MXL 990 polarization voltage boost to 60V?

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karloff70

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Hi everyone...

Waiting for my bargain basement MXL 990 to be delivered and thought I'd pipe up for some soon needed help....

The plan is to try Dale's M7 capsule in the MXL, but the polarization voltage in there is only 42V(?) I seem to read here. From some of the Gyraf G7 discussions I gather that Dale's M7 likes about 60V. Do I have to rip out the dc-dc converter for something else or is there another, sweeter way to raise the voltage to 60V...?

Any help deeply appreciated.....noob alert.... :roll:
 
You can add one more multiplier stage to DC/DC converter. I am not sure though, if I like the idea of using Schoeps circuit with LD capsules. IMO, it works much better with SD.

Best, M
 
Hello again Mark....

No good for LDC, no? Maybe I should just get it overwith then and sell/dump/spraypaint/blow up....the MXL and go for a G7 build with that M7 capsule coming my way......but then maybe it will do something useful with a couple of new caps a la 603.....or blow it up..... :green:
 
[quote author="karloff70"]Hi everyone...

The plan is to try Dale's M7 capsule in the MXL, but the polarization voltage in there is only 42V(?) I seem to read here. From some of the Gyraf G7 discussions I gather that Dale's M7 likes about 60V. Do I have to rip out the dc-dc converter for something else or is there another, sweeter way to raise the voltage to 60V...?

Any help deeply appreciated.....noob alert.... :roll:[/quote]

Keep in mind that the DC-DC converter cannot supply a lot of current. Your DMM most probably loads down the converter so you don't see the actual voltage it produces for the capsule (which hardly uses any current at all).
The actual voltage is probably higher than 42V as there is little point in putting in a DC-DC converter for a voltage you could get from P48 without a converter circuit.
The actual Schoeps circuit, which the MXL circuit more or less copies, produces 60V. So if MXL didn't screw up, you may already have a voltage in the vicinity of 60V.
 
Like Rossi said, a DMM-measurement can be misleading. Most DMMs have some 10 MOhm input impedance, which is not too bad. You could chain a few (say nine) 10M resistors to make a meter with 100M input 'impedance' for DC and multiply the reading by 10. Check the DMM manual for its input-load.

BTW, in a controlled environment with resetted caps you could measure the DC also by means of a capacitive divider, but that's unpractical here, requires still a high-Z DMM, isn't really necessary here since your DMM will range up to 60V anyway so is a different story altogether :grin:
 
[quote author="clintrubber"]Most DMMs have some 10 MOhm input impedance, which is not too bad. You could chain a few (say nine) 10M resistors to make a meter with 100M input 'impedance' for DC and multiply the reading by 10. Check the DMM manual for its input-load.[/quote]

Nice idea, Peter.

I'd be interested to know if there is any theory as to why the Schoeps-type circuit is not ideal for LDCs.

Still, there would be nothing to stop you if you wanted to replace the circuit board with some stripboard and install a new circuit.
 
Altogether amazed at the chiming in capabilities of this forum.....great stuff!

So that sounds like I might just try the M7 in the soon to arrive 990 after all. If it doesn't appeal, it will just come straight back out and wander into a G7 build instead...

Now the noob-admission..: what is a DMM? :roll:
 
[quote author="karloff70"]Doh! :oops:[/quote]
No problem, now & then there's abbreviated talk here without realizing these are abbreviations.

TX = tranny = transformer = iron

// = parallel

PSU = Power Supply Unit

OPT = OutPutTX = ...etc

LTP = Long Tailed Pair

CCS = Constant Current Source


And finally...

Boris = your brother ? :cool:
 
i just bought 2 990s. price was good so i couldnt resist. i thought i would do the gus 630 mod and maybe mess with the mesh and try removing the brass ring. see how each effects the sound. i have not recieved my mics yet but i thought the capsules in this mic were the same as the capsules in the 603. these capsules have been praised as very nice capsules for the price. looking at a photo of the 990 capsule i see it looks like the same capsule inside an MCA Sp1. does any one know if these are the same capsules?
 
I have both the MXL990 and MCA SP-1. the capsules are identical and are mounted in what appears to be a brass "baffle" ring.
I don't have a MXL 603 but, it's my understanding that it is also the same capsule (without the baffle ring).

ZAP
 
great! i hope they are the same. thanks for the info Zap. i hear you can do great things with the 603s. i now have one sp1 which i havent touched(mod wise) but dont use for anything,dont know what the hype was about? to each his own. now to find that the 990s have the same capsule! it's disapointing. but worse come to worse i'll make new "603" mic bodies and grills for them and use the 990 and sp1 bodies for some diy ldc mics
 
I think the MXL990/MCA SP-1 capsules can sound very nice on voice recordings. (I do mostly spoken word recordings)
I have modified 2 MCA SP-1 microphones and I am very pleased with the results. (The details on the mods are over at the Yahoo Micbuilders group)
I believe, when it comes to microphones, everybody has their own personal tastes on the kind of sound they prefer. I wouldn't shortchange these capsules right away. You might want to experiment further with them.

ZAP
 
Hi Zapnspark...

Just wondering if you did a mod to the voltage in your MCA/MXL's or just cap upgrades? Read somewher that Jim Williams did something to raise the capsule voltage in the MCA to make it more sensitive/give it better s/n ratio.

Maybe I'll just try that on my 990 and keep the M7 capsule for a G7.....

Anyone know how to do that and how much voltage the original capsule can benefit from?
 
I didn't try to up the polarizing voltage on the MCA SP-1.
I have done that to my Frankentube microphoness to improve the S/N ratio.
The changes I made were as follows:
1. Change the emitter follower output transistors to BC559C types.
2. Replace the 2SK170 JFET with a J305 JFET.
3. Change the bias resistor ratios for the J305 JFET replacement.
4. Changed to direct polarizing of the capsule. This gets rid of a 1 Gig
resistor and the (controversial) 1 nF ceramic coupling capacitor.
5. Changed the JFET to emitter follower coupling capacitors from .33 uF
to mylar .47 uF types. This was done to extend the low frequency response a bit.

I'm not familiar with the type of mods that Jim Williams makes to these microphones.
I have heard that people are very satisfied with the results of his mods.

ZAP
 

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