I just did some calculations on how much power the NAD will deliver in class A into 8 Ohms, with a bias current of 100mA. It seems it is a whooping 0,08W peak! I may try, over a period, increasing the bias current to 500mA to give about 1W class A. I did add to the cooling after all.

Note: I tried upping the bias current to 250mA, but the heat sinks were warmer than I liked, and I have backed down to 125mA.

Since I lost all my post on my transconductance amplifier projects, that ended up in an all N-channel MOSFET version of Nelson Pass First Watt F2 amplifier, I will summarize them here.

The above mess is the first prototype of a transconductance amplifier like the First Watt F2. It came to life after numerous SPICE simulations and chewing through OTA datasheets, Pass papers, and forum posts on DIYAudio.

It all started when I began experimenting with open baffle speakers, and got hold of some vintage SEAS full range drivers. It turned out Nelson Pass had been experimenting a with this kind of driver, and had written a paper on the subject.

Current Source Amplifiers and Sensitive / Full-Range Drivers

In short it seems that some full range drivers will benefit from being driven by a transconductance amplifier. A transconductance amplifier is a voltage to current converter with amplification. Our standard power amplifiers, are mostly voltage amplifiers, and will vary the voltage to the load, according to the input voltage. A transconductance amplifier will vary the current to the load, according to the input voltage. For a purely resistive load, this makes no difference as I=U/R, but a loudspeaker is not a purely resistive load. It is actually the current through the voice coil, that controls the force of the generated magnetic field, not the voltage. Because of this, a variable current source seems the most sensible way to drive a speaker. There is a catch though, since most amplifiers, are voltage amplifiers, most n-way speakers have their crossover designed for voltage drive, and will behave wrong, when driven by current. Nelson Pass has written a paper on how to design filters for transconductance amplifiers instead, I have not studied this very hard, since I am building this for full range drivers.

Current Source Crossover Filters

Nelson Pass had designed both his First Watt F1 & F2 as transconductance amplifiers, since I have a bag of IRF640 N-channel MOSFET's. I ended up modifying the F2 (First Watt F2 schematic), to use only N-channel MOSFET's and added a simple regulator, from Nelson Pass ZEN series.

The resulting sound, was good. Despite the two fans needed to keep the amp from burning a hole in the table, that it was lying on (class A, silver, custom made mains cord, 300B, nuclear reactor in the kitchen, mumble mumble). Despite the crude boxes the SEAS drivers had to put up with. Despite the insane amount of distortion, compared to most amplifiers. I have not tested this, but believe I can hear a change to the better, when driven with this amplifier. I have not yet tried correcting the speaker response as per Mr. Pass papers. I have simply decided that it sounded so well I want to finish it, and play with it some more along the way.

I had an installation of Lunar Linux, that I wanted to move from VirtualBox to KVM, and therefore had to convert the imagefile. I searched high and low, but found no up to date instructions on how to do this. | In the old days there seem to have existed a tool called "vditool", but now "VBoxManager" will do the trick of converting a VirtualBox .vdi image into raw format. Here are the steps that I took:

  • Find the UUID of the VirtualBox disk image:

    oblivion@mastermind ~/.VirtualBox/VDI $ VBoxManage list hdds 
    VirtualBox Command Line Management Interface Version 2.1.2
    (C) 2005-2009 Sun Microsystems, Inc.
    All rights reserved.
    UUID:         e4e316cb-ad9f-46ae-b15c-164b893371cb
    Format:       VDI
    Location:     /home/oblivion/.VirtualBox/VDI/lunar.vdi
    Accessible:   yes
    Usage:        Lunar (UUID: d249a972-f112-4cbc-91ce-389ce75e4fac)

  • Convert the .vdi file to raw format, using the UUID just found. Using VBoxManage's clonehd function the .vdi file is cloned into a raw image, in this case called lunar.img:

    oblivion@mastermind ~/.VirtualBox/VDI $ VBoxManage clonehd e4e316cb-ad9f-46ae-b15c-164b893371cb lunar.img -format RAW
    VirtualBox Command Line Management Interface Version 2.1.2
    (C) 2005-2009 Sun Microsystems, Inc. All rights reserved.
    Clone hard disk created in format 'RAW'. UUID: 5106c566-7188-4513-a416-73eb7a4e44a9

  • On my Gentoo Linux system, the converted image was saved in ~/.VirtualBox/HardDisks.

