Apex 460 Noise

[sethinspain]

Hey guys,

I finally got my hands on an Apex 460 circuit and have done the typical mods out there.  Its got a low pass and pad switch but fixed cardioid.  Other than that, it's spot on the Alctron tube mic.

Here's the schematic i'll be rerencing:

I am hearing a noise and I am not sure this is the typical noise a mic makes.  The self noise is a high 20dB, but I thought self noise is more like a hiss, or white noise type sound.  What I am hearing almost like a PSU type noise that is in the lower freq range. 

It is not a strong hum from maybe the body being improperly shielded or the psu tranny being too close to the circuit.  It's more like a light 100-400hz tone that almost sounds like machinery noise.  I have tried swapping tubes and the noise changed minimally, if at all.

I don't know if anyone has this mic and can confirm that this is normal or not.  It is definitely audible, and for my taste too much so to record.

I removed C9 and C10 which act as RF filters.  This sound doesnt strike me as RF which is high pitched.  Maybe I'm wrong...

I removed C8 (output coupling cap) and changed it to a film cap and I wired the trafo directly to the cap.  This actually seemed to help the noise a little bit.  I thought of possibly jumping the trace from R8 and wiring it directly to the cathode of the tube socket.

I also removed C7, and put in a better capsule.

Now the PSU is still stock but I just lowered the output voltage by changing the zeners.  I have read zeners can put noise in a circuit but I have one that uses them and it's noise free practically.  I could put a by-pass cap (tantalum) as I heard that can help.  Or I could also install an additional cap to filter more, maybe a 470uf 250V - I don't have a ton of space.

I realize also that the voltage may not even reach 120V which is the zener voltage rating.  I could lower it to be 112V which would for sure clamp down the voltage and possibly eat the remaining ripple, but at the expnse of loosing some volts.

PSU Schematic:

Any ideas how I can take care of this?

Cheers,
Seth

 

[BernaVM]

I have the exact same problem with the same setup. Have you found a solution yet?

 

[enginefire]

Are you sure it has a pad and low pass?  Apex 460 is usually a multi pattern mic without those options.

Did you buy an upgrade kit or just follow a summary of a bunch of different posts?  Did you remove the cathode follower (to make it like the original c12)?

My instinct says noisy tube, which tubes are you using?  I'd try an EH 6072a.  The GEs everyone raves about are kinda pricy for what it is imho.

Maybe work backwards?  Replace the original components one by one with tests in between to find out where things went sideways?  Any chance you lifted a trace along the way?  You could also test for ground continuity, I had troubles modding an xml mic once because I replaced the mesh.  Lots of noise or hum iirc.  Grounding the new mesh screen to the chassis with a bit of solder solved it.

 

[BernaVM]

Sorry, didn't read that part, mine hasn't got pad and low pass. I did bought not a kit, but all my parts on Advanced Audio and I haven't removed the cathode follower stage.

I'm already using a EH 6072A tube. Just checked the grounding on my mesh (i removed inner layer) but seems okay.

I'm experiencing some noise, plus my mic has some changes on sensitivity and tone. During takes, overall volume is reduced, top end to (get a duller tone) but noise is consistent on same level.

 

[Walrus]

The PSU shows 12.6v out for heater, but the valve is shown wired for 6.3v heaters....!

 

[BernaVM]

how do you wire the valve for 6.3v?

 

[pucho812]

 

[enginefire]

I think I read somewhere that the heater voltage on the schematic was a typo... Obviously break out your DMM and check what it actually is to be sure!

 

[bernatvm]

Just checked, schematic is as its posted above, but my PSU is feeding 6.3V on the heater line.

 

[bernatvm]

Here's an audio of my problem:
https://drive.google.com/open?id=0B5_3AsW9KPbLanFJYXhPdXFLQnM

There's the loud noise, then at minute 1:00 it gets quieter, but rises again at the end of the clip.
Excuse me for the TV on the background

 

[pucho812]

maybe it was the fact I was listening on my macbook pro speakers but I did not hear any noise but I also did not hear any t.v.  so I have to ask is the file correct? Probably something related to my set up but you never know.

 

[bernatvm]

Well, the audio its not normalized. I'm not hearing a super heavy noise, more of a too high noise floor to use the mic on a recording.

 

[bernatvm]

An example from a vocal I tried to record:

https://drive.google.com/open?id=0B5_3AsW9KPbLVHNiZzFCMjZCZDg

 

[enginefire]

I have some questions

What is your room like?  Is it an iso booth or just a room in a house?  Does it have a window to outside etc.?

For the 1st recording with no singing, were you walking around or touching the mic stand or anything?  I hear lots of low frequency rumbles that sound like footsteps thru a mic stand.  Are you using a shock mount?

Admittedly I didn't listen to the entire recording of (not so) silence, I just jumped around a bit.

 

[bernatvm]

The only noise one is at home, lots of noise from tv going on background, footsteps, chair noise, etc. Plus the mic was standing in a table because I didn't have a stand at home.

The one with the guy singing is on an booth. On that all the noise is from the mic itself.

Just ignore the audio without the singing. I just realized its nit a good audio to listen the problem, as you don't have any reference of how high the gain is.

 

[enginefire]

In terms of noise I don't really hear it on either.  The 1st one I heard background noise the second one I didn't really hear the noise you are talking about.

Try recording some vocals with a long period of silence so give reference to levels plus ample time to hear the noise.

 

[bernatvm]

I finally solved the noise problem. While screwing the transformer enclosure on the mic body i damaged b+ cable a bit, so it was making contact to ground every now and then ( I know, very dangerous, first build pls don't take that into account).

Now the noise floor is somehow correct (higher than other microphones I have). The problem is that the microphone is generating a VERY serious noise on the sub area, 20 Hz and below. By very i mean 35db or so over the noise in other frequencies.

I know, and I've done it, it can be solved with a hpf at 20 Hz on every track I record, but I want to solve this. Some other posts said that replacing zeners with lower voltage ones to reduce b+ can result in noise, so I added another filter stage after those (capacitance multiplier stage), but problem is still there.

Any ideas what can be the isue here?

 

[radardoug]

Like most posters, you give no reference to what your noise level is. What you need to know is what is the signal to noise ratio between the highest sound the mike can handle and the noise floor. As to lf noise, welcome to the world of condenser microphone design. If it was so easy, everyone would do it! The schematic is interesting in the values it has, 1000M on capsule centre going into 200M on grid. Do some experiments where you disconnect the capsule and feed tone into the preamp, and get a set of figures for the preamp.
Try putting 1M on the grid, does everything improve markedly? High grid resistors put the tube on the bleeding edge, but are necessary for condenser tube operation. You may need to select tubes, and the more tubes you have the more you can select from. A ratio of 100 bad to one good could be normal.

 

[bernatvm]

What is a normal voltage to feed into a tube amplifier circuit inside a mic?

 

[radardoug]

Start at 100mV and work your way up or down until the output clips.