Hi Guys, I do not believe there is a design flaw with the 460 microphone output circuit other than there is an economy transformer in the output. The output transformer has a ratio of about 8:1 this means that if the secondary of the transformer is terminated in a 150 ohm load the primary impedance will be 10K ohms. This is what is required otherwise headroom will decrease as the cathode of the tube is loaded down. Even though cathode follower circuits claim to have a resistance of around 500 to 1500 ohms the cathode should be loaded will no less than 10 times this impedance. With a 1ufd coupling capacitor the low frequency response would be about 3db down at 400hz with a 600 ohm load.
If I was going to upgrade the transformer then I would replace it with a similar 10K to 150 ohm transformer. The legendary Sony C37P tube microphone had a cathode follower output circuit and the output transformer had a ratio of 10K to 150 ohms which makes sense. The Sony has a -8db input pad and I remember using them on toms for a Jazz record once but with an original 460 it is possible to overload the tube/transformer circuit when used on close "rock" vocals.
The AKG C12 uses a 6072 configured as a single triode amplifier requiring a much higher output impedance from the plate of the tube. The 6072 has about 6db less gain than a 12AX7. Placing a 6072 in the 460 will cut the output level reaching the transformer by almost half reducing harmonic distortion in the transformer. This helps get rid of some of that 460 "grainyness". I also replace the transformer coupling capacitors in upgraded 460 microphones which helps with the transient response as I bypass a larger value electrolytic with a faster .01 ufd. I also remove a couple of output bypass capacitors that are there to reduce RF pickup in transformerless preamps but it is possible for these capacitors to reduce hf response with some preamps.
The major contributor to the sound of the 460 is the capsule and not all 460 capsules are made equal. This is the downside of buying "cheap" Chinese microphones. However, this is the advantage of buying a selected and upgraded microphone from us. I do not ship out selected 460 microphones that have what I deam as inferior capsules. APEX is very good with me regarding this as long as I acknowledge that there will be differences from capsule to capsule and from one side of the capsule to the other but we have come to an agreement on how much of a difference is acceptable. I usually have to go through about 3-6 microphones to find a pair that are close enough to be classed as a pair. Yet, these microphones have a frequency response which is within + or - 2db of each other and can sound quite good on their own. I find the APEX 460 microphones to be more consistent between capsules than the OKTAVA MK319 microphones that I use to sell.
We had two Telefunken U47 tube microphones during my years at Ocean Sound and one always sounded a bit different than the other but both sounded good. Even, with the U47 I remember sometimes putting on the foam pop filter to reduce sibilance because we loved the sound of the microphone but the vocalist was really sibilant. If you cut a record too hot an sizzly the record grooves could actually run into each other on transient peaks. However, with digital recorders these transient levels can just be normalized and high frequency transients reduced all in the digital domain. However, I still hear some over sibilant vocal recordings from time to time.
The 460 has about a 2db rise at 2.7khz and this can result in some vocals sounding too bright...especially "rock" vocals. However, the 460 will suite some vocalists and it especially seems to work well on female vocals and mellow vocals. However, even on "screaming punk vocals" the 460 never sounds "brittle" to me like some of the RODE microphones and other economical Chinese microphones.
The 460 was built to economically replicate the AKG C12 which does not have a pad. When the AKG C12 was designed no one ever attempted to use a C12 closer than 2-3' from a vocalist and a pad was not necessary. However, the 460 seems to sound much "smoother" with a selected 6072 fitted. The 6072 seems to reduce the gain "organically" and allow the 460 to be used in front of high SPL sources and loud vocals and retain its smoothness.
If you have a drummer with loud cymbals who likes to hit them hard and you used 460's as overheads with 12AX7 or even 6072 tubes tubes in a low ceiling environment (less than 14') then you will probably be hearing some even order harmonic distortion from the tube and output transformer. Probably this senerio of drum recording is a better fit for ribbon microphones or with a 12AT7 in the 460 reducing the gain at least another 3db. However, I have used 460 microphones over a jazz drummer who plays with finesse and got a very nice results without the cymbals sounding mettalic. I had the microphones in OMNI about 6' to 7' of the ground in a studio with 24' ceilings. The 460 is also very nice on acoustic instruments like acoustic guitar and strings but it can be a bit "grainy" on grand piano.
Interesting, how personal taste must be taken into account. I prefer the sound of the 460 on voice/over compared to the U87. I find the 87 to be a bit to upfront in the midrange and "spitty" but a lot of high pressure full on voice-overs require this ....however, I prefer the 460 on straight narration for most voice talent. If I want a more upfront, upper midrange voice/over I would use the APEX 435 which is a Cardiod large diaphragm with a fet preamp and discrete balanced outputs. The 435 sounds closer to an 87 in cardiod than the 460 and it sells for $79. It has about 2db more 7-8k than an 87 but this can be cut at the EQ if necessary.
I have an old friend of mine with big deep broadcast pipes he has a 460 in his home studio which he loves on his voice. He is bringing it over next week for me to upgrade and put in a 6072.
There are lots of microphones that look like the 460 and the Telefunken M-16 which is nearly identical except for the grey finish, Telefunken Logo and inferior response. This difference in response was probably the result of a slightly superior 460 capsule and tube minus a slightly inferior M-16 capsule and tube.
In my experience with the 460 microphones the tubes and capsules make the biggest difference in sound between each microphone. The capacitors make a more subtle difference and are not as dramatic as changing the tubes gain. The microphone preamp must also be able to take high levels on its input before distortion as the 460 can produce a robust output even with a 6072.
Best regards, Dave Thomas (Advanced Audio) DRTAUDIO
drthomas@telus.net