LM3886 Servo Design - 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 13th November 2008, 08:46 PM   #1
diyAudio Member
 
Join Date: Aug 2008
Default LM3886 Servo Design

So for my next project, I'm trying to design my own board (and learning to use Eagle at the same time). As the title suggests, it's a LM3886 with servos for removing DC offset. I used the datasheet for the LM3886 and the non-inverting servo design from AN-1192. I have finished a first revision of the schematic design.

Notes:
- there is no input shunt resistor as I plan on using a passive attenuator
- all capacitor values are in uF

Let me know what you think!
Attached Images
File Type: png servo.png (6.7 KB, 2735 views)
  Reply With Quote
Old 13th November 2008, 08:59 PM   #2
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
It's a no-no to have a floating JFet input. Tie it down with anything, 100k, 1M or whatever. You can get "unwanted" effects if the input of some reason leaves open.

D1 and D2 don't have to be schottkys, use regular diodes such as 1N4148,
__________________
/Per-Anders (my first name) or P-A as my friends call me
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote
Old 14th November 2008, 02:34 AM   #3
diyAudio Member
 
Join Date: Aug 2008
Ok, so I can add the input resistor.

The app-note lists "low leakage" diodes for the servo. It might help if I actually understood what they do in this situation.
  Reply With Quote
Old 14th November 2008, 03:03 AM   #4
infinia is offline infinia  United States
diyAudio Member
 
infinia's Avatar
 
Join Date: May 2005
Location: Californication
Quote:
Originally posted by peranders


D1 and D2 don't have to be schottkys, use regular diodes such as 1N4148,

Quote:
Originally posted by Redshift187


The app-note lists "low leakage" diodes for the servo. It might help if I actually understood what they do in this situation.


Hi D1,2 are protection for the integrator opamp. The chip amp outputs more than the opamp supplies (Vcc,Vee > +/-15 V), so it could turn on the LF411 substrate diode. Normally schottkys are prefered for this, but since R2 is so large to limit damage.
EDIT>signal diodes provide the lowest leakage, so are best here.
__________________
like four million tons of hydrogen exploding on the sun
like the whisper of the termites building castles in the dust
  Reply With Quote
Old 14th November 2008, 12:05 PM   #5
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
the input impedance of the non-inverting 3886 is very high.
If you add that Zin resistor that Peranders talks about then that will define the input impedance of the amplifier. It will also supply the input offset current.

You do not need an input buffer to feed the Zin of this amp. Delete it.

Now, choose whether the attenuator needs a buffer to drive the interconnect.
Similarly decide if your source needs a buffer to drive their interconnects.

The 411 is excellent for the servo duty.
I suspect many opamps can provide a far better buffer than the 411. Change it.

Some low power transistors (I'm not calling them signal transistors deliberately) are even better when connected as diodes than diodes are with respect to leakage.
I think it was Jung that discussed particularly good transistors for this duty.

Finally,
a DC coupled amplifier with servo control of the output offset is great as long as all components are working properly.
Add a DC detect and output isolation to protect your speakers in case anything becomes faulty. Consider also adding an input mute activated by that same DC detect circuit.
__________________
regards Andrew T.
  Reply With Quote
Old 14th November 2008, 01:04 PM   #6
diyAudio Member
 
aparatusonitus's Avatar
 
Join Date: Jan 2004
Location: Split, Croatia
A right way to protect an J-Fet servo opamp input which I've learned recently from ilimzn
Attached Files
File Type: pdf servo opamp protection.pdf (6.4 KB, 664 views)
__________________
Non é mai abbastanza...
  Reply With Quote
Old 14th November 2008, 01:22 PM   #7
diyAudio Member
 
Join Date: Aug 2008
Ok, starting on the input side. I've been reading more about servos, and it was recommended that servo amps have a low pass filter at the input so they don't oscillate at frequencies above the audio range. Does this sound like a good idea?

In the meantime I'll do more research about transistors for servo protection and DC detection circuitry.
  Reply With Quote
Old 13th January 2009, 02:05 PM   #8
diyAudio Member
 
Join Date: Aug 2008
Ok, finally got around to updating this. Here is the amp circuit with the shunt resistor, input pot, input buffer removed, transistors for op-amp protection.

In order to keep the schematics easier to read, I'm making the DC detector and protection circuit a separate schematic.
Attached Images
File Type: png servo2.png (6.5 KB, 2067 views)
  Reply With Quote
Old 6th June 2009, 09:21 PM   #9
diyAudio Member
 
Join Date: Aug 2008
Finally got back to working on this. I decided to put the voltage regulators on board, and keep the protection circuitry off board for this version. In version 2 I'll put the protection circuitry on board too.

Schematic:
Attached Images
File Type: png servo.png (8.9 KB, 1787 views)
  Reply With Quote
Old 6th June 2009, 09:29 PM   #10
diyAudio Member
 
Join Date: Aug 2008
Silkscreen and component placement:
Attached Images
File Type: png servo-silk.png (36.4 KB, 1710 views)
  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
PCB design of parallel of two inverted LM3886 with buffer and dc-servo. Roushon Chip Amps 74 11th April 2012 11:17 AM
Gainclone LM3886 DC servo peranders Chip Amps 10 24th March 2004 04:59 PM


New To Site? Need Help?

All times are GMT. The time now is 12:11 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