Little opamp/darlington amplifier - 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 6th November 2008, 12:33 AM   #1
b_force is offline b_force  Netherlands
diyAudio Member
 
Join Date: Jan 2004
Location: Rijswijk
Default Little opamp/darlington amplifier

I was looking for a amplifier with great THD results, but also not that complex.

Because I will only use it for measurements (THD, tests, etc etc).
5W-8ohm/10W-4ohm would be enough.
(maybe I will go to 10W @ 8ohm)

The idea is very simple, use a decent opamp (opa134/lm4562 etc etc) and put a current amplifier behind it.

I first tried a Mosfet follower but found out that the currect was very big.
So I tried a darlington pair, and I think it's better.

Click the image to open in full size.

This is simulated with the ZTX605/705. I think the ZTX651/751 would be better. (because of the current)

I have a few questions.
Is this going to work this way ?
What does C2 (1uF) do exactly ? (in the simulation it reduces the THD enormous ! )
Are there any other improvements ?
  Reply With Quote
Old 6th November 2008, 12:46 AM   #2
diyAudio Member
 
Stuart Easson's Avatar
 
Join Date: Oct 2002
Location: Santa Cruz, CA
Send a message via MSN to Stuart Easson Send a message via Yahoo to Stuart Easson
Default well...

I may be wrong, but I think your diodes are backwards, and you may find that a third one will lower the distortion further...

IIUC they are to provide a some bias voltage for the output transistors, and as they are drawn they are reversed biased by R4 and R5, and therefore not having the desired effect. I think perhaps you can see the effect of the excess bias voltage in the idle current of 635ma, shown on the meters for the voltage rails.

If you connect the output of the opamp to the junction of the diodes, reversed, you can avoid loading the opamp at idle and depending on it's characteristics things will be different. Adding a resistor from the output of the opamp to one rail or the other will force it to operate with the output in class A. Not all opamps like this, and you may get different results depending out which rail and the value of the resistor.

The cap across the diodes helps discharge the base current from the output transistor that is turning off, without it the output is much slower in transition from full one direction to the other.

HTH

Stuart
  Reply With Quote
Old 6th November 2008, 12:55 AM   #3
b_force is offline b_force  Netherlands
diyAudio Member
 
Join Date: Jan 2004
Location: Rijswijk
Thank you for your comments, let's change it and look at the results

I thought that the ZTX651/751 where also Darlington,s but I think that's a mistake.

are there other Darlington pairs that can easy deliver 2-3A ?

I think it is not necessary to operate the opamp in class-a because the THD of the opamp is also very low.
Maybe a improvement later


edit:
The diodes are connected the right way. If I connect them backwards the THD is much higher.
There is also no difference between 2 or 3 diodes in the simulation. (but that's not the real world)

What kind of heatsink can I use ?
  Reply With Quote
Old 6th November 2008, 01:25 AM   #4
diyAudio Member
 
Stuart Easson's Avatar
 
Join Date: Oct 2002
Location: Santa Cruz, CA
Send a message via MSN to Stuart Easson Send a message via Yahoo to Stuart Easson
Default not so fast...

Unless I am missing a key point...

The distortion may be higher with the diodes reversed, but the output transistors will not be passing >600ma of current. I think what you have right now is a very improperly biased classB amp. It only works because the models for the transistors are unrealistic and the bases are fed from 100k resistors.

What is the idle current with the diodes reversed, I think it will drop to almost zero, indicating a 'normal' class B amp, then you have to bias the output transistors properly to minimize the crossover distortion. If you want to make an amp with a classA output stage you need to arrange the bias conditions with more control and less dependence on the sims transistor model.

Good luck

Stuart
  Reply With Quote
Old 6th November 2008, 02:00 AM   #5
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
What you are doing here is similar to this:

http://sound.westhost.com/project76.htm

and this

http://sound.westhost.com/project113.htm

In your diagram, the diodes are the wrong way around - the circuit would not work in real life.

I've had good results with using a diamond buffer on the end of an opamp. See for example Per-Anders Sjöström's QRV05 circuit.
  Reply With Quote
Old 6th November 2008, 10:02 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Eline output transistors that can dissipate 1.5W, if you can keep them at 25degC, will not be man enough for an 8W output.

