MXL 9000 hum & mod

I've just bought MXL 9000.Searched forum and found some upgrading ideas.But my MXL 9000 has hum.First of all i have to get rid of hum.

Here is schema

http://i17.photobucket.com/albums/b70/mnatera/MXL9000.jpg

I removed R20-R21-C20-C21-L1-L2.I will change C6 to 1 uf polyand R7 to 2k2.But how to get rid of hum?It has stock tube.Maybe tube causes this issue?Like hearing air conditioner...

And which tubes can i use with this mic?

Thanks.

Here is update!

Aaaaand why exactly would you want to remove RFI-filtering components?

Although back to the topic at hand - do you have an oscilloscope? That might help in pointing out at least the area from which said hum stems.

I don't suppose the mic's power supply has a ground-lift switch for the XLR output, does it?

Khron said:

Aaaaand why exactly would you want to remove RFI-filtering components?

Although back to the topic at hand - do you have an oscilloscope? That might help in pointing out at least the area from which said hum stems.

I don't suppose the mic's power supply has a ground-lift switch for the XLR output, does it?

Click to expand...

Cuz i saw everyone made like this.Actually after removing them,i hear some mid-range.It's better now.Even i recorded acoustic guitar after removing them and get good result.

I have no oscilloscope...

By the way should i change C6-33uf to 1 uf or leave it free?Which one is better?

I'm gonna use the cliche parenting example - if everyone jumped off a cliff, would you do that too, just because "everyone did it"?

I can only wonder how much of the perceived difference is down to confirmation bias, though... ("I modded it, so i MUST hear a difference")

Hum's, more often than not, the result of some grounding issue (either casing components not making contact where they should, or ground loops or whatnot).

Khron said:

I'm gonna use the cliche parenting example - if everyone jumped off a cliff, would you do that too, just because "everyone did it"?

I can only wonder how much of the perceived difference is down to confirmation bias, though... ("I modded it, so i MUST hear a difference")

Hum's, more often than not, the result of some grounding issue (either casing components not making contact where they should, or ground loops or whatnot).

Click to expand...

I'm not tube & mic guy.I have to follow masters.

I can't see any contact.But tube and C3 is really hot.I guess it's normal.

Tube really hot... Got a multimeter, at least?

And just how close (physically) is C30 to the tube and/or R30/R31?

Khron said:

Tube really hot... Got a multimeter, at least?

And just how close (physically) is C30 to the tube and/or R30/R31?

Click to expand...

All components close to tube.

Here is photo

http://cdn.recordinghacks.com/images//mic_extras/mxl/9000-circuit.jpg

Changed R7 to 2k2 and increased the level!

I'm starting to wonder if perhaps C30 might've dried out or something...

Khron said:

I'm starting to wonder if perhaps C30 might've dried out or something...

Click to expand...

Should i change or remove it to improve low end response?

Improvement is my second goal actually.First i have to get rid of noise.I guess firstly i have to change tube.

Should i go with 12AT7 or change to equavilent tube?

What about Genalex gold lion 12ax7?

In case you haven't noticed, C30 is the filter cap for the fillament supply.

Khron said:

In case you haven't noticed, C30 is the filter cap for the fillament supply.

Click to expand...

Yes it's filter cap but i saw that effecting to low end in bad way.And i can hear there is useless low end with this mic.So boomy.

Is it 50/60hz or 100/120hz?
Does it go away when you turn off the PSU and before the mic loses power?

I'd check that heater filter cap C30 as well as need info about PSU schematic since that might be where the noise is coming from.

The cathode bypass cap sets low-end roll off point for one way to  cut low end via NFB. Make it a lower C value and test (like 4.7u). This also introduces phase shift around the corner frequency.

Cheers!
-jb

You never answered whether the power supply has a ground lift switch or not.

Is it 50/60hz or 100/120hz?
Does it go away when you turn off the PSU and before the mic loses power?

I'd check that heater filter cap C30 as well as need info about PSU schematic since that might be where the noise is coming from.

The cathode bypass cap sets low-end roll off point for one way to  cut low end via NFB. Make it a lower C value and test (like 4.7u). This also introduces phase shift around the corner frequency.

Cheers!
-jb

Click to expand...

Hmm...

Actually not like 50 Hz.Not deep as it.Maybe 100 Hz.I need to check it.

I will check which Hz and your other idea.Checking mic after closing PSU.

You never answered whether the power supply has a ground lift switch or not.

Click to expand...

Sorry.There is no ground lift switch.

You've got an audio interface and a DAW, right? There must be some free spectrum analyzer plugins out there, so you can see precisely "where" the hum is

Khron said:

You've got an audio interface and a DAW, right? There must be some free spectrum analyzer plugins out there, so you can see precisely "where" the hum is

Click to expand...

Yes,i have but i didn't check.Actually didn't think to do that.  :

Tomorrow i will.

By the way i'll change tube with quality Siemens 12AT7.Do you think that i can get any improvement?

If it's 100hz then it's probably a failed filter cap in the PSU on the B+ line. Look there and test the caps or just replace them (rate the voltage higher and next uF value up for your replacements). There's probably only a couple in there. Check that the size will fit or you can make them fit. That is expecting that they were under-rated in the first place and failed prematurely as a result.

100hz because the full wave rectifier creates double the line frequency noise which is then filtered and smoothed via the large caps in the psu.

The noise on the heater supply in this self bias circuit is less critical because it's a not a fixed bias type where the cathode operating point is derived - therefore no direct audio path.

And that B+ line is also connected directly to the output path / transformer.

If it's 50hz then is probably a grounding issue somewhere.

You can insert an EQ and make a tight notch boost cut at 50/100 and see which one affects the audio noise - if you're not sure.

Swapping another tube is also an easy test to see if anything changes so why not.

Hope that helps and good luck!

Cheers,
jb

Was it humming before you 'improved' it?