So i got the chance to work on one of these. Here's a schematic i came up with. It should be correct, not sure about transformer feedback path as i didn't want to try remove the transformer. The voltages are off due to error in LTSpice simulation, disregard them. B+ is 330v, zener is at about 130v, capsule at 60v. Transformer inductances are just for the purpose of the simulation, not correct.
A @qspart did his own reverse engineering, and built the circuit. There are some differences, not sure if one of us made a mistake or if there is variance between revisions.
What if you opened up a Manley Reference C and ...
Edit: C3 should be 100n, i entered it wrong in the schematic!
The circuit is interesting, they surely managed to tame the 12ax7. It is not particularly noisy, but i haven't measured exact noise figure.
Transformer ratio is 11:1 for audio out, and 14:1 for the feedback. R19 is connected to ground for -10db pad.
I injected various level signal from my audio interface to check for the circuit response and THD behavior. Just db values, no exact voltage values sorry.
The circuit is flat, and due to feedback there is not much tube sound to speak of. At -30db injection signal which would translate to typical vocal spl range it is squeaky clean. Typical THD for FET mics, not for tube mics.
At -20db still very clean, but the transformer starts to misbehave bellow 50hz. My standard topology tube mics have between 2% and 3% THD at this input level.
At -15 we get 1%THD at 1k. But above and below it's not the same story- At 50hz we have 5.3%
And at 4k we get 3.82%, and it gets worse above that.
Circuit wise the mic takes quite high SPL before it starts rapidly to go up in THD. Not typical for a tube mic.
Now the capsule. As i have mentioned before they use cheapo 30 $ (at best) capsule, and this is my biggest complaint having the price of the mic in mind. Very sharp HF boost, preceded by dip at about 5.5k. The dip is accentuated by the grille of the microphone. Good k67 doesn't have this dip, the rise is smooth. The dip contributes to the hyped and sharper Q HF boost.
Nothing special about this capsule, this is typical response found in many 100 $ range microphones.
No matter what the circuit does, is it tube or not, this is perfect example of something many call ''China sound''.
Green - Capsule on it's own, Blue - Frequency response of the microphone itself. Considering the type of body used, they have done great job to make it pretty transparent. Usually there are more deviations in this type of body.
Here's the Manley capsule against identical capsule taken out of Sterling Audio ST51. ST51 is red. Disregard the low end bump, it is the measurement artefact. This is the same capsule, manufactured or distributed by Feilo.
They do deserve some slack, the mic came out in 1991, so i doubt there was much knowledge about capsules and many alternate sources. Around that period popular mod for u87 was to remove the de-emphasis and bring back that HF response. This was still analog age, so extra HF was often useful.
Edit:
I almost forgot. The capsule hiZ connection, grid, and 300Meg resistor are soldered to the board, and the flux isn't cleaned. Not really visible, there are lots of it remaining on the board. Ironically, the lady owner from Manley mentions in a tube swap video to be careful not to touch the sensitive area with bare hands
One more issue it has is the fact that the capsule stand is made out of soft rubber, and similarly to Røde NT1 (black) it vibrates, and resonates substantially at LF. Here's a video demonstrating the potential issue.
My conclusion is that the mic is unique circuit wise, it will not give you typical tube mic behavior, no ''warmth'' or character. At high SPL the mic will start to sound very ugly, very quick. A mic at this price point deserves much better capsule.
A @qspart did his own reverse engineering, and built the circuit. There are some differences, not sure if one of us made a mistake or if there is variance between revisions.
What if you opened up a Manley Reference C and ...
Edit: C3 should be 100n, i entered it wrong in the schematic!
The circuit is interesting, they surely managed to tame the 12ax7. It is not particularly noisy, but i haven't measured exact noise figure.
Transformer ratio is 11:1 for audio out, and 14:1 for the feedback. R19 is connected to ground for -10db pad.
I injected various level signal from my audio interface to check for the circuit response and THD behavior. Just db values, no exact voltage values sorry.
The circuit is flat, and due to feedback there is not much tube sound to speak of. At -30db injection signal which would translate to typical vocal spl range it is squeaky clean. Typical THD for FET mics, not for tube mics.
At -20db still very clean, but the transformer starts to misbehave bellow 50hz. My standard topology tube mics have between 2% and 3% THD at this input level.
At -15 we get 1%THD at 1k. But above and below it's not the same story- At 50hz we have 5.3%
And at 4k we get 3.82%, and it gets worse above that.
Circuit wise the mic takes quite high SPL before it starts rapidly to go up in THD. Not typical for a tube mic.
Now the capsule. As i have mentioned before they use cheapo 30 $ (at best) capsule, and this is my biggest complaint having the price of the mic in mind. Very sharp HF boost, preceded by dip at about 5.5k. The dip is accentuated by the grille of the microphone. Good k67 doesn't have this dip, the rise is smooth. The dip contributes to the hyped and sharper Q HF boost.
Nothing special about this capsule, this is typical response found in many 100 $ range microphones.
No matter what the circuit does, is it tube or not, this is perfect example of something many call ''China sound''.
Green - Capsule on it's own, Blue - Frequency response of the microphone itself. Considering the type of body used, they have done great job to make it pretty transparent. Usually there are more deviations in this type of body.
Here's the Manley capsule against identical capsule taken out of Sterling Audio ST51. ST51 is red. Disregard the low end bump, it is the measurement artefact. This is the same capsule, manufactured or distributed by Feilo.
They do deserve some slack, the mic came out in 1991, so i doubt there was much knowledge about capsules and many alternate sources. Around that period popular mod for u87 was to remove the de-emphasis and bring back that HF response. This was still analog age, so extra HF was often useful.
Edit:
I almost forgot. The capsule hiZ connection, grid, and 300Meg resistor are soldered to the board, and the flux isn't cleaned. Not really visible, there are lots of it remaining on the board. Ironically, the lady owner from Manley mentions in a tube swap video to be careful not to touch the sensitive area with bare hands
One more issue it has is the fact that the capsule stand is made out of soft rubber, and similarly to Røde NT1 (black) it vibrates, and resonates substantially at LF. Here's a video demonstrating the potential issue.
My conclusion is that the mic is unique circuit wise, it will not give you typical tube mic behavior, no ''warmth'' or character. At high SPL the mic will start to sound very ugly, very quick. A mic at this price point deserves much better capsule.
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