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.

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.

First Watt F4 (part 1)

I don’t normally build class A amps in the summer because my apartment gets really warm, but this time is an exception. Partly because this summer in Copenhagen has been much more “class A amp friendly” (i.e. a lot colder!) than usual, and partly because this is a design I’ve been wanting to try for a very long time now.

The First Watt F4 is a classic Nelson Pass/First Watt design with JFET inputs and MOS-FET outputs. However, as with the other FW amps there is a twist here, namely that the F4 has no voltage gain. That means it’s essentially a buffer than can provide a full 25W class A output. What’s the point of that you might ask? Well, one point is that it can help get a better gain structure and that it’s possible to use some sources (such as DACs) which have a very high output. There are various other applications in the F4 manual as well.

Some will have spotted that the F4 boards are from the diyaudio store. They are good quality and a well-proven design, so I decided not to bother doing my own.

The chassis is sort of the usual from Modushop, but then not quite anyway. Partly because the heatsinks are predrilled 4U types from diyaudio (which did cost a bit more, but saved me drilling and tapping nearly 30 M3 holes) – because they match the boards 100%, and partly because I have decided to do a bit of “hacking” to make a non-standard size chassis (teaser! 🙂 )

So, in addition to the chassis hacking, I am also thinking about which preamp to choose to provide the voltage gain for this and obviously there are plenty to choose from, so it should be possible to come up with an intriguing combination for you guys 😀

Soundwise I also have quite high expectations because of my past experience with the First Watt F5 design – which I still consider one of the best sounding amplifiers I have tried in my home system – but let’s see if the F4 delivers on that front as well when it’s ready 🙂

(Yet another) anniversary!

Yes, it’s that time of the year again – and this year it’s the fourth anniversary of the blog 😀

Not a lot to say that I haven’t already said the last couple of years, but I still expect to continue writing as much as time allows. I am also still very excited and greatly appreciative of your questions and comments, so keep it up 🙂

Picture below is of what is (currently) sitting near the top of my project pipeline, namely four 4U diyaudio special-edition pre-drilled heatsinks. These are specifically intended to accelerate (as much as possible) the completion of my Pass VFET project as well as one other Pass project using boards from the diyaudio store that I have wanted to do for some time now 🙂

Project files: VFET PSU

What is it?
In response to a reader request, the project files for my V-FET PSU board shown here. Of course, this will also work for any other class A design you might think of, as it is a fairly standard CC-R-C configuration with onboard rectifiers and space for three 35mm snap-in capacitors per rail. On typical class A voltages that means you’ll be able to use capacitors in the 22-33mF range and the the onboard rectifiers are 15-25A plastic SIP types, which should be just fine for most applications.

Input and output connections are via FAST-ON tabs and there are two sets of output connections. Since we’re paying for the copper on the boards anyway, I’ve tried to keep as much of it as possible  with a top-side ground plane and the supply rails on the bottom. 🙂

How big are the boards?
The board measures 3.1” x 6.675” (app. 78 x 170 mm).

What is the status of the boards?
Since the prototypes worked fine I haven’t made any changes and the board is therefore version 1.0.

Does it use any special/expensive/hard-to-find parts?
Nothing worth worrying about really. The only possible exception is only really the rectifier which is in a small GBU-package. However, Mouser has them up to 25A (p/n 750-GBU2510-G) and they are available from many other sources in 10-15A variants as well.

Anything else I need to know?

  • If you want to use off-board bridges, bridge the AC and the DC-connections with as thick a wire as you can get through the holes. That should allow you to use offboard metal-cased rectifiers up to 50A. Since the average current draw of most class A amps is quite low and the surge ratings aren’t that different between package types I don’t see the need to use anything else than the plastic ones, but by all means complicate matters with offboard bridges if you must 😀
  • The four series resistors can be 3-5W types in parallel which should be plenty, even if you want to burn off a bit of voltage in them.
  • The (optional) 3W bleeder resistor discharges the two first capacitors while the LEDs will discharge the last ones. The series resistor for the LED can be a 1/2W or 1W type.
  • Last, but not least: Electrolytic capacitors in this sort of size aren’t to be trifled with, so make sure you mount them correctly and test the board properly before mounting it in your amplifier chassis.

