DiY AMI U47

[Melodeath00]

I would try small washers placed between the capsule mount and capsule, and locking washers for the standoff. Use as many as needed to secure the whole assembly.

-James-

Thank you for the suggestion. I will definitely try this!

The screws you are using are just to long, use some wire cutters and make them shorter!
The capsule moving like you describe is bad news simpley because of the very loud sound it creates
If you. Ant cut the screws a bit shorter buy some shorter ones.

Eric

Thanks for the reply! It definitely creates a loud sound when the capsule shifts around, but do you think that's what might be causing the clipping and popping/cutting out when I tilt the mic? I'll see if I have a friend who could help me cut the screws down.

 

[Moby]

I would try small washers placed between the capsule mount and capsule, and locking washers for the standoff. Use as many as needed to secure the whole assembly.

-James-

Thank you for the suggestion. I will definitely try this!

The screws you are using are just to long, use some wire cutters and make them shorter!
The capsule moving like you describe is bad news simpley because of the very loud sound it creates
If you. Ant cut the screws a bit shorter buy some shorter ones.

Eric

Thanks for the reply! It definitely creates a loud sound when the capsule shifts around, but do you think that's what might be causing the clipping and popping/cutting out when I tilt the mic? I'll see if I have a friend who could help me cut the screws down.

First do the washers trick (or shorten the screws as tskguy suggested)and if it still pops go with furher questions. It's impossible to gues what else can produce that sound but sounds like the mechanical problem.

 

[Melodeath00]

Cheers, Moby!

I'm headed to the hardware store soon.

In the meantime, as I stated, my U47 is reverse polarity from my SM57 and Manley. I want to fix this. I followed the AMI wiring diagram for the BV8 (http://www.tab-funkenwerk.com/id78.html) going to the correct XLR pins. Should I just rewire the XLR in the PSU, switching Pin 2 and 3, or should I switch the green and black wires from the transformer in the mic? Does it not matter?

 

[HellfireStudios]

Switching after the transformer is a lot like using a phase switch to flip polarity. I wouldn't think any harm could come from that.

-James-

 

[Melodeath00]

Switching after the transformer is a lot like using a phase switch to flip polarity. I wouldn't think any harm could come from that.

-James-

Good point. But would switching the transformer wires be problematic? I might actually be easier for me to do that. I can't see why it would be a problem, but my understanding of transformers is lacking. Maybe the AMI BV8 wiring diagram is incorrect/backwards?

What a frustrating evening I had last night.  In an effort to tighten the mount on the deck, and raise it by 3mm or so for proper U47 height, I bought some washers. They did not work for tightening. I think the screw hole in the wood dowel I used to modify the BeesNeez mount became weak/stripped, so it wasn't latching onto the screw properly, and I think it also remained loose because the bottom of the wood dowel is not perfectly flat. I got so fed up that I put a dab of superglue on the tip of the screw, pount the mount on, and now it's drying, but it seems to have worked in terms of wobbliness. The mount is much sturdier, but can still spin 360 degrees with moderate force. I am thinking a strip of hot glue down from the post of the mount onto the plastic deck would be enough to prevent it from rotating. Opinions?

In an effort to tighten the capsule to the saddle so it doesn't shift, I looked for some very small washers, but Lowe's didn't have them. Instead, they had big rubber washers, which I basically used as rubber sheets to modify, and I have pushed the screws through, and cut down to size. I think it will work. Essentially I fashioned my own very small rubber washers. The rubber will not have any affect on the 60V polarizing voltage going through the one screw that connects to the backplate, right?

Hopefully this will solve the mic-tilting audio cutout/clip problem. Perhaps the diaphragm was getting sucked into the backplate when it lost backplate-polarization voltage. I've heard about "diaphragm sucking," but I can't remember what causes it.

