AKG C12 A / B Version Build

[Matt Nolan]

Put something like a 47 ohm resistor (2W or preferably larger) in series with the pot, leave R2 and R3 the same.

You may find that pulling the heater down might fix your B+ and cathode voltages a bit, so I'd leave the B+ changes until after you've fixed the heater supply.

 

[Spencerleehorton]

I put in the 47ohm resistor, I could then just raise it up a bit and the H+ sat at 5.09v
cathode is 62.5v and B+ is 113v. I'm using a 330ohm resistor instead of the 560ohm.
This second mic is a fair bit down on output, hoping once I get B+ up to 120v this will be a little better!

I think the output low level might be because of the transformer I'm using on the second version.
On my first version I think I was able to parallel the secondary's so I only get 17ohm rather than on this second mic the secondary is 33ohms as the other wires for that transformer have a fault so I'm only using one set.
Also the B+ is 7v or so down but I don't know if the B+ will make that much difference in the output?
Also I have noticed the signal to noise floor is quite high on this second mic, when both levels are matched there is a noise level that you can visually see being there when recording, it's not noticeable when listening though?
Any ideas on these points?

Regards

Spence.

 

[Matt Nolan]

I put in the 47ohm resistor, I could then just raise it up a bit and the H+ sat at 5.09v
cathode is 62.5v and B+ is 113v. I'm using a 330ohm resistor instead of the 560ohm.
This second mic is a fair bit down on output, hoping once I get B+ up to 120v this will be a little better!

Both mics have 330 instead of 560, or just the second one? I would make them both the same. What was the reason to switch from 560 in the first place?

Raising the B+ will have little to no effect on the gain of the amp, but it might give you 0.5dB more from the capsule.

I think the output low level might be because of the transformer I'm using on the second version.
On my first version I think I was able to parallel the secondary's so I only get 17ohm rather than on this second mic the secondary is 33ohms as the other wires for that transformer have a fault so I'm only using one set.

Again, you should have the same arrangement in both mics if you want them to work as a matched pair.

Can you explain precisely what is going on with your transformers? There are two secondary windings? In one mic you have them in parallel, in the other you have just one of them in use? Having an open circuit secondary on there is not so good. Nor is having a different output impedance on each mic. Can you get a replacement transformer?

Also I have noticed the signal to noise floor is quite high on this second mic, when both levels are matched there is a noise level that you can visually see being there when recording, it's not noticeable when listening though?
Any ideas on these points?

What is the difference in gain needed to match the levels?

Use your scope and trace through each mic to find where the signal level is being lost. Inject a small signal at the grid, measure at the cathode, at the trafo primary, at the trafo secondary. Ideally, do this while the mics are loaded with the pre-amp inputs.

Also, set up a test tone through a speaker and put the mics as close together as you can get them, same distance from and pointing at the speaker. Measure the output on your XLR connection in the PSU.

But, to be honest, you should fix up your resistors and transformers first.

 

[Spencerleehorton]

HI Matt,

it seems i was mistaken before with thinking my mic txf were wired different ly, they both check out as 33ohm and 740ohm and yes both of them should have a secondary winding on both sides but im just using one winding per side.
both mics have 330ohm in them rather than 560ohm to bring up the cathode to 62v ish.
i have just setup my neve pre amp which i was using and one channel was at least 2 clicks down on volume, so the transistors and th gain stage on the lower channel needs looking at so i can match them. i ahve given the 2055 transistor a tweak so both are behaving the same in the neve at 22.66v and im going to now have a listen and see what it sounds like now.

regards

Spence.

 

[Spencerleehorton]

I've taken the Neve out of the chain and im now just going mic into mixer into interface and 1st mic is good level at half way gain with fader at unity.
The 2nd mic is at exactly the same level going in and within protools i have made a aux bus for both channels and i can see the 2nd mic is down about 4db, rising the slider approx that amount levels both the mics, so i think its around the 3-4db mark my 2nd is down.

this is very crude but hopefully helps!!!

meanwhile my neve now needs a good looking at as the channel levels are all over the place!!!! aaaghhhh!!!

regards

Spence.

 

[Matt Nolan]

it seems i was mistaken before with thinking my mic txf were wired different ly, they both check out as 33ohm and 740ohm and yes both of them should have a secondary winding on both sides but im just using one winding per side.

This still isn't clear Spencer. Do you mean each side has a second winding? i.e. two primary windings and two secondary windings? The primary is the side you drive signal into, the secondary is the side you draw signal from.

You shouldn't leave audio transformer windings open. If there are 2 primaries and 2 secondaries they should all be used. Either in series or in parallel in each case. Otherwise, you are not coupling efficiently through the transformer core, frequency response could be off and you might even start picking up interference.

You want to connect your transformer windings for roughly a 4:1 ratio. For example, if the windings are 4+4:1+1 then you want the primaries and secondaries strapped the same way. Measure or listen to determine which is nicer, series or parallel, my guess would be series for the higher inductance. If you wanted 8:1 then you would strap the primaries in series and the secondaries in parallel. If you wanted 2:1 then you would strap the primaries in parallel and the secondaries in series.

If you are unsure about the calibration of your pre-amps then hot plug the 2 mics between just one pre-amp, or do tests twice - first with mic A connected to channel 1 and B to 2, then with mic A connected to channel 2 and B to 1.

 

[Matt Nolan]

Don't trust mixer gain pots and faders. Use your oscilloscope to get an accurate measurement at the XLR.

