Wednesday, February 8, 2012


In the latter stages of 2011 there was an interesting thread that developed at DiyAudio about a new solid state (transistor) amp that used lateral Fets. I normally build tube equipment but decided to give this amp a try. Fortunately one of the main developers of the project, Hugh of Aspen Amplifiers, created and sold a pcb that made building the amplifier relatively simple. The first batch of 100 boards sold out before I could get my name on the list, but I managed to get onto the second list for another 100 boards.

Eventually my boards arrived as well as a bunch of parts from Mouser. I already had heatsinks and a 300 VA transformer that I could use, so the cost of the amp was probably under $150 for me, including the boards.

So here is my version of the amp with 2SK1058 and 2SJ162 fets. The base is just a 12" by 15" piece of plywood. There are two heatsinks on each side that I used years ago in my first amp, a Son of Zen. They are 7" x 5" x 2" approx. They barely get warm, even when driving the Apogees, but don't take this as an invitation to skimp on your heatsinks. Think large, particularly if your listening room gets hot in summer.

At this end, I have the RCA jacks, the speaker terminals and a couple of mono volume controls mounted on a piece of polycarbonate. I hate drilling and cutting metal so I use poly.

On top are some wire grates that were initially cake / cookie / biscuit trays. It makes it easy for the heat to get out. If there was one thing I learned from my Son of Zen amp, you need to be able to deal with heat. You could have roasted a chicken in that thing. We have a cat that likes to sit on warm objects, particularly amplifiers, so I need to protect it from itself.

The heatsinks came with pre-drilled screw taps which I used to mount the FETS. The holes did not match too well with the positioning of the FETs on the board so I had to separate the heat sinks. The green wire is for grounding.

The brown object in front is a russian MGB 2uf capacitor that I used instead of the recommended Auricap. I like the sound of these caps.

My old 300va 115: 24+ 24 toroid transformer. Ideally you should have two 150 VA toroids, but you can use a single 300 VA if that is what you have. You just have to be careful wiring the boards.

I've covered the terminal block with electrical tape to protect the cat.

This is a very good amp and very good value considering the cost of building it. At a rough guess, if you bought all new components, it would cost somewhere between $200 and $250 and it's not easy to build an amp for under $200. I've tried. The Fetzilla thread contains a Bill of Materials (BOM) and suggestions for where to source the parts. I found the building process pretty easy but I've done a fair bit of soldering over the past ten years. The thread has instructions for how to adjust the three pots to achieve the correct operating points and the process is pretty straight forward. I have quite a few cheap multimeters that I purchased from Harbor Freight and I used three of them to measure the voltages. I have found this article useful.

So enough about building the thing, how does it sound. For a while I used it on my old Apogee Stages (4 ohms) and for the last few days, on some old 16 ohm 15" Tannoy Monitor reds that I inherited from my father. I'll try some other speakers in a week or so.

In each case, this is one of those amps where you discover something new in recordings you thought you knew well. In my set up I get a deep solid sound stage and plenty of detail. I'm not aware of any grain which I sometimes hear in transistor / solid state amps. I wouldn't say it's got a tube like sound but instead it seems quite neutral to me. I am more used to my diy tube amps and they seem to have a more pronounced sonic signature.

For me, variation from one amp to another is not a bad thing. There are many good wines that taste quite different from each other and similarly there are many good amps that each sound different. What I can say is that this is an amp that I enjoy listening to and I look forward to each listening session.

My understanding is that the amp will deliver 50 watts approx and I would believe it. There is a real 'jump' factor with the Tannoys, particularly with classical piano. In other words, I think this amp will do well with most speakers out there.

If you are sitting on the fence wondering if this amp is for you, I would suggest giving it a go. 

Update - late Feb 2012

I was still getting some hum and mechanical noises from the 300 VA transformer. The noise didn't bother me too much while music was playing, but I knew I had to do something to eliminate it so that the amp could sound its best. While cleaning out my storage area I found a 160 VA toroid  (115: 24+24) that I had misplaced and remembered that I had an additional identical transformer in another project that I wasn't using. So I duly swapped the transformers and now there is no hum or buzz. 

Interestingly, I checked the operating voltages and the offset settled back to exactly 0 volts again. No pot adjustments were needed.

Without the the hum and buzz, the amp sounds wonderful and really seems to suit the Tannoy reds.

Now that I don't need to do any more to the amplifier itself and there doesn't seem to be much heat being developed inside the case that the heat sinks can't easily deal with, I will replace the grills with a solid wood top. I'll probably replace the base as well with the same wood.

The final version with Catty-Watty ensconced. She really likes to sit there.

No comments:

Post a Comment