Takstar CM-60 and CM-63

Introduction

The Takstar CM-60 and CM-63 are two SDC microphones that are very popular among microphone DIY enthusiasts. And for good reasons:

• They are very cheap and abundantly available.
• Yet, they offer features that many other budget SDCs do not offer, such as conformally coated PCBs for good moisture protection and RFI filtering (CM-63 only).
• The 22mm diameter body tubes are very solid and capsule threads are very well machined without burrs. There’s a variety of third-party 22mm capsules available as an upgrade (a.o. from 3U Audio, Mic&Mod, and Microphone-parts).

 

What surprised me is that they differ in so many ways: mechanically, electronically, and in finishing, even though they come from the same manufacturer. You would expect a manufacturer that offers its products so cheaply to standardize parts and modules to achieve greater economy-of-scale benefits. Even if the microphones were targeted for different purposes, one could still standardize components more than they actually did. Just have a look at the B9audio brochure, and you know what I mean.

 

Frequency Response graphs

I copied the Frequency Response graphs from the web and merged them into one picture, depicted below. I hope I can add my own FR measurements later, but I’m still not happy with my setup at the time of writing.

Although the graphs are not the same, both microphones have the typical high-end boost that is common with many (Chinese) microphones. And they also show the famous (or infamous) mid-range scoops before the high-end peak. Far from flat and disliked by many. The 3U capsules, and possibly also other similar aftermarket capsules, are much better. At least if you define a flatter Frequency Response as better. I have bought some 3U capsules and tested those together with my friend Boyd Timothy and we are both very pleased with the results. I would encourage you to visit his Youtube channel and listen to all the shootouts, comparisons, and blind tests that he recorded. Very educational!

Circuit Diagrams

Before diving into the modifications that can be done to the PCBAs and circuits, the Circuit Diagrams will be presented.

 

Both circuits are classic Schoeps designs, with some minor tweaks and changes. For a basic understanding of the Schoeps circuit, you can read more here or here. I will confine myself here to listing a few things that stood out:

CM-60:

• There is a provision on the PCBA to place trimmer P1, which adjusts the JFET bias. But it is not placed. More on this in the modifications section.
• There is also a provision for trimmer P2, but this one is also not placed. And you’d better not do that, because it makes no sense. But the holes can be used for another mod to increase the headroom, which will also be discussed in the Modifications section.
• The RFI suppression capacitors on the outputs measured 2.2nF. In most designs, you’ll find 22nF. I think the lower values are a better choice, reducing IM distortion when using the CM-60 on high-pitched instruments at high SPLs.
• A major difference between the CM-60 and CM-63 is the supply voltage powering the JFET impedance converter/phase-splitter circuit: on the CM-60, it’s just 7.2V, while on the CM-63 it measures 13.4V. This severely limits the headroom and max SPL of the CM-60 to ~124 dB SPL. Too low to use as a snare drum mic.

CM-63:

• Trimmer RV1 adjusts the JFET bias. But the adjustment, or lack of it, by Takstar is all over the place as everybody who cared to measure it mentions different values. More on this in the modifications section.
• Capacitor C13 is a switchable -10 dB pad. See also the Modifications section if you want to change it to -20dB attenuation.
• Unique in this price range is a decent RFI filter in the XLR output that prevents cell phone TDMA noises. Well done!

 

CM-60 and CM-63 PCBAs

An interesting detail of the CM-60 is the slide switch. It has a rather tall actuator and if one presses on the black slide switch, it makes the actuator tilt and harder to slide. But in the picture below, you can see that the slider is supported at both ends by two electrolytic capacitors.

For CM-63 PCBA pictures, I’m posting a picture that I took from this thread on Micbuilders@groups.io, copied under the MIT License. The author, Stephanie, was so kind to project all the Reference Designators on the parts, which eases repairing or modifying on the PCBAs.

An interesting detail of the CM-63 is the XLR connector. It has a spring-loaded contact, connecting pin 1 to the chassis in the shortest possible way, which is required for effective RFI suppression. Unfortunately, these XLR inserts are hard to find, but I found them on Aliexpress here. It’s just a shame that you have to buy 100 pieces at once 😒. 

