A Pair of TL/C Mics
A Pair of TL/C Mics
Friday, December 25, 2015
I built a pair of mics to explore the charge amplifier circuit, in this case transformer-less. The original charge amplifier mics were mostly single FETs driving a transformer like the KM-84 and the U87a from Neumann. For a modern version of that circuit, there are Microphone-Parts T-series kits. Checking out a T-47 and comparing it to my old mics got me thinking about a more modern version of the charge amplifier. I had a pair of K-5000 mics on hand to use for bodies and a pair of RK-12 capsules, so the mics were completed over last weekend. The circuit is based on the original K-5000 printed circuit board and described here.
The modified amplifier boards and DC-DC converter boards look like this:
Pretty easy except that dealing with SMT parts is a PITA. When assembled they look like this:
The RK-12 capsules fit right into the existing silicone shock mounts in the headbaskets. Easy.
I have another pair of K-5000 bodies with RK-12s and “Super Schoeps” low capacity front ends, so a comparo is in order. For grins, a Schoeps RK-47 in K-5000 body and a Schoeps RK-12 in a MXL 990 body were also compared to the new mics. So here are a bunch of frequency runs.
RK-12
#1837 (green) vs #1868 (yellow)
matched pair of TL/C mics
Cardioid
#1837 vs #1868
TL/C
Omni
#1837 vs #1868
TL/C
Fig. 8
#1837 TL/C
Card (green) Omni (yellow)
#1837 TL/C
Card (green) Fig.8 (yellow)
The sound check of these mics is here.
A few other measurements made on this circuit with a 100pF dummy capsule:
Gain vs feedback cap, no cap = 0dB;
10pF = -3.4dB; 25pF = -5.9dB; 48pF = -8.7dB; 68pF = -10.7dB; 115pF = -13.4dB.
@100mV (280mV p/p) out w/10pF, 2nd harmonic = 0.5%; @ 10mV out, 2nd = 0.05%
The page is getting long, so comparison with other designs continues on the next page.