With ClassAB amplifiers it is usual to have the output devices able to dissipate 2 to 3 times the maximum output power. This would indicate you need 15 to 30W devices.
__________________
regards Andrew T.
  Reply With Quote
Old 6th November 2008, 01:18 PM   #7
mlloyd1 is offline mlloyd1  United States
diyAudio Member
 
mlloyd1's Avatar
 
Join Date: Feb 2001
Location: illinois
the diodes are the wrong way around.
think about the actual function of the diodes.
you can certainly build it (instead of simulating), but we would recommend a current limited power supply and some spare output devices.
also, don't hook up your good speakers just yet either ...


mlloyd1
  Reply With Quote
Old 6th November 2008, 03:08 PM   #8
diyAudio Member
 
jerluwoo's Avatar
 
Join Date: Feb 2006
Location: Ashland,Ky U.S
Those Zetex devices would die a horrible death as output devices. The low distortion you see with the diodes wrong way round is because the outputs are operating in class A. Those poor Zetex darlingtons are cooking with 9+ watts of heat on them. Think they are only good for 2 watts at most, haven't looked them up but I think thats right. With the diodes the correct way round the outputs idle at cutoff which will show as very high crossover distortion, thus the reason you see a huge increase in your distortion sims. You need 4 diodes to get a correct forward bias using darlingtons. Remember a darlington is two transistors stacked together so they have two BE drops per device. Attached is a circuit that would be a much better starting point for your experiments.
Attached Images
File Type: gif opamp.gif (32.1 KB, 1064 views)
  Reply With Quote
Old 6th November 2008, 04:33 PM   #9
diyAudio Member
 
Stuart Easson's Avatar
 
Join Date: Oct 2002
Location: Santa Cruz, CA
Send a message via MSN to Stuart Easson Send a message via Yahoo to Stuart Easson
Default it's like there's...

...an echo chamber here or something...

Your idea is sound, but you need to understand the limitations of the simulation environment vs a real construction. There are physical boundaries the simulation is not honoring, in this case, the output transistors being cooked etc.

Even if you can find a huge set of darlington output devices, Motorola have 300w devices I think, they will not behave the way the simulation suggests they should. The pair will have dissimilar gain, Vbe etc that will seriously compromise the real world behavior of the amp.

If you are willing to look around a little more you can easily find a stable output stage onto which you can graft the opamp input. If simple is the goal, Class A will probably get you the best bang for your part count, but you need more power and heatsinks. Class B will need more attention to biasing detail, but has lower power needs and smaller heatsinks.

From the Passlabs arena you might try an output mosfet loaded by a CCS; very easy to get stable, simulates well and adds very little distortion as long as you set the CCS properly...

HTH

Stuart
  Reply With Quote
Old 6th November 2008, 06:29 PM   #10
diyAudio Member
 
ostripper's Avatar
 
Join Date: May 2008
Location: Albany , NY (smallbany)
This amp will most likely work, but having built a similiar one,
it will sound like garbage.

After having a heated discussion with our beloved carlos (DX)
about how crappy a darlington amp sounded, I decided to build
this:elector darlington amp
on a veroboard using 100w sanken devices from a dead sony
HT unit. The elector amp has a real Vbias generator and
LPT, so I figured it would sound OK.
Once it was built I almost conceded to carlos's opinions...
crossover distortion, harsh sound..
I was discouraged.

But, later, still wanting to give the sanken devices a home,
I came across the LM4702 and this circuit:LM4702 darlington amp
Much to my surprise, it worked and sounded very good!!
playing around, I subbed tip120/125's, a Real EF OP
section, and various other junkbox outputs into my prototype
and they all sounded better than the HT receiver.(same speakers,
same music.)

In conclusion, for the same price as a couple of HQ op-amps
the LM4702 (7-8 US$$) will give you stereo you can use and
enjoy while giving a home to all your orphaned darlingtons.

BTW.. In a future DIY project I'm thinking of a 4 OP device EF or
CFP (maybe even Thermal trak) variation of the 4702.
OS
  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
Darlington TIP142 & 147 power amplifier. Matthewong Solid State 18 30th November 2013 08:14 AM
is darlington pair is unsuitable for audio amplifier? boxedin Solid State 29 13th October 2006 12:00 AM
need darlington amplifier schematics mbates14 Solid State 7 2nd December 2004 12:54 AM


New To Site? Need Help?

All times are GMT. The time now is 05:03 AM.


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