Tuesday, November 4, 2008

Triode Connected EF86 linestage and headphone amplifier

This is an old post from my now-defunct Jenison Audio website.

EF86 Linestage: I've been messing around with the EF86 linestage for a number of years. This was based on the old Svetlana schematic available here. I've build several generations of this circuit, each better than the other. I initially started with a simple cathode follower using a 6SN7. Then I built the 'ultimate' version using tube regulated power supplies and a white cathode follower - various experiments with this made me eventually pull out the regulation and I preferred a simple choke and capacitor combination.

The final version of the schematic I built is here:

A note on the headphone outputs - this will work best with a pair of high-z headphones - a 32ohm pair will be just too difficult for the white cathode follower circuit to drive well. If you need to drive a pair of low impedance headphones I would recommend some loop feedback from the headphones out. This would reduce the output impedance and distortion. Something worth experimenting with at the very least. In any case I would also add a good bypass capacitor on the 470uF output capacitor - something like a 1 to 4.7uf film capacitor will help.

Power supply was a 6X5GT rectifier, cap/choke/cap/resistor/cap. I've kept this part vague since everyone has their favorite ways of building a PS and it turns out I never made a schematic for this portion of the circuit!

and the finished 'product' looks like this:

I ended up selling this linstage to a friend of mine who gets daily use out of it. He has rolled in several different types of EF86s and the best ones have been the GEC CV4085s and the Blackburn Mullards. However just about any EF86 sounds good. Gain is a touch high, which is why I built a switch so the cathode capacitors on the EF86 can be bypassed - this adds local feedback and reduces the gain. Not an optimal solution, but something nice to have if your amplifier is too sensitive.