vari-mu - which one and diferences?

Hey all

having only used a handfull of vari-mu's, still new to the options here.
I would like to attempt something like this and the forum offers quite a few alternatives:
- Sta-level
- 175B
- Altec
- PRR's
- PM670
- Fairchild
- Federal AM 864

I was thinking I'd like to try a single channel one for a start. I see Majestic has gone for the sta.
What would you do? what are the differences? Facts? "sound descriptions"?

If there's already a thread discussing differences between these please let me know/delete this thread.

Thank you

Don´t forget the Federal AM 864!!!
Is is slow and needs some modification for faster speed.
I build this and the PM660...... Federal wins in all application reffered to sound/colour despite it is Mono and my PM is stereo.
Perfect for 50´s 60´s sound!
Have not tried any other Vari-mu.
Build was quite simple, but I had some "motorboating" issues with faster speeds. But solved with a hi-cut in Cv-amp.

greetings,
Stephan

DerEber said:

Don´t forget the Federal AM 864!!!
I build this and the PM660...... Federal wins in all application reffered to sound/colour despite it is Mono and my PM is stereo.
Perfect for 50´s 60´s sound!

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Hi Steph

Thanks for pitching in.
I'm curious to see more owners/designers/engineers pitching in in the subtle differences between the designs of the mentioned units

When do you say the federal is slow, did you modify it already? if so how much faster?

João

This is a vast ocean that can't be easily summed.  There are many many threads here about this, some dozens of pages long.  Get some popcorn and dig in. 

Hey João,
For my taste I got fast enoth to put it on the Mono Bus for the singer and his guitar. And to get the tomsfill full on in the Overhead Mic.
Never meassured the actual timings. Slow (stock) is great for bass, but too slow for anything else. (again, my taste!)
I needed to reduce the Caps going to the CV-amps and leaving permanent lowcut for the faster settings (Sidechain/not in the Audio path)
Maybe different layout will lead to different sollutions, as the "motorboating" can also be caused by Power supply design if I understud right. For me it is a great compressor. If you decide to build one. feel free to ask me for my mods. It´s not much anyway. some caps and resistors.
There is a thread here that covers all.

original prr is easy enough, you can modify to taste--simple circuit

I'm still very much happy with Analag and Silent Arts' Poorman.

Having had some more experience in the pondering and building, more so, I think this is a relatively easy one to succeed with.
Assuming a simpler rather than more complex build done with some patience and care.
Preferably with shielded subs for the Edcors. And using the Rondo transformer which is a quiet one, no doubt about it.

I'm using my ones more subtly these days, with less obtrusive time constants  (ie. slower) and less GR (ie. < 15db )

--

If I was to do another pcb unit I would try the prr-176 build.
Something intriguing about the vari-m GR stage with DOAs around it.

I also think that Drip stuff is a great path to success. Probably the least risk. Which is important considering the outlay involved.

---

But for the truly adventurous, it has to be a point-point, adapted for parts at hand. Much learning of the craft in that for sure.
Lots of dollars and time but eventually, the skills follow. Least that's what I hope!

You have to be a little crazy to go down that path - like chasing tail down a rabbit hole  8)

But for the truly adventurous, it has to be a point-point, adapted for parts at hand. Much learning of the craft in that for sure.

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You just made me go one step closer to start an adventure. At least, thinking about it. But maybe not MU, something simpler

I also think that Drip stuff is a great path to success. Probably the least risk. Which is important considering the outlay involved.

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Risk you mean of getting hurt/damaging stuff or getting something working well in the end?

Again, the drip stuff alone has much to choose from and all of it is a much to big investment if you haven't played with them:
Sta
Altec
Urei

Analag?

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For me, anything DRIP is unnecessarily too expensive.  The boards are overly gigantic, many of his circuits are not terribly hard to build point-to-point, eliminating the board cost.  It's great if you know little, need a lot of room to work (and have the case real estate!), and need a really good assembly guide.  If you can sort out lots of things for yourself, and want to make modifications, DRIP can be limiting. 

The number one thing to investigate when you say 'vari-mu' is what the intended purpose is.  Some are low ratio compressors with slow time constants, some are meant to be high ratio limiters.  Very different beasts using similar control methods. 

emrr said:

The number one thing to investigate when you say 'vari-mu' is what the intended purpose is.  Some are low ratio compressors with slow time constants, some are meant to be high ratio limiters.  Very different beasts using similar control methods.

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Hi Emrr

Thanks for putting it into words. I guess you did nicely sum it up for me there.
I have some of the schems for the above guys and trying to make sense of that. I guess splitting them in those categories would be a start.

for me, TBH, looking for the first option. I still have access to a bloody crippling fast vari-mu.

off topic q - I found this whilst reading up on the PRR vari mu (took me to a you tube link, which took me to this video) https://www.youtube.com/watch?v=yKLygXX4_ys which project is this?

Looks like Analags PM670 ;D

jplebre said:

Risk you mean of getting hurt/damaging stuff or getting something working well in the end?

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I mean risk of getting something working. You can hurt yourself or anything else equally easily with any high voltage work

As already mentioned, Drip stuff is expensive and large but very well documented and if you have the dollars, and rack space, but owing to the documentation and parts specifications, it's imho the least risk to a good result.

If you are in the pp game, then it's open slather - there's all the legendary builds out there and with quite a bit of info provided by this good community

I think a good classification is the fast and the not so fast then the less complex to the more complex.

The fast is the fairchild style, you could do the drip (EXEPENSIVE++)  or the Poorman 660/670 in the pcb world -this is a sensible compromise on the original in favour of cost, power and speed while still maintaining more of the orginal than any other I'm aware of.

