Introduction
This amplifier started life as a post on John Broskie's Tubecad website. The original circuit is here and the post is here. The picture above is my own Moskido amplifier.
A friend was looking to build an amplifier at the time, so I suggested this design. He built it using cheap parts, to see how it sounded. At the time I was using some NuForce amplifiers which sounded pretty good or so I thought. But this amplifier trounced it completely. And what's more, it had a goddamn electrolytic capacitor in the output, as it used a single rail. So I decided to make one using the best parts.
Sidenote : After this was a success I wrote up the project on a well known diy audio forum. This thread ended up being one of the most popular posts on the amplifier forum, reaching 200 pages before it was deleted at the same time I was booted out of the forum. (No reason was given for this action, but the argument I was having with some moderators is clearly what upset them - apparently saying that interconnects sound different is heresy on that forum.
I coined the name Moskido at some point, being a combination of Mosfet and Aikido.
The design was developed over the years. The biggest change came when I went to DC coupled with split rail supply. Another change that produced significant improvement was the mod to go push-pull on the aikido driver stage.
This amplifier runs very hot. It's a class A monster but only puts out 50W - 72W per channel. But they are class A watts, which seem to sound a lot louder than regular watts. This amp is powerful enough to drive my Magnepan 3.6Rs to wife-annoying levels. As shown in the schematics, it's designed for a 4 ohm load.
But this design is easy to alter to suit your requirements, so if you want it to drive an 8 ohm load and make 20W, just let me know and I'll show you how. But don't think you can get mega-power out of this (unless you want to build a space heater.) To give some idea, if the bias is set at 2.5Amps, the output into 4ohms is 50W pure class A. But the amplifier is then dissipating about 400W at idle.
Overview
The Moskido is a fairly simple amplifier conceptually. It uses the Aikido stage for voltage gain. It uses the Mosfet stage for current gain as the devices are run as a source-follower. There is no negative feedback used. There is one coupling capacitor (originally I used Mundorf Gold/Silver but have changed to Obligatto Gold as they sound better to me).
As noted on the schematics, I used Vishay bulk foil resistors for many of the important resistors. I don't think I'd recommend that now as the axial Vishay bulk foils I used seem to be different to the current production, which seem to sound quite horrible in comparison.
The mosfets used are BUZ900 and BUZ905. These were designed for audio and are more linear and have better characteristics than some of the mosfets commonly used in amplifiers (which are often repurposed switching mosfets).
The Aikido stage is shown using octal 6SN7 tubes, but I think it would be better to make the Aikido using 9-pin tubes as there are many more types available.
Building
This is not a kit; it's a project for an advanced DIYer. And it's not suitable for your first project!
One thing that does make this easier is that you can buy either PCBs or complete parts kits for the Aikido - click here for more. The power supply and mosfet parts don't really need a PCB. That leaves the bias stage. Personally I'd make that up using some stripboard.
If you do decide to build, I'm happy to help with any questions that you might have.
Sound Quality
Very musical and punchy. You can vary the overall tone of the amp by altering the tubes, especially if you make using 9-pin tubes.
Schematics
Divided into four parts - Aikido voltage amplifier, Bias Circuit, Power Supply and Mosfet stage. Contact me if you'd like larger versions. You can right-click and save the images below. You can use these schematics to make non-commercial Moskidos.
