GAP 73 with Jensen mod

[WesAudio]

Great job Jim! I'm wondering about the input transformer too. Did you upgrade any other components also or just output transformer ?


Radek

[jbates]

I previously did the output transformer upgrade in mine, yet I used the Carnhill transformer. Would it be worth it to swap it again and go with the Jensen?

[Lights]

I assume this is a GAP73 mk1? Was any improvement made in the mk2 output transformer or is is still lacking the depth you mentioned?

[Dan Kennedy]

Any idea of the long term effects of the 75ma dc in the transformer?

I don't recall nickel cores being particularly happy getting magnetized.

[Schaap]

Originally Posted by Jim Williams

Best part is that lack of low end depth I've complained about in the GAP 73 is gone. It's deep and full. Mids are far more smooth, less sibilence on "s" sounds. It's a major improvement over the stock iron, it's "transformed".

Now I'm wondering about the input transformer...

I have a GAP 73 modded by GS-member 'API' and it has a Carnhill outputtrafo and a Carnhill VTB 9045 input transformer.
I like the sound with nice highs and also a full low end in a pleasant way.

[Lights]

Wow... I have to try this if the sibilance goes down 80%...

so where does one buy such a transformer and how much does it cost?

[nosebleedaudio]

Originally Posted by Lights

Wow... I have to try this if the sibilance goes down 80%...

so where does one buy such a transformer and how much does it cost?

Jensen Transformers..
I use the JT-123-ALCF in the JM-530 Mic pre..

[Lights]

Wow. That is pricey directly from Jensen--$115 and I'm sure shipping isn't cheap.

They can't be bought from other sources?

[nosebleedaudio]

Originally Posted by Lights

Wow. That is pricey directly from Jensen--$115 and I'm sure shipping isn't cheap.

They can't be bought from other sources?

Other sources?
Jensen sells direct, they make them...
My mistake, I use the JT-123-ALCF, for this application/circuit I see no real advantage of the JT-123-BMCF..Well it is DC coupled..so..

[Ike Zimbel]

I have some early 70's vintage, US made output transformers at the shop that may fit the bill. Let me take some measurements on them tomorrow and see if they will work.
Stay tuned...
Ike

[Lights]

Cool. Just didn't know if they ONLY sold direct or also through resellers. Many companies do both and are obligated to sell their product at MSRP direct, but the same item can be bought for half as much elsewhere.

[jensenmann]

I´ve replaced the original GAP Comp-54 output transformer in my unit, which is probabably the same POS than the 73 output-iron, with a Carnhill or Marinair - don´t remember exactly - and the result was pretty much the same as Jim described: lowend turned from shy into big. After that I replaced all other transformers with Carnhills. That made another step forward. Now it plays in the same sonic league as other famous diodebridge comps do. I only have to figure out a way to make the meter behave now...

[jbates]

Originally Posted by Jim Williams

Did those stop the sibilance? I lost about 80% just with that Jensen on the output.

If you want, I can bring mine down to you and we can compare. Mine has the Carnhill input and output transformers installed. PM me.

[LimpyLoo]

Jim Williams is so ****ing awesome.

Great contributor to Gearslutz.

I'll be doing these mods sometime soon...

[Schaap]

Originally Posted by Jim Williams

Did those stop the sibilance? I lost about 80% just with that Jensen on the output.

Yep, if not I had sold the GAP

[dogma]

Would the jensen fit into the 500 series? $260! I'll immediately change the trafo but some guidance here would be extremely helpful. They will be across my 2 buss so I'm looking from clean to dark. Money is very tight so these, with some tinkering are a godsend for me.

[manzini]

Originally Posted by LimpyLoo

Jim Williams is so ****ing awesome.

Great contributor to Gearslutz.

I'll be doing these mods sometime soon...

Also.

Another Jim Williams gift to gs members.

Thanks Jim.

[Lights]

Can one replace *just* the output transformer? Or does the Jensen require other components be changed?

[Lights]

Great to hear. Is the same true for the Carnhill output transformer? I read this post which made me wonder:

GAP 73 (Neve style) Mic Pre with 'real' Carnhill Transformers vs Original version
(specifically the part about the input load capacitor, but perhaps this was related to the input transformer as he changed both)

I'd love to hear a shoot out comparing stock with Jensen and Carnhill and competitor the three on the same vocal (preferably with lots of Ss) :-)

[Lights]

Hey folks. I'm planning to order one of these Jensen JT-123-BMCF transformers this week. Any words of wisdom before I take the $115 plunge? A local friend who is pretty good with gear is going to do the replacement for me.

Any other things I should look at while I have the hood open?

I'm not sure whether this matters but my GAP73 is the mark II, not the original.

[Sean Sullivan]

Will a Jensen JT-16-A work for the input transformer? There's very little information about the Carnhill transformers that are popular for these preamps.

I'm guessing 1 input transformer is for the mic, the other for the DI?

[manzini]

What happened Jim? (If you would seem appropriate to answer me)

Issue or wrong info, or not appropiate ?

I was following your indications with great interest.

[dualflip]

Seems like he got cold feet about giving away a free mod...

[abechap024]

Its not rocket science to figure out what kind of transformer will work as a replacment transformer. Surely ANY transformer will work (pass signal) but its all about optimization. Your pre is based on an old neve right? Just find out what ratio your transformer is and then get the Jensen one that is close. I would advise you but I haven't looked at the schematic recently and am on my phone, but the info is out there.

[Lights]

Hey folks. I have received my Jensen output transformer and also have a more Neve-like Carnhill mic input transformer enroute from the UK (model CA-18-VTT2378).

If anyone has any suggestions for other things to do while I'm under the hood, certainly let me know.

[abechap024]

Sweet. Replacing/upgrading the capacitors is usually a good way to increase clarity in a signal path. Depending on what types are already in there. And depending how far you want to dig into it...

[slenderchap]

Originally Posted by Lights

....Carnhill mic input transformer enroute from the UK (model CA-18-VTT2378).....

VTT2378 is a mains input transformer for valve gear ?

Colin
Audio Maintenance Limited - Support Zone

[PRH]

On the input side, you would need the Carnhill VTB9045 or VTB9045M (sheilded) transformer. The footprint of the pins on these two transformers is slightly different, and I don't know if one is preferable for the Pre73.

The appropriate Carnhill output transformer would be the VTB1148, which, if mounted on its side, will fit in a 1RU case.

These transformers seem to require a Zobel network. Refer to this document. There might be some creativity required in mounting the input transformer.

Note that I don't have a GAP Pre-73, but I have used the Carnhills in other projects.

Paul