DiY AMI U47

[Melodeath00]

Cool to see this thread continue on. A few comments:

I believe Wagner and Wunder use AMI BV8 transformers, and I've forgotten who else. Oh, Lucas of course. I've not encountered anyone who knows more about this transformer than Oliver Archut. BTW, he just posted an excellent article on his website about BV8. http://www.tab-funkenwerk.com/id146.html

Someone mentioned original materials for BV8 not being available. AMI offers BV8 with both NOS laminations and Oliver's modern lams of the same high nickel materials.

Oliver said on the Lucas forum that "EF802 is the closest glass tube to VF14 in specs and sonics", and that's what he uses in CS-4. I have one in my mic and it sounds godly. YMMV.

I found Oliver sells Thierch capsules for the same price that Thiersch sells them for directly, with lower freight and less wait too.

I posted the correct relay part number and the Mouser part number earlier in the thread. Works great, have it in my mic. Same one Oliver uses in CS-4. It was pretty easy to implement rear diaphragm switching from the PSU using the relay. I reckon it's a lot easier that sourcing the parts to do the switching at the mic head, and a hell of a lot cheaper too. Also, I did not measure ANY drop in heater voltage with the relay engaged or not. Apparently it must not draw too much current.

To the fellow with 136V from the PSU - must measure those voltages inside the mic, under load.

AusTex, thanks for chiming in again. Do you happen to have a part number for an internal switch I could use? It may be more cumbersome to use than a relay, but it will be easier to install and it's absolutely more versatile than the relay option. With an internal switch you could emulate both U47 cardioid and U48 cardioid (they are different according to Oliver, since they receive different voltages to the rear diaphragm, which affects the movement of the diaphragms, even if the rear is not passing audio in cardioid). I'm probably going to buy an EF802 in the next couple days. I wish I could get Oliver to reply about it. Did he ever answer you when you emailed about the slightly different circuit used in the Lucas with the EF802 compared to this U47 kit?

 

[sr1200]

Im interested to see if one cannot use just any simple switch as the schematic makes it out to look.

 

[Melodeath00]

I just bought a couple EF802 tubes on ebay. No idea when they'll get here, as they're coming from Germany, but we'll see how they affect the sound of my mic. I'm going to just swap the tube first, before I consider altering the circuit and capsule (been wanting to try a Thiersch for a long time)

 

[AusTex64]

Melo,

Here are some answers to questions posed:

<Do you happen to have a part number for an internal switch I could use? It may be more cumbersome to use than a relay, but it will be easier to install and it's absolutely more versatile than the relay option.>

I've not researched it, but any submini toggle switch with gold contacts and low resistance should work fine. I'm not sure I agree a toggle switch will be easier to install - the relay sure was easy, and doesn't make a loud "thump" when switched, like a mechanical switch attched to a mic body will. Maybe I'm lazy, but I often take note of how much more experienced builders than me such as Oliver Archut, David Bock and Mike Castoro handle things like this. All of them use relays in the head of the mic...

<With an internal switch you could emulate both U47 cardioid and U48 cardioid (they are different according to Oliver, since they receive different voltages to the rear diaphragm, which affects the movement of the diaphragms, even if the rear is not passing audio in cardioid).>

The mics we built are not like a "true" U47, in that the back plate carries the polarization voltage on historical U47, but the back plate is ground on our AMI U47's and the diaphragms are polarized, like a U48 or M49. I think you can accomplish what you describe by what pattern is selected when you switch the back diaphragm on and off, and you'll need to be able to switch the polarization voltage on and off independently, via a seperate switch, to get all the options you describe. Archut, Castoro and Bock don't seem to think this is worth the trouble, or they would most likely offer it on their mics. If you only switch the back diaphragm on and off and shunt the polarization voltage to ground (closest to historical 47 implementation we can get with the AMI circuit), you still have the option of using the non-switched cardioid pattern that the AMI kit offers as stock, as well as the other patterns too. A lot of people like one or two clicks toward figure 8, I've heard. I'll tell you my U47 sounds GODLY in "true cardioid" when wired as described by Oliver.

