Chameleon 7602 Input Transformer Mod

kraines · Post by **kraines** » Mon Jan 05, 2009 12:28 pm

Howdy Y'all,

Here's my first post, please be gentle.

I'd like to put a different input tranny on my 7602. I'm looking at Lundahl--LL1538XL, LL1550, or LL1576--because they are in the price range I'm shooting for.

Anybody done a mod of this type and have some suggestions?

Thanks!

idealfreedistribution · Post by **idealfreedistribution** » Mon Jan 05, 2009 7:14 pm

Don't have the Chameleon unit, but I'd use a Carnhill vtb9045, which is what is used in Neve clones (and the new Neves I believe) today. You can get them at Audio Maintenence Limited (you can also get an output transformer, and inductors for the EQ).

The Lundahls are nice but I think would give a cleaner sound, which may be what you want.

Craig

druedger · Post by **druedger** » Wed Jan 07, 2009 10:22 am

I have to second the Carnhill recommendation. The Lundahls will impart a different sound. Plus, the Carhill will be the same pinout format, which means you should be able to just drop it in place where the existing one is. The Lundahls would require some kind of daughtercard and hand wiring to connect it to the Chameleon board since the transformer pinout is different from the Lundahl format, I believe.

kraines · Post by **kraines** » Wed Jan 14, 2009 11:27 am

Thanks guys. I think I'll take your rec and go with the carnhill. I was hoping to have a US seller. Oh well.

I'll post my results when I get it going.

kraines · Post by **kraines** » Tue Jan 20, 2009 4:31 am

I've ordered the tranny and it is in transit right now. Ordering from the UK wasn't as much a pain as I thought it would be even though the website's checkout failed multiple times so I had to be really persistant.

I may take notes and photos and post my experiences with this mod.

Cheers

b3groover · Post by **b3groover** » Tue Jan 20, 2009 9:23 am

Please do!

kraines · Post by **kraines** » Tue Jan 27, 2009 4:57 am

I've got the tranny and am starting the mod. I'm not exactly a genius with regard to electronics so I'm trying to figure out the odd little circuit board on the top of the Chameleon's tranny.

It looks like there's a capacitor and a resistor between pins 9 and 6 with pins 7 and 8 and 2 and 3 bridged.

Does that look correct to anybody else?

Also, does anybody have a suggestion for the kind of cap I shout use on the Carnhill or if I need to at all?

rodabod · Post by **rodabod** » Wed Jan 28, 2009 6:53 am

That's a Zoebel (Zobel) network added to reduce transformer resonance at the top end of the audio band.

Check the values used in the Neve 1073 circuit - they'll possibly be the same. If so, keep the original cap if you want - it looks to be a decent film type.

kraines · Post by **kraines** » Wed Jan 28, 2009 7:46 pm

I installed the tranny without the cap and it sounds pretty good. It sounds great actually.

I fried the cap trying to get it off the circuit board and don't have a local source but I have a variety of caps ordered.

What should I expect to hear with the addition of the cap?

digitaldrummer · Post by **digitaldrummer** » Thu Jan 29, 2009 2:38 pm

looks like a 1000pF polystyrene to me. you can get them at Mouser.com.

nutrapuppy · Post by **nutrapuppy** » Fri Apr 10, 2009 9:56 pm

Hrmmm. the actual pin numbers do not match up on my Carnhill. I had to find which pin corresponds to what part of the transformer on the data sheet online, and trace it down from there.

Also, how are you mounting your Carnhill? The screw holes do not match up.

Word,
Nutrapuppy

nutrapuppy · Post by **nutrapuppy** » Sat Apr 11, 2009 6:29 pm

Secular_Tool wrote:I installed the tranny without the cap and it sounds pretty good. It sounds great actually.

I fried the cap trying to get it off the circuit board and don't have a local source but I have a variety of caps ordered.

What should I expect to hear with the addition of the cap?

Yeah, what should he expect to hear? Or not hear?

I am guessing it cleans up the tonality in the upper range, but what upper range? 10k and up?

rodabod · Post by **rodabod** » Mon Apr 13, 2009 9:09 am

For the Carnhill it'll be 180pF and no resistor across the secondary.

It reduces ringing at high frequencies, so possibly expect smoother sound and less phase issues.

nutrapuppy · Post by **nutrapuppy** » Tue Jun 23, 2009 12:24 pm

http://www.audiomaintenance.com/downloa ... _guide.pdf

scroll down. Just helping out since i had trouble finding the details.

Also my engineer drew this for me:

http://mindfart.com/pic/?carnhill

Kevin S · Post by **Kevin S** » Wed Jun 24, 2009 9:12 pm

The zobel is going to stop the rigging, that may be inaudible to your ears. rumor has it if it's left off it allows for some extra high end, but it's pretty much standard to have it. I would keep the 180pf (Silver Mica, Film) in the circuit

http://www.ams-neve.com/Sites/8/Files/D ... Issue5.pdf

Page 22 of 46 shows the schematic, locate the Mic input transformer (upper left) and the pin out's (which should be the same on the Carnhill) Depending on if any impedance switching happens for the High and low Z function you would set you input ratio by soldering jumpers to make that connection or a switch. I believe the secondaries are depicted in series as it's shown as an single coil where the primary is shown as two that are in parallel (low Z) or in series (High Z).

Primary imp in low = 300 ohm Z
Primary Imp in High = 1.2K ohm Z
Series Secondary = 4.8K Ohm Z

from this chart http://www.technicalaudio.com/reading/n ... o7113.html

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