Moskido - Hybrid Aikido/MOSFET amplifier

[Andypairo]

I use my Hybrid with Aikido as preamp and the power buffer in another chassis but they act as a integrated amp.

Mine has gain roughly half of E88CC mu (so about 16), enough for the power levels I need (my buffer works at +/- 32V, so about 50W/8Ohm max power) but you can use different tubes in the first place of the Aikido to suit the gain you need.

I see no point in using a Aikido as preamp and then a integrated amp with another Aikido as front end. I switched to this kind of hybrid because Aikido's gain as a preamp was too high (with my tubes).

Cheers

Andrea

[marsupialx]

First, I'd love to see the actual schematic, assuming there are little details that the TubeCad circuit takes for granted that I don't know to include. And you may have deviated from it, anyway. I started out to build this, then decided to do an Aikido into an F4 Pass current amp. Now I'm looking favorably at this again, for its simplicity.
Can one simply hook a pot (or stepped attenuator) onto the input and have an integrated amp? I emailed John Broskie that question some time ago but didn't get a response.
Thanks for any and all info. I love it when somebody is doing what I want to do that knows more than I.

[beau2317]

John has many many emails and he just can't manage to answer them all. I feel lucky if he answers one in five. But it's great to get input from the great man himself.

I am working on a complete schematic and will post asap. I did make some small changes, etc, and I will also include the PSU

I am glad that someone is interested. You will not regret building this amp if you choose to...

As to whether you could put a pot on the front and make an integrated amp. I have tried this and it works but maybe does not drive the amp to full power.

The aikido voltage amp has a gain of about 29 with a 6SL7/6SN7 (see table http://www.tubecad.com/2006/05/blog0064.htm ) so 2V p-p should result in 58V p-p being sent to the MOSFETs. Not sure the exact gain of the output stage, but would guess that it's a shade less than unity like most follower stages (correct me please anyone..). Let's say it has a gain of 0.9, producing 52.2V p-p at the output

Now according to my calculations 52.2V p-p is 18.45V RMS, which into 8 ohms gives about 42W

So you can see it's straining to deliver full power for a big amp like mine with 65V rail, but would work OK for a smaller amp with a 40V rail. Also it depends on whether you are running 4 or 8 ohm speakers.

[beau2317]

Here's the main amp schematic. Apologies for poor quality drawing.

Notes:

-- I added a level of filtering to the diode string (additional 10k resistors, capacitor)

-- zeners are for protection (so John says)

-- I used BUZ900/905 instead of 901/906 as they are slightly lower voltage (150v vs 200V) and cheaper.

-- there is a 1K resistor across the output I forgot to draw. This is just to make the output sit at 0V if there is no speakers attached.

-- you can adjust the bias current by altering the number of diodes in the diode string (more diodes, more current), and/or by altering the 100K resistors (larger resistance, less current)

One nice thing about this amp is that it does not suffer any nasty turn-on of turn-off noises.

[beau2317]

And here is the PSU

FRED bridge rectifiers, Jensen 4 pole caps, Tube transformer from PartsConnexion

MOSFET transformer from RS Components, 500VA

Common mode filter made from some ferrite RF chokes and 0.1uF

[marsupialx]

Thank you very much. Looks like I have my next project. I have 2 mono octal Aikido boards that I am starting to populate (12sl7 / 12sn7). Not ultimate, but affordable.
Because money is not available, this will be a long process, so I won't be reporting results anytime soon. I plan to build the SE version, as I already own 'the big rig' (Classe amps into Maggies) and am putting together my version of the cool low-powered amp with single full-range driver thang. I might have to build another Aikido eventually as a phono pre, as this system begs for vinyl. Thank you again!
Jeffrey Hill (marsupialx)

[JoshK]

Its very cool to see that it works so well and it looks pretty simple. Even a complete SS dumby like me can understand it.

I had scribbled an idea of replacing the front end of one of my UcD amps with an aikido stage. It seems quite possible, I just don't understand the output stage technology enough to know if I am overlooking something. The front end is just a simple 3 opamp instrumentation amp according to Hypex's website. I am still curious to give this a try at some point.

[beau2317]

Sounds like a good idea. My friend replaced the front end on his 300B monoblocks with aikido mono boards, and it worked a whole lot better.

John does very nice PCBs, mono or stereo, octal or nine-pin, at a very reasonable price. Linked from his website somewhere.

The aikido has quite a low output impedance and can be made to swing from a few volts to a few hundred volts. The choice of tubes to use is pretty large. There's a table on John's website giving resistor values, gain, etc, for just about every tube you could use. Go for it..it seems that most people who build aikidos become fans (or at least like them a lot). I don't recall ever seeing anyone who has built one and not liked it a lot.

[JoshK]

I have a pair of stuffed octal mono boards ready for a power supply and I have two sets of Bas's noval boards.

[beau2317]

Cool. Please post your results when you get them working..