Download design files here

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


VFET progress…

Well, not that much progress on the Pass VFET boards themselves – hopefully this weekend something will happen – but I have managed to make a PSU-board for them. Plenty of those around already of course, but being a) particular about dimensions and b) a bit particular about PCB colour matching I decided to roll my own instead 🙂

The design is a pi-filtered CC-R-C type with space for 35mm electrolytics, which at the VFET-voltage are available up to 27-33mF. As I plan to use the boards in mono-mode (one per channel) that’s actually enough energy storage to be a bit frightening. The Pi-resistors can dissipate up to 12W per channel which should be plenty (at least I don’t plan to go that high).

Also included are a polyester decoupling cap, a bleeder resistor for the two first electrolytics and a pair of LEDs which, apart from indicating power, also bleeds the last pair of caps.

As the pictures show, I’m still missing some parts but this project was never going to be a rush-job anyway so that’s just fine. The days in Scandinavia are getting noticeably shorter now, so saving projects for winter will not be a problem 🙂

Pass V-FET kits are here!

Forgot to post this a week ago when they arrived, but I managed to secure a couple of the Nelson Pass V-FET kits which I am quite excited about.

In short, this is a low-power class A amplifier based on some complementary Sony V-FET (SIT) transistors that have been out of production more or less since before I was born. The actual devices were bought as NOS (new old stock) by Nelson Pass himself and offered to the diyaudio community through the diyaudio store as a (more or less) one-off opportunity. I was lucky enough to register my interest early on and so managed to secure a couple of kits to keep me busy on those long Scandinavian winter nights when they come around 😀

There’s a big discussion thread on diyaudio and also an article on the FirstWatt website about the design, in addition to the information in Nelsons previous articles on SITs (also on the FW website). As usual, I don’t really need these and the class A heat is a bit impractical in a small apartment, but a limited-edition amplifier kit with unobtanium transistors that was developed by Nelson Pass himself was an opportunity I simply could not pass up (pardon the stupid pun 🙂 ).

The Firstwatt F5 is still one of the best amplifiers I’ve heard in my system so I have very high expectations for this new design. The lower power of the VFET could be an issue, but I’ll have to build it and try I guess – with my current speakers it should be OK and if not, I can always get a pair of very inefficient planar magnetic headphones instead :D.


Project files: The Zen Headphone Amplifier

What is it?
The board files for my Zen Headphone Amplifier “remake” shown here.

How big are the boards?
The board measures 3.575” x 3.75” (app. 91 x 95 mm). This is obviously for a mono-channel.

What is the status of the boards?
The boards are version 1.0. The prototypes seem to work well and there wasn’t really anything that needed changing in my view.

Does it use any special/expensive/hard-to-find parts?
Not really.

Anything else I need to know?
A few things:

  • Heatsinks: The basic type for me here in Europe is the Fischer SK129, but there are many manufacturers of this profile. The board is designed for the heatsinks to be soldered in place with pins, but screw-mounted versions might work as well. You can use 1″/25mm heat sinks, but in that case I recommend to tune the bias a little lower. My prototype measured app. 240 mA of bias and the heat sinks seemed to stabilise at around 55C in free air, which probably is a bit too much when the board is cased. So, either turn the bias down a bit and/or use taller heat sinks if your case allows for it.
  • Adjustments: Space around R10 and R12 is quite tight, especially with heat sinks/output caps taller than 25mm. In order to easily be able to adjust bias and balance of the amp, my suggestion would be that you don’t trim the leads of the two resistors completely flush but leave enough of the resistor legs that you can connect crocodile clips to them on the underside.
  • Output capacitors: The recommended value is 2 x 470uF from the original schematic, but if you’re using low-impedance headphones I think you should consider 2 x 1000uF instead. This is one place where I think “audiophile” capacitors can’t hurt, so look for Nichicon Muse (KZ/KW, FG/FW, ES etc.), Elna Cerafine/Silmic capacitors or similar. Bypassing the electrolytics with small film capacitors is easily done on the underside of the board if you want to.
  • Transistors: The Q3 footprint on the board is for a BC550C, but the original ZTX450 from the schematic can be used as well if it’s turned 180 degrees. Remember also to match at least the two Q2 FETs between channels as described in the build article. If you buy 8-10 of the IRF610 FETs you should be able to get a couple of very tight matches and the leftovers can be used for the current source (Q1).

