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!

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Reworking the ACP+…

Last weekend was this year’s “Burning Amp” festival in San Francisco. I wasn’t there (it’s a bit far from Denmark for a weekend trip…), but as usual there was a thread on diyaudio.

Burning Amp has frquently been a “launchpad” for new Nelson Pass designs and this year was no exception – the Amp Camp Pre (ACP+) was shown and the article is now on the FirstWatt website. As usual when Nelson releases a new design you sit up and take notice, but this one was just what I wanted to see (because there is only so many 25W class A amps you can use 😉 ). The ACP+ is a discrete preamp/headphone amp with the same basic architecture as a Pass J2 power amplifier. It’s discrete, doesn’t use a lot of components and runs from a single supply. The only fly in the proverbial ointment is that the amp uses P-channel JFETs for the input (either 2SJ74 or LSJ74), which are either impossible to get (2SJ) or just plain expensive (LSJ). However, I’m certainly not going to let that minor inconvenience stop me.

Nelson has of course done a board for the ACP+ already which will eventually find its way to the diyaudio store I’m sure. However, the original board breaks one of my rules because it has connectors on two edges. It also doesn’t look like the onboard RCAs are particularly good quality. As usual (I am tempted to say) I prefer a more modular approach, with the power supply, the amplifier, the volume pot etc. separated and so as I’ve done in the past I am going to have a go at redoing the ACP+ in modules instead. When I dig into the design I am sure i will be tempted to add a few changes, but let’s see. I expect I am going to build the original proposed linear supply, but an obvious candidate (in my mind) is a filtered IRM-module.

PCB order (hopefully) going out shortly, so with the usual shipping lead time this is going to be my X-mas present for myself this year 🙂

Picture of the prototype amp from the diyaudio-thread.

Testing the F3 amplifier…

Earlier this year Nelson Pass graciously started distributing batches of the Lovoltech LU1014 Power-JFETs for free to diy’ers. You could get 4 pcs. and only pay for shipping and of course that offer was hard to turn down – all the more so since I had been looking at trying an F3 amplifier at some point. There are no “official” group buy boards for the F3 at the moment and the few redesign/group-buy initiatives I have seen have been false starts, so I picked up a set of amplifier boards from ebay instead.

The F3 amplifier is one of Nelson’s “unusual” First Watt amplifiers, in that it uses a Power JFET as the gain device. Power JFETs are rare, and as a result for those of us who were a bit slow on the uptake the LU1014 came and went without me buying any. Otherwise the F3 isn’t a very complicated design, but it’s got normal First Watt class A heat levels and even-lesss-than-First Watt levels of output and gain. As a result i am not sure whether I actually have any practical use for the finished amplifier, but as a listening experiment I am still going to give it a try 🙂

The F3 is also a single-rail amplifier, which quickly led me to the realisation that I didn’t really have a PSU board suitable for a single-rail power amp. When you have a good “back catalogue” of designs then that’s quite helpful and so taking one of my existing class A power supply boards and chopping it up to create a single-rail version wasn’t that hard. The end result should hopefully – at some point – end up as a (nearly) dual-mono F3 amplifier (meaning a single transformer is used).

Another complication is that for reasons I can’t really remember I decided to try using 3U heatsinks for this build which may end up being a mistake – they are going to get very hot I guess. Anyway, for now it is an experiment and hopefully I will have it (electrically) completed by the Christmas break so I can hear what it sounds like before I put time and money into finishing the mechanical design 🙂

Project files: The J-Mo Headphone Buffer

What is it?
The project files for my version of Richard Murdey’s  J-Mo mk. 2 buffer with gain.

How big are the boards?

  • Amp: 2.45” x 1.975” (app. 62 x 50 mm.)
  • PSU: 2.35” x 1.975” (app. 60 x 50 mm.)

What is the status of the boards?
Both boards are version 1.0, meaning I have prototyped them and they work. However, I am still waiting for some mechanical parts for my own build so this isn’t final yet which means I have only done very basic tests.

Does it use any special/expensive/hard-to-find parts?
Well, the J-FETs are getting harder and harder to find but it isn’t impossible yet.

Anything else I need to know?

  • Can’t really think of anything. Be sure to read through the article on Richards website though, that contains most of what you need to know.

Downloads:
Download design files here

Related information:
See the original post for some more information and links. There is also a big discussion thread on diyaudio that may be of help.

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

The JFET-matcher

While building the Borbely amp from the previous post I had to measure and match a number of JFETs, both for the input circuit and for the current sources. I have done that before using a test setup based on a solderless bredboard and I used that here as well, but afterwards I started thinking if there was an easier way because I always have to rebuild the setup and it can be a little bit fiddly to use I think.

The result is this little PCB. It has screw clamps for input and output connections which means you can connect the leads from a multimeter directly – no crocodile clips and resistor leads required :D. There are connections for the power source and for volt-, amp- and ohmmeters (if you are measuring current sources with a resistor and not the raw Idss of the JFET). In addition, there are a few other features which I thought might be handy:

  • Silkscreen on the back of the boards to keep board connections to hand.
  • A jumper (marked “Idss”) to bypass the resistor/trimpot and measure Idss.
  • 2×3 pattern for a trimpot to allow connections of pots with both inline and staggered pins (soldered or via socket strips)
  • Connections for a switch to control whether the ohmmeter is connected or not. This means that you can connect the ohmmeter permanently to the board using the screwclamps. Then measuring is as simple as connecting the JFET, adjusting the trimpot to read the right current and then remove the JFET and pressing the switch to get the corresponding resistance reading – should help if measuring a larger batch of FETs.

The board measures app. 50x41mm and if there’s any interest I can easily share the eagle files. I also have a few spare boards (as I don’t really need 10 of these :D) so if  you just want a single board send me a message.