Class A gainclone/chipamp idea - 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 14th February 2004, 05:56 PM   #1
Steven is offline Steven  Netherlands
diyAudio Member
 
Steven's Avatar
 
Join Date: Oct 2003
Location: The Netherlands
Lightbulb Class A gainclone/chipamp idea

I would like to present the circuit below just as an idea. I have not build it, and it may take a while before I do, if ever. I am actually more interested in discrete circuits.

But, browsing the Chip Amps forum, I wondered if someone had ever put these LM3875's and LM3886's in class A. After a search I did not find such a thing, although some people connected a 1.5k resistor from the output to the negative supply, with the intention to run the amp a little in class A. I think that won't work, because the current sink is far too little, about 20mA. This is close to the actual quiescent current of the chip, leaving the lower output transistor just starving for current. Not good.

I think a better way is to use two amplifiers in parallel connected at the outputs via two 0.22 Ohm resistors. One chip gets a positive offset of around 220mV, the other an offset of -220mV. The DC gain is 1x, so the offset at the non-inverting input is just copied to the output: one output at 220mV, the other at -220mV. In this way the loudspeaker output will be at 0V DC, which is OK, and an idle current of approx 1A will flow from one output into the other. The amplifier with the positive offset will have its upper (positive rail) output transistor running in class A, the other chip will have its lower (negative rail) output transistor in class A. The other output transistors are not conducting at all. Having such an idle current of 1A, a maximum peak current of 2A can be delivered to the load (16W sine in 8Ohm). Using a power supply of +/-20V each chip is dissipating some 20W idle. This is acceptable.

The offset voltages are derived from the power supply by the two transistors. Although the base voltage will change a little with an unregulated supply, the collector voltage will be quite stable, due to the action of the 33 Ohm resistor, which compensates for Vbe modulation. This gives an improvement of about 20 times less ripple on the output (collector). The 22k and 12k resistors and the capacitor bring the collector voltage of 650mV back to 220mV with low noise.
The offset voltage has a negative temperature coefficient, which is good. The amplifier idle current will be slightly lower for increasing temperatures.

The accuracy of the gain determining resistors around the amplifiers should be 0.1% or better, otherwise the the fixed offset of 440mV between the outputs will change with output signal. This 440mV is required to keep the bias current flowing.

To get an accurate 0V at the output it is possible to replace R4 and R15 with a single preset potentiometer with the wiper to ground. Then it is possible to adjust the output exactly to 0V while keeping the bias current current of 1A unaltered (the 440mV will remain stable; only the center moves). An alternative method would be to use only a 220mV bias for the first amplifier and use a servo integrator with its input to the load for the second. This integrator will adjust the offset of the second amplifier (around –220mV) such that the output will be at 0V.

The waveforms shown are the voltage output on both the amplifiers (green and yellow) and at the load (blue) for a low output voltage. Ortherwise the offset is hardly visible.
The second waveform picture shows the output currents of both amplifiers (green and yellow) and the load (blue) at maximum output for class A operation. It is clearly visible that the output current of the amplifiers never cross 0A, that is the output current is unidirectional. In this case this is a condition for class A.

The amplifiers can be any chipamp you like, as long as they act like an opamp with both inverting and non-inverting inputs available. I didn’t draw any other connections, like the ground connection or mute connection for a LM3886. The power supply voltages and bias current has been selected with the LM3886 in mind though.

Steven
Attached Images
File Type: gif gainclonea.gif (21.8 KB, 4250 views)
  Reply With Quote
Old 14th February 2004, 06:24 PM   #2
diyAudio Member
 
Pjotr's Avatar
 
Join Date: Nov 2002
Location: Netherlands
Hmm Steven,

Interesting idea, but what about the build-in thermal protection of the LM3886’s (the “SPIKE” protection mechanism)? It will spoil your idea IMHO unless you want not much power from them. Probably you are better off with external power transistors biased in class-A. There was once a driver IC for it, the LM391. It is not in production anymore but the datasheet is still there:

http://www.national.com/ds/LM/LM391.pdf

It is intended for class-AB but with some mods to the bias circuit it can be used for class-A as well.

