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 (mirror link). This model is called the M/SLeVeL (???) but I found one mention of a PureAnalog “Variable Moo” model (mirror link) and this might be two of those linked together.

There is a Gearspace thread about PureAnalog offering a dual Sta Level in one box (mirror link) 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.

some other archived links regarding PureAnalog that I saved with Wayback Machine because there is just so little info:

• a sale listing of a dual vari mu by PureAnalog with good info in the listing description -- this one had Hammond transformers and appears to be closer to a true Sta Level
• link from dealer Vintageloop with some info on PureAnalog
• Gearspace thread about a mid/side box PureAnalog offered around 2009, probably a similar matrix as what is in my "M/SLevel" unit? (mirror link)
• unrelated testimonial from McHan about radio

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!!!

 

[z11111]

Took apart again today as I make moves to re-do the PSU as an external box with Ian's PSU PCB. Wanted to take high voltage measurements to see how they compared to the schematic included.

Mostly consistent, aside from the labeled 150vdc to the V1 and V2 plate resistors. That is only 89.5vdc (on both units).

Output transformers appear to be Edcor XSM series but label is removed. Input transformers are OEP A262 series (covered in A262CAN mu-metal shield) so unsure of part. It seems the builder may or may not have intentionally removed labels from these transformers to hide their value...

As mentioned, units sound great, even if the transformers aren't necessarily anything special. Used them on a mix this past week and they behaved just fine. Kinda crazy that this might have been sold for around $4000 USD new, but only used Edcor and OEP xformers... lol... Might swap the input xformers someday, but other things are a priority. I should probably trace the mid-side matrix switching at some point.

 

[rock soderstrom]

Thank you very much for the comprehensive information! Very interesting for someone who is currently working with vari mu compressors.

I think the build quality of the devices is dubious, and not just because they cost a lot of money.

For instance, fastening high voltage capacitors with plastic cable ties and what I consider to be inadequate galvanic isolation of the power transformer would make me lose sleep. Thank goodness this transformer "trick" doesn't work in Europe.

Did the builder really solder the input transformer to a potentiometer?

 

[gyraf]

To figure out OEP transformer type, simply measure DC resistances and compare to data sheets..

/Jakob E.

 

[z11111]

Did the builder really solder the input transformer to a potentiometer?
View attachment 128114

yes, big yikes haha…

As an addendum: I paid an extremely fair price for this piece, not “sight unseen”, assuming it would need some heavy rehab down the road.

 

[rock soderstrom]

In essence, this is an interesting circuit!

What I like most is the use of the 6SK7 as a control tube. Apparently you can use it as a kind of replacement for the 6386. Seems to work well and I really like US steel tubes in general, you get great tubes for little money because the hifi guys don't like them that much.

In addition, everything I've built with 6SN7 so far has always sounded good. I have enough of both tube types at home, and the transformers are also affordable. As I said, an interesting circuit.

z11111, what do you prefer to use these compressors for?

for reference the original Sta Level schematic.

 

[z11111]

Y

In essence, this is an interesting circuit!

What I like most is the use of the 6SK7 as a control tube. Apparently you can use it as a kind of replacement for the 6386. Seems to work well and I really like US steel tubes in general, you get great tubes for little money because the hifi guys don't like them that much.

In addition, everything I've built with 6SN7 so far has always sounded good. I have enough of both tube types at home, and the transformers are also affordable. As I said, an interesting circuit.

z11111, what do you prefer to use these compressors for?

for reference the original Sta Level schematic.
View attachment 128146

Yup, 6SS7 (or 6SK7, more current though) remote cut off pentode, can be bought new in box for extremely cheap and in my experience sound good. Smart idea. Like most metal can tubes, they can be very microphonic — the little dampener rings work well though.

I’ve used this as my main mix bus compressor on several records (here’s one) in the past year always in mid-side mode — not pushing it super hard or anything, it just does something special. The pots are awful, but it’s sort of “set and forget” once you dial it in the first time (good info in the attached manual, actually). I am building stepped switches to replace them.

Have used one in mono during vocal tracking a few times as well and it works like how I’d want a Sta Level to work.

Despite the shoddy build, I do still think this adaptation of the Sta Level / vari mu comp scheme has some cool implementations. Hence why I’m going to all of this trouble to rehab them!

 

[rock soderstrom]

Yup, 6SS7 (or 6SK7, more current though) remote cut off pentode, can be bought new in box for extremely cheap and in my experience sound good. Smart idea. Like most metal can tubes, they can be very microphonic — the little dampener rings work well though.

The double heater current for the 6SK7 is of course a good argument in favor of the 6SS7, no doubt about it. However, the 6SK7 is much easier to get on my side of the pond and is also cheaper. You can actually get NOS NIB quality for extremely little money, even from the big tube dealers.

I’ve used this as my main mix bus compressor on several records (here’s one) in the past year always in mid-side mode — not pushing it super hard or anything, it just does something special.

Sounds good!

A stereo application makes sense with this tube setup, as it is still very affordable compared to the 6386.

When I'm finished with the RS124 style compressor, I'll set up a test channel. Let's see how it fits into the vocal chain and harmonizes with the bass.

Have used one in mono during vocal tracking a few times as well and it works like how I’d want a Sta Level to work.

Despite the shoddy build, I do still think this adaptation of the Sta Level / vari mu comp scheme has some cool implementations. Hence why I’m going to all of this trouble to rehab them!

Makes sense to me. The pots are quickly replaced. I would actually rework the PSU a bit, partly for safety reasons as I said, but also to reduce the background noise and sensitivity to the surroundings. I think that will be worthwhile.

 

[rock soderstrom]

I’ve used this as my main mix bus compressor on several records (here’s one) in the past year always in mid-side mode

Almost forgot, it would be great if you would sketch out the MS encoder part as well. Of course only if you have some time to spare and feel like it.

Is there any reason not to do it in the box? What are the advantages of a hardware solution?

 

[gyraf]

As you have transformer in and out of this circuit anyway, the simplest would be to wire the existing ones as matrix/dematrix

(or, like in my commercial G22, use five relays to switch between LR and MS modes)

/Jakob E.

 

[z11111]

Almost forgot, it would be great if you would sketch out the MS encoder part as well. Of course only if you have some time to spare and feel like it.

Is there any reason not to do it in the box? What are the advantages of a hardware solution?

In the box is fine, and we have a mid side matrix box in our racks as well, I just think this is a cool feature to have it built in to the hardware

And yes, full PSU rebuild is in order and the main goal. I am going to build an external box using one of Ian’s “Lunchbox” tube PSU PCBs as a starting point for the filtered B+ and to rectify heaters, then going to regulate heaters locally using a Microchip LDO, maybe.

Still some R&D required for that adaptation, such as the original designer elevating the heaters (I think) using the same +24vdc for the control voltage?