My findings on polar pattern: You may remember I posted that I noticed the U47 seems to actually reject better from the side than it does from the rear in cardioid mode. So I did a little research, and it turns out the K47 capsule is almost "supercardioid" as a result of it's design, so it has a lobe of pickup behind the capsule. The 180 degree off-axis frequency plot for the historic U47 actually looks very "scooped," which is indeed how my mic sounds from the rear side. Interesting.

I'm planning to buy a SPST switch from Radioshack, and hot glue it to the acrylic deck so that I can switch off the rear diaphragm when i take the headbasket off. Do you think hot glue will be strong enough to hold a small switch onto a piece of plastic?

Cheers, everyone!

 

[AusTex64]

< Do you think hot glue will be strong enough to hold a small switch onto a piece of plastic?>

I like clear E6000 adhesive for mic work. You can get it at Michael's, the art supply store. Oliver suggested it to me, it's great.

 

[Melodeath00]

< Do you think hot glue will be strong enough to hold a small switch onto a piece of plastic?>

I like clear E6000 adhesive for mic work. You can get it at Michael's, the art supply store. Oliver suggested it to me, it's great.

I will look into that. Thanks!

An Update...

The mount is stiff, and though it rotates 360 degrees if I force it, it doesnt rotate under normal use, so I decided the mount the capsule. The rubber washers worked perfectly for getting the K7 snug on the mount, as well.

I fired the mic up, and again, I am getting the huge clipping sound when I tilt or move the mic. So the problem was not a loose K7 before. I decided to switch tubes just to make sure that was not the problem. I install a second EF800, same problem.

I open the mic back up and notice the wire from the capsule to the tube grid has become loose. I re-solder it, put the mic back together. The tube circuit is working (I can hear noise, and taps when I tap the body), but absolutely no sound from the capsule. I open the mic several times, and check voltages. 5.6V heater, 57V polarizing voltage. Everything looks good. Eventually, I realize I accidentally soldered the wire from the capsule to the tube cathode on pin 3, and not the grid on pin 2. D'oh!

Ok, so now I have audio again, but still the clipping when I tilt. In fact, I can hear a slight clicking sound when I tilt, and I start to wonder if that clicking sound is the capsule diaphragm physically moving. I take the headbasket off, and it looks like the front diaphragm is "sucked in" to the backplate. I have no idea why this would be happening, but when I shut off power from the PSU, I saw the diaphragm physically "un-suck" itself. Now I am worried that the K7 is damaged.

I head out to get something to eat with friends. When I come back, I fire the mic up again, and try to measure the polarizing voltage to see if the backplate is somehow not getting 60V when the diaphragm sucks in. Well, this time, the diaphragm is not sucking in, and the audio sounds good/normal. What the heck? I have fully assembled the mic now, but I have no idea what the problem was, or why it's gone now.

I re-wired the PSU Binder connector audio wires, so now the U47 has the same polarity as my other mics. And the washers for the mount worked perfectly for raising the capsule - it sits at 45mm now, and it looks "correct," so I'm very happy. However, I am very worried about having seen the mic diaphragm sucked in. Does such a position damage the capsule permanently? Again, I don't know what caused it, or why it seems to be working normally now.

I decided to forgo the switch for true cardioid for the time being. I just want a working mic, and I wasn't sure how much current and voltage I need to handle for the switch, so I don't know what switch to purchase. Any ideas? Is 3A, 250V enough? I have no idea how many amps a capsule puts off.

I also noticed, even with the PSU off (no power going to tube, etc), if I am live monitoring the pre/channel the U47 is plugged into, I can hear static as I plug the binder cable into the mic. Why would this happen? Is the cable not properly grounded maybe?

The mic makes a tiny clicking sound when I tilt it around in my hand. I haven't been able to determine the source, which I am trying to do because I want to know if something is wrong with the K7.

Thanks for any insight! Though I am a n00b and have many ignorant questions, I hope the overall story is at least entertaining to read!

 

[Melodeath00]

Ok I spoke too soon.

