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 🙂

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The “Whammy” headphone amplifier…

Although I did my own version of the Pass/Colburn “Whammy” headphone amplifier before there were boards available for sale in the diyaudio store (and before it was officially called the Whammy), I have still considered getting an original all-in-one board as well.

The cost of shipping from the US originally deterred me enough to do my own version, but a couple of weeks ago a board popped up on diyaudio from a fellow hobbyist in Europe, so I was able to get one at a reasonable cost. Unlike the diyaudio board this one is green (which I massively approve of) and also 2mm thick and plated with gold (ENIG) so it looks and feels really great. Because the board was thicker than usual and I knew I had to mount it in a big case I decided to go “all out”, use tall caps and heatsinks and maybe experiment with turning up the current compared to normal (haven’t do that yet though).

The power supply is running at 20V courtesy of some 7×18-regulators and a pair of green LEDs. This limits my choice of opamps, more or less to either the original OPA2604 or the (now-discontinued) LME49860 which is supposed to be a 22V-tolerant LME49720. Not sure if that is true, but I did chose the latter and I have no complaints about the sound. I might try the OPA2604 at some point instead since I haven’t listened to that since the comparison was an OPA2134 – that’s been a while. The output FETs are the recommended 2SK2013/2SJ313 which I already had matched pairs of, but obviously plenty of other options available that are easier to source.

Just like my clone version this one worked immediately after being powered up, but that is probably more to Wayne’s credit than mine 🙂 I don’t have a case idea just yet, so for the moment it’s going in a box until I come up with a plan for what to do next – still sounds great though 😀

Fame and fortune here I come!

Well OK maybe not, but it looks like I’ve made it to audioXpress magazine and surely that counts for something? 😀

I didn’t subscribe to their newsletter (I do now! 😉 ) so I found out a bit late, but they seem to have open-sourced the Borbely Hybrid Headphone-amp design (kudos for that) and referenced my post as one of the examples of the usage of the circuit. That’s really great and very much appreciated, but next time guys let me know in advance and I’ll provide you with a better board picture, ok? 😀

Since that discovery gave me time to reread the original post, I started googling for turn-on/turn-off protection and delay circuits and maybe it’s time to see if something can be done here? That the amp had dangerous levels of DC offset at turn-on and turn-off was more or less the only real flaw with an otherwise great and very interesting design. Google did provide some inspiration that I am going to look at and see if I can come up with something good. I was tempted for a moment to use a uC, but putting software in a Borbely-design seems inappropriate in some way 😉

Project files: PassHP headphone amp

What is it?
It’s the project files for the PassHP headphone amplifier from last week’s post and judging by the number of views since then they are eagerly awaited 😀
As mentioned last time, this design is a clone of the one from here and my version consists of a mono amplifier board and a stereo PSU board instead of the original “all-in-one” design.

How big are the boards?
The amplifier boards measure 2.95” x 3.0” (app. 75 x 76 mm.) and the PSU board measures 2.0” x 5.05” (app. 51 x 128 mm.).

What is the status of the boards?
Both boards are in version 1.0 as the prototype seems to work well and I couldn’t be bothered to make any cosmetic changes 😉

Does it use any special/expensive/hard-to-find parts?
Well, the recommended 2SJ313/2SK2013 output transistors are a bit hard to find, but there are plenty of substitutes available. This is a fairly simple design, so otherwise no problems.

Anything else I need to know?

  • Resistors: I’ve used RN60-type resistors which are rated 0.5W, but that probably isn’t necessary – at least not for all the positions.
  • Heatsinks: The heat sink profile is the one Fischer calls SK104 but there are many substitutes. The power dissipation isn’t great so even the small 25mm high version should suffice, but if you want to use bigger ones for cosmetic reasons that should be just fine 🙂
  • Transistors: I’ve used 2SJ313/2SK2013 output devices because I had them, but if you don’t then I recommend using IRF610/9610 or one of the other substitutes mentioned in the diyaudio build thread. The 2SJ/2SK pairs are now either very expensive or very fake (and sometimes even both!), so using parts that are still in production should be safer.
  • Optocoupler: In theory this is also substitutable for something else, but in all honesty I don’t know exactly how the optical bias-system works so it’s probably best to stick with the standard 4N35.
  • Gain: The default gain is app. 6 but that can be lowered or raised by tweaking the value of R4. In theory you should recalculate the BW-limiting capacitor across the resistor if you change the value, but in practice you’ll probably be fine unless you make major changes. My prototype version has a gain of 3 (R4 = 2k) and I haven’t observed any problems.
  • Opamp: My version uses a single-channel opamp which gives a bit more choice. Start out with something like the OPA604, OPA134 or LME49710 and then experiment from there if you want to change the sound.
    Most opamps have a max. supply voltage of +/-15V so as a starting point I’d recommend this as the supply voltage. If you want more voltage swing use the OPA604 which is good up to +/-22V.
  • PSU voltage adjustment: Just as in the original you can use LEDs to raise the output voltage of the supply above the regulator voltage (although I’ve ditched the resistor option). Using 7×15-regulators and green/red LEDs should give you around 17V output whereas using 7×18-regulators and LEDs should bump that to app. 20V. If you just want the regulator voltage as the output, remember to jumper across the LED pins and omit the capacitor.

