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R/E/P => Klaus Heyne's Mic Lab => Topic started by: afterlifestudios on August 11, 2016, 02:03:17 am

Title: RCA 44bx pair possible?
Post by: afterlifestudios on August 11, 2016, 02:03:17 am
I have an RCA 44bx that sounds great.  I have the opportunity to purchase another.  It needs some work, but I am wondering how close to a matched pair one could ever hope to achieve with these..

They are slightly different production models.  One has a low cut switch, the other has a low cut jumper.
Inside, one has a screen in front of the ribbon element that is "cross" shaped and has 1/8" holes with fabric across them in it.  The other has a straight vertical strip of fine mesh in front of the ribbon element.

Are these and other potential differences in production materials etc going to make it impossible for them to ever be a really balanced pair?  (Even if they were both re-ribboned and gone through.)

Title: Re: RCA 44bx pair possible?
Post by: soapfoot on August 11, 2016, 07:30:13 am
They will probably not be a "pair."

If the metallurgy of the magnets is different (and it did change over time, from revision B to BX), and if the orientation of the ribbon is different (it did move slightly within the gap, again from revision B to BX), and if the transformer is different in some way (the jumper versus switch would make me suspicious that it might be), re-ribboning alone will not make an exact matched pair.

The jumper instead of the switch suggests that you might have a 44B on your hands. Which is great; those are great mics. We have one of each at our studio, as well.

To tell the truth-- I don't often feel drawn to ask for stereo RCA 44s, so I don't think having one of each would be any great loss from my perspective. I tend to like the 44 on trumpet, trombone, baritone saxophone, sometimes double bass (in combination with a good condenser), sometimes in the heel of the piano in combination with a good pair of 87s or 67s, etc. Having two around is handy, but not necessarily as a pair... more like one on trumpet one on trombone, etc.
Title: Re: RCA 44bx pair possible?
Post by: afterlifestudios on August 11, 2016, 02:19:02 pm
I love this place...

Thank you Allen.  They are both indeed 44-BX models.  But upon further inspection, the second one was manufactured by RCA Great Britain Ltd.  Does anyone know if that means different transformers, and metals, magnets etc?  Or was it just an assembly operation?

Another difference is that the UK mic has some kind of "insulation fluff" stuffed in the cavities above and below the ribbon element.

Main sonic difference is there are more sub frequencies (below 50 hz) on the UK model.  General output level seems reasonably close from my speaking tests...  The US model is a little brighter, but I'm thinking that's more to do with ribbon tension.

photos attached
Title: Re: RCA 44bx pair possible?
Post by: soapfoot on August 11, 2016, 02:42:42 pm
Really hard to say; perhaps find a place to ask Wes Dooley what he thinks? Maybe contact him directly through AEA's site for an opinion? He's certainly got more knowledge of these mics in his little finger than I'll ever have.

To an extent some of it might depend on how close they have to match for you to consider them a "pair."

Like, recording an orchestra with them as a main pair is one thing; having them behind a funky upright piano on a rock record is something else entirely.

Title: Re: RCA 44bx pair possible?
Post by: mbrebes on August 12, 2016, 02:09:12 pm
Per a thread over on Gearslutz, most of the mods had to do with rerouting wires for decreased hum.  Per AEA's website, it also sounds like there was some kind of mod that reduced the structural weight but did not change the sound of the mic.
Title: Re: RCA 44bx pair possible?
Post by: afterlifestudios on August 13, 2016, 01:34:19 pm
Thanks Everyone. I got in touch with Wes at AEA.  A fine fellow, he got back to me right away.  Like mbrebes mentioned, the UK versions are essentially the same, with a few modifications to the physical routing path of some wires to minimize noise. (I believe that AEA used this version for their recreation, but Wes did not say so specifically in our correspondence.)  Wes figured the biggest difference in my mics will be from the two different "puff shield" varieties I have. Switching them both to a "cross guard" like in my first photo would likely be the way to go.

I adjusted the tension of the ribbon (terrifying) and they are getting pretty close. Still a little more sub frequency (maybe 60hz and down) on the UK version...

Title: Re: RCA 44bx pair possible?
Post by: afterlifestudios on August 21, 2016, 11:10:54 pm
Just an update on the RCA 44bx's.  After some research, I further adjusted both ribbons to a resonant frequency of around 25hz.  The response is now very closely matched.  Considering that one dates from 1949 and the other from 1951 and the fact that they were "born" on different continents, I think it's absolutely remarkable how similar they sound. 

