PureAnalog M/SLevel Vari Mu Compressor (manual + schematic)

[z11111]

Looking for any info or anecdotes about PureAnalog or Charles McHan. Starting this thread to preserve any information and documents that might exist.

Have this on loan from a friend and probably going to purchase from him because I’m really liking the sound of it a lot. This is a cool vari mu comp based on a Sta Level with various options for dual mono, stereo, or mid side and was built by a company (person?) called PureAnalog who seemingly disappeared without a trace. This model is called the M/SLeVeL (???) but I found one mention of a PureAnalog “Variable Moo” model and this might be two of those linked together.

There is a Gearspace thread about PureAnalog offering a dual Sta Level in one box designed by a certain Charles McHan and while there is some information about him as a person and legacy as a radio tech, it is very difficult to find any information about the company PureAnalog or anyone else associated. Any mention of McHan discusses that he was a master of modifying Sta Level comps and as you can see, the design of this piece is pretty much an Sta Level with different tubes and some mods, such as replacing the 6AL5 with a pair of diodes among others.

Anyhow, this unit has a detailed instruction manual which confirmed a “lifetime warranty” but no contact information and unfortunately Charles McHan passed away last year. Luckily, there was a schematic included so I have attached that here along with the manual for archival purposes, seeing as so little information exists about this company and its products.

So how does it sound? Really damn good and has an interesting, useful feature set. Pretty clean but it’s doing the thing -- a studio I used to work out of had the Retro Instruments Sta Level and I used that piece extensively so I'm familiar with the Sta Level functionality. One channel is a little noisy when you have it running pretty hot and the VU meter on the bottom unit is a little finicky, so it needs a checkup for sure. I haven’t cracked this one open yet to see what’s going on inside because we already have it working on a mix

 

[wlinart]

Look interesting. Also something special is going on in the PSU; they used one of the primary windings to tap the higher voltage from.

 

[z11111]

Look interesting. Also something special is going on in the PSU; they used one of the primary windings to tap the higher voltage from.

Definitely wasn't sure what was going on there. Interested to take it apart and see what I can find. I assumed it was the HV secondary but drawn weird but we'll see.

 

[aurt]

Definitely wasn't sure what was going on there. Interested to take it apart and see what I can find. I assumed it was the HV secondary but drawn weird but we'll see.

Sorry to resurrect a dead thread, but I had one of these on the bench a few years back. Nice sounding piece. Wish I'd had the schematic for it then. The one I had may have been a prototype or one-off. Well put together, though. It was mono, iirc, and in a standard sort of hobby chassis. Big meter on the front/top that the owner wanted me to make a scale for.

That looks like a secondary drawn to save space. It does look like some sort of voltage doubler, though. Don't see that every day. And I admit to being confused as to why you'd hook up the AC from the heaters to the diode biasing.

 

[alphasnk]

Wouldn't it be the old school way to do it where they connect two transformers: like 2 12VAC transformers, where the second transformer is connected to the secondary of the first transformer via its 12VAC, so its primary becomes the high voltage secondary.. The schematic dates from 2010, it wasn't as easy as it is now to find transformers with multiple secondaries for tube stuff..

 

[z11111]

Wouldn't it be the old school way to do it where they connect two transformers: like 2 12VAC transformers, where the second transformer is connected to the secondary of the first transformer via its 12VAC, so its primary becomes the high voltage secondary.. The schematic dates from 2010, it wasn't as easy as it is now to find transformers with multiple secondaries for tube stuff..

Took mine apart last week and that’s exactly what’s going on in there.

Mine are very interesting where if I’m using them as a hardware insert in Pro Tools at 88.2khz or 96khz (or higher presumably), I get what looks like visible ticking / clock noise on the waveforms that isn’t audible. This doesn’t happen with any of my other gear, tube or not, and happens on any AC outlet / circuit, any line conditioner (I use Tripp Lite and Surge X units), regardless if my interface (RME UFX+) is clocked to itself or to my additional converters over MADI. Gotta figure out what’s going on there as it works fine and doesn’t exhibit this issue at 48khz or 44.1khz as a HW insert… have to try it out on our other converters as well at my studio.