And then there's the issue of even HAVING more choices. I am blessed to have lots of choices of mics, mic pres, compressors and EQ's. Honestly, maybe too many. Often find myself caught up in "what should I use for this"? I've noticed it was a lot easier when I had one good mic, mic pre and compressor - that made real it easy! And you know something else? I made good sounding recordings that way. And faster too, by not fretting so much about signal path. Now I take 10 minutes to decide mic, pattern, pre, EQ and comp(s), then patch it up, get levels, etc.

<Did he ever answer you when you emailed about the slightly different circuit used in the Lucas with the EF802 compared to this U47 kit?>

Actually, I forgot to ask. I just sent that email. Will post the reply.

Robert in Texas

 

[Melodeath00]

Melo,

Here are some answers to questions posed:

<Do you happen to have a part number for an internal switch I could use? It may be more cumbersome to use than a relay, but it will be easier to install and it's absolutely more versatile than the relay option.>

I've not researched it, but any submini toggle switch with gold contacts and low resistance should work fine. I'm not sure I agree a toggle switch will be easier to install - the relay sure was easy, and doesn't make a loud "thump" when switched, like a mechanical switch attched to a mic body will. Maybe I'm lazy, but I often take note of how much more experienced builders than me such as Oliver Archut, David Bock and Mike Castoro handle things like this. All of them use relays in the head of the mic...

<With an internal switch you could emulate both U47 cardioid and U48 cardioid (they are different according to Oliver, since they receive different voltages to the rear diaphragm, which affects the movement of the diaphragms, even if the rear is not passing audio in cardioid).>

The mics we built are not like a "true" U47, in that the back plate carries the polarization voltage on historical U47, but the back plate is ground on our AMI U47's and the diaphragms are polarized, like a U48 or M49. I think you can accomplish what you describe by what pattern is selected when you switch the back diaphragm on and off, and you'll need to be able to switch the polarization voltage on and off independently, via a seperate switch, to get all the options you describe. Archut, Castoro and Bock don't seem to think this is worth the trouble, or they would most likely offer it on their mics. If you only switch the back diaphragm on and off and shunt the polarization voltage to ground (closest to historical 47 implementation we can get with the AMI circuit), you still have the option of using the non-switched cardioid pattern that the AMI kit offers as stock, as well as the other patterns too. A lot of people like one or two clicks toward figure 8, I've heard. I'll tell you my U47 sounds GODLY in "true cardioid" when wired as described by Oliver.

And then there's the issue of even HAVING more choices. I am blessed to have lots of choices of mics, mic pres, compressors and EQ's. Honestly, maybe too many. Often find myself caught up in "what should I use for this"? I've noticed it was a lot easier when I had one good mic, mic pre and compressor - that made real it easy! And you know something else? I made good sounding recordings that way. And faster too, by not fretting so much about signal path. Now I take 10 minutes to decide mic, pattern, pre, EQ and comp(s), then patch it up, get levels, etc.

<Did he ever answer you when you emailed about the slightly different circuit used in the Lucas with the EF802 compared to this U47 kit?>

Actually, I forgot to ask. I just sent that email. Will post the reply.

Robert in Texas

Great to hear from you Robert!

Firstly, unless I'm misunderstanding, in our AMI U47, the backplate IS polarized at 60V just like the historic U47. It isn't ground. The Front Diaphragm is tied to the tube grid, and the rear is tied to the polar pattern selector, which varies between 0V (omni) and 120V (bi directional). In a U47, 0V is sent to the Rear Diaphragm, that's why it's omni, or cardioid when you disconnect the rear. In a U48, the backplate has about 50V due to the 105V limitation of the original PSU (you need double the polarization voltage  sent to the rear diaphragm to get bi-directional).