Surprisingly the UK version seems to be slightly more susceptible to 60hz hum when the mic is positioned at certain angles.  Surprising, because the UK version is the model with the manufacturer's modifications to reduce hum by running the leads through the magnets to further shield them.  Any other thoughts about where I might look to reduce this hum?  (It is minimal, and if I didn't have the other mic I would probably think it was a "normal" amount.  But since I know it can be quieter, I'd like to get it there...)
Title: Re: RCA 44bx pair possible?
Post by: radardoug on August 21, 2016, 11:49:40 pm
Do both mikes set up with the same amount of gain on the preamp for equal input?
If the US one is then quieter, maybe wire the UK one to match it exactly. Maybe their mod didn't actually work.
Title: Re: RCA 44bx pair possible?
Post by: Kai on August 22, 2016, 04:41:06 am
Any other thoughts about where I might look to reduce this hum?
There are two basic kinds of hum being injected into a signal:

1. Electrostatic: A faraday cage avoids this, check if all metal parts are properly grounded and the cable shield is connected to the housing and grounded on the preamp side.

2. Magnetic (more likely): hum injected by parts of the microphone acting like a coil (Think single coil electric guitar).

Does the hum increase when you come closer to a mains transformer, and if you change orientation of the microphone?
The area covered by the connection wires from the ribbon to the transformer form a coil wich should be as small as possible.
The transformer should have a MU-metal shield.

If the transformer catches hum (short transformer primary as close as possible to its input wires to check) it can (partly) be compensated by catching inverted phased hum on the primary, the ribbon side.
Try re-orienting the connection wires.
Try reverting the transformer connections one booth in and output side (try primary/ribbon side first, if that helps do it on the output side too to regain correct phase).
Title: Re: RCA 44bx pair possible?
Post by: gtoledo3 on August 22, 2016, 02:40:58 pm
It is hard to see because of the screens, but it looked to me like the ribbon on the UK version was a little stretched out on the bottom half, from the look of the pleats on the ribbon.

Tightening it up is better than leaving it loose... while I wouldn't think of it as resulting in more hum typically, I also wouldn't be surprised if it did. Especially if the age of the ribbon is resulting in it not being as centered in the magnetic gap in some positions.
Title: Re: RCA 44bx pair possible?
Post by: boz6906 on August 22, 2016, 05:37:38 pm
The leads should be twisted to minimize inductive pick-up.

Also, I've had good results using both the CL-1 'Cloudlifter' and Cathedral 'Durham' head amps with my RCA 44BX (and my WE 639B pair as well).

The ribbons do like a higher input Z, 3k or better seems best for preserving the hi-freqs.
Title: Re: RCA 44bx pair possible?
Post by: afterlifestudios on August 24, 2016, 01:46:27 am
Do both mikes set up with the same amount of gain on the preamp for equal input?
If the US one is then quieter, maybe wire the UK one to match it exactly. Maybe their mod didn't actually work.

Yes, they are within 1db or so at most frequencies at same preamp gain.  Both mics are strapped for 250ohm output.

There are two basic kinds of hum being injected into a signal:

1. Electrostatic: A faraday cage avoids this, check if all metal parts are properly grounded and the cable shield is connected to the housing and grounded on the preamp side.

2. Magnetic (more likely): hum injected by parts of the microphone acting like a coil (Think single coil electric guitar).

Does the hum increase when you come closer to a mains transformer, and if you change orientation of the microphone?
The area covered by the connection wires from the ribbon to the transformer form a coil wich should be as small as possible.
The transformer should have a MU-metal shield.

If the transformer catches hum (short transformer primary as close as possible to its input wires to check) it can (partly) be compensated by catching inverted phased hum on the primary, the ribbon side.
Try re-orienting the connection wires.
Try reverting the transformer connections one booth in and output side (try primary/ribbon side first, if that helps do it on the output side too to regain correct phase).

Thanks Kai.  I will try this.  It seems like magnetic hum and it changes with orientation.

It is hard to see because of the screens, but it looked to me like the ribbon on the UK version was a little stretched out on the bottom half, from the look of the pleats on the ribbon.

Tightening it up is better than leaving it loose... while I wouldn't think of it as resulting in more hum typically, I also wouldn't be surprised if it did. Especially if the age of the ribbon is resulting in it not being as centered in the magnetic gap in some positions.

The photo does look a bit like that but not so much in person.  I think the tension is correct.  According to my findings, the resonant frequency should be between 20-25hz, which it currently is. The output is strong and it sounds very much like my other one except for a slightly louder 60hz hum.

The leads should be twisted to minimize inductive pick-up.

Also, I've had good results using both the CL-1 'Cloudlifter' and Cathedral 'Durham' head amps with my RCA 44BX (and my WE 639B pair as well).

The ribbons do like a higher input Z, 3k or better seems best for preserving the hi-freqs.

Which leads are you referring to that should be twisted?
Title: Re: RCA 44bx pair possible?
Post by: boz6906 on August 24, 2016, 12:20:31 pm
The leads from the ribbon, see picture  on Dr. Coutant's page:

http://www.coutant.org/pb31/

The idea is that twisting the wires together causes the inductance & capacitance to cancel one another, similar to Lintz cable.

