My M49 style mic

[telefunk]

jb,

did you figure out that pattern switching silence? I'm having similar thing with Oliver's U47 design.

What if you change the 1G grid bias resistor, will the capsule re-charge faster?

Did you measure if the plate voltage rises during the muted 30sec?

 

[0dbfs]

I've got these mic's in service at the moment and haven't had them back on the bench for a while. Next mouser order is going to bring me some PSU parts for these so I will do some additional tests & tweaks including the 1G grid resistor.

BTW, I've got one of these with the stock capsule (apex460 / acm12) and the second with the semi-recent c12 clone capsule gb. C12 clone is way better. No question. Stock is almost un-useable from a comparative standpoint so I'll be putting the second c12 clone in. Also ordered Tim Campbell CT12's for these. Looking forward to those in a big way.

I also have a pair of MK7's which behave the same re: pattern switching (their grid leak resistor is 100M though). My pair of mk47's do not do this however and I've got them on the bench right now working on a couple other fine tuning and mechanical details.

Any ideas are welcome and I don't mind researching and reporting back with any suggested tests. As a matter of fact, I intend on addressing the issue and will report back anyway but enjoy the discussion

Ciao,
jb

 

[telefunk]

Great! I'm trying to figure out what might cause the pattern switching to act this way...

While you're testing, check if the pattern switching affects the plate voltage...

 

[0dbfs]

Awesome, Thanks TFunk. I'll check that too.

It's not a deal breaker obviously but would nice to listen to sonic differences between patterns without waiting so long

I imagine that's what happening is that switching the pattern causes the backplate to lose it's charge (since that's what provides "-60V" to the front and the front is definitely losing audio.

I'll take more measurements in any case to see how various voltages change when the switching occurs.

Cheers,
jb

 

[0dbfs]

TF, Are you using a switched pattern selector that switches rear and front together (like original u47) or are you using a resistor ladder sourced from B+ to bias the rear diaphragm?

In both mics (actually four) they all behave this way and are using the ladder method for pattern switching. I also believe it only happens when switching towards omni. Switching towards bi s fine. Might be the other way around though.

Cheers,
jb

 

[telefunk]

TF, Are you using a switched pattern selector that switches rear and front together (like original u47) or are you using a resistor ladder sourced from B+ to bias the rear diaphragm?

In both mics (actually four) they all behave this way and are using the ladder method for pattern switching. I also believe it only happens when switching towards omni. Switching towards bi s fine. Might be the other way around though.

Cheers,
jb

JB,

I have the resistor ladder from b+. And I have exactly the same effect...

Does every MK7 act like that?

 

[abbey road d enfer]

When I switch towards omni (or power-up$ the capsule mutes until it charges. This takes several seconds. Maybe 30. Anyway, I can hear a buzz during that time when there  is no acoustic signal. Must be from the heater. Definitely in the noise floor once capsule is charged but wondering if others hear this in other builds.

When you switch the pattern, you apply suddenly a pulse of several volts to the head amp; during the settling period, the operating point is all over the place and in particular the stage has zero PSRR, so whatever hum residual is on the B+ rail goes directly to the output. I'm not convinced it indicates a defect in the heater voltage.

 

[0dbfs]

I will be taking these out of service for a bit, putting them back on the bench, and opening these up again "soon'ish" and further investigating the pattern switching issue as well as fine-tuning desired PSU voltages. Perhaps with some zeners and maybe further RC stages.

I am considering putting a cal-pot in place for the heater, cathode, and plate voltages. I might experiment and see how stable the voltages remain over time with a cal-pot.

Tim Campbell CT12 capsules & mounting-posts are done too and on their way to me now so I'll also be swapping those in too

More to follow!

Cheers and thanks for all the input!

-jb

ps. This means I can use the Group-Buy edge-terminated/c12 types for some other franken-mic's too. They do sound pretty good. I have several bodies available so maybe a km84 type circuit would go well with those?

 

[abbey road d enfer]

I am considering putting a cal-pot in place for the heater, cathode, and plate voltages. I might experiment and see how stable the voltages remain over time with a cal-pot.