The clipping is back. It seems to occur randomly, but I am able to "cause" it sometimes by tapping on the side of the mic. It occurs definitely in Cardioid, Omni, and Bi-Directional modes, and in fact, seems to sometimes occur just from switching polar patterns from the PSU as well. Does this help the diagnosis at all?

 

[HellfireStudios]

Try re-flowing all of your solder joints. It sounds like an intermittent solder joint or a broken PCB trace. You could try removing the body and looking for a physical change in the wiring (when you rotate it) that could indicate a break. Like I tell all my HVAC customers, if it's not acting up when I check it, then I probably won't find anything to fix... So be sure it's acting up before pulling it apart.

-James-

 

[Melodeath00]

Try re-flowing all of your solder joints. It sounds like an intermittent solder joint or a broken PCB trace. You could try removing the body and looking for a physical change in the wiring (when you rotate it) that could indicate a break. Like I tell all my HVAC customers, if it's not acting up when I check it, then I probably won't find anything to fix... So be sure it's acting up before pulling it apart.

-James-

Will do, tonight! I suspect something to do with the polarization voltages, since I did see a sucked in diaphragm at one point. Again, I'm a bit worried that that would cause damage to a capsule.

 

[0dbfs]

Does it pass the breath test?

Thx,
jb

 

[Melodeath00]

Does it pass the breath test?

Thx,
jb

Could you please elaborate what this is?

 

[0dbfs]

re: breath test

I googled and found this here:

http://www.neumann.com/forums/view.php?bn=neumann_mictec&key=1295618509&v=f

Put the microphone in a *very* quiet place, pull up the gain to maximum, and listen to the self noise of the microphone via headphones. It should be a regular "pink" noise [plus ambient noise, of course]. Then, breathe a few times at the capsule from very close, and listen again. If the self noise is now much louder, and sounds "irregular and rumbly" or "sputtering" or the signal "cuts out", it's possible that you'll need a new capsule [Mr. Kilim has the prices for Israel], and you should possibly think about re-negotiating the sales price.

Search around a little and you can find tons of info about the breath test and the what/why of what it's doing.

Basically, your testing the capsule for micro-holes and debris/contamination by introducing humidity in a controlled manner.

If you have another capsule (cheap doner or something?) you can also swap that in to confirm/deny/localise the fault to capsule or something else.

Thx,
jonathan

 

[Melodeath00]

Thanks 0dbfs. I will give that a try, and maybe re-install my tester K67 to see if I have any clipping problems.

One problem though: if I crank the gain so I can hear the mic noise, then if the mic DOES make the huge clipping sound, it's really going to stress my speakers and possibly my ears, no? Any way to avoid this? Maybe monitor through a software limiter?

 

[0dbfs]

You don't need to crank it that much (just normal). The discharge / wind / popping sounds are pretty loud compared to the typical audio when there is an issue.

It's an easy test to rule some things out and pops/wind sounds are certainly a related symptom.

I have also localized popping/wind sounds to remaining-solder-flux-contamination in the hi-z section and in one case swapping a styroflex cap in that area also fixed it. They are pretty sensitive to heat and can be damaged while soldering or cleaning w/alcohol.

Cheers,
jb

 

[Melodeath00]

Ok, I had some trouble recreating the problem today. I couldnt do anythign to consistently cause the huge clipping. Eventually, as I tilted the mic, I could start to hear what sounds like static inside the mic. I mean literally in the headbasket. SO I take the headbasket off, and the diaphragm is sucked in. The mic still makes sound, and it doesn't sound bad, but the diaphram is visually sucked in, and I couldn't get it go away until i turned the PSU off. I turned the PSU back on once the diaphragm was ok, and now I can't repeat any steps/actions to get sucked in again.

Breath test passed, and I wanted to recreate the problem before I switched capsules. Based on the fact that sometimes tilting the mic is causing some sort of static charge/discharge in the mic, I'm thinking a loose connection. However, if the diaphragm is getting sucked, it seems as thought he backplate is getting too much voltage, which would mean a loose connection around the voltage divider, and not int he high impedance section. Please correct me if I'm wrong. I'm back to opening th emic and seeing if I can spot anythign wrong.