So, in a U47 when you switch to cardioid, you have 0V on the rear diaphragm, and then it is disconnected from the grid after C1. In a U48, in cardioid you still have 100V on the rear diaphragm, and then it is disonnected after C1.

If you install a Relay, you are "stuck" with U47 cardioid only. Of course, how much difference in sound it makes is a big question. However, the fact remains that installing an internal switch inside of the mic gives you every possible choice, and seems easier to implement than a Relay, because all you need to do is glue a switch under the acrylic deck, and wire C1 to it. I have seen Oliver post online about how when you engage the cardioid-only switch inside of an M49, the manual states to leave the PSU on Omni. This is so you get 0V on the rear diaphragm, like a U47 as opposed to a U48.

With an internal switch you have these options:
Cardioid by remote pattern selection - 0V on rear, and still connected to grid. (3 or 4dB less s/n supposedly. Not "true cardioid," but more like C12 or M49 cardioid)
U47 True Cardioid (open the mic and disconnect the rear diaphragm with switch, with PSU set to Omni)
"Modded" U48 True Cardioid (open the mic and disconnect the rear diaphragm with switch, with PSU set to Bi-directional. Rear diaphragm receives 120V still, but does not transfer audio to grid. I say "modded" because we have a 120V PSU, not 105V like the original)

With a Relay all you have is U47 True Cardioid. Granted, you don't have to open your mic to get to True Cardioid, but you have less tonal options (if you care)

 

[Melodeath00]

Sorry it took me so long to post this info, trying to finish up a CD and working all the time....

What I mean by "shunting the rear polarization voltage to ground" is this: Mount a DPDT toggle switch on the PSU. One side will make and break ground on the pin from the Binder that goes to the relay ground wire, which will open and close the relay inside the mic, as described in my earlier post. The other side of the DPDT switch will go in line with the rear diaphragm polarization voltage. When the toggle switch is OFF (both diaphragms in operation), the polarization voltage passes right through the switch to the mic. When the toggle switch is in the ON position (rear diaphragm disconnected from circuit by the relay, AKA "true cardioid") the polariation voltage will go to a 10K 1/2 watt resistor attached to ground. What this will do is pull the polarization voltage down to where the rear diaphragm is not polarized, thus avoiding the problem mentioned about the rear capsule polaization still affecting the sound of the front capsule. Another benefit is if/when you switch the mic hot from "true cardioid" to normal multipattern operation, you won't get a loud thump in the audio, the polarization voltage (now disconnected from the resistor that runs to ground) will rise up slowly. You can watch it with a meter, it takes a second or so to go from O VDC to 105 VDC. Nifty.

Note that the relay can be wired for open or closed with the coil not energized, depending on what pins you choose to use on the switch side of the relay. I chose to wire it for normally closed (rear diaphragm connected) with the coil not energized. This is what I would call the OFF position. When the ground is made, what I'm calling ON/true cardioid, the switch is open. I suppose how one uses the mic would determine how you wire this. I figured I would be using true cardioid only some of the time, and more with both diaphragms on, but would now challange that assumption. Might do it the other way around, if I had it do over again. Probably doesn't make any difference anyway.

Final bit of info - here's the relay I used: Mouser part # 769-DS2E-M-DC6V

And a link to make it easy: http://www.mouser.com/Search/ProductDetail.aspx?qs=YINDDaGsG3E%252beqT3zlaH1g%3D%3D

You guys can thank Oliver Archut for this approach, I'm only the messenger.....  8)

I FINALLY understand this. I think. My question is, how does Oliver do this automatically? As far as I know, the Lucas PSU does not have a "True Cardioid" switch that is separate from the polar pattern rotary switch. Instead, the Relay automatically switches the mic to True Cardioid whenever you select cardioid. Am I incorrect? That seems to be what's indicated on RecordingHacks.

 

[pasarski]

Sorry it took me so long to post this info, trying to finish up a CD and working all the time....