 

[Spencerleehorton]

yes in each transformer there are four wires on each side, so it can be run 5:1 or 10:1 apparently.
when i first tested these txf they both had some problems so iw as going to send them back, i then forgot to and then this project came along. i then tested them again and one works perfectly with all windings working, the other one set of higher ohm doesn't work but both lower ohms do.
i will connect the one that tests ok in parallel and see what happens, dont know what to do about this other one but i will connect the primarys (33ohm) in parallel but i cant connect the secondarys as one winding is faulty and i get very strange ohmage readings?

regards

Spence.

 

[Spencerleehorton]

Well an end to a very event full day. A day of testing and refining, the C12a version 1 now sounds fantastic I have paralleled up all the wires on the txf and the mic sounds much better.
Version 2 txf  only has one winding on each side working so I have opted for a NTE4 neutrik txf, it the only small enough txf which I have lying around!
When I then tested the mic with the neutrik txf in it sounded great! Version 2'mic is still a bit down on output level but I'm going to do a few more checks, swap valves and see if higher output follows the tube which is in verison1.
Bring up B+ to 120v, change 200MB capsule polarising resistors for 1G resistors.

 

[audiophreak]

Hi All,
      Thank you all for this very useful info. I have an AKG nylon ck12 capsule and have built several large and small diaphragm Royer mods with excellent results. I find my self wanting to do a C12A clone.
    I have a couple questions .... the 15uF 75v Tant cap is impossible to  find , I've used Tants in comp builds - do they work as other caps in parallel ?  I'm wanting to put a 10uF50v and a 4.7uF 50v tants in Parallel , will this work ??  also the schematic shows 1000UF styro - hope thats a typo , looking at other similar schematics the styro cap in that position ranges from 500PF to 2000PF , so is it a 1000PF styro that belongs there ?? 

  Thanks in advance   

 

[TLRT]

Hi All,
      Thank you all for this very useful info. I have an AKG nylon ck12 capsule and have built several large and small diaphragm Royer mods with excellent results. I find my self wanting to do a C12A clone.
    I have a couple questions .... the 15uF 75v Tant cap is impossible to  find , I've used Tants in comp builds - do they work as other caps in parallel ?  I'm wanting to put a 10uF50v and a 4.7uF 50v tants in Parallel , will this work ??  also the schematic shows 1000UF styro - hope thats a typo , looking at other similar schematics the styro cap in that position ranges from 500PF to 2000PF , so is it a 1000PF styro that belongs there ?? 

  Thanks in advance 

Mouser has 15uf/75V wet tantalum in stock now ,although it is a pretty expensive.Styro is 1000pf. I have an original waiting for restoration(too much projects now)

 

[audiophreak]

Mouser has 15uf/75V wet tantalum in stock now ,although it is a pretty expensive.Styro is 1000pf. I have an original waiting for restoration(too much projects now)

Thanks TLRT !!

    Not sure what " Wet" is , but its $35    will my suggestion of paralleling a 10uf50v and 4.7uf50v work ??  cuz I have those left over from other projects  , not sure if splitting the voltage between the two works  :-\  and thanks for confirming the Styro value !!

 

[Spencerleehorton]

The 50v will be a problem, needs 75v at least here as ends up being just over 60v on that tant.

http://www.ebay.co.uk/itm/2-Mil-Spec-M39006-22-0167-15uF-75V-10-Wet-Tantalum-Capacitors-/231239767645?hash=item35d6f6d65d:g:xIkAAOxyrrpTf5lA

 

[Gus]

In reply 67 there are is a picture of tie wraps holding a capsule. 
I do not depend on tie wraps to hold anything  for any length of time I use them as a temporary holder.
Some white ones change color and get brittle and break. UV seems to speed up the break down.

After reading most of this thread what capsule did you use?

I like the home made bodies.

If I wanted a CF type microphone I would build a solid state version with a nice output transformer.  You could build an external power supply.

 

[audiophreak]

The 50v will be a problem, needs 75v at least here as ends up being just over 60v on that tant.

...  I didn't think the voltage thing worked like that but was looking for options , Thanks for the confirmation and the ebay tip !!

 

[Spencerleehorton]

What’s your thoughts on using this transformer for the C12a?

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F112882929558

 

[Spencerleehorton]

I’ve taken the plunge and bought the transformer, will report back on how it shapes up.

 

[audiophreak]

I’ve taken the plunge and bought the transformer, will report back on how it shapes up.

Hi !!
      wondering if you had the chance to try that transformer, and if so , how it worked out ?

  I have finished mine, I used the MXL 2001 body and an AKG Nylon Ck12 capsule with the Jensen DB/E which is a 12:1 per Royer Mod and the Royer PSU - It all worked, pick up patterns Omni to figure 8 all worked, but the B+ was 95vdc so all associated B+ voltages were correspondingly lower  and the overall signal output was very low but clean and sounding good.
  I had a GrooveTubes  PSU I picked up on ebay several years ago  and did a little " modificating "  and the voltages fell right in place , unfortunately had  the same results, low output from mic , then changed the 7586  tube and again , same results , clean audio and patterns working and all , no strage noises or hums or buzzes ... just very low output , like i have to have the mic pre at 3/4 gain to have a decent level ... so I am thinking maybe the mic output transformer  ...

any thoughts ?

Thanks in advance    ;D

 

[audiophreak]

pic of inside

 

[Spencerleehorton]

Hi,

I have two of these mic builds and in one I have a 10:1 ratio txf and in the other this 7:1 ratio.
There is an output difference but I think it maybe the valve differences rather than the txf.
I will keep experimenting and testing these as would like to end up with two matches mics for overheads as they work so well in blumeline.