bias in leach amp - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 30th January 2011, 08:54 PM   #1
diyAudio Member
 
Join Date: Jan 2011
Default bias in leach amp

yes its me again with yet another stupid question (at least i think so)

I have finally built one channel of the leach super amp, and dammit it works.

but i dont understand how to set the bias. I have read other peoples posts and some are setting it to 230 mA or so ???????. in the instructions im supposed to set the bias till my DC current draw reaches 100mA at the power entrance to the board (is this correct), but at this level i see 2 problems. the output transistors get ridiculously hot even when it is at idle. When looking at the output signal (no input or anything else connected but power) on the oscilloscope there is some sort of noise/distortion surrounding the flat line, if i turn it down to just below the threshold where the "noise" disappears the transistors run a lot cooler, but my draw drops to approx 30mA. what am i doing/not doing right

thank you once again for your help
  Reply With Quote
Old 30th January 2011, 10:40 PM   #2
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
Is the behavior the same when the input is shorted? It sounds like you are either getting some RF into the front end or you have an oscillation issue.

Pictures of your layout would be useful
  Reply With Quote
Old 30th January 2011, 10:45 PM   #3
diyAudio Member
 
Join Date: Jan 2011
Quote:
Originally Posted by BobEllis View Post
Is the behavior the same when the input is shorted? It sounds like you are either getting some RF into the front end or you have an oscillation issue.

Pictures of your layout would be useful
i havent tried shorting the input (im guessing you mean input signal to common ground) id love to send pics, but dont know how to put them up, or send them for that matter, is there an "attach" button i dont see on here

thanx!
  Reply With Quote
Old 30th January 2011, 11:10 PM   #4
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Having designed my own amplifiers I use my own method for setting bias.

I apply a small sine wave and monitor the output.
I turn the bias right down to start with then increase it until crossover distortion disappears on the scope. This method means the setting is not too little you get crossover distortion but not so much the heat generated is wasted.

If the problem is oscillation you will see it on the output.
__________________
PCBCAD50 software. http://www.murtonpikesystems.co.uk

Last edited by nigelwright7557; 30th January 2011 at 11:14 PM.
  Reply With Quote
Old 31st January 2011, 12:14 AM   #5
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
If you click on the "Go advanced" button there is an icon that looks like a couple mountains. That will insert a link to a picture you have uploaded to a website like photobucket or you can put in a link to your own website.

Down below in the additional options section is a button labeled Manage attachments.

Yes, connect the signal input to ground or connect it to a low impedance source as Nigel suggests. See if you can adjust the time base on your scope to find the frequency of the noise you see.

You may be seeing an oscillation due to lead length or dress. Another potential problem area is if the base stopper resistors are on the board and the leads to the bases of the output devices are more than an inch or so long. Better to mount them right on the base lead of the device.
  Reply With Quote
Old 31st January 2011, 06:07 AM   #6
diyAudio Member
 
Join Date: Jan 2011
dammit, you cant just attach a pic like everything else, i dont use photobucket, its been 2 1/2 years since i was on facebook lol

ok im going to go check out photobucket
  Reply With Quote
Old 31st January 2011, 09:33 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
manage attachments allows one to attach pics and diagrams to one's post.
If you upload your first 3 then you can repeat the process for a further 3 attachments.

Before you save them:
crop the pic to what you need us to see.
resize to as few pixels as allow us to see the details.
compress using an appropriate jpg, jpeg, png, compression system.

Then upload to DIYaudio.

Please do not use other servers. They are too slow and you eventually lose links.
__________________
regards Andrew T.
  Reply With Quote
Old 31st January 2011, 09:39 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default output bias setting

measure the bias current across the output emitter resistor, all 4 should be within 10% of total range (20 to 22mV) .
Leach tells you the voltage/current to set for Re=0r33

The Iq measured at the supply rails is the total of all +ve (or -ve) output bias plus quiescent current of the voltage amp stage plus the driver and pre-driver bias currents, plus the output offset current (if any).

Short the input.
Leave the output open circuit.
Set bias and then check Iq (volts across 1r0) and output offset (mVolts) and output noise (mVac)

Check what happens when you switch ON and OFF. You don't want big pulses of current going to your speakers at ON/OFF

Connect your Source and recheck everything with the Source switched OFF and again with the Source switched ON.
__________________
regards Andrew T.

Last edited by AndrewT; 31st January 2011 at 09:42 AM.
  Reply With Quote
Old 31st January 2011, 03:20 PM   #9
diyAudio Member
 
Join Date: Jan 2011
please excuse my horrific setup, but it is my first time, im nervous "tee hee"

1. the bias pot turned down
2. bias approx half way
3. time on scope slowed right down with bias at same level

(im reading from channel 2 if you can read my settings. (i really dont know how the hell to use that thing, but i understand it for the most part)
Attached Images
File Type: jpg IMG_1572.JPG (838.3 KB, 225 views)
File Type: jpg IMG_1573.JPG (889.1 KB, 191 views)
File Type: jpg IMG_1574.JPG (868.5 KB, 178 views)
File Type: jpg IMG_1578.jpg (263.9 KB, 175 views)

Last edited by rookie amp builder; 31st January 2011 at 03:28 PM.
  Reply With Quote
Old 31st January 2011, 03:58 PM   #10
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
You want to turn the time base the other direction to see the waveform - this is a HF oscillation. Start @.2us and work your way down to see an entire waveform if possible.

Are your base stoppers on the board or at the output devices? Those long leads are likely contributing to instability. The lead inductance and the input capacitance of the output devices form an LC resonator. Having the resistors at the base of the output devices lowers the Q of the circuit helping to prevent oscillation. Make the leads as short as possible. What devices are you using? The original devices were slower than many modern replacements. Faster devices are more likely to oscillate.

The only comment I have about your bench is clean up a bit to avoid accidentally contacting high voltage, whether that partially inserted mains plug on the left or the rail fuses on the right. We'd all love to have a bench that looks like a NASA clean room, but most look more like yours. Mine is often more cluttered.
  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
Floating Bias in Leach vs. Slone ?? sam9 Solid State 20 17th March 2004 02:56 PM
Leach amp bias diodes Borc Solid State 63 20th July 2003 03:52 PM
Leach Amp bias/star ground questions? hard times Solid State 2 14th September 2002 02:30 PM
Leach Amp bias current (what is yours set to?) BrianGT Solid State 3 21st February 2002 08:53 AM


New To Site? Need Help?

All times are GMT. The time now is 08:57 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