What I mean by "shunting the rear polarization voltage to ground" is this: Mount a DPDT toggle switch on the PSU. One side will make and break ground on the pin from the Binder that goes to the relay ground wire, which will open and close the relay inside the mic, as described in my earlier post. The other side of the DPDT switch will go in line with the rear diaphragm polarization voltage. When the toggle switch is OFF (both diaphragms in operation), the polarization voltage passes right through the switch to the mic. When the toggle switch is in the ON position (rear diaphragm disconnected from circuit by the relay, AKA "true cardioid") the polariation voltage will go to a 10K 1/2 watt resistor attached to ground. What this will do is pull the polarization voltage down to where the rear diaphragm is not polarized, thus avoiding the problem mentioned about the rear capsule polaization still affecting the sound of the front capsule. Another benefit is if/when you switch the mic hot from "true cardioid" to normal multipattern operation, you won't get a loud thump in the audio, the polarization voltage (now disconnected from the resistor that runs to ground) will rise up slowly. You can watch it with a meter, it takes a second or so to go from O VDC to 105 VDC. Nifty.

Note that the relay can be wired for open or closed with the coil not energized, depending on what pins you choose to use on the switch side of the relay. I chose to wire it for normally closed (rear diaphragm connected) with the coil not energized. This is what I would call the OFF position. When the ground is made, what I'm calling ON/true cardioid, the switch is open. I suppose how one uses the mic would determine how you wire this. I figured I would be using true cardioid only some of the time, and more with both diaphragms on, but would now challange that assumption. Might do it the other way around, if I had it do over again. Probably doesn't make any difference anyway.

Final bit of info - here's the relay I used: Mouser part # 769-DS2E-M-DC6V

And a link to make it easy: http://www.mouser.com/Search/ProductDetail.aspx?qs=YINDDaGsG3E%252beqT3zlaH1g%3D%3D

You guys can thank Oliver Archut for this approach, I'm only the messenger.....  8)

I FINALLY understand this. I think. My question is, how does Oliver do this automatically? As far as I know, the Lucas PSU does not have a "True Cardioid" switch that is separate from the polar pattern rotary switch. Instead, the Relay automatically switches the mic to True Cardioid whenever you select cardioid. Am I incorrect? That seems to be what's indicated on RecordingHacks.

2 pole switch.

 

[chaserose1]

I have been working on my AMI U47, Alternate tube with PSU pattern switching using the EF800, BV8R, and Thiersch M7 PVC capsule.  I know several of you guys have successfully completed this build using the 1 giga-ohm grid and capsule resistors.  I was wondering if someone could post the expected polarization voltages of a successful build at the capsule, each- the front, back, and center.  I am having trouble with mine and want to know if my polarization voltages are within reason.  Thanks for the help-- chaserose   

 

[pasarski]

I have been working on my AMI U47, Alternate tube with PSU pattern switching using the EF800, BV8R, and Thiersch M7 PVC capsule.  I know several of you guys have successfully completed this build using the 1 giga-ohm grid and capsule resistors.  I was wondering if someone could post the expected polarization voltages of a successful build at the capsule, each- the front, back, and center.  I am having trouble with mine and want to know if my polarization voltages are within reason.  Thanks for the help-- chaserose 

60V. You have to measure before the 1G resistors. Impedance is too high for a regular DMM at the capsule.

 

[chaserose1]

I have 64.12V at the voltage divider R6,7,5 (1 gig resistor), I can measure .78V at the center post, but this may not be accurate because of the impedance (?). On the back capsule which is adjustable remotely I have 0V-116V at R9 and at R10 (1 gig resistor) I have 0V to 10.7V, I can measure at the back capsule 0V-1.21V which again may not be accurate due to impedance (?),  I have a measurable -.311 V bias at the grid, again maybe not accurate due to the 1 gig (R1)-(?).  How do these measurements compare?  How do you know if you have a bad capsule?  Can the capsule be bad to the point that no output is seen on Pin 2 of the EF800?  I also measure 117.6V at the plate of the EF800 and have an adjustable heater voltage that is reading 4.98V    All I know right now is I have no output from the microphone pin 2 of the XLR

Any ideas that some would like to share on what I might try?