Cheers
  Reply With Quote
Old 14th February 2004, 07:30 PM   #3
Steven is offline Steven  Netherlands
diyAudio Member
 
Steven's Avatar
 
Join Date: Oct 2003
Location: The Netherlands
Hi Pjotr,

The idea is just a general idea, not only for an LM3886. I am normally more interested in discrete amplifiers, but wanted to add something to the Chipamps forum. Otherwise they will never have the benefits of class A.

Nevertheless I think the SPiKe protection leaves room enough for class A opeartion. Notice that the power rails were lowered to a mere+/-20V. At 20V collector-emitter more than 2.5A collector current is allowed for a LM3886, even at Tc=75*C. AT Tc=125*C it is still slightly less than 2A. So a bias current of 1A will not be a problem.

The circuit uses two amplifiers in parallel, remember, for only 16W output.

Steven
  Reply With Quote
Old 14th February 2004, 08:06 PM   #4
Pedja is offline Pedja  Serbia
diyAudio Member
 
Join Date: Jun 2002
Location: Belgrade
The offset could be the problem. Not in the term of the basic operation (it will, certainly, work), but 220mV offset could easily alter the sonic performance. Opamps often do not sound very well with it.

Pedja
  Reply With Quote
Old 14th February 2004, 08:14 PM   #5
Account Disabled
 
Join Date: Apr 2003
Location: US
Default Re: Class A gainclone/chipamp idea

Quote:
Originally posted by Steven
I would like to present the circuit below just as an idea.

Steven

I think it will work. I proposed something similar like this on the solid state forum. Essentially, you want to intentially introduce a DC offset to the input to "float" the output and allow it to happen only in one half of the rail to avoid cutting off of output devices -> class a.

Introducing a current source is as good of an idea on paper but in practice it is tougher to get a current source at high amperage.

Another way to configure the same thing is to use two chips, and apply the same DC offset to both chips in exactly the same fashion. Except that in the 2nd chip, you use no signal (or if you want, use the other input for a quasi-balance set-up). This way, the difference between the two chip's output is exactly your desired audio signal -> like one of those X-setup.
  Reply With Quote
Old 14th February 2004, 11:17 PM   #6
Steven is offline Steven  Netherlands
diyAudio Member
 
Steven's Avatar
 
Join Date: Oct 2003
Location: The Netherlands
Quote:
Originally posted by Pedja
The offset could be the problem. Not in the term of the basic operation (it will, certainly, work), but 220mV offset could easily alter the sonic performance. Opamps often do not sound very well with it.

Pedja
Actually, I don't see that problem. An amplifier like the LM3886 has quite a big common mode input range. For that amplifier it will almost certain make no difference at all whether both inputs are at 220mV (or -220mV) instead of at 0V. If that would be a problem then a difference of 220mV in supply voltage (positive versus negative) also would be a problem, and it does not. The only real difference is that the output is also at 220mV, creating an additional DC output current. That might change the sonic behaviour, but that is by intention, to make it class A.

Steven
  Reply With Quote
Old 14th February 2004, 11:20 PM   #7
Steven is offline Steven  Netherlands
diyAudio Member
 
Steven's Avatar
 
Join Date: Oct 2003
Location: The Netherlands
Default Re: Re: Class A gainclone/chipamp idea

Quote:
Originally posted by millwood

Another way to configure the same thing is to use two chips, and apply the same DC offset to both chips in exactly the same fashion. Except that in the 2nd chip, you use no signal (or if you want, use the other input for a quasi-balance set-up). This way, the difference between the two chip's output is exactly your desired audio signal -> like one of those X-setup.
I don't get it. Do you propose to have a big DC current through the load?

Steven
  Reply With Quote
Old 14th February 2004, 11:40 PM   #8
Account Disabled
 
Join Date: Apr 2003
Location: US
Default Re: Re: Re: Class A gainclone/chipamp idea

Quote:
Originally posted by Steven


I don't get it. Do you propose to have a big DC current through the load?

