Lowering SD Condenser noise, Pearl CMR-55H / CR-55.

[mrelwood]

Hello.

I've got a pair of these physically large, C1000-style mics. They have a marvellous sound on acoustic western, but they have a very high noise floor. I believe CR-55 is the same as CMR-55H.

Any suggestions how to get the noise lower? There are no transformers, all components are showing, except for the bass roll-off switch.

For me, those blue tantals are yelling "replace me", but I have very little experience in modding mics. The orange drops say "1-35", the trannies are fuzzy, but I believe the upper says "A1015 Y 16", the other "C1819 Y 16".

Here You can get some idea of what's going on with the connections. The black and red on the very right come from optional onboard 9V battery. White and red 1/3rd from the right goes directly to output, as does the blue ground. The two whites and two reds go to bass roll-off switch; in flat mode the reds are together, and white ones are together.

Thanks for any ideas!

 

[rodabod]

Do you have a photo of the whole microphone?

Some of those Pearl mics are quite interesting - I'm surprised the noise floor is so high though.

 

[bcarso]

That guy in the middle on the far left is a FET I'll wager. Probably a Toshib* part like the other two (the 2SA1015 and probably a 2SC1815; odd that they are both the lowest beta codes ("Y")).

The FET, if Toshib*, is probably a 2SK266. Although it doesn't have the lowest voltage noise it has low capacitance compared to lower noise parts, so the loading on the capsule will be less and the signal voltage higher. You might still see an improvement with a lower noise higher C part, e.g. 2SK170, but there will likely be effects on the frequency response.

One thing of note: the FET's gate lead terminates on the lossy circuit board. Especially if there has benn contamination or just high humidity the leakage currents can be substantial for this kind of work. Other designs have the wiring in the air or connected to teflon standoffs.

If the character of the noise is low frequency this could be one of the issues.

It's a simple enough circuit, once you have the transistor pinouts, to trace and make a schematic.

 

[Gus]

3 transistors make me think a Schoeps type circuit 1 n channel fet and 2 PNPs. It looks like there is no DC to DC converter.

Schoeps might be a BC something FET

As posted the noise could be from the board mounted gate lead of the fet. Any moisture or dirt and the noise gets bad.

Leave the tants in for now find the noise first. In most microphones there is forming volatges across the caps so the "sound' of electro caps is not bad.

The capsule could be the cause of noise if it is dirty I would check this first.

Can you give us a list of the parts and part numbers?

Could this be the source of the Scott D circuit?

 

[Svart]

if you zoom in on the backside of the board you also see that all the traces are covered in solder flux. clean this off also.

 

[gyraf]

If there's room for an on-board 9V battery, then this is most probably an electrect microphone. And many electrects are inherently noisy, so there may or may not be anything to do about it.

I think that you should expect that the designer / manufacturer has done everything possible +/-3dB to keep noise down already.

Jakob E.

 

[mrelwood]

Okay, so I'll try to clean it as thoroughly as I can and see if that helps. If I decide to try lifting the FET connetions, what should I use? Would a stripped shielding for a cable help?

And how do I clean the capsule? Sounds really dangerous....

Here are the full pictures:

 

[gyraf]

First of all, compare your mic's to the factory noise target figures - or a known-good specimen of the mic. Chances are that there's nothing wrong with your mic other than the compromises decided in design - and then no cleaning would help at all. In this case, you'd be better off using the mic for it's intended purpose, whatever it may be.

If your mic noise is significantly higher than what is expected from this design, then first try the mic electronics with the capsule replaced by a small polyester cap, some 10 or 22p to emulate capsule. This will show you if noise is from capsule or electronics.

Don't clean, replace or rebuild anything if you're not certain of what you're doing - after all, optimizing mic performance is not an easy task at all - and Pearl normally knows what they're doing...

Jakob E.

 

[rodabod]

[quote author="gyraf"]If there's room for an on-board 9V battery, then this is most probably an electrect microphone. And many electrects are inherently noisy, so there may or may not be anything to do about it.
[/quote]

I think it is a true condenser. Pearl provided 9V power options for most of their true condenser mics;

"CR 55
Condenser microphone with linear frequency response and anti-feedback design. It is recommended for simply all purposes. It comes with a 12 dB pad and a switchable bass-cut, an integrated circuit impedance converter and transformerless output. Powered by phantom or 9V battery.

