Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Chip Amps Amplifiers based on integrated circuits

lm3886 servo help
lm3886 servo help
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 7th December 2017, 02:01 PM   #21
AndrewT is offline AndrewT  Scotland
R.I.P.
 
Join Date: Jul 2004
Location: Scottish Borders
Measure pin7 to pin4.
If it reads 45V, that's 9volts above absolute maximum rating, you may have damaged the lf411.
Check the pin out of the 79L15 and how you have inserted it.
__________________
regards Andrew T.
  Reply With Quote
Old 7th December 2017, 02:13 PM   #22
tomchr is offline tomchr  Canada
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Calgary
Are the decoupling caps for the local regulators close to the regulators?

Also note that the pinouts for the positive and negative regulators are different. Pin 2 of the package is the lowest voltage on the regulator die. Thus, for the positive regulator pin 2 is GND, whereas for the negative regulator it's -Vin.

Tom
__________________
Modulus-686: 240W/8Ω @ <0.00025% THD. Modulus-86: 40W/8Ω @ 0.000054% THD. DG300B & Other Tube Circuits.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
  Reply With Quote
Old 7th December 2017, 03:10 PM   #23
Drummer 35 is offline Drummer 35  Spain
diyAudio Member
 
Join Date: Oct 2016
Location: Madrid, Spain
Well, after some debugging I found that removing diode across pins 2 and 3 of the 79L15 did the trick. I think I put it wrong way and let the ps -v go directly to the 411 pin 4.

At the moment I get the required voltage without the diode, so I think Iīll leave it this way.

Tom, the decoupling caps are close to the regulators, so no problems here.

Andrew, exactly, I got around 47v between pins before solving the problem, but not sure if they are toasted. Iīm measuring offset between pins 9 and 10 and I got like 0,2-0,3mv with and without sound source playing. You think the 411s are working or not.

Whats the best procedure to measure offset?

Thanks again for your help.

Last edited by Drummer 35; 7th December 2017 at 03:19 PM.
  Reply With Quote
Old 7th December 2017, 04:38 PM   #24
tomchr is offline tomchr  Canada
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Calgary
The diode across the regulator is recommended to make sure that any capacitors on the load side are drained of charge through the diode rather than through the regulator when the circuit is turned off. I suggest soldering the diode back in. Just make sure you turn it the right way.

Tom
__________________
Modulus-686: 240W/8Ω @ <0.00025% THD. Modulus-86: 40W/8Ω @ 0.000054% THD. DG300B & Other Tube Circuits.
Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
  Reply With Quote
Old 7th December 2017, 07:09 PM   #25
Drummer 35 is offline Drummer 35  Spain
diyAudio Member
 
Join Date: Oct 2016
Location: Madrid, Spain
yeah Tom, I did.

Now trying to figure out if the servo is working or not.

Help is welcome.

Thanks!!
  Reply With Quote
Old 8th December 2017, 07:48 AM   #26
AndrewT is offline AndrewT  Scotland
R.I.P.
 
Join Date: Jul 2004
Location: Scottish Borders
measure the output offset and measure the opamp output voltage.
report here.
__________________
regards Andrew T.
  Reply With Quote
Old 8th December 2017, 09:58 AM   #27
Drummer 35 is offline Drummer 35  Spain
diyAudio Member
 
Join Date: Oct 2016
Location: Madrid, Spain
Some updates.

Tested the amp with no 411s and the offset goes to 12mv and 20mv on the other channel.

Put back the 411s and Im reading 0,3mv and 0,8mv. Turn the volume pot and the offset goes up, and then drops again to those values, so I think itīs working.

Had to change some resistors to set a gain of 34db since my sound source is too low on voltage.

Im stunned with the sound, and even after increasing the gain, I canīt hear any distortion or quality drop.

Andrew, Tom; not only in this thread, but all your posts in that forum helped me a lot.

Thanks!!
  Reply With Quote
Old 8th December 2017, 10:17 AM   #28
AndrewT is offline AndrewT  Scotland
R.I.P.
 
Join Date: Jul 2004
Location: Scottish Borders
measure the opamp output voltage on both channels.

output offset is measured with no input signal and the input loaded with a zero ohm dummy plug. The output is disconnected from the speaker/load.

Then it is worth checking with the input connected with a cable but an open end and then with a Source connected first with volume turned down to zero and then at ~ -6dB ref maximum (no speaker or it might be too loud). If the output is near clipping when vol is set to -6dB, then you have far too much gain in the amplifier.

+34dB gain for a 20Vac output (50W into 8ohms) needs only 400mVac as Maximum input. That's a very sensitive setting !
__________________
regards Andrew T.

Last edited by AndrewT; 8th December 2017 at 10:23 AM.
  Reply With Quote
Old 8th December 2017, 10:20 AM   #29
Drummer 35 is offline Drummer 35  Spain
diyAudio Member
 
Join Date: Oct 2016
Location: Madrid, Spain
Quote:
Originally Posted by AndrewT View Post
measure the opamp output voltage on both channels.
Will do.

How much should I read?
  Reply With Quote
Old 8th December 2017, 10:32 AM   #30
AndrewT is offline AndrewT  Scotland
R.I.P.
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by Drummer 35 View Post
............

How much should I read?
If the inherent output offset were zero, then the opamp output would be near zero mV ref audio ground.
But we know the servo is applying some correction (injecting current) to bring the output offset from tens of mV to <1mV
That implies some opamp output voltage.

If the output voltage is over half of opamp rail voltage, then you should be looking at changing the injection resistor value.
If it's already near rail voltage, then you have a serious problem and need to re-design the servo and/or the power amplifier.

But your correction is very small so I would expect the opamp output voltage to be less than 1000mV ref audio ground.
In which case you could be looking at increasing the injection resistor value.

The injection resistor bypasses the normal feedback resistor. Any noise, or distortion, or remaining audio signal that passes from the opamp through the injecting resistor is applied to the -IN input and gets multiplied by the amplifier gain.

A larger injection resistor reduces the unwanted signal relative to the normal feedback signal.
A too large injection resistor makes the opamp work harder and in the extreme forces the opamp output towards the supply rail, after which it cannot provide an further correction.

BTW,
A DC coupled amplifier needs speaker protection. The DC servo simply minimises the output offset when everything is working correctly.
You NEED speaker protection for the rare occasion when something is not working correctly.
eg.
window comparator monitoring the opamp output voltage and turning ON a warning beacon/klaxon when opamp output voltage exceeds 50% of rail voltage.
a second window comparator monitoring opamp voltage and shuts down the amplifier when output exceeds 90% of rail voltage OR output offset detection that isolates speaker when a threshold is exceeded.
__________________
regards Andrew T.

Last edited by AndrewT; 8th December 2017 at 10:47 AM.
  Reply With Quote

Reply


lm3886 servo helpHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
LM3886 gainclone DC servo belyakove Chip Amps 24 16th December 2017 09:16 PM
LM3886 Servo Design Redshift187 Chip Amps 67 3rd September 2017 11:13 AM
LM3886, XO, Buffer and Servo jackinnj Chip Amps 3 30th October 2008 12:14 AM


New To Site? Need Help?

All times are GMT. The time now is 06:35 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki