GroupDIY

General Discussions => The Lab => Topic started by: sethinspain on October 13, 2013, 03:34:04 AM

Title: Apex 460 Noise
Post by: sethinspain on October 13, 2013, 03:34:04 AM
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:
(http://www.deeringamps.com/workbench/images/apex460_mic_lg.jpg)

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:
(http://www.deeringamps.com/workbench/images/apex460_ps_lg.jpg)
Any ideas how I can take care of this?

Cheers,
Seth
Title: Re: Apex 460 Noise
Post by: BernaVM on August 31, 2017, 05:15:51 PM
I have the exact same problem with the same setup. Have you found a solution yet?
Title: Re: Apex 460 Noise
Post by: enginefire on August 31, 2017, 07:41:17 PM
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.
Title: Re: Apex 460 Noise
Post by: BernaVM on September 01, 2017, 07:40:56 AM
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.
Title: Re: Apex 460 Noise
Post by: Walrus on September 01, 2017, 10:12:37 AM
The PSU shows 12.6v out for heater, but the valve is shown wired for 6.3v heaters....!
Title: Re: Apex 460 Noise
Post by: BernaVM on September 01, 2017, 10:34:51 AM
how do you wire the valve for 6.3v?
Title: Re: Apex 460 Noise
Post by: pucho812 on September 01, 2017, 10:48:26 AM
(http://valvewizard.co.uk/heater1.jpg)
Title: Re: Apex 460 Noise
Post by: enginefire on September 01, 2017, 10:57:07 AM
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!
Title: Re: Apex 460 Noise
Post by: bernatvm on September 01, 2017, 10:58:56 AM
Just checked, schematic is as its posted above, but my PSU is feeding 6.3V on the heater line.
Title: Re: Apex 460 Noise
Post by: bernatvm on September 01, 2017, 01:27:13 PM
Here's an audio of my problem:
https://drive.google.com/open?id=0B5_3AsW9KPbLanFJYXhPdXFLQnM (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
Title: Re: Apex 460 Noise
Post by: pucho812 on September 02, 2017, 03:47:26 PM
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.
Title: Re: Apex 460 Noise
Post by: bernatvm on September 02, 2017, 03:51:21 PM
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.
Title: Re: Apex 460 Noise
Post by: bernatvm on September 03, 2017, 06:52:08 AM
An example from a vocal I tried to record:

https://drive.google.com/open?id=0B5_3AsW9KPbLVHNiZzFCMjZCZDg (https://drive.google.com/open?id=0B5_3AsW9KPbLVHNiZzFCMjZCZDg)
Title: Re: Apex 460 Noise
Post by: enginefire on September 03, 2017, 09:08:50 AM
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.
Title: Re: Apex 460 Noise
Post by: bernatvm on September 03, 2017, 10:19:29 AM
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.
Title: Re: Apex 460 Noise
Post by: enginefire on September 03, 2017, 10:32:39 AM
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.
Title: Re: Apex 460 Noise
Post by: bernatvm on December 27, 2018, 11:12:59 AM
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?
Title: Re: Apex 460 Noise
Post by: radardoug on December 27, 2018, 03:53:10 PM
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.
Title: Re: Apex 460 Noise
Post by: bernatvm on December 28, 2018, 05:55:02 AM
What is a normal voltage to feed into a tube amplifier circuit inside a mic?
Title: Re: Apex 460 Noise
Post by: radardoug on December 28, 2018, 03:26:23 PM
Start at 100mV and work your way up or down until the output clips.
Title: Re: Apex 460 Noise
Post by: squarewave on December 28, 2018, 08:27:18 PM
It could be tricky to try and feed signal in directly because the input is such high impedance. Meaning you cannot actually connect a probe to the grid without affecting the impedance.

Place the mic 1 meter away from a monitor speaker and play a pure tone into it. Try and do it in an isolated / quiet place (make a couch cushion "isolation chamber" maybe). Raise the monitor output until the mic output starts to clip (use lower mic pre settings to make sure it's not the mic pre clipping). Look at the fft / spectrum in your DAW and raise the monitor level until you see the 2nd and 3rd harmonics jump. That's the clipping point. Then increase mic pre gain until you get full-scale digital on the ADC. Then, without touching anything else, turn off the tone and record / listen to the silence critically and compare to decide if you really have a problem.

