kambo

premap front for 60 ohm mic
« on: August 07, 2019, 08:05:51 PM »
i have couple of B&K mics coming over to test.
also, one gift from a famous retired producer... his mono B&K setup in mine now  ;D but he is hanging on to his GLM preamps  :-[
anyway,
my mic pres all have 150ohm input transformers, except NPNG mic pre is electronically balanced.

what is the best way to connect these 60 ohm mics to any mic pre without altering the sound....
i have seen some 1k pot tricks, but how exactly would i know that i am hitting the right impedance!
how would i measure, if i have a matching impedance ?
not interested in if sounds good term... i will be recording over 20Khz so i wont be hearing anything  ;D
thanks


squarewave

Re: premap front for 60 ohm mic
« Reply #1 on: August 07, 2019, 08:13:01 PM »
It's not crystal clear that you would need to do anything. Running a 60 ohms source into 150 ohms probably works just fine. Maybe use a pre like a Neve with Low-Z switch that switches the mic transformer primary to parallel.

Re: premap front for 60 ohm mic
« Reply #2 on: August 07, 2019, 08:15:42 PM »
On your transformer coupled amps, build out the 60 ohms impedance to 150 or so ohms with resistance. 
On the NPNG, leave it as is.

kambo

Re: premap front for 60 ohm mic
« Reply #3 on: August 07, 2019, 08:22:24 PM »
It's not crystal clear that you would need to do anything. Running a 60 ohms source into 150 ohms probably works just fine. Maybe use a pre like a Neve with Low-Z switch that switches the mic transformer primary to parallel.

neither i am crystal clear , application engineer will be bringing the mics, i wanted to make sure it will all work properly...
good idea re Neve preap. i have one, without a switch, i should add one tonight :)

kambo

Re: premap front for 60 ohm mic
« Reply #4 on: August 07, 2019, 08:24:34 PM »
On your transformer coupled amps, build out the 60 ohms impedance to 150 or so ohms with resistance. 
On the NPNG, leave it as is.

thanks... 60 ohm in middle, and say 150 - 10k on pin 2-3, kind a like a pad, without gain reduction ?


Re: premap front for 60 ohm mic
« Reply #5 on: August 07, 2019, 08:34:13 PM »
thanks... 60 ohm in middle, and say 150 - 10k on pin 2-3, kind a like a pad, without gain reduction ?
j

No not like that.  The mics are fine with the load of the pre amps, but the transformers will probably be designed around a 150 ohm source for best frequency response.  So, two resistors of, say, 49r from each leg of the mic signal into the transformer.  Not a pad, just a build out of impedance.
Having said that, if you really are recording sources upwards of 20kHz then you may have other issues with the response of the transformers but...
I believe there's an app note on Jensen's site showing what you need to do if you're still not sure.
Congrats on the B&K's 👌

kambo

Re: premap front for 60 ohm mic
« Reply #6 on: August 07, 2019, 08:43:14 PM »
j

No not like that.  The mics are fine with the load of the pre amps, but the transformers will probably be designed around a 150 ohm source for best frequency response.  So, two resistors of, say, 49r from each leg of the mic signal into the transformer.  Not a pad, just a build out of impedance.
Having said that, if you really are recording sources upwards of 20kHz then you may have other issues with the response of the transformers but...
I believe there's an app note on Jensen's site showing what you need to do if you're still not sure.
Congrats on the B&K's 👌

ok got it, thank you... 

eventually i wont be using any transformer coupled micpre, as recommended by this producer dude...
new B&K's have their own preamps with PSU and conditioner (what ever that is) and filters to cut below 20Khz and more!
will c

thanks all!



JohnRoberts

Re: premap front for 60 ohm mic
« Reply #7 on: August 08, 2019, 12:36:19 PM »
You may not need to do anything.

FWIW back before wireless mics became inexpensive and practical for movie location recording they used something like 50 ohm mics so they could drive thousands of feet of mic cable without audible HF loss (due to accumulated cable capacitance). 

The solid state mic preamp will likely be less bothered... Adding series resistance to the mic output could add noise.

You don't need much of a step up transformer to get in the ballpark for typical mics (1.4x or so turns ratio).

