A gem for Christmas…

The other of Richard Murdeys “gem” designs (the first one was here) that I have worked on is the Sapphire headphone/line amplifier. Now in version 4, I’ve looked at it before but I didn’t have a compelling reason to start building anything. However, when Richard “upgraded” it to version 4 before the summer it caught my eye again (through here) and so I finally decided to start my own version. This was a while ago now, but due to some problems with my board orders (the first one didn’t show up at all and the second one just took nearly two months to arrive…) it’s been a lot longer than I expected.

The Sapphire is a current feedback design which can be built to drive low- or high-impedance loads, meaning it can be used as both a line preamplifier and a headphone amplifier. It’s fully-discrete circuit but uses standard parts that are easy to find and it’s actually a fairly simple build. Once again, for “my” version I set out from the published Eagle-files (v4.1) with a view to make minor changes but just like the Emerald it didn’t really hold. My board is smaller than the original and some of the routing is different because I used a “splayed-pin” footprint for all the transistors (I try to buy all my TO92s on tape, so the inline pins footprint makes everything much easier).

The only real changes I’ve made to the electrical design is the addition of a couple of indicator LEDs to show that the board is powered on. I find this quite useful for troubleshooting, it looks nice and there was space on the board for them 🙂 While I’ve been faffing around with PCB orders from China, Richard has made small tweaks and released v4.2, but that’s of course fine – if you want guaranteed boards with support etc. you should be buying his anyway 🙂

My prototypes are one of the “high-bias” configurations because I mainly have low-impedance headphones. That’s probably fine, but the small heatsinks get seriously hot (app. 65 degrees C) and the offset is also higher than I would have liked. However, I think that’s more to do with my iffy transistor-matching that any serious issues with the board layout itself. I’ll probably give it one more go in January and see if I can get the amp to behave as I want them to and then post the board files in case anyone wants to do more tweaking 🙂

Project files: The RJM Emerald RIAA

Last week I showed my version of the “Emerald” RIAA design by Richard J. Murdey. The Emerald is a neat little design: It has switchable gain and load for MM/MC, if you use good components it’s got a very accurate RIAA-curve, and of course with just two opamps per channel as the active devices, it’s very easy to build.

Richard has graciously shared the Eagle-files for his version and so it seems only right that I do the same here. Richard is also selling boards from his website, so if you want something that is proven to be working and comes with support then I suggest you buy your boards from him instead.

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A real gem…

I’ve already had my hands on a couple of designs by Richard J. Murdey, mainly here and here. However, Richard has quite a few other interesting designs on his site and one of them I’ve had my eyes on for a while, namely the “Sapphire” which is a line/headphone amplifier. Unfortunately my first order of Sapphire boards never showed up and while I was working on the Sapphire I also had a look at the Emerald MM/MC RIAA. Those boards did show up and so while I wait for the reordered Sapphire boards to (hopefully) show up soon, let’s look at the Emerald instead 🙂

The Emerald is very simple RIAA-design with two opamps per channel and selectable gain for both MM and MC cartridges. The RIAA correction is a bit special as described in Richards write-up and the design also includes an on-board discrete voltage regulator.

Richard has graciously shared the Eagle files for his designs so that was what I started from, but what was originally meant to be minor touch-ups ended up taking me a bit further away from the original than I expected. Although the schematic is still (more or less) the same, I’ve made quite a few changes to the board so that it now looks more like one of mine and uses the parts footprints that I normally use – the latter is absolutely intentional, the former a bit less so. I’ve also done quite a bit of rerouting – which I think of as optimizing the layout, but others might disagree – and managed to shrink the overall board dimensions a little in the process as well.

I haven’t really sacrificed anything from the original, except maybe by downsizing the footprint for the output cap a little bit, but that’s hopefully an acceptable compromise for most people. Project files (probably) coming shortly 😀

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.

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.

J-Mo buffer/headphone amp

A couple of months ago I stumbled upon the discussion thread for Richard Murdey’s J-Mo Mk. 2 headphone buffer on diyaudio.com. Richard runs RJM Audio (which you should definitely check out if you haven’t already) and is also the man behind the Szekeres VE design that I built a while ago (see this post).

I am not sure why the J-Mo caught my eye, probably the simplicity of it, but it did. Richard already has a nice PCB for the design, but as I thought I could make it a little smaller, substitute some parts for more commonly available sizes and also improve the versatility a bit I decided to “roll my own” boards instead 🙂

I downloaded the eagle-files that Richard graciously provide for download, ripped up the original PCB file and got to work. The result is here in dual-mono format with amp and regulator on separate PCBs similar to the original. Apart from the revised layout my changes are relatively minor, mainly different component footprints where I thought it made sense and of course different heat sinks.

The rectifier bridge PCB is from my second Gainclone design and was perfect for making the unregulated DC voltage that the Zener regulator requires. The bottom plate in the pictures was just one that I had left over from another project and was suitable for mounting the boards. There is no wiring in the pictures, but the boards were wired up and it does play wonderful music (although feeding it with wonderful music seems to be required… :D)

As usual I didn’t really listen for a long time while it was sitting on my desk in “prototype mode”, but what I heard was good enough that I have ordered a proper mounting plate (i.e. one that fits in a standard enclosure), a transformer and some chassis part so I can give these great boards a “real” home 😀

Project files: Szekeres VE headphone buffer

What is it?
This is a board for my version of Richard Murdey’s Szekeres VE (for “virtual engine”) buffer. This design is based on the original buffer concept by Greg Szekeres published at headwize a long time ago (c. 2002). The Szekeres buffer was one of the original headwize designs that I was very interested in when I started building amps due to its simplicity, but I never really got round to it. When I stumbled upon Richard’s revised version I had to try it – just to see if it was as good as I had imagined all those years ago (well, almost 🙂 ) Oh, and be sure to check out Richard’s site for some other very interesting DIY designs as well.

How big is the board?
The board measures 3.925″x1.95″ (just under 10×5 cm.), so just small enough for a 5×10 cm board service at itead.

What is the status of the board?
The board is in version 1.1. I have tested v1.0 and made a couple of alterations to the board as there were some minor space-issues here and there, but nothing serious.

Does it use any special/expensive/hard-to-find parts?
Yes. The power resistors are TO-220 types, Caddock MP-915 or MP-930 or equivalent and they are quite expensive. The FETs are cheap but need to be matched, so buy 10 pcs. or so. The heat sinks are Fischer SK129 types (or any one of many equivalents)

Anything else I need to know?

  • The layout is pretty tight so the mounting sequence is quite important, especially if using heat sinks taller than 25mm. Start with mounting the small resistors on the board. Then loosely assemble FETs, power resistors and heatsinks before mounting them one by one on the PCB. The mounting sequence should be to first to solder the heat sink pins and then the FET/resistor pins in place before tightening the FET mounting screws. Then move on to the next heat sink.
  • There is only room for small capacitors on the board so a regulated PSU is recommended (several options available from this site 😉 )
  • The FETs are the IRF510 types recommended in the article. The FETs need to be matched as described in the article as there is no way to null DC offset.

Download design files here

Related information:
Note: Always read the “intro post” for additional important information about my designs.
Refer to the RJM audio site for BoM and information of component selection.