If you still think you have a problem, you will need to characterize the noise more descriptively (hiss, whistle, hum, etc). Post a screenshot of the spectrum / FFT (use the highest sample rate + averaging if your DAW supports specifying that sort of thing so that the resolution is better).
Title: Re: Apex 460 Noise
Post by: radardoug on December 29, 2018, 12:45:30 AM
Its not a problem to feed a signal into the grid, however the generator will have low output impedance. Put a .1 uf cap in series with the hot lead from the generator, and this will keep the d.c. conditions on the grid the same.  Note though that the generator will load this point from a noise perspective, and so noise figures would be misleading with the generator connected but turned down.
If you use a 1M grid resistor, and the unit is quiet, but when you go back to the 200M resistor its noisy, then it could be the 200M resistor, or just the difficult high impedance conditions the tube is working in.
Title: Re: Apex 460 Noise
Post by: bernatvm on December 29, 2018, 02:38:46 PM
Thank you all for your suggestions. I don't have any signal generator other than my small audio interface at home, nor I have any monitors. Next week I'll go to my college electronics lab and do all the tests, electronic and acoustic. Will pots all screenshoots here so you can help me.

I think the problem I have right now is solid transmision of low frequency (i have 20db of <20Hz over my noise floor). Only way to prove that is by injecting some tone to the circuit, as the solid transmision should not be there if there's no capsule to pick it up.
I suspect that because when I hold the mic with my hand this 20db become 40 ??? . I also have a mesurement mic, so I will do a bi-channel freq and phase analysis too.

Title: Re: Apex 460 Noise
Post by: squarewave on December 29, 2018, 05:05:04 PM
I suspect that because when I hold the mic with my hand this 20db become 40 ???
Sounds like a grounding issue. Check resistance between pin 1 of the far end of the cable and mic body and basket. Be aware that it can be difficult to find a good connection on anodized or smooth plating surfaces so you might need to fidget with the probes a little until you get a properly low reading. The resistance should be 0 of course (or whatever your meter displays when the probe tips are shorted together).
Title: Re: Apex 460 Noise
Post by: bernatvm on December 30, 2018, 08:59:17 AM
How are you supposed to deal with the heater ground or "return" and the ground of the b+/signal? Where should they meet. I didn't mod any of this but maybe stock mic was wrong wired in the first place?
Title: Re: Apex 460 Noise
Post by: squarewave on December 30, 2018, 02:40:50 PM
The rule is that currents should run anti-parallel so that magnetic fields created by AC tend to cancel. Meaning the heater ground wire should return right next to the supply wire. If the cable actually has separate wires for pins 4 and 7, you could try using them to separate the two grounds. Meaning separate 4 and 7 and connect 4 (and 3?) to pin 9 of the tube. But then in the power supply circuit you will need to add a largish resistor between the two grounds just to hold them at the same DC potential (elevating the heater supply can also significantly reduce noise).

However, I don't think any of that is the problem because a) your heater supply is already pretty seriously filtered DC so it should be quiet and b) the circuit doesn't have enough gain to amplify the heater noise that much (compared to a guitar amp for example). Unless you find a heater wire running close to the grid wire, I think your problem is elsewhere. Note that tube stuff is inherently a little noisier then something like a JFET circuit.

If you're just listening to silence at full volume, you will hear some hum. Make a recording of 10 seconds of vocalizing or guitar or whatever followed by 10 seconds of silence. Put a solid signal into the mic and adjust the mic pre level so that you're recording at +4dBu or whatever you normally do. Then listen to the recording with 10 seconds of sound and then 10 seconds of silence. Can you still hear the hum during the 10 seconds of "silence"?
Title: Re: Apex 460 Noise
Post by: bernatvm on December 31, 2018, 04:49:40 AM
I measured the headgrill to the far end on the cable. I get 2.6ohm on pin 4 and 1.7 ohm on pin 7. While checking your explanation i realized the psu schematic on first post is wrong. Here's the actual psu
Title: Re: Apex 460 Noise
Post by: squarewave on December 31, 2018, 10:35:24 AM
I measured the headgrill to the far end on the cable. I get 2.6ohm on pin 4 and 1.7 ohm on pin 7.
The fact that you get different readings even through the two pins are connected together at the other end means that your probes are simply not making good contact. Keep trying ...
Title: Re: Apex 460 Noise
Post by: bernatvm on January 03, 2019, 01:20:15 PM
I went to the lab earlier today and took at the voltages where the cable join the mic pcb with the oscilloscope. Didn't have time for any of the other tests. Here are some pics of the readings, I think there are no problems.

Heater
(https://i.imgur.com/1yKbU7q.jpg)

B+
(https://i.imgur.com/nMoO75J.jpg)


I also attach an FFT of the noise. Snapshot is taken in a silence moment on a speech track. The gain is what I would usually use in that situation.
(https://i.imgur.com/exHta7x.png)
Title: Re: Apex 460 Noise
Post by: bernatvm on January 07, 2019, 11:48:30 AM
The fact that you get different readings even through the two pins are connected together at the other end means that your probes are simply not making good contact. Keep trying ...

