How the sweet mother do I build functional Mic Pre's from these cards?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

vineyardgray

Well-known member
Joined
Jun 12, 2008
Messages
123
So on a whim, I bought what appears to be a 40 channel line console.   No power supply came with.   If one were inclined, there is space to fill the entire top "panel" with custom pres.  Unfortunately, I can't get this beast down the stairs into my studio, so now the decision about what to do with it begins:

Helm_Console.JPG


It looks like the summing amps and output drivers were op-amp style cards:

NonXFRM990.JPG

NonXFRMB.JPG


And a couple with transformers:

XFRM990A.JPG



So, I'd like to make some pre's from these amps.  The board the op-amps are mounted on says 990, but they don't appear to have inductors like the 990's I've seen:

990close.JPG


990etch.JPG


But they do appear to be "standard" pin-out op-amps, so I'm guessing the easiest way for me to operate them would be to remove them from the cards they are in and pop them into another mic-pre project.  

Opinions?

MR
 
Maybe an earlier version 990, or a DIY, the design is public domain, after all. Feed them +/-15VDC, if you want to go higher you should change R10/R11 according to the pdf on John HArdy's site. FWIW, a JH990 should have one more transistor somewhere.  Maybe compare this to the schematic on JH's PDF. I think many of the components would be the same. What is strange is that they one on the card seems much better built than the one in the huge pic in the dropbox folder. 

Who is/was Helm Audio? 

Those cards look like generic amps, meant to be used for everything, from mic pres to line amps, to summing, etc. The one with an output trafo, all you need is a 1:2 input trafo and minimal rewiring to get it working as a mic amp. Footprint looks like the little round Cinemag, Jensen, altran, etc. We did a group buy on some Altrans last year, they work very nicely with 990s.  Measure the pin distances and compare to the Cinemag 75101APC and any Jensens that share the footprint, to make sure, but I bet they fit. Just make sure you get a 1:2 or 1:4 or something like that, not higher.

Not sure if it's even possible to get this thing back together, but if all you want are micpres, it might be more practical to take the opamp and trafos off and stick them on something that will more easily give you other necessary features (phantom, pad, polarity invert, etc).  OR you could use these cards with a JLM go btween or whatever it's called. I mean you could do everything with this card, but it would be much cleaner to use either another preamp board or the JLM module.
 
Yeah, since these babies are already built, and I have the edge connectors, I think a JLM Go-between might just be the ticket.   I'm guessing I could use the JLM attenuator too and that would solve how to implement that.   All that might be left then would be to tie in an input transformer as you suggest and then go.

I have no idea who Helm Audio are, but judging from the inconsistent components, the silk screened panels and the jumpered to bejesus PCB's, my money is on DIY.   I mentioned Helm Audio mostly as a half-baked attempt to see if perhaps the builder was lurking around here.  It is quite an accomplishment though, with all the EQ's in the board looks really nice.   It is kind of sad to tear it down before even hearing it.

Speaking of the EQ's (I'll post more on them later) they appear to be a State Variable design (a la Steve Dove).  I have to rack a pair up yet, but I can't wait to hear them.  

IMG_0103.resized.JPG
 

;D

 

Latest posts

Back
Top