A cheap fantasy….

Yes, it’s a not a very good joke – sorry! 🙂

Haven’t done one of these “cheap kits” for a while, but I am possibly getting a bit more picky. However, this particular board was around 20 USD for a kit (excl. tubes) and so the risk was manageable. It’s supposedly a clone of the famous Matisse “Fantasy” preamp which uses 5670 tubes. I wouldn’t know to be honest, but I have been intrigued by low(ish)-voltage tube circuits for a long time and so I took the plunge.

I’ve replaced some components (mainly the capacitors and the volume pot), but there was still plenty of kit parts left to salvage to make it worthwhile (tube sockets, semiconductors, heat sinks etc.) over buying just a blank PCB. The caps probably would have worked just fine, so that replacement was mainly cosmetic. One notably exception was the 80V rated electrolytic in a part of the circuit that sees rectified 55-60VAC – a little too close to the limit for my liking.

The tubes ‘ve bought from an Eastern European seller and they are 6n3p-E which is supposedly a long-life Russian version of the 5670. The circuit runs off 50-60VAC, so it’s what I would call a “medium-voltage” project. As usual, the instructions that came with the kit were poor (especially if you can’t read Chinese 😉 ) but with a bit of care it wasn’t a problem to put together.

Sound quality? Well, as usual with these projects I’ve only really done basic bench testing for now and so the only thing I can really confirm is that it produces sound. One of the things I did notice though was that it seems to be a well-behaved circuit. By that I mean no big turn-on/turn-off transients, no excessive noise and no microphonics from the tubes. I might just have to splurge for a proper transformer and case for this anyway…

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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.

WCF amp completed

It’s been quite a while since I really finished a build. Sure, lots of small steps forward on lots of other projects, but nothing really finished to that fantastic moment where you can actually hear music for the first time 🙂 (or the slight less fantatic moments where you see smoke, a broken fuse or simply nothing at all… 😀 )

This is the just the WCF headphone amp I build a while ago (see this post) which has now been cased up. Instead of putting in a lot of effort on DIY mechanics I took a shortcut and bought the bespoke ebay-chassis for the design. It’s not bad, obviously fit and finished has been seen better, but given how I normally struggle to do the mechanical stuff it was still worth it. And anyway, this was a bit of a “sideline” project anyway. The transformer is from Audiophonics because they had an “off-the-shelf” model with the right specs which also was about half the price of a custom one from Toroidy.

Listening at the moment with a newly-purchased (but second-hand) Arcam rDac as the frontend and this amp is still not bad at all for the modest outlay I think 🙂