This http://www.calpot.com/
or that http://calpotnews.com/ ?

 

[0dbfs]

Option #2, then I'll try to see how far I can get working Option #1 into this build.


-jb

PS: That reminds me of another little project I want to put together with one of the doner mic bodies. Modify the head-basket with a nice tight-yet-breathable three layer mesh, hollow out an XLR cable and perhaps install an aquarian-processing-chamber in the PSU chassis. I'm sure it will get some use in the studio as well as provide plenty inspiration. I'll post photo's a I gather the materials.

 

[abbey road d enfer]

Option #2, then I'll try to see how far I can get working Option #1 into this build.


-jb

PS: That reminds me of another little project I want to put together with one of the doner mic bodies. Modify the head-basket with a nice tight-yet-breathable three layer mesh, hollow out an XLR cable and perhaps install an aquarian-processing-chamber in the PSU chassis. I'm sure it will get some use in the studio as well as provide plenty inspiration. I'll post photo's a I gather the materials.

Make sure you load it with helium!

 

[0dbfs]

Finally got one of these going in their final/anticipated versions last night with the following:

Tim Campbell CT-12.
ACM1200 / Apex 460 body.
ACM1200 / Apex 460 PSU modded w/0.5W - 60V zeners for 120V B+.
AMI/TAB T49 output trafo.
AMI/TAB M49 circuit w/5840 & 10uF cathode bypass cap, and series diode to drop heater to 5.65V.

CT-12 sounds great. Definitely better than the Chinese C12 clone. I'm finishing up the second one tonight with PSU mods and CT12 installation. Very happy with the results so far and looking forward to using them in the studio.

I had some wind noise and such after initially swapping the capsule in and ended up cleaning up the Hi-Z area as well as swapping out the front/back 1000pF styro as well as the backplate filter 10,000pF styro to sort it out.

Also found a non-heat-shrunk termination pin in the stock tube-mic-cable supplied with the original mic-system which was randomly shorting (H+) from the PSU and causing intermittent dropouts.

Still getting the capsule muting when switching towards omni so will take some measurements tonight when I am working on this one and see if I can sort anything out.
I suspect that the capsule back-plate is discharging/recharging for some reason when switching towards omni.

I have taken note that measuring my capsule back-plate voltage is not exactly 1/2 (should be 60V but measures 55V) when I measure @ the divider node before the 1G connection to the back-plate. I may try to better match the divider resistors to get exactly 1/2 of B+. Same thing with the pattern switch. Cardioid, and Omni are not exact wrt B+ and 1/2 B+ so may try to match for those positions to make the patterns more accurate in those critical (IMO) positions.

I have inquired to AMI/TAB regarding what differences I may expect if I were to swap in their BV11R which is described as "extending lows and mids".

AMI/TAB's BV11R is a $485.- upgrade per/tx so.... So it would give me a couple T49's to play with some more... I guess my lesson here may be to go with the higher quality parts in critical positions in the first place so I don't end up spending more when I eventually decide to upgrade anyway...

It's always a fun/learning experiment in any case!

Cheers,
jb

 

[telefunk]

Still getting the capsule muting when switching towards omni so will take some measurements tonight when I am working on this one and see if I can sort anything out.
I suspect that the capsule back-plate is discharging/recharging for some reason when switching towards omni.

Everybody seems to have that muting when switching toward Omni. I never got rid of it, just got faster recovery when I changed the grid leak resistor to the original value.

Do you think it is a problem?

 

[0dbfs]

Not really a problem except for muting the audio... I mean it would be nice to listen to the difference quickly but it seems to take the same amount of time for it to ramp while switching the other way.