Also, make sure pin 1 of your output XLR has continuity with the mic body, don't put your meter on pins 2 & 3, might meltyour ribbon...



Title: Re: RCA 44bx pair possible?
Post by: Kai on August 24, 2016, 12:47:54 pm
The leads from the ribbon, see picture  on Dr. Coutant's page:
http://www.coutant.org/pb31/
The idea is that twisting the wires together causes the inductance & capacitance to cancel one another, similar to Lintz cable
In the picture from the link above, (I placed it here for convenience), you can see that the wires taking the signal from the top of the ribbon are doubled, a clever idea.
They are running symmetrically left and right from the ribbon, so two coils with opposing winding directions are formed.
This is to cancel out pick up of inductive hum.
It has nothing to do with capacitance or skin effect, which is not an issue in such a low Ohm environment.
The third wire was carefully positioned and fastened with laquer.
Might be to pick up just the right amount of hum to cancel out what is taken by the transformer.
Title: Re: RCA 44bx pair possible?
Post by: boz6906 on August 24, 2016, 03:20:24 pm

I'm not sure I agree that C isn't involved.  And because the two leads carry signals that are 180 out of phase, the physically opposed coils would add, rather than subtract Z.

Twisted pair was initially develped by Ma Bell for audio and is still useful for hi-freq data cabling:

"In high bandwidth data cabling, two conductors in close proximity are, in effect, plates of a capacitor. Accordingly, capacitive reactance between the two conductors tends to shunt out the high-frequency signal. Similarly, each conductor has inductance and at higher frequencies the inductive reactance is high. Inductive reactance is a series phenomenon, so it tends to weaken the signal. This unfortunate fact limits the upper frequency of signals that a given conductor can convey.
An effective remedy, employed in category as in Cat 5, Cat 5e (enhanced), Cat 6 and Cat 6a (augmented) cabling, is to use twisted pairs of conductors, as in the near-by illustration. How can twisting conductors mitigate reactive loss?
The answer is in the strategy of differential transmission. The technique originated in nineteenth century telephone technology when high-powered, brush-type motors became commonplace. These motors were problematic, especially in large cities which at the time were great centers of manufacturing, because they generated significant electrical noise. The noise tended to inductively couple into long telephone lines. Telephone engineers devised the strategy of line transposition as a countermeasure, where lines crossed and changed places typically every third pole."

http://www.testandmeasurementtips.com/new-articles/mitigating-harmful-effects-capacitance-inductance/
Title: Re: RCA 44bx pair possible?
Post by: Kai on August 25, 2016, 03:34:18 am
I'm not sure I agree that C isn't involved.  And because the two leads carry signals that are 180 out of phase, the physically opposed coils would add, rather than subtract Z.
If you carefully examine the picture you see that the twisted cables are connected in parallel, both making the same link from the top of the ribbon to one of the transformer's input tabs.
Only for magnetically introduced hum they carry an out of phase signal (which cancels out at the transformer input), as the two coils formed between ribbon and either wire are running in opposite rotation.
This is the reason they did it that way.

Regarding stray capacitance: the ribbon is a signal source close to zero Ohms. Any C having an effect in the audio range would need to have a size in the uF range. This is by far not possible using some wires that short.
If you assume 0.1 Ohm for the ribbon you would need a 83 uF cap load to effect 20 kHz, equaling about 1000 km of twisted pairs.

The site you are citing is about transmission lines (e.g telefone lines of several km's lenght or Giga-Hz frequencies of network connections), and ONLY for those.
The loss effects described cannot be observed in normal audio range - studio distance situations.
Not to mention some cm's of wire inside a mic.

Looking at audio studio interconnects as transmission lines is a widely spread misconception.
Even then people usually don't follow the rules for transmission lines, e.g. proper termination.
They're just being sold over-expensive cables.
Title: Re: RCA 44bx pair possible?
Post by: afterlifestudios on September 29, 2016, 01:21:39 am
Just to report back...  I replaced the ribbons for both my 44's and installed the cross shaped "puff guard" into the UK version.  (Thanks to the fine folks at AEA for recreating one for me...)  Both mics are wired now for 250 ohm output impedance.  I twisted the leads on the UK version.  All in all they sound remarkably similar.  Output level is the same for each.  The overall tonal quality is that unmistakable 44bx larger than life sound.  The noise floor is very similar for both, and they can both be used on quiet sources, (especially with my BN2A as the mic pres.  So much gain...)  I'm calling this one a success!  Thanks for all the help.

- Just an aside, when dismantling the UK version for cleaning, I didn't take careful notes and reinstalled the lower magnet with the polarity reversed.  Essentially cancelling the output of the ribbon...  Took me a few minutes (or more) to figure that one out!!