Project files: The Borbely non-hybrid headamp

To supplement the original Borbely tube hybrid headphone amplifier are here the files for the solid-state version as described previously here. Have fun!

What is it?
This is my version of Erno Borbely’s non-Hybrid headphone amplifier as shown by audioXpress. As discussed in my previous post, having made the tubed version the solid-state version was so easy it was a bit of a no-brainer to do that as well. Most of the comments below are actually the same as for the tubed version (except where they are obviously not 😉 ).

How big are the boards?
Each mono board measures 2.9″ x 3.6″ (app. 74 x 91 mm.) making it slightly smaller than the tubed version.

What is the status of the boards?
The board file is version 1.1. I have built version 1.0 and it works as expected but I’ve just made some minor touch-ups afterwards.

Does it use any special/expensive/hard-to-find parts?

• Power FETs: I decided to make the boards fit 2SJ313/2SK2013 FETs because they are mentioned as substitutes in original article and while still a bit difficult to get (watch out for fakes!) they are still easier to find than the original 2SJ79/2SK216s. This means that this layout is not useable with the original devices because they have a different pinout. You would however (most likely) be able to use IRF510/IRF9510 or IRF610/IRF9610 pairs instead. I haven’t tried this myself, but at least the pinouts match.
• Small-signal transistors: I replaced the originals (2SA872/2SC1775) with BC550C/BC560C variants that aren’t quite as low-noise as the original types, but much easier to get. If you prefer “audiophile-approved” (near) unobtanium parts at all cost, then I believe the 2SA970/2SC2240 would work as well if they are turned 180 degrees to compensate for the different pinouts.
• JFETS: The input FET pair is a single 2SK389 or a pair of 2SK170s. They should be the BL or V version (which doesn’t make it easier to find them) and the problems of getting real JFETs are well-documented already. As mentioned in the first post the LSK170s are still available (from the diyaudio store among others) but a more future-proof solution would be to tweak the layout for a Linear-systems dual JFET that is still in production and available from other sources. I can’t be bothered to do that though, but if you want to have a go yourself be my guest 🙂

Anything else I need to know?

• The original circuit is differential input. However, my layout grounds the negative input as standard to make the input single-ended only. I normally wouldn’t make a simplification like this, however it avoids a very long and unsightly trace through the ground plane. Since I expect that SE-input is how most people would use it anyway this was an acceptable compromise for me (but of course it might not be for you 🙂 ).
• The component designations are identical to the original in the article, so my BoM is simply annotated with comments and package information etc.
• When you mount components, note the power supply decoupling caps sort of block access to the heat sink screws. I suggest you mount the caps as the last step after mounting heatsinks and output transistors.
• Just like it’s tubed sibling, the solid-state amp has a DC-spike on the output when it’s turned off. However, two comments:
  1) On my test sample the DC spike gets to 2.5V or so, which should be much less severe than the tubed version which in my experiments went more or less fully to one of the supply rails.
  2) While doing this layout I re-read the build article and there is actually a mention of this issue if the tubed version has its heater supply removed so maybe a bit of experimentation here can minimise the issue – something which obviously isn’t possible with a solid-state amp that doesn’t have a heater supply. My suggestion would be that you build and test the amps before deciding whether protection is needed, but plan for including output protection all the same.

Downloads:
Download design files here

Related information:
Read the original build article on audioXpress for more information about the design and a great walkthrough of component values etc.

Note: Always read the “intro post” for additional important information about my designs.