  • Convert the image to QEMU qcow2 format using qemu-img:

    oblivion@mastermind ~/.VirtualBox/HardDisks $ qemu-img convert ~/.VirtualBox/HardDisks/lunar.img -O qcow2 lunar.qcow2

From the utter silence of this command springs lunar.qcow2, ready for KVM!

The beast has been tamed. It sounds good too, although my headphones are cheap Sennheisers. The headamp is enclosed in a Hammond enclosure, using the aluminium casing as heat sink, it gets hot. It makes for a nice way to keep the coffee warm by placing the cup on top of the amplifier.

Q6, Q8, Q15, and Q17 are the output transistors, and needs heat sinks, I have not calculated the exact size needed, but the bigger the better. I use the aluminium chassis of a Hammond 1455N2202 with the transistors mounted on the bottom, and it gets warmer than I like. Never test the amp without proper heat sinking in place, as the output section will most certainly release the precious blue smoke.

Power supplyI have used a 2X12V toroid, a rectifier bridge, and 4700uf on each supply rail. Each channel draws about 200mA. 

GroundingSome attention is advised when wiring the ground, my first ground layout resulted in a nasty saw-ish 50Hz hum from the amplifier. I now connect the separate grounds from the input RCA connectors to their respective ground pads, next to the input pads. The power ground are connected to a star ground between the 4700uf power supply capacitors, as well as the ground wire from the headphone jack.

Files: SE Class A headamp schematic and PCB SE Class A headamp schematic and PCB (Cadsoft Eagle)

In the 3020i there a two thermal fuses (E401, E402) in the output circuit, you can take these out if you are certain, that you will never short circuit the output. On the earlier original model I believe this protection is done using a relay, and an integrated circuit, I think you will be able to rip out the relay in these versions.

I just ripped out these thermal fuses and replaced them with a piece of SILVER (you have to shout that word to make sure it's stays audiophile) wire, that my good friend Peter gave me.

I do believe this further cleaned up the bass output, of the thing, so much that I will have to find means to reconnect my sub woofers soon. This means that the bass output is now at "normal" and acceptable levels. I'm not working at optimal listening conditions as a friend of mine is playing Battlefield right beside me, but I do believe these thermal fuses had a higher sonic impact, than I would have thought, then maybe again it's the SILVER.

Build the last simulated version of the SE class a headamp today, on breadboard. It works as an amplifier, it is class A, it is hot, and it is oscillating. I have to read a little more about taming the beast, did try some tricks, and it helped. Also I have to try some other transistors in the output stage, I have two different brands of BD139. I was unable to measure the hFE of the ones of which I have the most, even though the other brand measures fine. Since this was a prototype, I used the strange ones, maybe they are bad to the bone.

NOTE: I found this thread at diyaudio.com read through, it's really nice stuff: Pure Class A Single End Amplifier Idea!

I have had this baby sitting on a shelf for a couple of years, getting more and more sick, to the point where it would skip through a CD in about 1 minute, very annoying. I needed something for my DAC to compete against and tried the SONY CDP-228ESD I had around, this uses a SONY digital filter and 2xPCM58P-J, but it was really no match, and I have never liked the sound of the thing. I must say, that I like the construction, this SONY, is surely made to be above average, but for me it didn't do the trick sound wise.

Back to the DCD-3560, I tried the adjustment procedure outlined in the service manual, with no success. So, back to the conclusion, that the laser was dying, fast. The laser is a KSS151A, from SONY, and of course out of production! Costing on the wrong side of 100EUR it wasn't really an option to just buy a new one. Secondly, things are a little more complicated with the KSS151A than a most other lasers I have seen, throwing out a lot of money, only to wreck the player completely, didn't seem like the way to go Until

I realised, that the SONY machine had the same laser, unfortunately, this means, that I've killed it, not in cold blood, but still the SONY is now very dead. After sweating 5 litres of nervous sweat, loosing a screw in the actuator, and readjusting the DCD-3560, which by the way, went smother with the new laser. The tracking gain was impossible to get right, with the old laser. Well the DCD-3560 is running smoothly again. Now there is nothing to do but hope the laser will last a long time.

DENON DCD-3560 service manual

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