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

CJ

Re: premap front for 60 ohm mic
« Reply #8 on: August 08, 2019, 07:12:03 PM »
replace input xfmr with one that has taps,

If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

kambo

Re: premap front for 60 ohm mic
« Reply #9 on: August 08, 2019, 07:50:21 PM »
replace input xfmr with one that has taps,

sowter 9045, lundahl 1538xl



kambo

motorboat : Re: premap front for 60 ohm mic
« Reply #10 on: August 09, 2019, 11:07:17 PM »
so, the B&K mic is starting to motorboat after a min or so!
tried couple of different pres! same!
its a 30 years old mic and PSU... some caps must be goon bad i guess! but
i have no idea if it would be in PSU or the capsule preamp... damn!


JohnRoberts

Re: motorboat : Re: premap front for 60 ohm mic
« Reply #11 on: August 10, 2019, 01:14:01 PM »
so, the B&K mic is starting to motorboat after a min or so!
tried couple of different pres! same!
its a 30 years old mic and PSU... some caps must be goon bad i guess! but
i have no idea if it would be in PSU or the capsule preamp... damn!
Motor boating is generally a very LF oscillation involving some feedback loop (often through PS).

Check all power and ground connections for possible marginal continuity.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

kambo

Re: motorboat : Re: premap front for 60 ohm mic
« Reply #12 on: August 10, 2019, 03:10:28 PM »
Motor boating is generally a very LF oscillation involving some feedback loop (often through PS).

Check all power and ground connections for possible marginal continuity.

JR

it starts with a click/pop sound! then poporprporporporpro,,,,
when i start scoping/measuring around, its going away, i think i am discharging a cap, while i am trying to measure...
then its coming back 2-3 min later...
so, its going back !

Re: premap front for 60 ohm mic
« Reply #13 on: August 10, 2019, 04:52:40 PM »
Which model of B&K did you get ?

Id try placing the mic/capsule somewhere warm for a day or two  , it may have some moisture in it .

kambo

Re: premap front for 60 ohm mic
« Reply #14 on: August 10, 2019, 05:06:51 PM »
Which model of B&K did you get ?

Id try placing the mic/capsule somewhere warm for a day or two  , it may have some moisture in it .

2619 pre, 2804 PSU,
temporary 4134 capsule, until i get 4135-8... 

Re: premap front for 60 ohm mic
« Reply #15 on: August 10, 2019, 07:55:06 PM »
Thats basically the transistorised version of the earlier cathode follower mic ,
Single ended bnc connectivity , battery powered supply ,
a high grade 1:1 600 ohm ,unbalanced to balanced config might do you into your recorder.

kambo

Re: premap front for 60 ohm mic
« Reply #16 on: August 10, 2019, 08:24:37 PM »
Thats basically the transistorised version of the earlier cathode follower mic ,
Single ended bnc connectivity , battery powered supply ,
a high grade 1:1 600 ohm ,unbalanced to balanced config might do you into your recorder.

i am confused now... 
i thought these mics has 60ohm output, and i kind a sort a need to match it...
shall i connect the mic out  to 1:1 600 ohm  trx first then to a mic pre  ???

Re: premap front for 60 ohm mic
« Reply #17 on: August 10, 2019, 08:37:27 PM »
Yes , single core screened to the transformer ,then balanced out from there to your pre-amp.

kambo

Re: premap front for 60 ohm mic
« Reply #18 on: August 10, 2019, 09:44:34 PM »
will sure try this! thank you

kambo

Re: premap front for 60 ohm mic
« Reply #19 on: August 20, 2019, 10:42:59 PM »
Which model of B&K did you get ?

Id try placing the mic/capsule somewhere warm for a day or two  , it may have some moisture in it .

update:
looks like this worked...
wired a Jensen 11PT in front... that seems to be working too :)


 

Related Topics

  Subject / Started by Replies Last post
1 Replies
1160 Views
Last post April 02, 2005, 04:57:08 PM
by Gus
27 Replies
14527 Views
Last post March 17, 2006, 01:30:26 PM
by toby
5 Replies
1809 Views
Last post April 21, 2010, 08:49:31 PM
by cletus monroe
21 Replies
7744 Views
Last post March 04, 2012, 11:38:44 AM
by zayance