How to Pass a little time ...cont

Looking at the first pic, I never would have guessed the chassis was cobbled together bits. Amazing what people with talent can do...

 

I think this is going to be the last solid state amplifier I build, I would have stopped with the one I use currently but I started collecting the parts for this amp prior to COVID and the pandemic put a damper on the development and availability of this amp which is what I had intended to build all along – my current amp, the balanced M2OPS was a very pleasant accident and I have been very happy with its performance. The amp in question is a derivative of the PASS XA25 – called the XA252SIT – it is the creation of the crazy ZenMod, naturally inspired by NPs design. It is a beefy 40W class A amp - full details available here ( link ) - I am building the SIT version with the industrial TH51s



I picked up the output devices a very long time ago – they are now virtually unobtanium , and have picked up parts as I saw them over the past few years. Dual 12A 24VAC transformers for a full dual mono implementation. Mosfet switched output protection for very quick shutdown in case of failure and to protect my speakers ( I used the same boards in my F4 - they are very well designed, not cheap but significantly cheaper than a new pair of speakers). A slow start and timed startup board to support both latching and momentary power switches. I picked up 8 beer can capacitors ( 50V 56000uF ) with the idea of building a CRC supply filter but they were massive and filled the entire chassis , so I switched gears to using a mosfet regulator for both positive and negative rails.

I had planned to build this one in a prefabbed chassis as a “keeper” amp , but my plans may be thwarted by the massive heat dissipation requirements of the output devices.


I picked up a non-working Sumo Andromeda II that was quickly filled by building the power supply





All rails settle to + and – 33VDC which is perfect for the circuit.


These are the heatsinks I had originally bought for the amp and have put them next to the original andromeda sinks for comparison.



And in relation to the chassis and power supply.




A long way to go , I thought I might have had more time over the holidays but family time was a priority and there are many a cold night ahead as we nestle down for winter in the midwest.

I have a few implementation decisions to make but this will evolve as I complete the amp itself and start to test just how hot these heatsinks get. This beast is going to weigh in close to 60lbs with these beefy heatsinks , so need to think about the practicalities too.

..dB

 

I'm still debating on building this amp, despite having no time to do so and already having built a wonderful SissySIT with the THF-51S (highly recommend this design from Zen Mod!)


I'm halfway through a Wolverine (ultra-low THD;Class A/B) build, which is essentially a wire with gain project; and, I really don't need more amps, but the XA252 SIT looks awesome.

Stoked to see your progress and read any impressions you may be willing to share.

 

I very nearly picked up the wolverine boards but i always gravitate back to Class A , hence the XA252 - I will compare to Sissy when done

 

Curious about your thoughts on the Wolverine when you're done. I have the boards and silicon but that's about it right now.

 

I am both easily distracted and what I would call an episodic diyr

I may have mentioned this in the past but I HATE chassis work - my intention for the XA252 was to pick up the sumo donor and be done but I clearly hadn't considered the heat dissipation and got beefier heatsinks - now why didn't i just buy a 5Ux500 chassis from the DIYstore and have a pretty ( albeit generic ) case .... well I may just enjoy torture.

some progress on the big XA252

more chassis wrangling - wasn't sure how much airflow i wanted at the front - leaning toward the narrower gap which will be filled in with perforated plate - this allows for creating a heatsink panel on the back to handle the mosfet based speaker protection circuit.






amplifier boards complete but will only mount them once I have a chassis that can be assembled



and while I was waiting for time and motivation to strike , I found myself reading about the Holy Grail - now who could resist the lure of the grail

apparently not me

psu:



initial test:



It powers up perfectly, settles to the recommended bias but I have yet to apply a signal to hear it play music.


more will surely happen in the next episode of frenetic activity ..dB

 

Quick update. I "cooked" the holy grail for a few 8 hour runs, all stable and offset in the mV with bias stable at 1.5A.

Music on the test rig sounds very promising. Next step is to install in my main rig and let it do what it was built to do.

Copy from diyA:

cranked through a few favourite test tracks and am suitably impressed - easily holds its own against my reference build - fully balanced 50W class A (M2OPS) which I have found does the best job of driving the maggies out of all the amps I have built thus far ( M2X, F4, SissySIT, Aleph 30, LuDef, XRKs 100W A/B, Baby Huey-PP EL84 )

l

It plays plenty loud but I keep things turned down typically for mid 60dB and spikes up to 80, really exceptional spatial separation, good heft to the bass, layering etc - all the things you look for in high end audio technicalities.

 

I know I posted my first attempt somewhere but I couldn't find it so here goes to keep it all in one place:

I stumbled across a YouTube video some time ago of a guy making a small planar speaker pod and he had posted all of his files for others to give it a go.

I ordered the flexible PCBs from PCBWay and printed up the driver frame along with some magnets from AliExpress.

Flexible PCB



3D Printed driver assembly



test fit



Assembling the Drivers









donor DT880








As I said - this is the second attempt, the first one worked but had some driver buzz , so I switched out the paper gasket for some 1mm rubber gaskets and drilled and screwed the halves together as the magnetic force pushed open the tape I had used to hold the first round.

The assembly into the Beyer donor frame was to allow me to see if there was any value tinkering anymore and to my surprise - it doesn't suck. Vocals have that natural planar timbre bass is surprisingly present for what essentially is a dipole being held in open air. It has an odd echoey vibe to it but that is likely because it is completely undamped. Both drivers measure 54ohm but they are not very sensitive - only tried with a dongle thus far and enough to get a sense that there is reasonable promise but not able to shine. I will secure the wires to the frame and not risk the solder pad being ripped off the membrane and do some more testing and try and see if I can dig out my old measurement rig.

Definitely holds promise and might even be a way to make a lot of drivers for a line array ( although GR planars are so cheap, probably not worth it )

..dB

 

Instead of the DT880 shell, try a Beyer Custom One Pro shell with the sliding bass vent for funsies. I did that years ago with a Fostex transplant but this looks more promising.

 

I merely grabbed one of the many carcasses in my "bin" and as luck would have it, the front baffle plate needed very little dremel work to accommodate the diy drivers.

.. dB

 

Just a brief update, made a balanced cable in case I needed to drive this with a speaker amp, no such need luckily. These little drivers put out an incredible amount of bass. I have damped them really heavily and they are still putting out more bass than I would typically want from a headphone. Beautiful thing is that the bass is crisp and responsive. I cranked them up (while sitting on the table) and they just get louder, no distortion that I can hear easily.

If I had bought these for a few hundred, I would be happy with them, as it is, just amazed at how such a thick / heavy driver can perform like this.

Still room for finessing the sound with additional tuning but this was all a concept exercise, so done for now..dB

 

Slow progress with the beast - after a brief challenge because I missed a small jumper to tie the gate of the SIT to the bias circuit. ( which resulted in all current possible being drawn by the SIT - 4A at 5V on variac



BUT it lives! running at 1.9A bias at 30V rails

..dB