The sound of two transistors switching

[Tube_Dude]

Originally posted by millwood

it will not, unless the feedback pick up is before the resistor.

The resistor here are only increasing the open loop output impedance...but it don't change nothing in the issue of the CCS forcing a enriched bias...

[janneman]

Originally posted by millwood


the resistor is there so that even if the opamp doesn't have emitter resistors, you will still not get a DC offset at the output.



it will not, unless the feedback pick up is before the resistor.

Sorry Milwood, I don't follow you on the first point. I cannot see that this does anything on the offset, except make it worse (the feedback being less effective). How do you calculate that the resistor should exactly drop half the output voltage?

On the second point, Jorge is right. The feedback will try to get more output voltage, but with say 12v supply, and 6 v across the resistor, that only leaves 6 v on the load. In fact, the feedback will try so hard that you probably end up with hard clipping.

Jan Didden

[millwood]

Originally posted by janneman
On the second point, Jorge is right. The feedback will try to get more output voltage, but with say 12v supply, and 6 v across the resistor, that only leaves 6 v on the load. In fact, the feedback will try so hard that you probably end up with hard clipping.

Jan Didden

essentially, you raise the "true opamp output" (the output pin on the opamp, just before the resistor) to half of its full output. After the bias current going through that added resistor, the output after the resistor is exactly zero, as it should be.

Say that you have an opamp that goes from -12v to +12v, and CCS at 1ma. You drop in a resistor of 6k in there. The voltage drop on the resistor will be 6v, and the output of the opamp will be at 6v, and the feedback will see 6v - 6v = 0v.

The maximum swing this particular setup is able to achieve is +6v and -6v (from +12v to 0v at the opamp output pin).

By comparison, say you used a 9k resistor. The opamp output pin will idle at 9v. Its maximum swing is from 3v (12v-9v) to -9v (0v-9v). or +/- 3v assuming music is symmetric on average.

as you can see, as far as the feedback is concerned, it is dealing exactly with the same amp. Albeit the amp's swing is reduced because you want it to work in class A, thus only one of the two transistors will be conducting.

[Tube_Dude]

Originally posted by millwood


By comparison, say you used a 9k resistor. The opamp output pin will idle at 9v. Its maximum swing is from 3v (12v-9v) to -9v (0v-9v). or +/- 3v assuming music is symmetric on average.

as you can see, as far as the feedback is concerned, it is dealing exactly with the same amp.

In this point you are correct!!

Albeit the amp's swing is reduced because you want it to work in class A, thus only one of the two transistors will be conducting.

In this second case the resistor don't do any change!!

PS : I must leave for a while...i go to hear my tweked digital amp in a friend house that have some Rehdeko speakers....3 speakers full range with 104dB W /m....
Anybody know this speakers??

[dmfraser]

Any op-amp based on the LM324/LM358 architecture (quite a few are - its the most common op-amp in the world) have a Class-B output stage. It is a common trick to put a 6.8K resistor from the output to the negative rail to bias these into Class-A operation. If you look at the schematics of cheap Asian auto equalizers, you will usually see this resistor on each op-amp. It is apparantly cheaper for them to do this than to use a decent op-amp.

When this resistor is added, the LM324, which was never meant for audio use, experiences an improvement of 6db to 20db in THD products. However, the mediocre noise performance does not improve. I have the feeling that the LM series op-amps in this person's CD player are based on the LM324 design and it is indeed possible to get a sonic improvement by doing this.

However, a good op-amp like the LM833 or OP275 will see essentially no improvement and may actually get worse.

Note - LM324 is a quad device. All the others mentioned are dual op-amps in an 8 pin package and are pin for pin interchangeable.

Our research shows that for a dual op-amp for audio it is hard to beat the OP275 from Analog Devices with the LM833 not too far behind. If you need a quad device, we recommend the LM837.