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How to tell if this amp is in class A or AB ???
How to tell if this amp is in class A or AB ???
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Old 10th November 2017, 01:06 PM   #11
woody is offline woody
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How to tell if this amp is in class A or AB ???
This depends on the power supply voltage, the idle current through R3 and R4 and the load resistance. For a nice easy to understand amp circuite you might look at Nelson's A40 article. It's a simple circuite but takes some big hardware that is a big transformer and BIG heatsinks.
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Old 11th November 2017, 01:41 AM   #12
xrk971 is online now xrk971  United States
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How to tell if this amp is in class A or AB ???
You might try from a starting point similar to your topology, but known to work well. I have built this and it works very well. In stock form as designed, it runs a very low bias current of maybe 3mA. I changed R7 and R8 from 10R to 0.5R and the bias went up to 35mA, at which point I think for most listening at normal volume levels with headphones, is Class A. When driven to higher output levels above 35mA, it goes out of Class A.

Headphone Amplifier

Click the image to open in full size.

Here is my build:
Click the image to open in full size.

But, I think this topology may be close to what you are looking for: an opamp driving a push pull complenmentary output stage. The oapmp keeps the dc offset under control and it never goes beyond 1mV or so.
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Old 11th November 2017, 03:30 AM   #13
piz1 is offline piz1  Italy
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Quote:
Originally Posted by xrk971 View Post
You might try from a starting point similar to your topology, but known to work well. I have built this and it works very well. In stock form as designed, it runs a very low bias current of maybe 3mA. I changed R7 and R8 from 10R to 0.5R and the bias went up to 35mA, at which point I think for most listening at normal volume levels with headphones, is Class A. When driven to higher output levels above 35mA, it goes out of Class A.

Headphone Amplifier

Click the image to open in full size.

Here is my build:
Click the image to open in full size.

But, I think this topology may be close to what you are looking for: an opamp driving a push pull complenmentary output stage. The oapmp keeps the dc offset under control and it never goes beyond 1mV or so.
Thanks i really appreciate it!

Do you think that this kind of approach can go well for higher power?
I see no heatsink on the BDs, are they still cool?
Do you have the schematics of the power supply on the right?

You build looks nice and clean, mine is a tangle of wires...
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Old 11th November 2017, 03:59 AM   #14
Lazy Cat is offline Lazy Cat  Slovenia
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Quote:
Originally Posted by xrk971 View Post
..and the bias went up to 35mA, at which point I think for most listening at normal volume levels with headphones, is Class A. When driven to higher output levels above 35mA, it goes out of Class A.
Wrong.

Squeezing one rail side to zero - at that moment the output current measures 70 mA (see Kirchhoff’s circuit laws) and your amp goes to Class B. It is the double value of bias current that makes amp leaving Class A. Mooly also clearly stated this in post #10.
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Old 11th November 2017, 04:10 AM   #15
xrk971 is online now xrk971  United States
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How to tell if this amp is in class A or AB ???
Right, 70mA - even better.
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Old 11th November 2017, 04:18 AM   #16
xrk971 is online now xrk971  United States
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How to tell if this amp is in class A or AB ???
Quote:
Originally Posted by piz1 View Post
Thanks i really appreciate it!

Do you think that this kind of approach can go well for higher power?
I see no heatsink on the BDs, are they still cool?
Do you have the schematics of the power supply on the right?

You build looks nice and clean, mine is a tangle of wires...
The PSU is the Nazar shunt regulator circuit. Prasi made those so you will have to contact him to see where the schematic is. I have operated it with a simple linear 7815/7915 regulators following a CRC from a trafo. Even that works well enough without audible noise.

When biased at stock (default) value of 3mA no heatsink needed. But at 35mA. Definitely needs a moderate heatsink as its about 4.2w dissipation.

To get higher power requires bigger PSU rail voltages. You either need to get special HV opamps (which I recently found out from Mooly that LT makes 150v p-p opamps. ). Then use transistors with higher current capability, bigger heatsinks, etc. but I think the basic opamp might be able to drive a bigger transistor just fine if you pick the right one capable of higher currents. You can also bootstrap a common +/-15v opamp to get higher swing. But again, need to check to see if opamp can drive enough current.
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Last edited by xrk971; 11th November 2017 at 04:29 AM.
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Old 11th November 2017, 04:41 AM   #17
Lazy Cat is offline Lazy Cat  Slovenia
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Quote:
Originally Posted by piz1 View Post
Do you think that this kind of approach can go well for higher power?
Wrong approach.

Iíve kindly posted you two schematics on which you should study hard and make amplifiers out of them. On the way youíll learn a lot and maybe someday become apprentice of amp designer.
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Old 11th November 2017, 10:24 AM   #18
DF96 is online now DF96  England
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Quote:
Originally Posted by Lazy Cat
I’ve kindly posted you two schematics on which you should study hard and make amplifiers out of them. On the way you’ll learn a lot and maybe someday become apprentice of amp designer.
Your posts in this thread so far have shown anything but kindness. Maybe someday you will become apprentice of person with good manners?
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Old 11th November 2017, 11:30 PM   #19
Lazy Cat is offline Lazy Cat  Slovenia
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Originally Posted by DF96 View Post
Your posts in this thread so far have shown anything but kindness. Maybe someday you will become apprentice of person with good manners?
Not likely. Shouldn't be tempted to post anything anymore, makes no sense.
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Old 12th November 2017, 01:53 AM   #20
piz1 is offline piz1  Italy
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Originally Posted by Lazy Cat View Post
Wrong approach.

I’ve kindly posted you two schematics on which you should study hard and make amplifiers out of them. On the way you’ll learn a lot and maybe someday become apprentice of amp designer.
Okay, wrong approach but you did not explain why...You post a sch from a datasheet and a sch with three mistakes, again i know that it isn't a good sch (mine) because there is no CCS, no negative-feedback etc. Is a push-pull stage without anything else and i did it only to figure out when class A switch to AB.

I wanted to find a good method, a sort of "algorithm" that (by measurements with a scope or something like) tells you when the OPS switch to AB, nothing more nothing less.


Last edited by piz1; 12th November 2017 at 02:09 AM.
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