Carnhill VTB9045 on Pm1000

[iampoor1]

Been doing some investigating on using Carnhill VTB9045's as PM1000 input transformers instead of The Ga80071.

My first question is simple....since the PM1k's need a center tap, I would use both coils in parallel for an input impedance of 300...correct? If they were wired in series, then there would be no center tap?

The original input transformer is a 300/2.7k with a load resistor of 4.7k. The Carnhill would be 300/4.8k. Would I raise R7 to 10k and then adjust from there? I know it sets the input impedance (and since its a higher value would raise noise).

I know with a 300ohm input impedance the front end attenuation will not need to be changed, however the rotary switch also adjusts the gain of the second two transistor amps. Would these need to be changed due to increased voltage gain of the Carnhill?

Your thoughts are much appreciated. Looking forward to gaining some real world insight on a topic that is fairly foreign to me!

I know Rodney at OWA does these mods, so its possible, just not sure what the best implementation would be.

Thanks

 

[CJ]

to create a CT you wire in series,

you have a PM1000 schematic?

 

[iampoor1]

to create a CT you wire in series,

you have a PM1000 schematic?

Good goly I need to go to bed earlier before writing these posts....  :-[

Yes, I have attached the input section. Whole file is too big to upload. If you would like the rest of it for your records Ill put it on dropbox tomorrow...

 

[CJ]

i believe you could ditch the CT and use 5.11 k resistors to pins 2 and 3 of the xlr,

this would allow you freedom on the pri wiring of the xfmr, parallel or series

stay up late enough and you can rewrite the laws of physics, been there done that, 

online humiliation is less painful than say, Bill Clinton and his girlfriend,

you know what i'm sayin?  ;D

 

[mjrippe]

Center tap is used to supply phantom power on the PM1000, so you'd need to change that if you eliminate the tap.

 

[iampoor1]

i believe you could ditch the CT and use 5.11 k resistors to pins 2 and 3 of the xlr,

this would allow you freedom on the pri wiring of the xfmr, parallel or series

stay up late enough and you can rewrite the laws of physics, been there done that, 

online humiliation is less painful than say, Bill Clinton and his girlfriend,

you know what i'm sayin?  ;D

Why 5.11k? Isnt 68k "standard"? Or is that only for differential inputs?

Right, so then I would have my 300ohm input impedance!
Now, as far as choosing R7, this is a "damping" resistor (that loads the secondary) and is usually set to twice the "average" secondary impedance correct? I know that Ill need to futz around with the value, but Im looking for other potential issues I might run into (and unfortunately, I dont have the "late night" excuse right now!)

hahaha! Tell me about it, every "good" idea I have seems to happen then.  ;D

Absolutly! Its the beauty of the internet! Plus I can tell all my friends that I got corrected by Ben Franklin. 

 

[iampoor1]

anyone?

 

[shabtek]

6.8K or close x2 >/=1% match

 

[iampoor1]

okay, I have gotten my brain wrapped around the phantom power.  8)

But what about the different impedence/ratios? Im assuming ill just need to try and see what happens? I am presuming the loading resistor will have to be changed,

 

[JW]

Bumping this from LONG go. I'm curious whether you can go the other way. Using the Yamaha GA80071 as a (1073ish) input? I think the lowest ratio you can wire it up for is 300:2.7K  so a bit more gain on the input. I have a bunch of these transformers and wonder what they'd be like as an input to the Neve circuit for a mic.

 

[capacitorless]

Necro-bump (since it's Halloween). Same here, wondering if there's any experience using a GA80071 in a 1073-ish circuit? I have one sitting here in a lonely PM2000 channel strip to play with. The op amps are present as well

 

[iampoor1]

Oh boy...thanks for the bump guys...great reminder of how much I've learned in the past 8 years.