 

[Melodeath00]

I opened the mic up to check my solder joints. While the mic was open, I swapped the tube out for the original tube I used, as I believe it was quieter.

All solder joints look ok, but I noticed one of the joints carrying the rear diaphragm remote polarity voltage was very close to the mic body, which would short it out. I sort of pushed it further in to prevent shorting. Since then, I haven't been able to recreate the clipping and diaphragm sucking. I don't know what the remote polarity has to do with the front diaphragm, but it would explain some quick shorting out of a lot of volts going to the rear diaphragm (except in Omni of course, hmmm).

Again, I don't know for certain that this was the cause of the issue, or that the connection was even touching the mic body. All I know is that's the only thing in the mic I touched besides changing the tube, and the mic is behaving now. I'm a bit scared the issue will pop up again, of course, so we will see. And I'm still curious/worried if temporary diaphragm sucking damages a capsule.

After some testing, I was indeed correct that the original EF800 I used has much less noise than the second. I also compared the frequency response of the U47 with the two different tubes just using my voice, and the 2nd EF800 has the mid-range presence boost I expect in the 3k to 7kHz area I expect for a U47, while the quieter tube is smoother in the mids. I need to do more thorough testing, and use pink noise instead of my voice to be sure that presence boost is there, but I found that very interesting. Unfortunately, that tube is unusable due to noise (it's at least 6dB louder noise). And changing the tube is not as easy as I hoped. I get worried I will accidentally knock a solder connection loose while I'm wiggling the tube around.

I will post vocal and acoustic guitar clips soon, comparing the U47 to my Manley Ref Card

 

[Pip]

I opened the mic up to check my solder joints. While the mic was open, I swapped the tube out for the original tube I used, as I believe it was quieter.

All solder joints look ok, but I noticed one of the joints carrying the rear diaphragm remote polarity voltage was very close to the mic body, which would short it out. I sort of pushed it further in to prevent shorting. Since then, I haven't been able to recreate the clipping and diaphragm sucking. I don't know what the remote polarity has to do with the front diaphragm, but it would explain some quick shorting out of a lot of volts going to the rear diaphragm (except in Omni of course, hmmm).

Again, I don't know for certain that this was the cause of the issue, or that the connection was even touching the mic body. All I know is that's the only thing in the mic I touched besides changing the tube, and the mic is behaving now. I'm a bit scared the issue will pop up again, of course, so we will see. And I'm still curious/worried if temporary diaphragm sucking damages a capsule.

After some testing, I was indeed correct that the original EF800 I used has much less noise than the second. I also compared the frequency response of the U47 with the two different tubes just using my voice, and the 2nd EF800 has the mid-range presence boost I expect in the 3k to 7kHz area I expect for a U47, while the quieter tube is smoother in the mids. I need to do more thorough testing, and use pink noise instead of my voice to be sure that presence boost is there, but I found that very interesting. Unfortunately, that tube is unusable due to noise (it's at least 6dB louder noise). And changing the tube is not as easy as I hoped. I get worried I will accidentally knock a solder connection loose while I'm wiggling the tube around.

I will post vocal and acoustic guitar clips soon, comparing the U47 to my Manley Ref Card

You need to check all your solder joints and also make triple sure that nothing is shorting to the body that could cause it to shock someone! Now if your mic is properly grounded it should blow a fuse before it causes harm. But?! The capsule sucking in is also not good at all and means some how the voltage to the capsule is jumping way to high. It also will result in a permanantly damaged capsule if it happens to often.

The tube thing does not surprise me at all. When it comes to tubes and mics you can go through a bunch before you get on that really is right for the job. Thats why so much is mentioned regarding tube selection in these forums.

Good going though your most of the way there. Its making noise a truly a very promissing sign.

 

[Melodeath00]

I opened the mic up to check my solder joints. While the mic was open, I swapped the tube out for the original tube I used, as I believe it was quieter.

