Apex 205 ribbon mic + transformer mods...

[rindigo]

interested))))

[terence]

Here at last are my recorded notes of moding the Apex 205. Apologies to anyone who was holding their breath for this – as you know life happens.

This is primarily about swapping the stock transformer for the Lundahl 2912. I also retensioned the ribbons (I can get back later on that) and did some basic tests for ribbon resonance afterwards (all cudos to Mike Joly for his guidance in various theads).

First a preamble, which I think is necessary as this guide may only be of use if your mic resembles mine in orientation: On my first attempt, as I mentioned previously, I discovered that having installed the transformer the polarity was inverted compared to my other mics. I re-checked Michael Joly’s Hometracked blog on Moding the 205, which, although it doesn’t specifically go into transformer mods, has some photos of the mic taken apart and I wanted to see if I had put things back the right way. It became clear from those photos that the side illustrated as the front, the PCB side of the ribbon assembly, was reversed in my microphones. In mine the transformer faces the front: Compare the shots below with “Step 5: Reassemble” in Mike’s blog.

I discovered it is actually impossible to put the ribbon assembly back the “opposite” way because of various screw locations and sizes in the head basket and the bottom casing (although I confess I did try!). Fortunately I had made a photographic record of my activities when I took the things apart – a reassuring reality check.

I had a total of 3 mics that were assembled like this, one of which I sent back for unrelated reasons (SNs: 09070349, 09070359, and one within 10 digits of those). The number of mics involved and the physical relocation of screws makes me suspect this is more than a trivial error in the final stages of assembly. Having said that, I have no explanation for these anomalies of construction. The bottom line is I do now have a great sounding pair of Apex ribbons, polarized in the right direction.

Okay, so if your Apex 205 looks orientated like mine when you first crack it open (see Fig 2), my wiring instructions should give you a correctly polarized microphone. These notes and figures therefore refer mostly to the second installation - when I got I got the mics wired up properly. Also, the second time around, I had a chance to re-think the physical installation of the transformer, which IMHO is an improvement on the first.

On the first attempt I left the transformer unshielded and simply attached it to the metal cross-bar of the ribbon assembly with double sided tape (see Fig 1). This worked well generally, but, 1) I decided I wanted a more secure anchor for the transformer and 2) I could detect the faintest 60Hz hum from both mics plugged into an AEA TRP preamp cranked to maximum gain in a quiet studio. It was almost imperceptible and right at the noise floor, and to be honest probably would not be an issue for most people in most recording situations. In an electrically noisy studio with lots of high-power equipment around it might be a more significant issue. So in my second installation I decided to shield the transformers. This may seem like overkill to some, given that the outer case and headbasket is already grounded. But the fact that transformer shielding is used in this mic (and perhaps in most if not all other ribbon mics) makes me think the shielding is there for a reason. Possibly ribbon motors are sensitive enough to pick up any stray EMF from the transformer itself, since no balanced line is perfect. I don’t know, nor am I an electronics expert, but after my second transformer installation that included a homemade shield the hum went away. You may in fact be better off with the Lundahl 2913 which is shielded. (Incidentally, to save anyone the effort of trying, I did try to fit the Lundahl transformer into the Apex shield canister, but it was too large. Even if you manage to tape the cap on, the whole thing is then too tall to fit into the Apex body).

I have described the modification with illustrated figures, which I hope are self explanatory, so won’t comment further. I’m assuming most people know how to check microphone polarity. If you want check this but unsure how, I’m happy to share my 2 cents. Any other questions, comments, please let me know.

[Michael_Joly]

Nice write up Terence!

Re: hum issues - yeah, a shielded transformer is better. But a potentially significant source of hum is wire dress from the top of the motor to the bottom. These two wires must be parallel to the ribbon, in the same plane, secured to the frame, and twisted together with its accompanying black wire at the bottom. Doing so can reduce AC hum pick up by 10dB or more.

[martymart2]

Yeah, Terence, nice documentation!

I bought a pair of 205s in late '07 and the xfmrs were still facing the rear as in MJ's descriptions. And, of course, you are right MJ about the wire dressing, as I had to go back and fix mine due to hum pickup.

mm

[terence]

Thanks for the thumbsup responses, and the info on the wires to the ribbon. I really paid no attention to that aspect, but all seems fine at the moment. I did wonder why the ribbon motor was double wired, and pretty beefy wire at that. It's kind of amazing to me how sensitive these things are to what seem like relatively minor details in construction. I guess with all that gain needed to drive them even tiny changes in the electromagnetic environment are amplified.

[terence]

Originally Posted by terence

Thanks for the thumbsup responses, and the info on the wires to the ribbon. I really paid no attention to that aspect, but all seems fine at the moment. I did wonder why the ribbon motor was double wired, and pretty beefy wire at that. It's kind of amazing to me how sensitive these things are to what seem like relatively minor details in construction. I guess with all that gain needed to drive them even tiny changes in the electromagnetic environment are amplified.

I finally redressed the wires running down the sides of the ribbon motor, as others have suggested. I actually used a little ring of sticky tape at a midpoint to adhere each wire against the vertical strut of the motor assembly, so now they run nice and parallel to the ribbon. This is really icing the cake, but I'm convinced it has improved the noise floor slightly more; there was occasionally the vestiges of a 60Hz hum, but now nothing, except the faint soft white noise natural to this design when I crank up the TRP. I'm very pleased!

[gecsafunk]

Hi Terence,

Your description is very useful for me. Thanks for it!

I have a question:
Did You use any kind of insulation material at the ends of the Lundhal transformer (between transformer and your homemade shield)?

Thanks,
gecsafunk

[terence]

Originally Posted by gecsafunk

Hi Terence,

Your description is very useful for me. Thanks for it!

