Hi Klaus,
thank you so much for your reply
IF and this is a big if, the circuit and component values are 100% identical between the original and the copy mics, increasing the coupling capacitor five fold would considerably increase the level of very low frequency content in the copy mic.
But I am skeptical that we are actually comparing similar circuits. The transformer alone is such a beast to duplicate, let alone the capsule...
Yes, component values are almost 100% identical.
C12, C13, C14, C15 and C16 capacitors are 2200pf instead of 2000pf, which is negligible.
C17 is 100pf, a pretty standard value in U67
and C9 is 4,7uf 160v.
I traced the circuit and all the other components have exactly the same values as the U67 schematic.
Transformer is a Lundahl LL1965B.
Capsule is a 3U Audio "flat K67 type", it's a capsule made similar to the K67 but with less holes in the backplate to achieve a flatter and with less high boost than a standard K67.
This choice of capsule it's a bit strange since the C17 value is 100pf, I don't see the point because it will just make the microphone darker and not take full advantage of the Emphasis/De-emphasis circuit that is part if the U67 sound
You can easily substitute the 4.7mfd cap with with a 1.0 or 0.5mfd and listen to the results. Your ears will tell whether that is an improvement or a detriment.
I will do that for sure, will try the stock 4,7uf value versus U67 1uf or 0,5uf value and see which I prefer.
I will have to replace stock C9 4,7uf capacitor anyway since its voltage rating is only 160V, and at power up and power down that capacitor is hit with a much higher voltage than it's rating.
In addition, you can also call the company that makes the copy mic, and test their willingness to engage a curious customer on this level of inquiry.
It sure will tell a lot how serious they take their customers' intelligence and how much they respect inquisitiveness.
I did that actually, I sent them an email. They told me they would get back to me but it has been 2 weeks and no reply.
Maybe they didn't like my inquiry or maybe they will reply at some point, I don't know yet.
But I'm pretty sure they are not much interested in customer inquiries about the circuit of their products and are more interested in just marketing and selling.