Friday, 29 April 2011

The journey to the Pass Labs F4

Having set a path forward with the headphone side of my audio setup, I thought it appropriate time to look back at the stereo setup. I am quite satisfied at the current crop of speakers sitting in boxes, ready to be mounted onto cabinets. With a move to a home with a more fitting stereo room, I think it would make an ideal start in comparison of the speakers.

In this same vein, I have started to review the amps. I have a range of 1W and 8W tube amps ready to be assembled and tested. At the moment, most of my speaker drivers are in the 95 db+ range, and can be considered efficient, with the exception of the Jordan JX92s.

Reading through the various articles about amplifiers in the voltage and current domains, my conclusion (which is very similar to many others) is that tubes are excellent voltage gain devices. They work primarily in the voltage domain. To get them to power speakers, one has to convert some of the voltage into current, hence step down transformers. The output transformer takes, typically, hundreds of volts and milliamps, and transforms that into tens of volts and potentially tens of amps. This is then able to drive a speaker.

Though I have not much been interested in MOSFETS or transistors for amplifying devices, there seems to be a movement towards using them where their strength lies, in the high current domain. In a sense, they operating in tens of volts and tens of amps (or thereabouts). Most popular power amps are of the transistor type, and are able to produce hundreds of watts of power.

The debate between what is better; tubes or transistors, I think will never be concluded, as it is a function of taste. Each to their own. So far, I have experienced primarily tubes, and I like them not only for their sound, but their nostalgia and appearance. For me, a glowing red tube is so much more attractive than a massive heat sink. The fundamental problem, taking aside taste, is one of power. Tubes cannot really go above 100W, and if we limit ourselves to the single ended affair, probably not above 30W, sensibly.

I think what I, and other SET fanatics crave is more power. The only way to achieve that is to operate the tubes at an ever higher voltage and current. I have not found tubes that really work well in 16W plus in SE operation that operates at less than 800V. The 6C33 is close, but operates at a very high current of around 400 mA. The high current represents its own problem in transformers and chokes.

Fundamentally, what I have come to understand is nothing new, or earth shattering. However, it was important that one reach that conclusion in one's own time and way. My tastes at the moment favour; Single tube SE or one tube pair PP, both in Class A. Further, my goal would be 32 Wrms of clean power, irrespective of speaker efficiency.

Now that I am at this point, I find that I have two paths to move forward with. First path is with a high voltage SET, like the GM70, 211, 845 or similar transmitter tube. The other, a hybrid solution mating a tube front end with a transistor output stage.

Since I have ruled out high voltages, that only leaves the second path, one of a hybrid approach. Having only casually explored the hybrid path, I have come across the Moskido design, which is a hybrid of the Aikido design with a MOSFET output stage. The other design is one of a booster amplifier, after a tube output amp. Having further researched this idea, I came across two interesting solutions; DIY Hifi Supply Fusion module and the Pass Labs F4 impedance converting amplifier.

The Fusion module seems to be very easy to implement, with the addition of a low voltage power supply and the insertion of the module. It looks like a current amplifier that is probably in parallel with the speaker. It probably includes some kind of Class D chipamp operating as a current amplifier.

The F4 is more interesting. Now only has it got some really good reviews, it is a similar approach as with Andrea Ciuffoli in his "100W Hi End Hybrid" design. Any which way, they are both interesting approaches and designs. The F4 has a cult following in the DIY Audio community, and there are PCB boards available as well as volumes of forum postings about this. For me, that makes it a safer design, as it can be supported by a range of people who have built it.

Having come to this conclusion, I ordered 2 sets of the PCB boards from the DIY Audio Forum store. Also ordered the matching number of sets of the active devices from Tech DIY. I hope to start the build soon, and share what I have research and progress to date.


  1. Hi,

    I'm Bob, the developer of the moskido. I just wanted to make a small comment about the similarities between the Moskido and the F4. If you look carefully you will see that the output stages are almost the same in many ways. The source follower output. The use of the same precision shunt regulator for bias (TL431).

    I have looked carefully and my opinion is that the moskido output stage is superior because:

    - it doesn't have an electrolytic cap in the signal path, as the F4 does (C1 and C2).
    - the F4 uses cheaper mosfets. The moskido uses mosfets that are designed for audio and are more linear, and have less input capacitance.

    This is why it needs the jfet input stage.

    Of course, Moskido doesn't have a cult following. But I have compared it to Pass Labs amps, including the X350.5 and in the opinion of the X350.5's owner and others at the shootout, the moskido was better.

    Unfortunately there are no PCBs available.

    What would make a good amp imho is an aikido front end with an F4 backend. You could even leave out the input jfets and the electrolytic caps as the aikido would be able to drive the mosfets no problem

  2. Bob, thanks for your input, I have looked into the Moskido in the past, but it is hard to find the 'current' design and information on it, as it is archived through such long threads in the DIY Audio Forums. I'd love to host your information on my blog, to be a central repository of the information, schematics, parts lists, builds info, instructions, etc. I also understand that you are in Australia as well?

  3. This comment has been removed by the author.

    1. Are you sure about C1 & C2 in the F4 being in the signal path? Perhaps I'm wrong but aren't they bypass caps for biasing Q1 & Q2? Besides, Nelson Pass, who should know, says the F4 is DC coupled Also, isn't the F4 considered a follower amp, since its voltage gain is less than 1? If so, then despite what Charlieemb
      said about Pass MOSFET amps being susceptible to Miller capacitances here this would then not apply to the "follower" F4, according (and rather ironically) to Lynn Olson.