I keep trying and getting similar results. I'm gonna recheck all the wiring between the 7pin and the pcb in the mic. Also will try with a new cable, stock is bad quality anyway...
Title: Re: Apex 460 Noise
Post by: squarewave on January 07, 2019, 12:56:20 PM
I keep trying and getting similar results. I'm gonna recheck all the wiring between the 7pin and the pcb in the mic. Also will try with a new cable, stock is bad quality anyway...
Measure without the cable between pin 1 on the mic and the basket. Measure the cable separately. Divide and conquer. What does your meter read with the probes directly shorted? Mine reads 0.3 ohms which seems to be the limit of my DMM. But I have nice probes. If your probes are corroded or dirty, you might have to clean them to get good readings. If you really are getting 2-3 ohms between the basket and earth ground at the wall, that's not good. That could be a source of noise. Try grounding the basket separately using a jumper to something that you know has a solid ground. Is the noise reduced / eliminated?
Title: Re: Apex 460 Noise
Post by: bernatvm on January 07, 2019, 02:35:21 PM
Measure without the cable between pin 1 on the mic and the basket. Measure the cable separately. Divide and conquer. What does your meter read with the probes directly shorted? Mine reads 0.3 ohms which seems to be the limit of my DMM. But I have nice probes. If your probes are corroded or dirty, you might have to clean them to get good readings. If you really are getting 2-3 ohms between the basket and earth ground at the wall, that's not good. That could be a source of noise. Try grounding the basket separately using a jumper to something that you know has a solid ground. Is the noise reduced / eliminated?

My meter is reading 0.5 ohm with probes shorted. Cable mesurements gives me 2.4 - 0.5 = 1.9 ohm on pin 4 and 1.3 - 0.5 = 0.8 ohm on pin 7. From both pin 4 and 7 on the mic conector to the grill I get 0~0.1 ohm unstable reading.

I did a test today and unsoldered the capsule. I still get the same exact noise profile, so grounding should be the issue. It seems that the 7 pin cable is not ok...
Title: Re: Apex 460 Noise
Post by: squarewave on January 07, 2019, 03:29:06 PM
Sounds like a wonky cable. Are the cable pin / wire connections soldered? If yes, try re-wetting with a little bit of new solder. If not, and you have the right tool for removing the pins from the housing, you could try removing them and re-crimping the pins.
Title: Re: Apex 460 Noise
Post by: bernatvm on February 28, 2019, 07:05:57 PM
Recived the new cable today and still same isues. It looks like the microphone is amplifying the lows and sub a lot more than the rest of the frequencies and the noise gets a lot louder on those...

I'm a bit lost, don't know what more to do.
Title: Re: Apex 460 Noise
Post by: radardoug on February 28, 2019, 11:46:12 PM
Where is your studio located? Are you in a high traffic area? That spectrum curve looks typical for a noisy environment.
Thats what the high pass filter is for, to clean off all the low frequency crud you dont need.
Title: Re: Apex 460 Noise
Post by: bernatvm on March 01, 2019, 02:40:57 AM
I doubt it's ambient noise what the mic is picking, I'm prety sure it's its self noise
Title: Re: Apex 460 Noise
Post by: Rocinante on March 01, 2019, 03:20:03 AM
I worked out of studio that had 'dirty' electricity. There were nice surge protectors to protect and clean up some of the electrity but there were a few sockets that always created noise. Tube mics were effected the Most.   When the mic was plugged into a clean socket it was great but when using one of the 'dirty ' sockets it was a hum machine.
Title: Re: Apex 460 Noise
Post by: bernatvm on March 04, 2019, 02:00:18 PM
I've checked the mic in different places (different plug, different rooms and different towns) and I have the same problems everywhere
Title: Re: Apex 460 Noise
Post by: radardoug on March 04, 2019, 02:20:28 PM
Does the 0 dB on your spectrum display indicate true maximum signal level? Then the LF noise is 50 dB down. Higher frequencies are 90 dB down, which is probably as good as it will get. Are you sure its not power supply noise or bad shielding?
Title: Re: Apex 460 Noise
Post by: bernatvm on March 04, 2019, 04:10:27 PM
I checked psu with oscilloscope and seemed right. About shielding, what could I do to check the shielding?
Title: Re: Apex 460 Noise
Post by: radardoug on March 04, 2019, 06:33:00 PM
So how much ripple did you see on the scope?
Title: Re: Apex 460 Noise
Post by: bernatvm on March 04, 2019, 07:00:02 PM
I think this is normal ripple for a good PSU

I went to the lab earlier today and took at the voltages where the cable join the mic pcb with the oscilloscope. Didn't have time for any of the other tests. Here are some pics of the readings, I think there are no problems.

