Amplifier with jfets and laterals

Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
Heres another project of mine from last year, that i dug out, dusted off and fixed up.

A amplifier using jfets in the input stage and laterals in the output stage.

This amplifier is unique from my other projects in the sense that its dc coupled, which means there are no capacitors in the signal path to cause low frequency rolloff, instead this amplifier has frequency response down to DC.

The overall design is based off of several commercial amplifier topologies including D Self's "blameless" mixed together, and with some of my own ideas mixed in.

The overall schematic:
http://i.imgur.com/XSIEd.png

And a picture of the actual unit:
http://i.imgur.com/uEp0M.jpg
 

Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
These Jfets are obsolete since decades ago, i was lucky to find three on ebay.

I think you can still get the 2SK389 from digikey, mouser or farnell, its in a different package but should still be a suitable replacement.
 

Ron H

Joined Apr 14, 2005
7,014
The 2SK332 dual jfet was used in a commercial amplifier from the 80's that i got for free from a friend in germany.


http://www.datasheetarchive.com/dl/Datasheets-30/DSA-581429.pdf <-- this is the only datasheet i've found so far.
I don't see any matching specs.
Matching may not be critical. With a closed loop gain of 28.5, you'll get 28.5mV of output offset per mV of input offset (if your current mirror is perfect - it won't be). If the transistors are matched within 20mV (my arbitrary guess), that's 590mV of output offset. with a 4 ohm load, you'll have 150mA of current imbalance in your output drivers. Will that cause significant distortion? I have no idea. Does a current bias cause a problem with speakers? I don't know.

EDIT: What's the purpose of the RC network on the input?
 
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Thread Starter

Experimentonomen

Joined Feb 16, 2011
331
To remove unwanted rf from the input, without it you can sometimes hear radio stations when using long audio cables, i've heard stories of guitar amplifiers picking up radio stations from other countries.

This amplifier with no signal is dead silent and measured dc offset on the speaker outputs are 10-25mV.

It has strangely also has better psrr than my bjt amps which audibly hum on idle.
 
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