Takstar CM 60 suggested mods/improvements (?)

[Voyager10]

The CM-63 electronics is better in most aspects. The JFET stage runs on 13.4V. Much better. But the bias adjustment was totally off...

The CM-60 can be fixed to use a higher voltage (in this schematic, raising the value of R10 will do it). But I do wonder whether it's really worth saving $10 over the CM-63, given that you need to fit a bias pot as well (plus the RFI stuff is harder to fix, and you won't have a pad switch).

 

[MicMaven]

Are these actually recorded at the same time? To me the mic position sounds different.

They were recorded one after another at the same position and distance from the speaker/player with exactly the same flat software settings. Crazy eh? James

 

[pmfalcman]

1000pf is commonly seen between the capsule and the FET.

But, on the photo I see 100nJ marking, which, to my knowledge, is 100 nF 5 %.
Hence my question.

 

[jp8]

The CM-60 can be fixed to use a higher voltage (in this schematic, raising the value of R10 will do it). But I do wonder whether it's really worth saving $10 over the CM-63, given that you need to fit a bias pot as well (plus the RFI stuff is harder to fix, and you won't have a pad switch).

On the CM-63 schematic, R6 is the same resistor as R10 on the CM-60 schematic and is 3k3 instead of 150 Ohms. That should fix the low voltage on the JFET circuit. And, for somewhat lower noise, the JFET can be replaced by a 2SK209-Y (mind the Y). I did not try that, though. But it should work. I bought the CM-60 and CM-63 just to serve as donor bodies and as a reference to compare my own PCBAs with. I did not intend to modify the stock PCBAs. Replacing the two resistors on the CM-63 was all I did.

Jan

 

[HerbertR]

On the CM-63 schematic, R6 is the same resistor as R10 on the CM-60 schematic and is 3k3 instead of 150 Ohms. That should fix the low voltage on the JFET circuit. And, for somewhat lower noise, the JFET can be replaced by a 2SK209-Y (mind the Y). I did not try that, though. But it should work. I bought the CM-60 and CM-63 just to serve as donor bodies and as a reference to compare my own PCBAs with. I did not intend to modify the stock PCBAs. Replacing the two resistors on the CM-63 was all I did.

Jan

It is not entirely clear form me why changing the R10 150Ohm from the DC-DC input converter should bring a higher voltage to the JFET source. The voltage measured at about 5-6V on the JFET is relatively low and is, as I understand, provided directly and unregulated from the phantom power via R11 6k8 and on to R6 2K. The DC converter in my case also provides about 46V before the R12 1G Ohm connection and the "big" filter capacitor 100nF to provide the capsule voltage. I think to increase the JFET voltage the only way would be to lower this "main resistor" R11 6K8 (the voltage drop is about 27V across the resistor, drawing about 4mA for the whole circuit), but this would also affect the DC converter input voltage at the same time. With the R9 2K, there is still a drop of about 2 - 2.5 V/1.2 mA across the JFET voltage rail. Theoretically, the R11 should be reduced to 5K, which would only drop about 20V and give an "optimal" 12-13V at the FET after the R6 2K ohm. In parallel, the R10 150 Ohm would also have to be increased to about 3K (2.7mA) to hold the 7V in order to avoid a burst of about 14.5V at the DC converter after the R10 150 Ohm. Or am I wrong?

But the good thing is that the microphone seems to work very good even with just 5 V on the FET and some mods.
Yes, the C7 capsule capacitor to the gate is 1000pF on the CM60. I cannot say whether this is a C0G SMD. The switch to Styroflex is noticeable fuller a note imo, the 2SK117 also feels noticeably more transparent and nicer in the low end. The 100nJ Cap C2 is a Capsule Voltage Filter Cap. Nothing super special. Regards

 

[kingkorg]

You are ignoring the zener guys. I remember changing ZD1 to higher value, and adjusting R11. Can't remember the values off the top of my head. I get better noise performance as well with ca.60V on the capsule. However original Schoeps has voltages in this range at the drain 5-6v or something.

 

[Voyager10]

It is not entirely clear form me why changing the R10 150Ohm from the DC-DC input converter should bring a higher voltage to the JFET source. The voltage measured at about 5-6V on the JFET is relatively low and is, as I understand, provided directly and unregulated from the phantom power via R11 6k8 and on to R6 2K.

Think of R11 and R10 as a voltage divider, where one end is at +32V or so, and the other end is fixed at 6.8V by the zener ZD1. Increasing R10 reduces the voltage divider's ratio so the R10/R11 junction will then be at a higher voltage.

(The zener passes as much current as is necessary to keep the ZD1/R10 junction at +6.8V. Think of it as 'absorbing' all the surplus current which comes through R10 but isn't needed to power the DC converter).

 

[HerbertR]

Think of R11 and R10 as a voltage divider, where one end is at +32V or so, and the other end is fixed at 6.8V by the zener ZD1. Increasing R10 reduces the voltage divider's ratio so the R10/R11 junction will then be at a higher voltage.

(The zener passes as much current as is necessary to keep the ZD1/R10 junction at +6.8V. Think of it as 'absorbing' all the surplus current which comes through R10 but isn't needed to power the DC converter).

You are a "fox" ... I completely overlooked that, thanks!!!