Measurements and observations

Of both microphones, I measured some key characteristics and listed them together with some observations in the tables below. As said before, no Frequency Response charts yet.

 

 

There are some good things to mention, but also some areas where Takstar could improve their products with little or no extra costs involved. This is what I notice in many products of Chinese origin: they are seldom properly engineered and are not cost- or performance-optimized.

Modifications

On GroupDIY, many modifications to the CM-60 and CM-63 have been discussed. Some are more useful IMHO than others, but that’s up to you to decide. From the observations and measurement data listed above, we can already derive some useful improvements. These include:

1. On the CM-60, increase the JFET circuit supply voltage on C12 to the same level as on C12 on the CM-63. This will lower distortion and increase the dynamic range significantly.
2. Although the self-noise is not terrible, it can be improved, probably by a few dB. But there are limitations, as we’ll see.
3. The JFET bias must be properly trimmed for minimum distortion and improved headroom.

These modifications, and more suggestions for improvement, can also be found in this thread on GroupDIY. There is a nice summary of mods in post #30. Among others, these mods involve replacing SMT parts on the PCBA, which may not be everyone’s cup of tea. But with the right tools and after having built up some experience, you’ll notice that it is actually easier to work on SMT parts! I will dedicate a webpage to this someday.

Let’s go through all the mods one by one.

• On the CM-60, replace R8 by a 3k3 resistor. It can be soldered in the holes of P2, which is not placed. This should raise the voltage on C12 to the same level as on the CM-63.
• Replace the 1G resistors with 2G, or higher if you can get them as SMD parts. These resistors are a major contributor to the low-end noise, as you can read in my KM84 circuit description. The only drawback that I see is that you’ll have to remove the conformal coating, leaving exactly the most sensitive circuit nodes unprotected. With the coating still on and soldering through the coating, it might leave charred residues on the PCBA surface which are conductive. I would only do this mod if you thoroughly remove charred coating debris, and clean the area with IPA. And if possible, coat the parts again, preferably with some acrylic coating. Clear nail polish seems to work well too, but I have no experience with using that.
• Replace the JFET with a 2SK209-GR for less noise and more gain. This should work, of course, but the same notes about the coating described above apply here too.
• Now that we’ve replaced the JFET and raised the supply voltage, we can re-bias the circuit. On the CM-60, remove R3 and R4 and place a 500k trimmer. Search for Bourns 3362U-1-504RLF on the web or at Mouser. Measure the supply voltage on C12 (CM-60) or C1 (CM-63) and adjust the Source voltage to 1/4 and on the Drain to 3/4 of the supply voltage. If you have the means to do THD measurements using e.g. a soundcard and REW, you could also adjust for minimum THD or symmetrical clipping. These points are very close, but the 1/4 and 3/4 adjustments should get you sufficiently close to both.

There were several other mods discussed in the GroupDIY thread, but IMHO the ones listed above are the most important ones. E.g. it was proposed to change the input capacitor. But the one on the PCB is an excellent C0G type. And when swapping it for another component, the only thing that you achieve is killing the moisture protection of the conformal coating.

The best modification you can do to the Takstars is to swap out the capsule with one of the 3U capsules or one from e.g. microphone-parts.com or micandmod.com. Note: I’ve bought some 3U cardioid capsules, and I’m very delighted with them. I bought the 1.5mm centre pin versions, though I assume the 0.5mm should fit also. I don’t have hands-on experience with the other two.

A blind test where both these Takstars are being compared with several KM84 clones was recorded by my friend Boyd Timothy (aka joulupukki on GroupDIY) and shared on GroupDIY here. You will also find other interesting mic shootouts in that same thread and on his Youtube channel. You should definitely check out these blind tests!

Now that we’re talking about  KM84 clones: The KM84+ circuit that I developed and which was used in the the blind tests, I first designed for the CM-60. And a further development of this circuit, which I called the KM84++, I first made for the CM-63. Descriptions of both circuits can be found here.