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 Post subject: Moskido Amplifier
PostPosted: February 25th, 2019, 10:27 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 264
Just finished assembling a tube/MOSFET hybrid amp based on John Broskie & Bob Prangnell "Moskido" concept, using a 6CG7 gain stage, a 6N6P White cathode follower, cap coupled into a Pass Burning Amp complimentary output MOSFET source follower stage. I had originally considered an octal based tube front end (6SN7 + 6BX7), but did not have space or heater current for the 6BX7. This was a rebuild of my Pass First Watt F-4 clone (same chassis and LV power supply), but with new output/bias boards, mono HV B+ and heater supplies for each driver stage channel, and a point-to-point wired Aikido board.

I am still letting the amp settle in before pushing the bias hotter, but figure I should be getting around 30W Class A into 8 ohms, 60W Class A into 4 ohms. Hooked it up to speakers and a source this morning before work, and it plays music just fine! Planing to put it into the system in the next few days on the bass cabinets, but would like to listen to it full range with a pair of Aria 5R speakers to see how they sound together. These are 4 ohm speakers that really never worked well with SET amps.

David
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 Post subject: Re: Moskido Amplifier
PostPosted: February 26th, 2019, 9:29 am 
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Joined: March 12th, 2013, 11:12 am
Posts: 482
You got a lot squeezed in there! I don't remember hearing the 5Rs, how low do they go?


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 Post subject: Re: Moskido Amplifier
PostPosted: February 26th, 2019, 11:58 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 264
The Aria 5Rs go down to around 60Hz, as I recall. They are a 2-way D'Appolito design with a pair of Focal 5K013-L woofers and a Raven R-1 ribbon tweeter. Needs subwoofers (which of course I have). I did hook up the Aria 5R speakers to the amp, and on first listen, they seem to sound alot better than they did with a SET amp powering them. I remember finding that a PP amp (PP2A3) worked better with them than a SET 300B or even my GM70 SETs, but I think both were with the 8 ohm tap instead of 4 ohm. But this is with only 1 hour worth of playing down in the basement and not in the listening room, and near field besides. Need to let the amp cook a while longer to get the bias setting as hot as I dare (nice to finally have a temperature probe and an multi-meter that reads temperature).

David


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 Post subject: Re: Moskido Amplifier
PostPosted: March 6th, 2019, 10:58 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 264
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I have the Moskido amp in the listening room along with the Aria 5R speakers and subs. Although I originally intended the amplifier to drive a bass cabinet as part of the horn speaker setup of recent posting. I was curious on how it would do full range, particularly since it was sound so promising in the basement with just the Aria 5R close field. Overall sound is very clean, detailed, yet still having tube warmth. The speakers do disappear well, particularly if arranged so the tweeter is ear height, and the focal point is slightly in front of the listener. I had started with a GE 6CG7 in the voltage amplifier position, but found a Russian 6N1P seemed to give more life to the sound. Currently using a 1965 Novisibirsk 6N1P-B VI instead of the Svetlana version (Voshod?) of a more recent vintage (which I tried first).

I am still tuning the system, but the Aria 5Rs sound SO much better on this amp instead of tube amplifiers in the past. I am finding myself making excuses TO listen, rather than excuses NOT to listen (which, I am afraid, was the case with the horn setup). But this is probably due to novelty, so will see what happens in a few days or weeks when I start being bothered by a fault.

David


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 Post subject: Re: Moskido Amplifier
PostPosted: March 6th, 2019, 11:56 am 
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Joined: January 15th, 2015, 7:19 am
Posts: 820
Location: Baltimore MD
Wow that's a great looking room
would love to hear it too


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 Post subject: Re: Moskido Amplifier
PostPosted: April 6th, 2019, 9:56 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 264
Somehow, my last post from this afternoon did not get posted. But the gist of it is that the Moskido is for keeps. I have been listening to the setup with the Aria 5R speakers w/subs, and it is pretty amazing. The setup is clean sounding, very resolving, but still has a musical presentation. The soundstaging is fantastic, the speakers totally disappear. I also appreciate that I can listen at lower levels and not miss anything. I have spent nearly two decades down the SE triode path, with high efficiency speakers, and it is surprising to find that a mosfet/tube hybrid amplifier works better, on a pair of speakers I had dismissed 15 years ago. In fact, they were loaned about to friends, the last being Stuart, who was pretty disappointed with them, though it did not help that one of the Focal kevlar drivers were damaged. I restored them a year ago, and did not like how they sounded with SET amps. However, they love a great Class A Mosfet amp, driving the 4 ohm impedance with aplomb. A real tell that something is right is when you cannot stop listening, and find any excuse you can to listen and enjoy the music.