Download design files here

Related information:
See the original post for some more information and links to the build article for this design. After posting I actually also managed to find the original headwize article cached here – amazingly it seems that most of the headwize library has been kept intact there! 🙂

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


The Zen of Headphones…

Oh no, sorry – it’s the Zen for headphones 😀

This is another project from the early days of my DIY-career that I have “resurrected” by remaking the PCB. It’s an adapted version of the Zen amplifier by Nelson Pass and optimised for driving (primarily low-impedance) headphones.

The design was developed by an Italian diy’er called Marcello Pellerano back in 2002 and you can still find the original discussion thread here. In addition to the diyaudio-thread, the project was also later published as a project article on Headwize, the precursor to Head-Fi. Sadly the Headwize project repository is no longer available, but it seems the full text of that article is available here as well.

Fun fact I: Marcello’s project was the first time I ever saw the enclosures from Hifi2000/Modushop and therefore what originally led me to start using these cases.

Fun fact II: Although I haven’t copied the EQ PCB that is described in the build article, I actually own a pair of the original Grado SR-325 headphones that the amp and EQ were originally designed for. Even without the EQ, I can’t really think of a better excuse for getting the Grados out of storage and listening to them once again 🙂

My main changes compared to the original design is to use onboard heatsinks and some different footprints for the various capacitors. Especially on the output side I’ve scaled up a bit, because 32 ohm Grados aren’t as low-impedance as they were 13 years ago. Many modern headphones are lower impedance than that and so making space for bigger output capacitors seemed worthwhile.

I haven’t copied the PSU either, partly because I already have a few designs that can be used instead and partly because there are just so many other options out there now – more on that later!

Sound quality: We’ll get to that later on as well since I’ve only done bench testing so far, but it definitely works and first impressions are quite positive.

The Great B1-binge…

I’m obviously a fan of the Pass B1 design but the last couple of weeks have seen the arrival of no less than three more B1s which is a bit much, even by my standards… 😀

Not sure how this really came about, but it must be something like this: A while ago a saw an ad on a forum for a B1 clone board (the original type which is pretty much the same as the original pasty board). It was quite cheap and so I bought it. Since I now use both analog and digital I thought I could build one with two inputs (because my previous one only has a single input).

Shortly after I received this board and had started populating it, I realised I already had a partially-assembled board of another clone design that was basically only missing the input and output caps in order to be ready (yeah I know, I should keep a list or something… 😀 ). Because of the size of the onboard electrolytic caps, the board I had would fit nicely in a 40mm high enclosure, whereas the new board would require a bit more internal height, i.e. an 80mm chassis.

More or less the only thing I don’t like about these B1 boards is the fact that you have to “air-wire” the input switch, which means criss-crossing the inside of the chassis with long wires carrying the input signals. The obvious solution is to use a relay, so I went ahead and made a couple of small adapter boards to accomplish this. With a 24V relay all that is needed to switch the relay is the supply voltage to the B1 so it makes for very simple connections.

The last design was prompted by a reader email asking if I knew of a B1 with more than 2 inputs. The answer was “no”, but I then decided to build a source selector to match my own B1-board. The selector is a simple relay-based type with four inputs but it should work just fine. As I had some PCB mounted RCAs I made a board version for those, but also a more universal one without onboard connectors (not shown).

I’ve tried to put all of these in fairly nice cases, but it’ll still be a bit of a challenge to decide which one to keep as my “personal reference” 🙂