The No.1 Website for Pro Audio
Familiar Pre-amp?
Old 20th November 2009
  #1
Here for the gear
 

Familiar Pre-amp?

There's a mammoth Rauland-Borg mixing console I got from maybe early 60's...maybe 50's...I'm not sure. This is a mic pre module from it.

Does the circuit look familiar at all? It runs complete with AB Type J 5k pots and a few VU meters. Pretty cool looking.

Can anyone fill me in on history of this thing? (or if the circuit compares to anything in the same decade...) The company now only seems to do education and health industry stuff.
Attached Thumbnails
Familiar Pre-amp?-mvc-003f.jpg  
Old 20th November 2009
  #2
Lives for gear
 

looks like one half of a ba183 module, but most likely a knockoff of a v474A

this company copied alot of the german and bogden stuff (or made circuits simular to.)

the company got assemuatled....they are called Biamp now.
Old 20th November 2009
  #3
Lives for gear
 
ulysses's Avatar
There's no way that board was made in the early 1960s. Early-mid 1970s or later.
Old 20th November 2009
  #4
Here for the gear
 

You're probably right. I had no idea what decade this thing was connected to.

I kept thinking...Ok...there are transistors, so it must be after 62...but yea.

Was teh v474a a common pre-amp of the day? Was Rauland Borg (or Bodgen for that matter) known as quality gear? My history is obviously not great with any of this. I'm trying to learn as much as I can...

Thanks so much!

RaH
Old 20th November 2009
  #5
Here for the gear
 

Also, what is it that makes you think mid 70's or later?

Thanks

RaH
Old 20th November 2009
  #6
I see an IL cap, some axial tantalums, some to-92 transistors and a Riechenbock or Jensen looking mic transformer.

Some nice Toshiba discretes would open it up and lower noise. Replacing the carbon comp resistors with Dale CMF55's would also help the noise floor.

Jim Williams
Audio Upgrades
Old 16th December 2009
  #7
Here for the gear
 

Hey Jim, do you think it's worthwhile to upgrade this mic pre? It seems to be foolishness if this is just a cheap 1970's version of a behringer mic pre.

What would it cost for parts? Your labor?

Thanks
Old 22nd December 2009
  #8
Lives for gear
 
ulysses's Avatar
I think it would be a great piece to tinker with. It's a simple discrete circuit, and probably not worth much, so it's a great educational opportunity. The transformer and the low parts count in the amplifier will probably mean some nice coloration that you don't want to mess with too much, but Jim's right that you could update some components to lower the noise floor and perhaps expand the frequency range a bit.

The first thing you should do is draw out the schematic by hand. You'll learn a lot just doing that! Look up the transistors to find out their pin-outs (you might find them at digikey.com or mouser.com if they're not discontinued, and they'll have data sheets there). Post your schematic here when you're done, and we'll see if it looks right and make some more observations then.
Old 22nd December 2009
  #9
To do it up right, I would first characterize it with some tests on the Audio Precision.

I would measure amplifier noise, gain, bandwidth, THD, IMD without the transformer. Then I would replace the transistors with some .5 nv Toshiba bipolars I use here. The resistors would be swapped to Dale CMF60's.

Then I would measure the noise floor again. If it's much lower, a lower ratio input transformer migh be an improvement. I suspect a 10/1 ratio is being used. A 5/1 Jensen JT-13KC might be in order. The extra 6 db of gain loss could be made up with a bit more amplifier gain. That might be possible as the bipolars have large hfe's and very low noise and THD.

Once those tradoffs are optimized, great coupling caps would finish it off. It could be great, it could suck testicles. The only way to find out is to try.

Jim Williams
Audio Upgrades
Old 22nd December 2009
  #10
Gear Addict
 
Minion's Avatar
 

Those look a lot like some Bogen pre amp modules I got off e-bay a few years ago , They have the same Input transformer type and a very simular looking card , The ones I have had screw terminals for the Mics which I switched over to XLR , Mine also didn"t use Transistors but used Signet NE5523"s and Ran off of 24v DC with 24v Phantom power ... I racked up 4 of my cards in a Box with a 30v PSU and they actually sound really good , No hum or hiss and 54db of gain with Hi/Low cut filters , sond really good on drums ....


Cheers
📝 Reply
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
🖨️ Show Printable Version
✉️ Email this Page
🔍 Search thread
♾️ Similar Threads
🎙️ View mentioned gear
Forum Jump
Forum Jump