Mooly's MMAMFM Monoblocks - A build log

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Rails of that value are perfect, all that higher voltages buy you (unless you go significantly higher and with multiple output pairs) is more heat. 37, 47 or even 57. Yu will not notice a difference (apart from the heat) in a domestic setting.
 
After getting over my expensive mistake, things are progressing well.

I changed the 10uf capacitor at the signal input to the speaker protection to 47uF and I now have about 3.5s delay at turn on.

I tested the Vu meters. They light up nicely with the 10vac secondary from the power transformer. Perhaps more orange the yellow with the lamp on?
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I ran a sine wave from my sig gen through it to see how the calibration was:

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A 9v sine wave gives 55% power.
If I am doing my maths right, that means 100% is 16v which seems a bit low given 50w into 8R is 28.3v . I will test this more once I have the amps up and running and can run a larger signal through the meter.
Not the end of the world if it is as this would mean the meter would move more at lower volumes where the amp is likely to spend most of its life.
However I need to assess the possibly the meter would get damaged the amp was ever driven to full power.

I also got my new proper psus almost all soldered up pending parts from mouser. I discovered that I was 1 capacitor short on my prefered class x2 0.1uf caps for the rectifier C,RC snubbers. Also after testing the speaker protection with a no name 47uF bipolar from our local brick and mortar. Now that it is tested I have some nichicons on order.
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Now that these are (almost) done I can start mounting things properly in the chassis and working out a mounting solution for the transformers.
Once mounted I can work out the proper lead lengths for the transformers and then start termination and dressing them.
 
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It verks! Noise!
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It's been a busy couple of weeks to get some noise out of some test speakers.
I had a bit of extra time time this last week as I did something to my right Achilles tendon so I couldn't train all week.

Output boards all soldered up:
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These boards integrate the output Thiel network and protection relay.

Amp board and PSU mounted up to the heatsink and partially wired up.
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It always amazes me how long it takes to get the wiring all neatly cut, twisted, terminated, braided etc.

Here's the pair almost ready to go:
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Right channel wired up ready for power:
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Startup went without a hitch. No magic smoke or blown fuses.
Voltages at key nodes in the circuit were as they should be.
DC offset was 0.2mV on the left channel and 0.6mV on the right channel.

Biasing was nice and smooth. The bias was set to 100mA via 44mV measured from drain to drain. Across all 4 combinations of pairs and the voltages varied up to 2mV so the lowest pair was set at 44mV.

After biasing it was time for test speakers and noise.

Now that I know they work basically as expected, tomorrow night I will get out my scope and sig gen and start torturing them to see if they scream.
 
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The first round of testing was to find clipping.

Left channel into 8 ohm:
Left Channel.png
Left channel into 4 ohm:
left channel.png

Right channel into 8ohm:
Right channel.png
Right channel into 4ohm:
right channel.png

I think the slightly lower measurement into 8ohm on the right channe than the left channel, was me backing off clipping a bit too far before taking the screenshot.
Given that the measurements into 4ohm are almost identical.

Serendipitously, these output numbers correspond very well with the Vu meters.
 
Well,

I've been living with the pair for the last week. Impressions are this is a damn fine amp.
Definitely the best solid state amp that has been in my system so far. It's been a few years since I have fired up my KT88 monoblocks so I can't compare to a tube power amp.

Excellent all day listening amps. Never fatiguing, harsh or brittle.

Compared to the last amp I build a Rod Elliot P3A. It has a similar character but is much better.
The Mooly (I think we should just call it "The Mooly" from now on) has a bit more warmth or sparkle. It bit more of that special sauce:

The Eagles - Lyin' Eyes. The P3A just didn't quite put the chills up mine spine when the chorus comes in. But the Mooly bloody well does.

The Eagles - King of Hollywood. As the guitars fade in at the beginning, with the Mooly, the guitar harmonies stack up into something special. Not every amp can do the special part in my experience. My P3A has a bit of it. My LM3886 doesn't.

Deadmau5 - I Remember. The synth is rich and luscious. The big synth reverb before the vocals kick in goes on forever. Makes me feel like I'm in the middle of a bush rave seeing pretty colours.

Audiomatic - The Chosen Path. Oh, how I love the synth.

Jeff Buckley - Hallelujah. Got me in the feels. Once again, the hair on the back of the neck.

Compared to the P3A there is a bit more resolution or detail too:
Ben Folds - University A Cappella! - The Luckiest. I can pick out each singer in the chorus just a bit easier than the P3A.

AfroCubism - A la luna yo me voy. The Cacophony behind Company Segundo is made of distinct, un-muddied instruments. I can almost figure out what the hell is going on in that recording studio.

It would be interesting seeing good distortion measurments on this amp. My guess would be quite low distortion but a strong 2nd harmonic.

It sounds fantastic. Anybody wanting something circa 50w and class AB should build one.

My new favourite amp. I was very happy with my P3A. Now it might find a new owner.
 
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After living with them for a couple of weeks I am very happy with their sound.

Time to finish them.
Tonight I made on some extra brackets to secure the tops of the heatsinks to the chassis. A little bit of extra rigidity never hurt nobody.
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I will need to drill some some extra holes in the front and back panels. By the time that is done and test fitted that is probably another evening gone.

After that comes:
Painting the chassis.
Finishing the mahogany front panel.
Making moulds for the power transformers and epoxy potting them.
Making some copper plaques for the back of the amps with the power information, fuse etc.
I am also thinking of printing some copies of the schematics as stickers and putting them inside the side panels of the chassis for future reference.

Probably a few weeks work here before I get to button them back up for good looking all nice and purdy!
 
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After living with them for a couple of weeks I am very happy with their sound.
Well that's the main thing :) and these really do have the knack of sounding really good no matter what you throw at them. They are not one of those amps where only a very select few recordings actually sound good... everything sounds good and 'listenable' I find with these.