Project files: The Borbely Hybrid headamp

What is it?
This is my version of Erno Borbely’s tube/MOS-FET hybrid headphone amplifier. As discussed in a previous post I found this design a while ago and made plans to do my own board for it at some point. For a long time it was just another point on a long to-do list, but one random Saturday a couple of months ago I just somehow got started and managed to finish it shortly afterwards. There are only minor changes compared to the orginal. Apart from what is described below, it is mainly space for RN60-type resistors for all positions and use of bigger (and more easily available) heatsinks.

Note: I don’t really recommend building and using this as-is (see explanation below), but I decided to publish the board files anyway because I think it is still an interesting circuit. I am definitely not ruling out trying to make it useable at a later state with some soft of turn-on/turn-off delay circuit,. but for not that is not a priority for me.

How big are the boards?
Each mono board measures 3.2″ x 3.6″ (app. 81 x 91 mm.)

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 (note the rather large caveat mentioned below though!!), but I had swapped the labels on Q2 and Q5. I also corrected the footprint for the two CRDs to make it easier to fit axial types which are the most commonly available.

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. If anyone want’s to have a go at this I’d be happy to donate one of my spare sets of boards to “the cause” – just drop me a line 🙂
  • Small-signal transistors: I replaced the originals (2SA872/2SC1775) with BC550/BC560 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.
  • Tubes: You can pay a fortune for branded ECC86/ECC88 variants or you can use something cheaper, i.e. chinese/russian equivalents. I chose the latter and bought some 6N23P-types from ebay. These seem to work fine, but note that the 6N1P which is another oft-recommended substitute for ECC88 will not work in this application. Also: One of the tubes I tried gave me a steady-state DC offset of around 1V that I could not get rid of whereas the other channel was fine. I replaced the tube and redid the adjustment and then it was fine as well. This could be a fluke, but I think you should buy more than a single pair of tubes just to be on the safe side.

Anything else I need to know?

  • Important: As described in some of the diyaudio-posts I read about this design it has one major flaw and that is serious DC-offset at power-up and power-down. Both my boards are after adjustment within +/-10mV when they are fully warmed up and stabilised, but during power up they both swing the output to very close to one of the supply rails (meaning 20V or so) and stay for such a long time that I think it would be fatal for headphones. There are a couple of solutions to this, either an output capacitor or a delay circuit of some sort. I don’t have time to try either at the moment, but if you do so please report back.
  • The original circuit is differential input. However, my layout grounds the negative input as standard to make the inout 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 🙂 ).
  • When you mount components, note the resistor that is supposed to be mounted on the back of the board. This is much, much easier to do if you solder it before mounting the tube socket and not afterwards…

Downloads:
Download design files here

Related information:
See the original build article 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.

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Another Borbely clone – almost…

You may remember this post about a clone of one of Erno Borbely’s design. Now there’s another one! …but unlike the last one, which was green, this one is red! 😀

It is actually a combo of amp and PSU from ebay seller jimsaudio. The amp is not really a pure Borbely design, but instead a Borbely JFET frontend mated to a (Walt Jung) diamond buffer output stage – which in itself doesn’t really matter if the sound is OK. Quite like the last one though, it is full of parts that are now pretty much impossible to find. However, 20 years of “collecting” parts can actually be helpful sometimes :D.

The power supply is also a “bastard” design – a tweaked version of Borbely’s take on the Sulzer regulator I think. However, Erno has definitely left his mark here as well – even the regulator uses some very hard to find transistors – and I can’t be the only one that winces a little at putting “unobtanium” devices like 2SK170BL/2SJ74BL JFETs in a power supply? OK, it was only one pair of each and I had an odd number anyway, but still it hurts a bit…. ( 🙂 )

Anyway, if the results are worth it then I can live with it and initial impression is definitely that it is. The sound is smooth and detailed without any noise or hum, even from just a simple bench test. Now all that remains is the usual waiting period before I get around to making the rest of the mechanical stuff for the casing – shouldn’t be more than a year or two 😀

PS: While looking around the internet I found another Borbely design for a tube/FET hybrid line stage running at just +/- 24VDC. It comes in two variations, one intended for line stage duty and the other one beefed up a little to drive headphones. The differences are small though, so it should be possible to put both variations on the same PCB. I am thinking that there could be a way to redesign this with transistors that are still available – maybe not 100% the same performance, but it would be worth a go I think 🙂

Work in progress….

It’s been a while since I have posted here, but real life is still getting in the way of my build time. At the moment this means I am mostly starting up new projects, because for some strange reason I can always find time to start new projects – even when time to finish the old ones is nowhere in sight 😀

I have been shopping a little at diyinhk.com recently and bought several DAC boards and power supplies, so one a few DACs are in the cards for sure 🙂 Also, one or two gainclones might make an appearance as well at some point. One reason for this is that I rediscovered some pictures of the Audiosector “Patek” amp a couple of weeks ago, so guess who has been scouring ebay etc. for reasonably-priced copper bars lately? 😉

Lastly, the Borbely and the Le Monstre front/rear panels have arrived and I am pretty happy with those, so now I am just waiting for my transformer order and a free weekend and then at least that should be sorted out.

However, I have to say that even with not much new going on there are still plenty of people that find their way here. The blog was at 15k page views in the beginning of January and it is now over 21k views total – I can’t complain about that, so thanks a lot for stopping by 😀

A bit more complicated than usual…

Most of my projects tend to be quite simple designs. Apart from the obvious reasons of making the builds smaller, easier and cheaper, I very much like the thought of being able to make something play music with just a few parts. However, every so often I find myself attracted to designs that are more complex. One of these is this headamp/preamp.

This a variant of a couple of well-known designs by “Mr. FET” Erno Borbely, yet another of the greats of audio design. Sadly, the Borbely webshop closed up a couple of years ago, but many of the designs are still used today. This one is a discrete JFET input/BJT output headphone/preamp coupled with a Sulzer-style series-regulated discrete voltage regulator (phew! 😉 ). The boards were purchased from ebay seller al_tsankov in Bulgaria. The boards are decent enough in quality but very importantly, they come accompanied by detailed and clear instructions and BoMs/schematics you can easily read and understand – a welcome change from many of the chinese ebay sellers to be honest…

As with (nearly) all of Borbelys designs, this one incorporates some parts that are quite hard to find these days (Toshiba JFETs). I happen to have a small stock of these, but if you don’t this layout incorporates the option of using BJTs for the input as well (which is also why the board isn’t fully populated in the pictures). The circuit is fairly complex for what it does  (but no doubt, as Einstein said: “as simple as possible, but not simpler” :D) with constant current sources, LEDs to keep noise down and so on. Building it wasn’t too difficult as long as you concentrate on the parts and check everything before fitting. Turning something like this on for the first time is always with a sense of trepidation for me because you know the troubleshooting is going to be a pain if it doesn’t work, but in this case everything was fine the first time 🙂

As you can see, the boards are assembled and tested and working fine. All that is missing is final case assembly when the front and rear panels arrive from Schaeffer (FPX for the Americans) in a couple of weeks.

EDIT 24th Jan. 2014: I got a message from Alex (the PCB designer) correcting my text a little. The Sulzer regulator is a series-type, not a shunt-type as I had written above – sorry. Alex also asked me to attribute the regulator design, i.e. the modifications to the original Sulzer circuit, to Angel Despotov of Analogdomain which I am of course happy to do 🙂