RJM Audio Sapphire Desktop Headphone Amplifier
 

This is the new discussion thread for the Sapphire amplifier.

Previous threads,

183588-headphone-amplifier-drop-replacement-phonoclone-3-vsps300

160460-rjm-audio-gainbloc-preamplifier-mk-i

are redirected here.

Information about the kit group buy is at this thread.

Do I see a "diamond" buffer? :up: Very good design RJM. I've tried this with very good results. :)

Regards,
Venci

Yep, LH0002 buffer, lifted directly from the datasheet... with one little twist:

I realized that the National circuit works as it does specifically because it is an integrated circuit. All the transistors in the same package mean they all share thermal equilibrium. Forcing everything to be at the same temperature keeps the bias current steady at the desired value. When building a discrete version it is important to have the driver and output transistors bolted together. If you don't, you have to modify the circuit to steady the bias currents electrically, which adds complication and breaks the output-driver symmetry.

Yes, the other option is to replace the bias resistors with CCS. BTW in my design I didn't use the emitter output resistors.

Regards,
Venci

Can you suggest a simple short circuit protection for this diamond buffer?

Yes, I think I can: put 33R in series with the load. This limits the output current into a short to 300mA, 200 mA into a 16 ohm headphone load.

Or perhaps a 250 mA fuse in series with the load...

What you need depends on what you are primarily interested in protecting: the output transistors or your headphones.

Nice proto build. Why the heavy rectifier bridges?

They were cheap on eBay, easy to mount with the 4mm center hole, easy to connect with the push on lugs. And they look cool.

Primarily output stage if shorted to ground. Although both features would be nice to have.

Though it should have been something practical than electrical, that is why I asked. Thanks.