I have a question:
Did You use any kind of insulation material at the ends of the Lundhal transformer (between transformer and your homemade shield)?

Thanks,
gecsafunk

Yes I did in fact, just in case the possibility of the metal core of the transformer becoming grounded would cause a problem. I can't give a rational explanation for that, other than transformer cores aren't usually grounded to my knowledge. All I did was apply some insulating tape to the exposed Llundahl core before wrapping on the shield foil. I used a kind of clear yellow cellulose tape used in electronic workshops (you can just make it out on the pre-shielded transformers top-left of Fig. 6) simply because it was handy, but I'm sure anything electrically insulating would do the job.

It's great to know that this stuff was useful to someone else. Good luck!

[gecsafunk]

Originally Posted by terence

Yes I did in fact, just in case the possibility of the metal core of the transformer becoming grounded would cause a problem. I can't give a rational explanation for that, other than transformer cores aren't usually grounded to my knowledge. All I did was apply some insulating tape to the exposed Llundahl core before wrapping on the shield foil. I used a kind of clear yellow cellulose tape used in electronic workshops (you can just make it out on the pre-shielded transformers top-left of Fig. 6) simply because it was handy, but I'm sure anything electrically insulating would do the job.

It's great to know that this stuff was useful to someone else. Good luck!

Thanks for the information!

gecsafunk

[ben_allison]

So are these mods basically what MJ does?

Are there any component swaps?

[2b-3]

That's the beautiful simplicity of these mics. Those are the components: ribbon - transformer - xlr out

[ben_allison]

Originally Posted by 2b-3

That's the beautiful simplicity of these mics. Those are the components: ribbon - transformer - xlr out

*boner*

Ok, wow, looking at those pics more closely... there's really nothing else in there besides the motor and a transformer... huh...

[jimjazzdad]

terence asks: "Has anyone tried this ribbon mic mod recently? I did and discovered the Chinese manufacturer has completely rotated the internal components (since MJ did that post) so that the transformer is now facing the front. The ribbon motor assembly cannot be orientated any other way as the two screws that locate the headbasket (the small one and the large knurled one) are different sizes. I followed Lundahl's instructions and my best knowledge for wire and pin connections and found these modded Apex's had opposite polarity to all my other mics. "

I just finished working on a pair of Apex 205s for a friend - new wiring with the recommended "humbucking" geometry, new ribbons (1.8u), new Cine Mag CM9888 xfmers, removed excess screens and baffles and added foam to the body cavity. They sound good to me, but I don't have a lot to compare them to (other ribbons, that is).

One has the motor oriented with the "transformer facing forward", the other has the "transformer facing back", so the motors are oriented 180 degrees different. I assume the "polarity" is defined by the North/South orientation of the two pole magnets and which end of the ribbon is connected to the "high" side of the transformer? How do I measure the outputs of these two mics to determine if they are in phase with each other? I tried swaping the phase on one channel of the preamp, but I'll be damned if my less-than-golden ears can hear any difference. We want to use these mics as a Blumlein pair, so I imagine getting the phase right is important....thanks for the help!

[Michael_Joly]

Originally Posted by jimjazzdad

...How do I measure the outputs of these two mics to determine if they are in phase with each other? I tried swaping the phase on one channel of the preamp, but I'll be damned if my less-than-golden ears can hear any difference. We want to use these mics as a Blumlein pair, so I imagine getting the phase right is important....thanks for the help!

Put both mics up in front of you, ribbons the same distance from your mouth. Pan each mic to center. Listen in headphones. Set the gain to be the same for both mics. Say: "Shhhhhhhhhhhhhhhh" while flipping the phase in / out. You will get a strong null when the mics are out of phase and the volume will increase when they are in phase.

[jimjazzdad]

Originally Posted by Michael_Joly

Put both mics up in front of you, ribbons the same distance from your mouth. Pan each mic to center. Listen in headphones. Set the gain to be the same for both mics. Say: "Shhhhhhhhhhhhhhhh" while flipping the phase in / out. You will get a strong null when the mics are out of phase and the volume will increase when they are in phase.

Thanks Michael, that's a really easy way to check the polarity / phase agreement of a couple of ribbon mics! Don't know why I was hurting my brain trying to decypher the ribbon output based on the magnet orientation in the motor, etc. etc. As always, KISS principle triumphs!

BTW, really glad that you are still bestowing your knowledge and experience on this forum, despite some of the negative energy that has been around....

Best,

[smashprod]

Originally Posted by terence

You may in fact be better off with the Lundahl 2913 which is shielded. (Incidentally, to save anyone the effort of trying, I did try to fit the Lundahl transformer into the Apex shield canister, but it was too large. Even if you manage to tape the cap on, the whole thing is then too tall to fit into the Apex body).

Has anybody tried putting in an LL2913? Does it fit?

Thanks-
David

[ben_allison]

So, for people who have done this mod... how on EARTH did you strip the thin wires coming from the 2912?

They're impossibly thin. I tried stripping one and it just broke, with hardly any force applied. Then I tried to use a flame to melt the insulation, but all that did was make the wire itself super brittle.

Very frustrating. Now both of the thin leads on my tranny are essentially useless.

[trevjonez]

Did you try put the sleeved wire into a ball of molten solder? Some wire strip well that way because it melts the insulation without oxidizing the wire since the wire is in the solder. Just a thought I have only used the 2913 with a pcb

[ben_allison]

Originally Posted by trevjonez

Did you try put the sleeved wire into a ball of molten solder? Some wire strip well that way because it melts the insulation without oxidizing the wire since the wire is in the solder. Just a thought I have only used the 2913 with a pcb

The word from Lundahl (along with my own experience) is that the insulation won't melt as other insulation does.