Polar Pattern Uni (Cardioid)
Frequency Range 15 ... 22000 Hz
Sensitivity 8 mV/Pa
Impedance 200 Ohms
Max. Sound Pressure Level
with pad 150 dB
Dimensions 43 Ø x 220 mm
Weight 0.410 kg"

A very good website:

http://members.aol.com/mihartkopf/

 

[gyraf]

I don't think 9V would be adequate polarisation voltage for a SD-mic capsule - and I don't see any power conversion circuits on the PCB's pictured. Power conversion would probably also drain a 9V battery all too soon to be practical. My guess is that the

integrated circuit impedance converter

means electrect capsule. But then again, Pearl has made some unconventional designs over time.

I still think the best guess on the noise, judging from the pics, is that it's an electrect mic. Manufacturers more often than not tries to disguise this fact.

Jakob E.

 

[mtt-tom]

Hi all,

this mic was made by (or for) the japanese Drum-Company Pearl (not the swedish company PEARL) in the early 80's and definitively aimed for home recording purposes. Electret condenser type. Just to make that sure.

Regards,
Tom

 

[gyraf]

Aah.. everything makes sense then... Thanks, Tom.. :thumb:

Jakob E.

 

[bcarso]

Heh. And I notice the noise spec is conspicuous by its absence...

 

[rodabod]

[quote author="mtt-tom"]this mic was made by (or for) the japanese Drum-Company Pearl (not the swedish company PEARL)[/quote]

Ahhhh. That explains it. I remember thinking that Pearl made high quality true condensers, but that explains it.

Do any of you guys remember the square-shaped condenser diaphragms that they used to make? Weird.

 

[bcarso]

[quote author="rodabod"][quote author="mtt-tom"]this mic was made by (or for) the japanese Drum-Company Pearl (not the swedish company PEARL)[/quote]

Ahhhh. That explains it. I remember thinking that Pearl made high quality true condensers, but that explains it.

Do any of you guys remember the square-shaped condenser diaphragms that they used to make? Weird.[/quote]

I would have said that sounds as bad an idea as square cone speakers, except I got word at a dinner party Saturday that Infinit* has a new system coming out, that works pretty well and uses rectangular cones.

 

[rodabod]

Have a look here:

http://www.pearl.se/estart.htm

 

[Marik]

[quote author="bcarso"][quote author="rodabod"][quote author="mtt-tom"]this mic was made by (or for) the japanese Drum-Company Pearl (not the swedish company PEARL)[/quote]

Ahhhh. That explains it. I remember thinking that Pearl made high quality true condensers, but that explains it.

Do any of you guys remember the square-shaped condenser diaphragms that they used to make? Weird.[/quote]

I would have said that sounds as bad an idea as square cone speakers, except I got word at a dinner party Saturday that Infinit* has a new system coming out, that works pretty well and uses rectangular cones.[/quote]

Why do you think so? Most of electrostatics and planars have square shaped diaphragms and some of them (read: properly designed) can sound stunning.

Pearl attributes advantages of the rectangular capsules to "better" resonant modes. I always hear only very good reports from users. Part of the reason might be a PP design.

 

[bcarso]

[quote author="Marik"]

Why do you think so? Most of electrostatics and planars have square shaped diaphragms and some of them (read: properly designed) can sound stunning.

Pearl attributes advantages of the rectangular capsules to "better" resonant modes. I always hear only very good reports from users. Part of the reason might be a PP design.[/quote]

If you look at the modes and look at stiffness to mass ratios for loudspeakers at least, it looks worse than conical structures.

I know there are proponents of electrostats, but the bigger they are and the flatter the worse they beam. Transparency is much touted but I think the directivity makes them much worse than decent cone-based systems.

I think Beveridge had the right idea with the structure in front of his diaphragm, and probably got the benefits of both worlds as long as breakup modes weren't excited. I recall Keith Johnson suggesting that that would be a good idea for microphones, suitably scaled down. I imagine it would work well for ribbons too.

 

[Scodiddly]

Hard to say without seeing the back of the actual capsule, but I suspect an electret with the built-in FET (like Panasonic). Those are noisy just because of how the FET and the gate bias are handled.

 

[rodabod]

You can mod the FET on the Panasonic capsules.

I wonder if these can be modded?