Downloads:
Download design files here

Related information:
You really should chew your way through the diyaudio-thread for information about the amplifier. As mentioned this version was mostly because I did not like the original form factor. If you just want a functioning amplifier then I strongly recommend that you buy one of the “real” boards from Wayne Colburn via DIYaudio (or wait a few weeks for when the boards show up in the diyaudio store).

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

New PassDIY headphone amp…

One of the (few) PCB-projects that I have been able to spend time lately on is a version of a new PassDIY headphone amp developed by Wayne Colburn. I’ve been looking at this from when it was first posted (after all, most diy’ers watch with interest when Nelson and his gang are up to something new 🙂 ) When the design was finally released a few months ago and Wayne started offering boards I didn’t jump on it immediately though.

First off all there was a question of cost – paying $35 in shipping for $25 worth of PCB is a bit annoying although I probably would have survived that. Secondly, the “all-in-one” form factor has some very obvious benefits, but also some significant drawbacks to me. You’re generally tied to one particular chassis, one particular transformer, one specific potentiometer etc. and I wanted more flexibility.

So instead of getting a ready-made board I started thinking about making a more flexible version by splitting the board up into smaller sections – two mono amplifier channels and a separate supply board to which you need to add a separate volume control and a separate transformer. The design is simple and so there were no major issues and my protoype fortunately worked the first time.

As usual I have only done quick bench testing right now, but the design seems to be solid (no doubt more to the designer’s credit than mine 😀 ). There is no DC-offset worth mentioning although it spikes a little when you turn the power off, and even in a “birds nest” test setup with wires all over the place, the amp was completely silent. As far as I can see, the bias is spot on and stable as well.

Project files coming in a few weeks when I’ve had time to compile them 🙂 In the mean time, you can still get the original boards through the thread and the original Gerbers are there as well if you want to get your own boards made instead.

Project files: The last of its kind…

…for a while at least 😀

What is it?
The last (and smallest) version of my EL2k buffered headphone amp using NOS Elantec 2008/2009 buffer ICs. This is the smallest version designed for 1.5″ heat sink profiles as described here. The two other versions are of course also still available (here and here):

How big are the boards?
The board measures 3.95″ x 1.5″ (app. 100 x 38 mm.) and is obviously a mono amplifier channel.

What is the status of the boards?
I’ve called this board version 1.5 as it is a redesign. Apart from the redesign work described in a previous post, the circuit is identical to the other published files.

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

  • The EL2008/EL2009 buffers have been out of production for 10+ years. They can still be found and I don’t think you need to be especially concerned about fakes – there can’t be a lot of demand for these anymore – but of course no guarantees. The main risk is probably that instead of NOS parts that you get used parts that have been pulled from old equipment. This is annoying, but should be OK.
  • The heat sink profile is the same as the original, Fischer SK68, in 37mm length. Easy to get in Europe, but I’m not sure about elsewhere.

Anything else I need to know?

  • I’ve had to mount the buffers on the side of the heat sink that has an M2.5 slot and not an M3-slot. This isn’t a problem as such because there’s no need to isolate the tab, you’ll have to remember to buy M2.5 screws for mounting 😀
  • Otherwise this is a bog-standard buffered opamp circuit and there isn’t much that can go wrong 🙂

Downloads:
Download design files here

Related information:
Be sure to read the original posts for additional information and tips.

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

Project files: The (modified) EL2k headamp

What is it?
The board files for the new “medium-sized” version of the EL2k buffer/pre as shown a few weeks ago. The smaller 37mm board version will follow in a while.

How big are the boards?
The board measures 3.95″ x 2.0″ (app. 100 x 51 mm.) and is obviously a mono amplifier channel.

What is the status of the boards?
I’ve called this board version 1.5. Apart from the redesign work described in the last post, the circuit is identical to the originally published v1.1 files.

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

  • The EL2008/EL2009 buffers have been out of production for 10+ years. They can still be found and I don’t think you need to be especially concerned about fakes – there can’t be a lot of demand for these anymore – but of course no guarantees. The main risk is probably that instead of NOS parts that you get used parts that have been pulled from old equipment. This is annoying, but should be OK.
  • The heat sink profile is the same as the original, Fischer SK68, in 50mm length. Easy to get in Europe, but I’m not sure about elsewhere.

Anything else I need to know?

  • I’ve had to mount the buffers on the side of the heat sink that has an M2.5 slot and not an M3-slot. This isn’t a problem as such because there’s no need to isolate the tab, but some swearing will likely ensue when you sit there on Sunday afternoon and realise you don’t have any M2.5 screws to hand 😀
  • Otherwise this is a bog-standard buffered opamp circuit and there isn’t much that can go wrong 🙂

Downloads:
Download design files here

Related information:
Be sure to read the original posts for additional information and tips. You should be able to reuse the linked BoM as well.

