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Kaneda preamp?

T

teddy_pardo

Trade: Teddy Pardo
Can anyone tell what are the two trimmers VR1 and VR2 used for (I suspect for getting 0V at the output), and how exactly to set them.


kaneda.gif


Also, are there any "improved" Kaneda designs? I believe that it can be improved by replacing the LTP resistors by current sources, as well as the 4.7K resistor at the emitter of the output transistor. Obviously good voltage regulators will help too...
 
... yes, it's essential that this amp is fed from a very pure voltage source since the PSRR is rather low. It may be a good idea to get the original article also.
 
The trimmers are for adjusting the offset (VR1) and operating point of the VAS stage VR2.

My first guess would be to set them up by adjusting VR1 for zero offset at the block output. Then trim VR2 so that the two points marked 0.6V have an equal voltage - which will indeed be somewhere close to 0.6V. You could then go back and check if VR1 needs tweaking, but any changes should be fairly small.

As you point out, replace the resistors feeding the LTP and the output emitter follower by current sources would push the negative supply PSRR up a lot - the positive rail PSRR should not be too bad as it stands, as the second stage is differential, and so rejects most of the PSRR residuals.
 
Which currents would you set for the current sources? I'd guess around 1mA for the LTP and 10mA for the emitter follower, makes sense?
 
Roughly, yes, but note the 5.6K in the -ve rail, which drops a bit of voltage. I think the EF runs at about 35V/4.7K = 7.5mA. The input pair probably runs at about 0.8mA, but you can adjust Vr2 to suit.
 
The operating points of the JFETs are about 300 uA each, for the VAS PNPs around 600uA each.

There are methods for the adjustments:

either closed loop, like above:
first set VR1 that output is near 0 Volts, say between -1 and +1 Volts (that makes sure you are near the solution).
second VR2 so that the points marked with 0.6V do sum up to 1.2V,
then set VR1 that both points are equal

or you can open the global feedback loop (either remove one leg of the 51k resistor, or short the gate of T2 to ground):
first set VR1 that the points marked 0.6V are equal.
then set VR2 that output is zero.

The second method should work spot on without iteration.

there is a lot of DC gain, so it is quite normal to have 0.5V offset, and it is also normal to have an offset change after Power-On of say 1V in either direction.
If you have some air movements, you will notice some DC offset change.

The output cap should be of best quality.

best regards, Hartmut
 
Set VR2 to make the amplifier have lowest distortion. A distortion meter is necessary. Just set the VR2 to half value (1k) if not equipped.
 
Hello everyone, I have heard good things about this preamplifier, but I have never heard of it, someone has it and can provide their conclusions, I want to make one.
I was going to install a filter at the input, with the INA 1650, but to take capacitors, I think it could alter the sound, greetings.
I am trying to put the schematic, but it does not leave, I do not know what problem there is, but well looking at the manufacturer come several examples.
 
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