Heater
(https://i.imgur.com/1yKbU7q.jpg)

B+
(https://i.imgur.com/nMoO75J.jpg)


I also attach an FFT of the noise. Snapshot is taken in a silence moment on a speech track. The gain is what I would usually use in that situation.
(https://i.imgur.com/exHta7x.png)
Title: Re: Apex 460 Noise
Post by: radardoug on March 04, 2019, 09:59:17 PM
You shouldn't be able to see any ripple.
Title: Re: Apex 460 Noise
Post by: bernatvm on March 05, 2019, 02:28:00 AM
Even at that scale? Those pics are taken at 10mV/division
Title: Re: Apex 460 Noise
Post by: bernatvm on March 05, 2019, 03:01:30 AM
Shorted pin 2 of the to ground and the noise is gone. That makes me think the problem is between the capsule and the tube. Do you agree? What could it be?
Title: Re: Apex 460 Noise
Post by: JohnRoberts on March 05, 2019, 10:33:41 AM
Scope measurements are relative to the ground lead, so check where that is connected?

JR
Title: Re: Apex 460 Noise
Post by: bernatvm on March 05, 2019, 10:42:47 AM
Scope measurements were taken referenced to the chasis of the microphone
Title: Re: Apex 460 Noise
Post by: radardoug on March 05, 2019, 02:55:37 PM
Shorted pin 2 of the to ground and the noise is gone. That makes me think the problem is between the capsule and the tube. Do you agree? What could it be?
Pin 2 of what? the microphone cable or the output cable?
If pin 2 is a ground, then your problem is fixed.
Title: Re: Apex 460 Noise
Post by: bernatvm on March 05, 2019, 02:59:07 PM
Pin 2 of the tube, sorry. It's signal input from the capsule to the circuit
Title: Re: Apex 460 Noise
Post by: radardoug on March 05, 2019, 09:27:25 PM
Yes, well that will certainly stop the noise. Try disconnecting the capsule and everything but the high value grid resistor. See if it is noisy. Then substitute a 1 meg resistor for the high value resistor. See if it is noisy.
Title: Re: Apex 460 Noise
Post by: Mylithra on March 05, 2019, 11:05:22 PM
You're sure youre not working with a microphonic tube?
Out of curiosity, why'd you pulled the 2 filter caps in the 12v PS?
Title: Re: Apex 460 Noise
Post by: bernatvm on March 06, 2019, 02:06:27 AM
You're sure youre not working with a microphonic tube?
Out of curiosity, why'd you pulled the 2 filter caps in the 12v PS?

I tried three different tubes all with same results, tube might not be the problem.

About the caps, which ones are you referring? I don't think I pulled any of them in the 6.3V (actually a typo in the psu schematic)
Title: Re: Apex 460 Noise
Post by: davemascera on March 06, 2019, 10:54:36 AM
I tried three different tubes all with same results, tube might not be the problem.

About the caps, which ones are you referring? I don't think I pulled any of them in the 6.3V (actually a typo in the psu schematic)

At the beginning of this thread doug said the ratio of bad tubes to good might be 100:1.

He also said earlier on to sub out the 200M with 1M and see if the noise improves and if it does, it may very well just be that the tube isn't up to the task.
Title: Re: Apex 460 Noise
Post by: Mylithra on March 06, 2019, 12:55:54 PM
About the caps, which ones are you referring? I don't think I pulled any of them in the 6.3V (actually a typo in the psu schematic)

In the first post I saw this:
I removed C9 and C10 which act as RF filters.

In the PSU ,these are the larger filter caps in the DC heater supply. Are these not the caps you're referring to?

Title: Re: Apex 460 Noise
Post by: bernatvm on March 06, 2019, 01:00:36 PM
Quote
In the PSU ,these are the larger filter caps in the DC heater supply. Are these not the caps you're referring to?

I was reffering to C9 and C10 on the microphone, filters on the transformer output.
Title: Re: Apex 460 Noise
Post by: bernatvm on March 07, 2019, 05:14:36 AM
At the beginning of this thread doug said the ratio of bad tubes to good might be 100:1.

He also said earlier on to sub out the 200M with 1M and see if the noise improves and if it does, it may very well just be that the tube isn't up to the task.

I tried the 1M resistor. What I got was the expected hpf on the signal, but the noise was still the same and went down to the sub area at a very similar level than before.
Title: Re: Apex 460 Noise
Post by: davemascera on March 07, 2019, 02:08:51 PM
Did you ever resolve the pin 7 and 4 grounding problem? As in make sure that they're at the low measurable limit of your ohmmeter with the new cable?

Have you measured the resistance of the head basket to other parts of the mic body itself? Could it be possible that the headbasket or mic body isn't quite as conductive as it should be? Try wrapping the whole thing in tin foil lol.

Try measuring a wire and making sure it reads that same as your probes shorted, attach it to PSU ground and touch the other side straight to the mic. You might get some sort of hum obviously but like it's worth it to see if the noise goes away.

Just some ideas.

EDIT: Also, why not try throwing those RF filter caps back on? Maybe something is heterodyning?