All solder joints look ok, but I noticed one of the joints carrying the rear diaphragm remote polarity voltage was very close to the mic body, which would short it out. I sort of pushed it further in to prevent shorting. Since then, I haven't been able to recreate the clipping and diaphragm sucking. I don't know what the remote polarity has to do with the front diaphragm, but it would explain some quick shorting out of a lot of volts going to the rear diaphragm (except in Omni of course, hmmm).

Again, I don't know for certain that this was the cause of the issue, or that the connection was even touching the mic body. All I know is that's the only thing in the mic I touched besides changing the tube, and the mic is behaving now. I'm a bit scared the issue will pop up again, of course, so we will see. And I'm still curious/worried if temporary diaphragm sucking damages a capsule.

After some testing, I was indeed correct that the original EF800 I used has much less noise than the second. I also compared the frequency response of the U47 with the two different tubes just using my voice, and the 2nd EF800 has the mid-range presence boost I expect in the 3k to 7kHz area I expect for a U47, while the quieter tube is smoother in the mids. I need to do more thorough testing, and use pink noise instead of my voice to be sure that presence boost is there, but I found that very interesting. Unfortunately, that tube is unusable due to noise (it's at least 6dB louder noise). And changing the tube is not as easy as I hoped. I get worried I will accidentally knock a solder connection loose while I'm wiggling the tube around.

I will post vocal and acoustic guitar clips soon, comparing the U47 to my Manley Ref Card

You need to check all your solder joints and also make triple sure that nothing is shorting to the body that could cause it to shock someone! Now if your mic is properly grounded it should blow a fuse before it causes harm. But?! The capsule sucking in is also not good at all and means some how the voltage to the capsule is jumping way to high. It also will result in a permanantly damaged capsule if it happens to often.

The tube thing does not surprise me at all. When it comes to tubes and mics you can go through a bunch before you get on that really is right for the job. Thats why so much is mentioned regarding tube selection in these forums.

Good going though your most of the way there. Its making noise a truly a very promissing sign.

I agree, Pip, something is very odd with the diaphragm sucking in. Every time I check with a volt meter though, I'm getting ~57V polarization. I've never gotten anything higher than that. if the polarization voltage is 57V down by the circuit boards, there's no way it's jumping up in the high impedance section. It would have to be a loose connection in the voltage divider, right? I've checked the solder there thoroughly. It looks ok to me, but I'm not incredibly experienced.

I never got shocked from touching the body either. There is continuity between the mic body and the ground.

Here are some clips comparing my Manley and U47. I got the mics pretty close together, but it is somewhat difficult with LDCs, of course.

Voice
https://dl.dropboxusercontent.com/u/1579861/U47%20Manley%20-%20A.mp3
https://dl.dropboxusercontent.com/u/1579861/U47%20Manley%20-%20B.mp3

Acoustic Guitar
https://dl.dropboxusercontent.com/u/1579861/U47%20Manley%20-%20Guitar%20A.mp3
https://dl.dropboxusercontent.com/u/1579861/U47%20Manley%20-%20Guitar%20B.mp3

 

[Melodeath00]

After another two days, I still have not been able to re-create the clipping, so everything appears ok for the time being. I am uneasy that I was not able to figure out what was causing the problem.

I did notice that when I turn the PSU off, the audio clips. Is that normal/expected?

There is a lot of handling noise when I hold the mic. I'm going to do some testing against other mics to see if it's out of the ordinary.

How important is it to connect the shielding in the Gotham cable to the Ground pin of the Binder connectors? Would this solve cable handling noise?

I am basically considering the mic done, except for the following list of items.

To-do list:
-Try more EF800 tubes
-Install internal true cardioid switch
-Further secure capsule mount (it tilts the tiniest bit when you tilt the mic, which you can hear as a tiny, tiny thud int he sound. Shouldn't be an issue at all unless you're singing with the mic in your hand and moving all around, though haha

In the meantime, check out the clips I posted in the post above!

And here's a nice little picture