Steven

yes and no. Creat a big DC offset right on each amp. so each amp will be working on its positive half only (you can do negative half but for discussion purposes just stay with the positive half for now). one amp will have just the DC offset, the other the DC offset + the signal. The load will bridge the two outputs so you get nothing but the signal.

Think of each amp as a transistor in the differential pair. One has the input signal on its base and the other doesn't. And the load is cross the collectors of two such transistors.

Or one of those Zen amps with unbalanced input.

The only purpose to have the 2nd amp is to get rid of the DC offset on the load.
  Reply With Quote
Old 15th February 2004, 01:00 AM   #9
Pedja is offline Pedja  Serbia
diyAudio Member
 
Join Date: Jun 2002
Location: Belgrade
Quote:
Originally posted by Steven
Actually, I don't see that problem. An amplifier like the LM3886 has quite a big common mode input range. For that amplifier it will almost certain make no difference at all whether both inputs are at 220mV (or -220mV) instead of at 0V. If that would be a problem then a difference of 220mV in supply voltage (positive versus negative) also would be a problem, and it does not. The only real difference is that the output is also at 220mV, creating an additional DC output current. That might change the sonic behaviour, but that is by intention, to make it class A.

Steven
Steven,

It is not a problem with the common mode input range. I said the circuit will work. I have tried to point out the one possible soundwise problem with the configuration you have proposed. Opamps often do sound different with or without or with more or less DC offset. Some decent sounding opamps sound terrible with a few hundreds of mV of DC sent to one of their inputs. At the other hand, certain difference between the positive and negative supply voltages is indeed usually quite benign. I think at this moment I can not give you much good reasons why is this so. Note that I am usually making equipment to please the ear, not the theory. I use the theory to realize what I have heard but I am not always (if ever) quite successful with it. I could be probably more successful with theory, but only deliberately neglecting what I hear.

Pedja
  Reply With Quote
Old 15th February 2004, 11:57 AM   #10
Steven is offline Steven  Netherlands
diyAudio Member
 
Steven's Avatar
 
Join Date: Oct 2003
Location: The Netherlands
Quote:
Originally posted by Pedja

Some decent sounding opamps sound terrible with a few hundreds of mV of DC sent to one of their inputs. At the other hand, certain difference between the positive and negative supply voltages is indeed usually quite benign.
Pedja,

Thanks for your comment, but I do not understand it. I still think this is just a matter of input common mode range. If you apply let's say 200mV DC to one of the opamp inputs, the other input has to follow because of feedback. Otherwise the output would be sticking to one of the supply rails. Then you have two inputs and the output at 200mV (DC gain is supposed to be unity, as in the circuit shown), which is perfectly OK for an opamp, as long as it fits in its common mode range. If the output would be AC-coupled then the push and pull output currents would be the same as if the ouput would have been at 0V level, or any other DC level. The only difference would be that the maximum voltage swing would be different towards the positive supply rail as to the negative rail. For most opamps the difference in dropout voltage to the supply rails is already more than 200mV between the positive and the negative rail. I think this is not issue here.
As I said in my previous post, the only real difference that a DC offset brings at the output is when the output is not AC-coupled but DC-coupled and a big offset current is flowing. I can imagine that an opamp (chipamp, gainclone) will sound different then, because the push and pull output currents will not be equal anymore on average. But that was exactly the intention of the proposed circuit.
Please note that issue here is current offset, not voltage offset. The 220mV voltage offset has just been applied to get 1A current offset for class A operation, via the 0.22 Ohm resistor. I could have got a 1A current offset by connecting a 1A CCS to the output of the amplifier and kept a DC output voltage of 0V. An offset voltage wil not move an amplifier into class A, only an offset current does.

Steven
  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
Hum to noise problem with class A chipamp ionomolo Chip Amps 5 2nd June 2008 04:48 PM
What other components to complement a gainclone/chipamp? peepsalot Chip Amps 9 15th October 2007 09:14 AM
chipamp.com gainclone vs audiosector.com? fusion Chip Amps 2 2nd July 2007 04:29 PM
DAC + bridged gainclone idea... MWP Chip Amps 10 23rd July 2004 06:18 AM


New To Site? Need Help?

All times are GMT. The time now is 07:44 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