Thx,
jb

EDIT:
Just looked at an original m49c schematic... duh... There are a couple EQ and feedback components in there that are not in Oliver's conversion schematic plus he used 1G's where they had 100M and 150M's... That's fairly significant I think (feedback & EQ cap's). For instance, a 600pF from plate to gnd (LPF) as well as two .01uF's connected as a capacitive divider from backplate to GND with the divider node connected to the post-output-cap/trafo-pri node with a 5M resistor in-between (NFB)... Then the approx. 8pF "tuned" plate to grid connection (RF suppression)...
If I've got parts in stock I may play with some of that tonight too

Cheers, jb

 

[dmp]

I'm curious how you end up adapting to the 5840? The 'Royer' circuit uses a plate R=100k and a Cathode R=750. What did you use?
(R4 R3 in the linked M49 schematic)

 

[0dbfs]

I used this schematic:
http://www.tab-funkenwerk.com/id89.html

Which is a 2k2 cathode bias and a 100k plate resistor that yields about -1.5xxV at the grid WRTK and a little more than 40V at the plate (i'll measure tonight). The original m49c w/ac701k uses 100k rp and 2k2 rk with a 25uF bypss. I used 10uF bypass.

Cheers,
jb

 

[dmp]

So that would mean the tube is biased quite a bit hotter than in my build using a R=750 for the cathode. I'd like to understand better how this affects the operation of the mic.

 

[0dbfs]

So that would mean the tube is biased quite a bit hotter than in my build using a R=750 for the cathode. I'd like to understand better how this affects the operation of the mic.

I have not tweaked and tested the bias points yet but here are some notes from Oliver regarding bias when I queried about biasing for symmetrical clipping. My original plan was to inject signal into the grid and bias for symmetry but have not gone that route yet... It sounds quite good as is but I may play with this to better understand the effects. Hard thing is that it takes a little while of using the mic across a range of sources and then mixing for a bit for me to best understand the subtleties.

Here are Oliver's comments:
http://prorecordingworkshop.lefora.com/2012/01/31/calling-all-bulldog-builders/28/

Hello Jonathan,

for the PSU mod, you have two options,
A replace the two 75V Zehners with 60V (5W), that is the easiest way but you still have the Zehner noise.
B add an 27K (1W) in series with a 10K trim pot a 10uF (or larger) as final filter to ground. You can adjust (depending on the tube) the range  115 to 125V and have a 6dB better noise floor compared to option A.
A 5K cathode resistor in parallel with a 5K trim pot will give you a wide sound range with reference to bias, for most tubes the capsule won't be able to push the tube in to clipping so the goal of having a symmetric clipping buffer is not needed at all. But the bias point will change high end as well as low end response so playing with the bias can help to find the tubes sweet spot.
Best regards,
Oliver

Cheers,
jb

 

[mata_haze]

So that would mean the tube is biased quite a bit hotter than in my build using a R=750 for the cathode. I'd like to understand better how this affects the operation of the mic.

I have not tweaked and tested the bias points yet but here are some notes from Oliver regarding bias when I queried about biasing for symmetrical clipping. My original plan was to inject signal into the grid and bias for symmetry but have not gone that route yet... It sounds quite good as is but I may play with this to better understand the effects. Hard thing is that it takes a little while of using the mic across a range of sources and then mixing for a bit for me to best understand the subtleties.

Here are Oliver's comments:
http://prorecordingworkshop.lefora.com/2012/01/31/calling-all-bulldog-builders/28/

Hello Jonathan,

for the PSU mod, you have two options,
A replace the two 75V Zehners with 60V (5W), that is the easiest way but you still have the Zehner noise.
B add an 27K (1W) in series with a 10K trim pot a 10uF (or larger) as final filter to ground. You can adjust (depending on the tube) the range  115 to 125V and have a 6dB better noise floor compared to option A.
A 5K cathode resistor in parallel with a 5K trim pot will give you a wide sound range with reference to bias, for most tubes the capsule won't be able to push the tube in to clipping so the goal of having a symmetric clipping buffer is not needed at all. But the bias point will change high end as well as low end response so playing with the bias can help to find the tubes sweet spot.
Best regards,
Oliver

Cheers,
jb

so...ehm....PCB? KIT? ehm....  ;D

Mattia.

 

[0dbfs]

Painted the bodies today.
Six coats of Dupli-Color Wimbledon White.
Baked @ 170F for 20min between coats and wet-sanded/polished with 2000 grit.
Finished with four clear-coats, more baking, and polishing.