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

Project files: Universal Mini-preamp

A few weeks ago a reader was commenting on simple buffers/preamps and also asked about ebay-kits to use since I haven’t posted anything with a volume control yet. That got me searching to see what was actually out there and very quickly came the realisation – “I can do this better” 😀 Not sure if I did, but I at least tried 🙂

What is it?
A very simple opamp-based buffer/pre with an onboard volume control that can be used as a “buffered volume control” with a power amplifier module, a real preamp with or without gain or even a “CMoy”-style headphone amp. The board has space for a DIP-8 dual opamp, polypropylene input caps and a full-size Alps volume control and still manages to be very compact. I’m showing the board now as I already have a couple of applications for it in the pipeline myself which you will see later 🙂

How big are the boards?
2″x2″ (app. 51×51 mm) – a theslowdiyer standard size (TM) 😉

What is the status of the boards?
The board file is v1.0. I’ve built a prototype and everything seems to be fine.

Does it use any special/expensive/hard-to-find parts?
None, really. You can get what you need from Mouser/Reichelt and similar places and most of the component values aren’t that critical anyway.

Anything else I need to know?

  • The opamp should be a dual-type with standard pinout. My recommendations would be either the LME49720 (sadly discontinued in DIP) or the OPA2107 (still available but fairly expensive), but there are loads of other options. The board layout should be suitable for using adapters as well (for DIP/SO-8 singles or SO-8 duals) and if you want to go all-out there’s even a discrete option from Burson that should fit as well.
  • The only surface mount components are the optional (but recommended) 1206 bandwidth-limiting caps on the bottom – otherwise it’s through-hole all the way.
  • The PCB should be happy with just about any (regulated) dual power source – linear PSU, switching PSU or even a pair of 9V batteries.

Downloads:
Download design files here

Related information:
Even though this is a basic opamp circuit and I can just about draw the schematic and recite the parts values from memory, I went back to look at it once more to try and read up on the theory behind. If you aren’t very familiar with the basic schematic already I can absolutely recommend the old but still excellent articles from Headwize/Head-fi member Tangent here and here. Tangent’s pages also have a ton of other useful information and although the site isn’t updated any more (and it’s quite old) there’s still plenty of good stuff even for inexperienced diy’ers.

If you are more technically inclined then probably the best resource is the “Opamp Applications Handbook” from Analog Devices and edited by Walt Jung.

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

April fools…

No, this isn’t really a joke as such. However, there’s something strangely appropriate about posting this today I think 😀

It’s an “extended” version of my ZenHP amp, but I went a little overboard and added the gigantic polypropylene caps I purchased on my last trip to Japan as the output caps. As mentioned then, I’d want to test if using film capacitors on the output made any sonic difference compared to the electrolytics that are normally used. We shall see how that works out later on….

The downside of this (perceived) audiophile greatness (…) is that in order to make everything fit in a 2U/350mm enclosure I basically had to cheat a bit on the PSU. It’s either going to be an internal IRM20 switching type from Mean Well as shown or a very simple linear one that can be fed from an external transformer. Obviously having an amp as big as this requiring an external PSU is a bit stupid, but hey – it’s an experiment! 😀

Also, since it’s an experiment I’m not going to order fancy front and rear panels for this amp yet. Once I’m through travelling for work in a couple of weeks I’ll have to do a bit of metalwork of my own instead. Not much else missing before it’s ready to play though, but with my current workload it might still take a while to do.

PS: I’ve you see any good audiophile April fools jokes online, feel free to post a link in the comments section.

aprilfools

High voltage…

Yes I am still here, but it’s another busy period for me at work so updates to the blog are correspondingly few and far between. As usual when I don’t have a lot of time for diy I still somehow manage to start up new projects. Even with less than 48 hours at home in a weekend, there’s still time to do a little soldering to relax and unwind 😀

Among the overdue projects I’ve managed to start up lately are some amplifiers for my Stax electrostatic headphones. This is actually more than a little overdue, because I haven’t had a Stax amplifier for nearly a year now and so the headphones I have aren’t getting any use which is a shame really.

The pile of half-assembled boards in the picture actually consists of the following designs, all by Kevin Gilmore:

– A pair of KGST tube amp boards and matching 350V PSU

– A mini-version of the KGSSHV amp and matching 400V PSU

– A version of the KGSSIC/“Carbon” amp and matching 400/450V PSU

Most of the boards were all acquired through various group-buys on the head-case.org forums, but Kevin graciously keeps the Gerber files for all of his designs available for free download as well.

I’ve soldered more or less all the parts I have available, so still to do are:

1) Order remaining parts (working on that – since it’s also possible to do from hotels after work :D)

2) Figure out the mechanical stuff (mostly done, but still needs a bit of work – and some tools I don’t have regular access to)

3) Select and match a pile of semis (saving that one for a rainy day 🙂 )

4) Finish and test boards (as quickly as possible)

I’m not really used to high-voltage stuff so I am being extra careful with these boards. Just like when you move up in frequency, moving up in voltage means that things that were not previously issues suddenly become very important. Fortunately I have a working variac again (fixed after stupidly blowing a fuse in it a few weeks ago) which makes testing much easier – not to mention safer all round.

These aren’t the only Gilmore-designs I’m working at the moment by the way, but the rest involves much more pedestrian voltages 😀

staxboards