M49C circuit details / questions

[OneRoomStudio]

The "treble cap" is indeed audible. The LPF in your graph already starts between 2-3k.

This cap works well with AC701 since it has a very detailed and excited top end. Other tubes can sound very dull and dead with it.

It does start at 2-3K, but it's very shallow. It's only down <0.5dB at 20K. At 10K, it's down less than 0.1dB. You must have better ears than me to hear it.

 

[pasarski]

It does start at 2-3K, but it's very shallow. It's only down <0.5dB at 20K. At 10K, it's down less than 0.1dB. You must have better ears than me to hear it.

I definitely couldn't hear that. But it introduces some distortion, especially at high frequencies, that could be heard.

 

[tomas.borgstrom]

It does start at 2-3K, but it's very shallow. It's only down <0.5dB at 20K. At 10K, it's down less than 0.1dB. You must have better ears than me to hear it.

I’ve experimented a lot with this. Our ears are very sensitive. (BTW I’m not a hifi golden ears guy. I’m very critical and I know that our brains can easily fool us).

I definitely couldn't hear that. But it introduces some distortion, especially at high frequencies, that could be heard.

I agree, a low pass filter is not the only thing that is going on.

 

[Roman Beilharz]

Interesting thread, as I intend to start building a M49c clone right now and I don't want to implement the calibration input, either. Still I don't see one of Khron's original questions answered here - or maybe I didn't catch it?! It was:

"...would the 2meg R12 still need to exist, and be connected to the pattern adjust voltage?"

So: Do I have to throw in 2M between POL and the bottom of C3 regardless or not?

Thank you in advance for clarifying!

 

[OneRoomStudio]

Interesting thread, as I intend to start building a M49c clone right now and I don't want to implement the calibration input, either. Still I don't see one of Khron's original questions answered here - or maybe I didn't catch it?! It was:

"...would the 2meg R12 still need to exist, and be connected to the pattern adjust voltage?"

So: Do I have to throw in 2M between POL and the bottom of C3 regardless or not?

Thank you in advance for clarifying!

I did not include that in my mic (I essentially built Oliver's version in post #12), and I am very happy with how mine turned out.

 

[OneRoomStudio]

The "treble cap" is indeed audible. The LPF in your graph already starts between 2-3k.

This cap works well with AC701 since it has a very detailed and excited top end. Other tubes can sound very dull and dead with it.

I owe @tomas.borgstrom an apology. I thought the effect of this cap would be inaudible due to the shallow nature of the curve, but I now think I was wrong. I tried it out in my M49 (built around a 5718/EC71, based on Oliver circuit), and I found the differences to be very audible. I made two recordings. While I refer to them as "Microphone A" and "Microphone B," they're actually the same microphone through the same preamp and cable, in the same position, etc.

Obviously, this is not a scientific comparison, since it's (by necessity) a recording of two different takes, but I think you'll find the difference is pretty clear (listen to how "sharp" or "crispy" they sound). Lets see if everyone can guess which is which.

Again...the only difference is the presence or absence of "C6" from the original schematic (a 600pf - or 560pf in my case, from plate to ground). Side note, the noise/hum in the background is an air purifier I didn't bother turning off, not microphone noise.


View attachment Mic A.wav

View attachment Mic B.wav

 

[garp]

A is brighter ... but not only ...