calrec talkback info

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zgoheen

Member
Joined
Mar 21, 2006
Messages
12
Location
chicago
i picked up a pair of calrec talkback modules (tb1365) and was thinking about racking them up. i have the schematic but i don't know enough about circuit design to make a guess as to how they might sound. they seem to be based around a pair of fairchild 5532N opamps and have lundahl in/out transformers. are these going to sound any good or should i just part out the lundahls and make a DI box?

heres the schemo:
http://i66.photobucket.com/albums/h261/zgoheen/calrectb1365schem.gif

thanks,
:zach
http://www.mywerethey.com/zach
 
It will definitely sound good, but have a limited gain adjustment range. Use the gain control circuit from the Focusrite ISA110 around the first opamp stage, and you should be good to go. You probably won't need the attenuation part before the opamp, just the gain resistors in the feedback path.

http://img.photobucket.com/albums/v172/gyraf/ISA110_mic_input.gif

Jakob E.
 
alright. unfortunately my knowledge here is limited so how would i specifically go about implementing the ISA110 gain control to this circuit? would i replace the entire feedback path on the calrec with that of the 110? also, i've seen people here mention the input section of the 110 before as being something to use on other op amp designs. whats the significance here?
 
You need to make R7 switchable. You may search/google for "noninverting opamp" or so, that'll give you all the math and perhaps a handy online calculator.

I'd replace all 22 uF caps with higher values, say 220 uF--otherwise both bandwith and low-frequency distortion will be mediocre for audio. The 10 nF needs an upgrade to 100 nF as well. And while you're at it, replace the 2200 pF with a 220 pF and the 1 nF with a 100 pF.

Funny that they use coupling caps at the input despite the transformer--any reason for this?

Samuel
 
[quote author="Samuel Groner"]
Funny that they use coupling caps at the input despite the transformer--any reason for this?
[/quote]

Before looking at the schematic, I guessed there might have been a high-pass filter, but after looking, I can't see why either.

I think I'd maybe either try it without them, or swap them for non-polars or bypassed normal electos.

I've seen these talkback modules before on Ebay (and at work too). I think some of them have a nice Calrec condenser attached, but it has a permanent bass roll-off which you'd want to remove if possible.
 
[quote author="gyraf"]the caps are there to compensate for opamp bias current generated offset voltage - which gets annoying at high gains..[/quote]

Which opamp would this offset voltage be coming from? The caps are wired to the primary, so I can't see where this voltage could come from?...
 
aah.. I was thinking of the caps around the opamp in the isa circuit.. but yes, the caps at the calrec schematic transformer primary serves no really necessary purpose.. I'd just short them out.

Jakob E.
 

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