015, 04:20 PM   #6
Philfr is offline Philfr  France
diyAudio Member
 
Join Date: Dec 2012
Hi,
Quote:
Originally Posted by bentsnake View Post
.
At the same time I wonder. Is there some specific design goal that calls for the changes you made to the "Typical Application" circuit in the data sheet?
.
Maybe you are right, but I build several LM3886, with "Typical Application" and other by changing some value or quality componant, I prefer "tweaking".
My previous amp :
LM3886 "fullrange"
Everything is changed regarding typical schematic, and it's sound better (for my taste)..
And this has been possible with the help received here !
Phil.
  Reply With Quote
Old 25th March 2015, 07:53 PM   #7
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by Philfr View Post
Hi all,
Planning to build LM3875 amp with pcb from Chipamp..
Non-Inverting LM3875 Stereo Kit | Chipamp Electronics
To drive small "monitor" speakers (6 Ohm), I dont need much power, maybe 5/10 watts.
First time building LM3875, I will use toroid 80Va 12 vac, and snubberized PSU :

Click the image to open in full size.

Amp schematic :

Click the image to open in full size.

About Cs (1500 uF), not sure it's the good value for my use ?
The goal is to have a very smooth response (500 to 4000 hz), sounstage is not a problem, just tone amp.
Your advice ?
Phil.
DC coupled.
Do you intend installing output offset detection?
Do you intend installing load protection?

No RF attenuation.
No HF supply rail decoupling.
No output inductor with damping resistor.

You have shown Cz connected to the NFB connection and to the Signal Hot connections. These two should actually be connected to the Signal Return circuit, but you have omitted that.
__________________
regards Andrew T.

Last edited by AndrewT; 25th March 2015 at 07:55 PM.
  Reply With Quote
Old 25th March 2015, 10:33 PM   #8
diyAudio Member
 
Join Date: Jul 2014
Quote:
Originally Posted by Philfr View Post
I prefer "tweaking"
Works for me. Like I said before, fire another broadside and no prisoners. Or I might not have said exactly that, but I would have if I'd thought of it at the time.
.
  Reply With Quote
Old 25th March 2015, 10:52 PM   #9
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
If you want better stability near clipping, I suggest adding Cc, Rf2, and Cf as shown in attached schematic.

If you'd like your amp to be able to drive capacitive loads (long speaker cables, for example), I also suggest adding a Thiele network (L || R) in series with the output. I use L = 2 uH; R = 1.5 ohm. 15 turns of AWG 18 (1.0 mm diameter), 20 mm long wound on an AA battery is about 2 uH.

I suggest putting the snubber across the transformer secondary. That's where the diode switching transients occur. 100 nF across the secondary will reduce the frequency of the ringing to the point where it doesn't couple anywhere. You can optimize a snubber there if you feel like it, but in my experience, it makes no difference on the supply output (or amp output for that matter). This is the subject of great debate, so others may disagree...

With a 12 VAC transformer, you'll end up with about +/-16 V on the reservoir caps. That works well for a low-power LM3875, LM3876, or LM3886.

~Tom
Attached Images
File Type: png LM3875schematic.png (115.7 KB, 115 views)
__________________
Modulus-86: Composite amp achieving 0.00014 % THD. Damn Good 300B, Novar Spud, 21st Century Maida Reg., Filament Reg., etc.
Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

Last edited by tomchr; 25th March 2015 at 10:55 PM.
  Reply With Quote
Old 26th March 2015, 08:04 AM   #10
Philfr is offline Philfr  France
diyAudio Member
 
Join Date: Dec 2012
Hi all,
At first I would build a minimized GC, but I realize that it can work perfectly !
I add Cc 220 pF (I have not 180 pF), but it's very close.
About Cf and Rf2 like Tom advises, I will try without and with, to hear the difference ?
At beginning, I build amp on wood board, like this :

Click the image to open in full size.

So, it's easy to change something..
I agree, low-power with LM3886 works well, I have one working with 17/0/17 volts..

Cable are 1,5 m lenght, so Thiele network is not needed.. On the pict above I have.

Tom, you advise add a cap 100 nF across secondary, I thought polypro quality ?
Pcb from Chipamp are the same as Audiosector :

Click the image to open in full size.

Phil
  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
LM3875 Audio sector ChipAmp Insight (and other useful info) Transconductance Chip Amps 8 9th December 2013 09:56 PM
LM3875 chipamp board incl power supply lgreen Swap Meet 1 11th December 2009 06:23 PM
Chipamp for sub, parallel lm3875 or lm3886 coolbeer Chip Amps 20 11th December 2009 04:29 PM
Newbie question regarding LM3875 chipamp Rev 3 PCB colinB Chip Amps 6 27th September 2006 08:41 AM
Completed and cased LM3875 chipamp for sale. CJ Paul Swap Meet 0 10th July 2006 07:30 PM


New To Site? Need Help?

All times are GMT. The time now is 11:39 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2015 DragonByte Technologies Ltd.
Copyright 1999-2015 diyAudio

Content Relevant URLs by vBSEO 3.3.2