Thanks,


   

 

[Moby]

How do you know if you have a bad capsule?

First, test your electronics withouth capsule. You can simulate the capsule with capacitor (search at forum for details). Then, when you confirm working circuit try with capsule. If it doesn't work you have a problem with capsule or maybe polarization voltage.

 

[AusTex64]

<I FINALLY understand this. I think. My question is, how does Oliver do this automatically? As far as I know, the Lucas PSU does not have a "True Cardioid" switch that is separate from the polar pattern rotary switch. Instead, the Relay automatically switches the mic to True Cardioid whenever you select cardioid. Am I incorrect? That seems to be what's indicated on RecordingHacks.>

The Lucas PSU has an expensive custom switch on the pattern selector PCB that is not included on the AMI U47. Thus the need for the DPDT as described in my post about the relay and PSU mods.

 

[AusTex64]

I have 64.12V at the voltage divider R6,7,5 (1 gig resistor), I can measure .78V at the center post, but this may not be accurate because of the impedance (?). On the back capsule which is adjustable remotely I have 0V-116V at R9 and at R10 (1 gig resistor) I have 0V to 10.7V, I can measure at the back capsule 0V-1.21V which again may not be accurate due to impedance (?),  I have a measurable -.311 V bias at the grid, again maybe not accurate due to the 1 gig (R1)-(?).  How do these measurements compare?  How do you know if you have a bad capsule?  Can the capsule be bad to the point that no output is seen on Pin 2 of the EF800?  I also measure 117.6V at the plate of the EF800 and have an adjustable heater voltage that is reading 4.98V    All I know right now is I have no output from the microphone pin 2 of the XLR

Any ideas that some would like to share on what I might try?

Chase, have you seen my posts in this string about how the AMI supply is wired, and the mic layout with voltages? If not, those might be helpful.

Robert in Texas

Thanks,


 

 

[chaserose1]

Thanks for the replies, yes I have read through the forum, this one and others, I am now  thinking I have failed in my layout and execution and will tear my build down and re-execute using a layout that will pay paticular attention to the Hi-impedeace details that have been described in this and other forums, silver wire etc.  I have changed the PSU connector wiring which was really helpful with the info from this forum.  Still if I use the 1 gigaohm resistor (R5) from the voltage divider should I still see 60V at the back plate-- I am confused on this issue.  The original U47 schematic shows a 100M ohm and a voltage written in on the schematic of 63V or so on the back plate.  Also I see Robert from Texas has replaced R8 to 51K (AMI schematic indicates 30K)-- is this necessary with the EF800 tube, should I change this accordingly--thoughts?  Thanks guys--

I have built other DIY projects successfully but this is my first microphone and for it not to work is humbling.

Chase from Oklahoma

 

[dmp]

Measuring at R6,7,8 is the correct spot for measuring the polarization. If you measure after the 1G, the multimeter will most likely pull the voltage way down and you won't get an accurate reading.
(multimeter impedance is typically 1Meg, which is << than 1G)
So, your polarization seems OK.

The change from R8 (30k to 51k) and R6,R7 has to do with the B+ voltage. If you are using 105v, use the R values on the schematic. If you want to use a 120v B+, then the changes to the resistances just follow ohms law to get 60v for the capsule polarization and the same plate voltage on the tube.

I also measure 117.6V at the plate of the EF800 and have an adjustable heater voltage that is reading 4.98V    All I know right now is I have no output from the microphone pin 2 of the XLR

This seems too high for the plate voltage. It looks like your tube is not conducting. Check all the tube connections and if that is verified try a different tube. When the tube is conducting the voltage on the plate should be less than B+. The grid to R2,R3 is the bias and should be slightly negative, ~1v.

