Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Je55]

Hi Klaus,

As I'm sure you can recall I recently purchased a semi vintage c12 as a fixer upper.

Power Supply / Switcher, All Original
C12 Body / Bell, All Original
Capsule Re-Skinned Original
Inside frame and components  - Telefunken USA

When I purchased the mic, I shot it out at Paramount studio's against their C12 and their C24 and noticed that it was down two clicks on a 1073 (10db or so)

Since then I gutted the mic and restored all components with schematic accurate values and NOS original parts. Just using MOX resistors for the shunts until I can source better. NOS PIO/Tar MP Russian Caps 0.5 / 1uf which I can switch back and forth easily (picked up a box of 40 or so). Have also tried other coupling caps. I also replaced the Fixed Bias circuit which was missing.

I reversed the capsule as the rear sounds a little more open and slightly smoother. I am to source an original Golden Gate.

At this point down 15dbs from my Sony c800g

So I tried,

• Calibrating Heater at tube to 6.1-6.3v - no difference.
• Switching Tubes, Several 6072 Five Stars - no difference.
• Reversing the the Haufe T14/1 Left/Right terminals in case it was not bi directional - No Difference.
• Purchased AMI Transformer and installed - Up 1-2db.
• Checked Cables for cross contamination and impedence, - 0.8ohms max and no signs of cross contamination.
• Installed XLR on power box and terminated them from going to switcher. No noticeable difference

I'm getting

• 117v B+
• 64v at Plate
• 6.3 at Heater
• -0.8 at Bias
• Cables are max 12ft

I can't for the life of me work out where these db's are lost. I understand that a healthy c12 will not be as hot as a healthy c800g or u47 but it is definately still at least 10db's down. The only thing I havn't touched (other than reversing it) is the capsule.

Suggestions would be greatly appreciated.

Jesse

[klaus]

You really went out of your way to dig deep in your troubleshooting. And, I am sure, your general knowledge of these mics has advanced tremendously in the process.

My experience with significant output loss as you describe in your specific circumstance (mostly aftermarket parts and circuit, re-diaphragmed capsule) can generally be attributed to two causes:

1. capsule (my prime suspicion)
2. the loss of a thousand cuts.

As to 1: skill levels for doing a re-diaphragming, even in the year 2012, are still all over the map, and, in my opinion generally still not very good (Tim Campbell's efforts excluded). It is fairly easy to lose a good 6dB with a shitty reskin job. Besides, you may also check that the diaphragms move freely and are not sucked in by the polarization voltage (a common occurrence with bad reskin jobs). I also find that lead outs need to be carefully looked at: do the long M1 screws that connect polarization voltage to the backplate connect fully? Are the contact eyelets that these screws affix corrosion-free?

For a true confirmation of the state of your capsule, nothing beats a temporary substitution with a known good original AKG CK12 capsule, no matter the vintage.

As to 2: An additional 6dB loss can easily be accumulated through several component or circuit conditions. For example, is the Haufe T14/1 a genuine spare part or was this part of the C12 copy mic? The two are not the same. Then, your fixed bias values and setup may be off. Just for checking, try temporary substitution with a cathode bypass on the tube (2.7KΩ, 47mfd).  Then bump up the B+ to 120VDC.

[Je55]

Klaus,

Thank you,

Unfortunately I don't have a spare ck-12 laying around to try, I will order one from Tim and can at least use it later in a 251 build. In the mean time I'll check the backplate connections and make sure to go over the points you raise. I'm fairly sure I have a good connection though as there are two M1s in parallel entering the backplate and to my vision there seems to be zero sign of corrosion.

The T14/1 was a Haufe original supplied to Telefunken USA by Haufe, its a new rendition, I will probably stick with the Tab Funkenwerk T14 transformer. By the way when testing across the secondaries (Red) on the Haufe I get 64ohms and with the Tab I get 22ohms. Are my primaries and secondaries the right way round?

RE: temporary cathode bypass, as tempting as this is my thoughts are that the loss of output existed before my modifications which was cathode bypass at the time. Its always worth a try though.

Is there an easy way to configure B+ without replacing resistors in the PS? I have the PS jumper set to 125v but power in my studio is hot at 123v.

Cheers

Jess

[Kai]

An additional 6dB loss can easily be accumulated through several component or circuit conditions.

Klaus, do you know the overall gain of a healthy C12's preamp?
I mean from capsule input to transformer output, both sides with defined termination?

I think this figure is easy to measure, even with simple equipment, and would save a lot of guesswork if known.

Regards
Kai

[Je55]

Bare in mind that I can't measure above the shunts due to the impedance but i can measure anything below 40megs.

[klaus]

Klaus, do you know the overall gain of a healthy C12's preamp?

As far as I recall from past measurements, the amp gain (not including capsule) on a C12 is around minus (-) 3dB.

[Kai]

As far as I recall from past measurements, the amp gain (not including capsule) on a C12 is around minus (-) 3dB.

A figure in the usual range of most condenser mic amps.

Bare in mind that I can't measure above the shunts due to the impedance but i can measure anything below 40megs.

I'm not talking about impedance / resistor measurement but audio gain.

Feed a sine wave (about 1kHz) with a level of ca 100-200mV into the amp input where the capsule is connected.
Use a series capacitor of ca. 1-5nF/100V for the connection.
You can leave the capsule connected for this basic measurement.

Measure the amp output voltage with the output shunted (loaded) with a 1-2kOhm resistor.
This voltage should now be 3dB down (= 70%) from the input signal.
So if you feed in 100mV you should get 70mV out.

You should screen the mic amp (which is partly open to feed the signal in) with a bit of aluminium foil to avoid measuring stray introduced hum.
To check the hum/noise of your setup switch off the input sine (leave all connections as they are), output level should go down to less than 10%.

Regards
Kai

[J.J. Blair]

re: Vintage T14/1 v. the new Haufe ones, I have not perceived a difference in gain.  I simply find the new ones to be more reliable.  LOL.

Anybody else with experience on this?  Or even the difference in sound between the two?  I have not measured the against each other, but I'm not hearing a noticeable difference between the two, at least on the types of sources where I prefer a 251.

[klaus]

My understanding is that the T14/1 made for Telefunken/USA had additional bass response, to compensate for a bass-shy capsule they were using?  At least, that is my recollection from the beginning days of Tele/USA's ELA M251 copies.

I also seem to recall that Oliver Archut made forum statements to the effect that Haufe's materials, both core and wire, are no longer the same as was used in the original 1950s/1960s T14/1.