Definitely some mad scientist point to point construction. Looks like he was using whatever CE Dist was probably selling at the time (their house brand can cap and such). Not very impressed with component choice considering this was a several thousand dollar piece at one point in time, but hey it definitely sounds great!

The mode switch and pots have been getting scratchier. Turns out the pots are these pathetic little dual 16mm things, so I blasted them with F5 and they’re better now but I’m gonna replace them with stepped switches once I get the proper resistor values to make dual 1Kohm. (Ian Thompson Bell has a great pdf with the values somewhere on the forum; thanks Ian)

I know those CE cans don’t always hold up great. Will probably do some work at some point to beef up the PSU.

 

[flyingeyepro]

Looks very well made......

 

[gyraf]

Attention: if I understand the pictures correctly, this is outright dangerous.

It looks like we have here a 115+115v:6+6V transformer, where one 115V primary is mains input and the other is used for HT.

While this "will work", it completely ignores the need for safety insulation between primaries and secondaries in a mains transformer

Yes, it will work for supplying your power, but you have a significant exposure to mains-to-chassis disasters waiting to happen. I wouldn't even consider this for a quickfix - even in a region with mandatory GFI relays everywhere..

That said, I do hope I'm reading those pictures wrong, and that there's a HT isolation transformer hiding in there somewhere

/Jakob E.

edit: If my analysis is right, you need to have another power transformer made. Something like 115V : 6V/12VA + 6V/12VA + 115V/12VA

A manufacturer like Toroidy.pl can do this well and not at all expensive at one-offs - get their "audio" type, and you'll have even less trouble with stray hum fields..

 

[z11111]

Attention: if I understand the pictures correctly, this is outright dangerous.

It looks like we have here a 115+115v:6+6V transformer, where one 115V primary is mains input and the other is used for HT.

While this "will work", it completely ignores the need for safety insulation between primaries and secondaries in a mains transformer

Yes, it will work for supplying your power, but you have a significant exposure to mains-to-chassis disasters waiting to happen. I wouldn't even consider this for a quickfix - even in a region with mandatory GFI relays everywhere..

That said, I do hope I'm reading those pictures wrong, and that there's a HT isolation transformer hiding in there somewhere

/Jakob E.

edit: If my analysis is right, you need to have another power transformer made. Something like 115V : 6V/12VA + 6V/12VA + 115V/12VA

A manufacturer like Toroidy.pl can do this well and not at all expensive at one-offs - get their "audio" type, and you'll have even less trouble with stray hum fields..

Yes, pretty sketchy right? Since my last posting I have been looking into options to rebuild the PSU! Thanks for the tip on Toroidy. Been a minute since I looked but Antek might also make something that could work.

 

[z11111]

Alright, after about half of a year in service as-is with a tube swap at one point because some of the stock 6SN7s got noisy / microphonic (they were relabeled 6H8C from Russia), these units have stopped behaving as they should. The right channel has developed a strange oscillation around 5 kHz and has intermittent crackle even after swapping tubes (again). Both units are pretty damn noisy as well and seem to behave a lot different depending on what power outlets I connect them to. Maybe the most sensitive pieces of gear I've ever owned.

Time for some surgery / component improvements / probably entirely new PSU... have my eyes on ideas from Ian's Tube Lunchbox PSU.

On a quick glance, the Antek AS-2T200 looks like it'll provide what I need to power both units (4x 6SS7s and 4x 6SN7s between both, around 90mA plate current total and 3A heater draw according to datasheets). Rectified voltage should be around 280vdc unloaded? Just doing some quick guesstimates before actually diving in. Antek AS-1T200 might be fine too? (100VA vs 200VA, price difference and size is negligible if I'm putting it in an external floor box)

All of the above being said, these do sound really, really great when they're working right!!!