 

[chaserose1]

Thanks DMP, I will double check

 

[Melodeath00]

I received two NOS Telefunken EF802 tubes. They are from the early to mid 1980s, and are in fantastic shape. I have only done some basic testing, but so far, in the same circuit, it sounds a little bit brighter than the EF800. I am not sure if I want to adjust the bias cathode resistor and plate resistor to follow the Lucas schematic or not. I have a feeling those circuit adjustments will slightly darken the tube, but it is impossible to say without trying it. We're talking about raising the cathode resistor by 10 Ohm, and lowering the plate resistor by a few kOhm.

AusTex64, did you see my reply about backplate polarization and true cardioid? Also, do you have any idea how Oliver underheats the EF802 in the Lucas CS4? I am underheating it the same amount he underheats the EF800 according to his PSU.

 

[Melodeath00]

I have performed a much more scientific test on the EF800 versus EF802 tube. I took an already-recorded audio signal (an acoustic guitar recording), and played it through one studio monitor. I put the U47 with EF802 about 1ft away from the monitor, and recorded the speaker playback of the acoustic guitar with the mic. I took some physical measurements of the mic position relative to the speaker to make sure that after I swapped tubes and put the mic back in the shockmount, that it would be in the same position, or as close as humanly possible.

So, I swap tubes to the EF800 and record the same signal playback.

I then slightly changed the position of the mic. I I twisted it a tiny amount, and lowered it about 1/4", then recorded the same playback. I did this because I wanted to see how much effect exact placement has, and how much alteration I could expect if I accidentally moved the mic a bit despite my measurements when I swapped from EF802 to EF800.

Here are my findings. The EF800 is about 0.6dB louder than the EF802, which I accounted for in my DAW before comparing. The EF800 and EF802 are not exactly the same. They don't cancel in a null test. In fact, the EF800 compared to EF800 with slightly altered mic position nulls MORE than the EF800 and EF802 do. It was somewhat surprising the mic position had less effect than I anticipated.

Anyway, while the EF802 and EF800 are apparently not identical, they sound so similar that I simply can't tell them apart in a blind test. This is very interesting to me. I'd be very curious how the sound changes if I adjusted tube heater up to the full 6.3V. I just measured at the tube, and the EF800 is being heated at 5.67V, so there is a slight underheating going on.

Does anyone know what "Rk" stands for in a tube data sheet? I am referencing the EF802 and EF800 PDF files here: http://tubedata.milbert.com/sheetsE2.html

 

[Melodeath00]

I am planning to start experimenting with this mic component-wise. I am interested in trying carbon comp resistors instead of the current metal oxide I have in there now.

Question... I cannot find any carbon comp resistors above about 68Mohm. I need values up to 150Mohm. Can I just put 3 resistors in series, as long as I have the space in the mic to do so, or will this not work quite right? Secondly, if carbon comp resistors supposedly have a "sound," will 3 resistors in series to reach 150M have the same "sound" as one 150M resistor?

Instead of going with all carbon, should I only use carbon for select spots where the audio passes? If so, which do you recommend? The 100k plate resistor and the grid resistor? Does audio pass through any of the other resistors?

In terms of relay versus internal switch. Is there any reason I couldn't use a typical mic body switch inside the mic, glued to the plastic deck of the kit? I'm talkign abotu the 2-way switches you can get to use as a pad or a pattern switch. Is a relay more advisable? A relay would be more work to incorporate (I'd have to mod the PSU), but it would be easier to operate. I have read I need a "latching type" to keep noise down. However, I also need one that is powered by 5.67V (the heater voltage in my mic). Will a 6V relay work with 5.67V?

Thanks!

 

[0dbfs]

Here are some 200M:
http://www.ebay.com/itm/200-Meg-High-Voltage-Resistor-TRW-MVX-1-1-2-Tesla-Qty-20-resistors-/111226374531?pt=LH_DefaultDomain_0&hash=item19e59bc983

Cheers,
jb