Now, in full confession, the MTM speakers still are vertically "challenged", and I listen on axis, with the tweeters at ear height. So sweet spot is a bit locked in, but that is where I listen anyway, so who cares!. Just me and the cats don't care (and my wife is somewhere else in the house).

Pretty damn happy and foot is tapping.

David


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 Post subject: Re: Moskido Amplifier
PostPosted: April 6th, 2019, 11:33 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 174
:-) I just Googled your speakers. I was curious what tweeters those were. 1st hit was your for sale ad.
viewtopic.php?f=7&t=822

Glad they're working out.


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 Post subject: Re: Moskido Amplifier
PostPosted: April 7th, 2019, 10:08 am 
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Joined: February 28th, 2013, 1:19 pm
Posts: 264
The tweeter is a Raven R-1 ribbon tweeter. It was SOTA back 20 years ago, and was copied by Fountek, Aurum Cantus and others.

I need to withdraw my For Sale ad. Never got any takers anyway.

David


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 Post subject: Re: Moskido Amplifier
PostPosted: April 7th, 2019, 11:57 am 
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Joined: December 14th, 2013, 2:19 pm
Posts: 673
As David correctly said, I didn't like them. I was in a small bedroom and using substandard amplification just to create some music.

I had the chance to hear them, refinished with good drivers and the Moskido.

I agree with David. They are very pleasing to listen to. The Moskido is a winner. I'm anxious to hear it with different speakers. If it's as good as I think it is, building one wilm move way up my amp project "bucket list ".

Well done David!


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 Post subject: Re: Moskido Amplifier
PostPosted: April 7th, 2019, 8:55 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 264
Here is what I did for the MOSKido amplifier It is really simple in principle.

1. Buy the Pass Burning Amp complementary output and bias boards from DIYAudio.
2. Also, buy the Universal Power Supply board from DIYAudio.
3. Find an online source (Ebay) for matched output Mosfets. IRFP240 and IRFP9240. I found a source that matched them extremely closely, so that the current and device temperatures are very close in operation.
4. Order parts from your favorite supplier and build according to the Build Guides.
5. A 4U Deluxe chassis with heatsinks (also from DIYAudio) helps with a clean build and is the correct size for a stereo amp.
6. Antek 400 to 500 VA 18V or 20V (I used 20V) toroidal transformer. You can even go higher to get more voltage swing in the output (i.e., more power), but watch device temperatures. My output stage supply is +/-24VDC

7. As far as the driver stage, the easiest way is use the Broskie 9-pin Aikido boards with the output setup as a White Cathode follower. You want to cap couple it to the MOSFet output stage, I used a 5uF cap for a really low cutoff. I am using a 6N1P input and 6N6P White Follower.
8. Design and build a B+ supply for the driver stage, around 300VDC. I built dual mono supplies, choke "regulated". You can find compact boards from Broskie. I used AC heaters, however used a high current common mode choke and caps to build a filter.

I built my driver board point-to-point on a piece of copper clad glass epoxy board. You can have a much more compact layout point-to-point, and space is at a premium in a 4U chassis. Might be advisable to go with the 5U chassis.

This is the general outline on how I built the amp. Not going into details, since my approach is modular, and each of the modules have their own documentation. There are some things on the White Follower to pay attention to, you are not driving a very low input impedance, so it helps to have a plate resistor of around 1100 ohms, which will optimally load the White follower to drive a 7K input impedance push-pull at twice the supply current. Or at least according to my calculations.

If any of you are interested, I can go into more detail, and you are free to stop by to see (and hear) my build. If my weekend schedules permit, I will bring the amp to the next meet when that happens. Maybe at my house...?

David


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