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Stasis DIY

chiily

chiily

PFM Special Builder
Over on DIYAudio Nelson Pass fixed a member's Threshold Stasis amp with a new Front New PCB, re-worked with a few changes. The thread is here https://www.diyaudio.com/forums/pass-labs/363701-stasis-front.html

Nelson then released the PCB as Gerber files for anyone else to build and fix their Stasis amp. As as happens on DIYAudio Zen Mod took the cct, re-laid it out with a couple more tweaks and added Gerbers for a 6 way and 8 way Output stage. The scene was set for a build...

I ordered the PCBs for the original cct, ZM's update and both the 6 and 8 way output stages; 10 of each so feel free to contact me if you fancy a set.

Progress so far...

I've build the FE boards

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And the output stages. I went for 6 way in the end. The amp will be driving a pair of ESL57s and I don't need oodles of watts. With that I'm running the boards on about +/-32VDC to keep everything in line with the 57s.

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I chose a Modushop 4U, 400mm, Dissipante case with a little milling to the front panel

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I drilled and tapped the heatsinks and mounted both output stages and the front ends.

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The soft power on module I've chosen has a standby and power led output, so the three holes will be standby LED, toggle switch and power on LED. In fact the soft power on module is another DIYAudio creation. Thread here https://www.diyaudio.com/forums/pow...c-mains-relay-includes-soft-start-h9kpxg.html

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On the PSU side I'm trying out a pair of high power Cap-MX boards, again for DIYAudio. This long thread here https://www.diyaudio.com/forums/power-supplies/297921-jumas-easy-peasy-capacitance-multiplier.html

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The drivers will be bolted to the bottom of the amp's case, the heatsinks are there as I'm not 100% sure what the amp's bias is going to be yet - Nelson's line is, "set the bias until the OS heatsink's sit at 50degC after an hour with the lid on."

Again, I have some spare PCBs for both the soft power on and the Cap-MX.

I also decided to use two pairs of active rectifiers because I had the boards and bits :)

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There's another pair lurking in the build box :)

I'm stuck there atm, waiting for the transformer, 2x25 @ 750VAC to arrive from Airlink; ordered early April and nothing as yet. I've tested the Cap-MX boards and the rectifiers, set the bias pots to maximum...

Exciting to hear this amp when built.

Gosh, the pictures have come out big...oops...
 
I recall Stasis sounding very nice! Best I heard was with huge Infinity speakers. I have a Threshold brochure somewhere which I think was intended for dealers etc and is the glossiest brochure I've ever seen!
 
I've got four more amps in the pipe, and was actually worrying what I would do after that. This might be an interesting one to build.

Of course I need to finish the PFM Special as well, and I always appreciate the reminder in your profile thumbnail. Unfortunately, I have to wait until the local woodworking club's shop opens back up. (We're still in lockdown here.) I'm going to use the passive crossover first, but I'll soon receive the MiniDSP 4x10 so that I can eventually try going active. (I'm hoping to place them in the corners of my office, and I know the Special's passive crossover was designed for free space use.) In the mean time, the MiniDSP will act as a DAC+Pre between my computer and NCC200-upgraded NAP140.

So much fun!
 
Thank you all. I find building amps very therapeutic and my need for therapy is most acute atm.

I'm toying with the idea of mocking up a copy of the Threshold Stasis label, 2mm brushed aluminium. Would that be heresy?

Also, the FE boards are crying out for an independent, regulated PSU. Just got to make sure that the FE rail voltages are higher than the OS ones.
 
Arkless Electronics said:
I recall Stasis sounding very nice! Best I heard was with huge Infinity speakers. I have a Threshold brochure somewhere which I think was intended for dealers etc and is the glossiest brochure I've ever seen!
Click to expand...

Well if it can beat the KSA50 I will be very impressed. I still need to find the right value resistor in series with the fan to quieten is down a little.

The Stasis cct is suppose to have a unique sound, from the reviews I've read.
 
chiily said:
Well if it can beat the KSA50 I will be very impressed. I still need to find the right value resistor in series with the fan to quieten is down a little.

The Stasis cct is suppose to have a unique sound, from the reviews I've read.
Click to expand...

I doubt it will beat the Krell.
 
chiily said:
I still need to find the right value resistor in series with the fan to quieten is down a little.
Click to expand...

It will almost certainly use a shaded-pole motor - look for another way to achive your aim. Life isn't that simple.

Besides - the VAST majority of noise from a fan comes from the blades - some designs are far worse than others. If the blade assembly is plastic you may be able to trim it to achieve less noise. A die-cast assembly might also work better.

Fans almost invariably quote a noise figure, how bad is that for your current unit?

Maybe more than one fan rather than just one?
 
Yup, the world of cooling fans is expansive. You might want to look to the PC world, as there are many quiet options available there.
 
The KSA50 has main AC fan, a Papst model bolted to the bottom of the heatsink tower. I did try a 12v PWM fan for a while but is was quite loud, running hard to keep on top of the burning output stage :) I found that it didn't have enough grunt to push air through the heatsink fins leading to lots of spill over.
 
chiily said:
Well if it can beat the KSA50 I will be very impressed. I still need to find the right value resistor in series with the fan to quieten is down a little.

The Stasis cct is suppose to have a unique sound, from the reviews I've read.
Click to expand...


Looks like it doesn’t use global feedback, so distortion will be higher than normal and damping factor a bit lower. I have a pass SONY SIT amplifier and was wondering why it sounded a bit different to my class A amplifier looking at the output impedance the pass had a lower damping factor, adding a resistor to equal them up and I cannot say which is which.
 
I final finished this Stasis amp; I should say nearly finished, as the earthing needs reworking. Anyway a few pictures.

I did end up ditching the CapMx boards as the amp didn't sound quite right with them. A more traditional PSU, CRC, 80kuF pair channel does sound better to my ears.

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Both channels bias to about 210mA per transistor, six transistor pairs pair side gives about 2.5A per channel. The 4U heatsinks lift themselves to about 50degC :) Rail voltage is +/-35VDC.

Not had too long in front of it atm. It doesn't have the "grunt" of the KSA50, but it has much more finesse. Timing between the two is about on par - plenty of foot tapping.
 
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Suffered a small set back when the resistors, 0.5R, in the PSU's CRC network of one channel suffered a bit of a melt down and went pop. There must have been some level of in rush as they were only dropping 0.7V at a current of 1.4A, much less then their 3W rating...though I had noticed that they were getting very hot in normal operation.

Anyhoo I replaced them with three parallelised 0.47R, 3W resistors on each side. I changed them on both PSU boards to be safe. Rebiased the amp to 50degC in the top of the heatsink and all is good.

This is certainly a very impressive amp. Different character to the Krell. The Stasis is very neutral, but doesn't trade this against pace, or enjoyment. It reproduces sounds as they should be, in the right place at the right time, but again doesn't trade this against playing music.

You know sometimes an amp produces so much detail but doesn't play music? Well this amp plays cohesive detail wrapped in musical enjoyment. I'm really surprised more people haven't tried the commercial variant.

Edit: beguiling is the right word for this amp.
 
Mike Hanson said:
Yup, the world of cooling fans is expansive. You might want to look to the PC world, as there are many quiet options available there.
Click to expand...
I use several 12 volt computer fans, run at around 5 volts* to cool my valve amps. Almost silent, but I had to try a few to find the best ones. I managed to get a bunch from the tip, before they stopped people picking them out.
* I experimented to find the lowest voltage at which they would actually start. They would run even lower, but I wasn't prepared to try to make a PSU with a higher initial voltage.
 
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