You can also go for a pp version, either a full one, with tube psu or a 'cheat' pp hybrid, with pcb psu and the rest pp.
Simplfying further still and you get less brutal sidechain versions  ie. slower and cheaper eventually using discretes.

In the non fairchild style,  meaning those with post-GR makeup amp and being generally slower, there are the pcb hybrid types, like the prr-176 which is doa+ tube GR stage  or the full tube types.

For the full tube types, there are the interstage transformer types, like the RCA BA6A or the 175b for example or the ones without, like the 436 style or the Sta-Level style.

If you are going cheap, then you probably want to eliminate interstage transformers - which would mean 436 or sta-level.
436 is simplest I know of - a good one to pp for your first go.

They're all good!

A PM670 is hard to beat I think for value in the pcb world.
A PRR-176 probably gets you most of the way there in an even more compact pcb package.

A PP Altec 436 is a good starter. A Sta-level, even more interesting.
The PP 175b and BA6A is getting more elaborate again. But no less desirable.

Beyond that, its the domain of the experts! Obscure federal, rca and all the other old greats.

The toppermost of the poppermost of course is the fairchild style with brutish sidechain.
Either pp or pcb. Have to have at least one of them
Only the least sensible should apply!

Fair warning  - they are addictive. You think tube amps are but that pales in comparison to varimoo-itis

The original PRR vari-mu falls into the categories of "slow comp" and "extremely simple". It's excellent for experimenting and learning about so-called vari-mu -- if that's what you are after. Mine needed quite a bit of time and love though to finish it (I added tons of frills and thrills, including some pretty useless functions). Anyway:

- few components (all readily available)
- two mono channels but potentially stereo linkable
- B+ is "only" 100V (so comparatively "safe")
- very cheap to build (<USD150 incuding simple case)
- the sidechain is low-voltage solid-state, so it's very easy to mod
- ATT/REL only and a "THR" (but not really a threshold, more like for balancing differences in tube gain)
- it's a slow comp, so good for leveling (it adds maybe "glue" and "sheen" -- God knows how much I hate such words )
- p2p is really the way to go (I had boards for it, but almost all components are off-board by now!)
- it is very simple, so noise can be an issue and a real headache (grounding and PSU!)

Script said:

The original PRR vari-mu falls into the categories of "slow comp" and "extremely simple".

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did you stay with the TL072 (IIRC that's what it used?)?  Ever try a beefier amp (maybe a chipamp?)

very good information on these last few points! thank you guys!

So is it safe to assume that:

Brutal sidechain, faster, "fairchild" type:
- Fairchild (660/670)
- Poorman 660/670 (Silent arts/analag "cheap-to-build" fairchild style)

Post-GR make up, slower:
- RCA BA6A (transformer interstage)
- UREI 175b (transformer interstage)
- Altec 436/... (no transformer interstage, simpler)
- Sta-Level (no transformer interstage, more complex)
- PRR's (hybrid w/ solid state sidechain and more versatile - eg. easily modifiable)

alexc said:

Fair warning  - they are addictive. You think tube amps are but that pales in comparison to varimoo-itis

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not safe to assume any of that.  go find the manuals for all these devices, and read the ratio and timing specs. 

From quite a bit of usage of a PRR vari-mu variant I can tell it sounds least like the usual vari-mu "is it really on?" glue compression. It's a hard effect compressor and I don't think it's particularly slow either. It has a strange blocky attack reminiscent of guitar stomp boxes and a bit unique (and hard to use) as such.

Fairchild 670 sidechain is brutal on paper only. It's one of the most transparent compressors ever made besides the mastering units by Jonte Knif.

Poorman 670 is it's own sound much closer in performance to 175b than a Fairchild 670. The number is misleading.

For the very first vari-mu I would build an Altec 436 type point to point project due to the simplicity. It's also a great testbed for different types of time constant networks and represents a generic vari-mu type sound if there can be one.

Nice observation and "sound description" of the PRR sound. Yeah, technically it is fast:

"attack times from 100mSec to 1mSec..."
"release times 1Sec to 50mSec"

But in use, on fast ATT times it indeed really sounds rather like it's "choking" or something, rather like an "effect" (if overdone) and not so useful for program material maybe. So, I ended up hardly ever using fast ATT times on it. In fact, quite the opposite. And (I should have mentioned!) I attenuate the sidechain signal down (after A/R) for milder gain reduction. So for me it works better as a "slow" comp (in usability and sound).

Hm, coming to think of, I have had the PRR176 boards for ages now but haven't started building it yet. Probably because I expect it to be a bit similar...?

Script said:

Hm, coming to think of, I have had the PRR176 boards for ages now but haven't started building it yet. Probably because I expect it to be a bit similar...?

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Only I/O is different with all those options and PSU is knit-and-go. But the core vari-mu (we should really always say trans-conductance) stage and sidechain are exactly like the original board, besides the metering set up from a 175 that doesn't affect sound. Compression is exactly like the original.

And I agree, the compression sounds best with very mild settings. I haven't found a place where it wouldn't work, just that the usable control range is quite delicate.

Kingston said:

Fairchild 670 sidechain is brutal on paper only. It's one of the most transparent compressors ever made besides the mastering units by Jonte Knif.

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If we're talking about the real thing, I'd disagree.  Transparent is not a word I'd use to describe a 670 and it's definitely far from the most transparent.

I've yet to hear one of Jonte's units in person, hopefully soon.

Cheers,
Ruairi