GAP 73 (Neve style) Mic Pre with 'real' Carnhill Transformers vs Original version

[anguswoodhead]

I have 2 of the GAP 73 Neve style Mic Pres.
Both brand new.

1 has been modded with 'real' Carnhill Transformers (Mic input and line output).
Plus a few other tweaks to get it sounding 'right'.

The other is factory original.

Just posting photos for now but I am planning on doing a series of audio tests between the 2 units to ascertain differences etc.

Stay tuned.

[anguswoodhead]

"installed the carnhill mic and line out transformers. The output transformer mounts on the same centres as the original. The original mic transformer was mounted on the pcb. It was required to drill two holes on the side panel to mount the carnhill. I've countersunk these and used hex head black screws so that it is neat.
One significant variation from the original circuit was in the type of caps used in the main signal path. I've replaced these with the same capacitor type that Neve originally used ( tantalum capacitors ).
I replaced the input load or zobel cap with the correct one for the carnhill transformer.
I tweaked the output stage bias to squeeze max level / min distortion from the output stage.

some performance data :

post mod compared to pre mod:
gain stays the same, noise stays the same
max output level @ 1% distortion improves by 3dB into a 600R load, no significant change into a bridging load

distortion at very hot mic levels distortion at 20Hz improves from 2% to 0.11%, this is a result of a better mic transformer
frequency response with original transformer exhibits a rising response of near 3dB at 40kHz, the modified unit freq response is within 0.3dB from 20Hz to 40kHz.
square wave testing shows an underdamped mic transformer whilst the carnhill with the correct loading cap is critically damped and shows no ringing or rounding.

I have replaced the front panel screws with neater looking black cap head screws. "

[dysenterygary]

which caps did you replace, and what did you replace them with?

[anguswoodhead]

Originally Posted by dysentarygary

which caps did you replace, and what did you replace them with?

The caps were replaced to match the same type of caps as used in the original Neve 1073.

[dysenterygary]

ok, so it was the same values, just using tantalum capacitors?

[anguswoodhead]

Originally Posted by dysentarygary

ok, so it was the same values, just using tantalum capacitors?

[anguswoodhead]

Some U87i tests - my voice.
Mic held in my hand.

[anguswoodhead]

More U87i tests - my voice.
This time in a shock mount with a pop filter.

[anguswoodhead]

Mann M11 clips - my voice.
Mic in a clip, no pop filter.

[anguswoodhead]

I will try to do some Bass Guitar clips soon.
I will use a DI thru the Mic In with 48v to power the DI.
Also some Piano tests.

[anguswoodhead]

Ribbon tests - my voice.

[living sounds]

Wow, huge difference! Sounds great with the mod. Which Carnhill models did you use?

[anguswoodhead]

Originally Posted by living sounds

Wow, huge difference! Sounds great with the mod. Which Carnhill models did you use?

CA-18-VTB9045 - Mic Input (10468 re-issue)

CA-18-VTB1148 (narrow board version) - Line Output (LO1166 re-issue)

I wanted to use the real re-issue Carnhills for maximum authenticity.

[World Studios]

Good fun! The modded one sounds a lot more open and clear, but with less low end, perhaps?

[anguswoodhead]

Once I get a chance to run Piano and Bass thru them the differences should show up a bit more I reckon.

With previous tests on Bass the unmodded Gap was too woofy and wooly in the low end so that would make sense.

[anguswoodhead]

Piano - single channel using a Coles 4038 Ribbon. (x 2)
The Mics were setup beside each other outside the lid of my grand piano.
2 takes were done - both takes tracking both the original and unmodded GAP73 at the same time.
The mics were swapped around between takes to avoid colouration from the room to bias the test.

Bass - 4 seperate passes trying to play the same thing at the same volume.
First 2 takes were using the DI in on the GAP - original and modded.
The second 2 takes where thru a Millenia DI then Mic in to the GAP original and modded.

I am definately hearing the difference now - the Carnhill's just sound warmer and more rounded to me.
A nicer mid-range all round.
The Chinese trannies sound to have some ringing to me which comes thru as bite in the high mids.


BTW - I am selling both original and modded GAP 73's in Australia.
Original - AU$450
Modded - AU$750
Plus $20 postage anywhere in OZ.

[living sounds]

Part of the difference may also be attributed to the caps. Properly biased tants can be much better, and they're part of the Neve sound.

On the piano the difference is especially huge, so much more balance and clarity.

[dysenterygary]

those clips are great, thanks for your input! I just bought 2 pre73's that I'm on my way to modding. They sound awesome, but I can't wait to hear them modded. I think your piano clips are the best at showing the differences. the modded version just sounds much more crisp and like the piano is in the room.

[anguswoodhead]

[MacOz]

Hey guys...this mod definitely makes the leap from cheaper chinese ringing transformer to carnhill smoothness....

and I agree - more pronounced on certain sources than others and piano reveals it the best......

(or maybe it was your very professional assistant engineer Angus )

Mac

P.S. Mac = Cameron

[anguswoodhead]

Hey Cameron
Welcome to Gearslutz mate - a real forum - LOL.
Thanks for your help today - was cool to hang out.
And I am definately keen to work more with you in the kit dept.
The session on Sat starts at Midday - you are welcome (I cleared it with the client).
Bring any 'nice' gear you have by all means.

[Tom Hakala]

could you please do some tests with Y-cable. Would be really nice!!

[anguswoodhead]

Originally Posted by Tom Hakala

could you please do some tests with Y-cable. Would be really nice!!

Not gonna happen dude.
I think the tests I am doing are conclusive enough.
I used the modded unit on a session over the weekend with good results.
I am hoping to do one more test with Bass.
Mic'ing up a Bass amp which is LOUD with both units on the pair of 4038's.
I am expecting the stock GAP73 to 'crap out' on the bottom end.
(At least that's what my tech reckons will happen)

[GeorgeHayduke]

Wow. I imagine this will become a popular mod. Much more focus in bass, much more pleasant on vox and piano. And the ringing cleared up.


Good job.

[Tom Hakala]

Originally Posted by anguswoodhead

Not gonna happen dude.
I think the tests I am doing are conclusive enough.

y-cable would solve the differences that come from different takes. It would be a great way to test how much mods bring to the sound.

[anguswoodhead]

Originally Posted by Tom Hakala

y-cable would solve the differences that come from different takes. It would be a great way to test how much mods bring to the sound.

I hear ya mate...
The problem is that you lose the direct relationship between the mic and the pre-amp.
Also if the mic needs 48v you can't have it coming from both pre's - so there is a difference there also.
The only way to do it properly is with a transformer isolated split like the midas or Klark which I don't have - sorry.

[anguswoodhead]

There has been a lot of interest in my modded GAP 73's,
And rightly so - they sound a lot better than the stock ones.
For those interested you can also buy a rack mount to hold 2 of the GAP73's.

[Randy666]

Hello anguswoodhead,

I wrote you an email concerning the modding of the pre73.

Does the exchanging of the ouput tranny bring the most bettering?
Or is the exchange of the input tranny and the tant cap absolutely necessary?
What do you think?

If I would also change the input tranny - which kind of cable should I use zu connect the tranny with the board?

Thanks a lot!

Randy

[anguswoodhead]

I think it is like most things in audio.
Every little bit helps.
It is because of all 3 alterations that the modded unit sounds so good IMHO.

[Randy666]

Alright,

I have just ordered the carnhills und so I am looking foward modding the red devil :-D

Did you have to drill additional holes into the housing to build in the input tranny?
And - once again - which kind of cable should I use?

Thanks
Randy