ORS87 results
As I like measuring stuff, I've built an "ORS87" testbench (originally in
this thread) using the ORS87Plus PCB (
here) designed by
@Khron .
Initial measurements are without the feedback / frequency shaping network, for simplicity. Looks like this:
View attachment 135027
The circuit was run off a bench power supply (48V in series with 8k2, connected to S1-/S2+) and the output was taken from P+ (via an OPA1641 buffer, see earlier posts) to a mic input.
Input was connected to BP, via a 68pF capacitor. The FET Q1 and feedback cap C4 were socketed. D1 is 33V and R7 is 20K, as per the ORS87-Plus BOM.
There are
lots of things to play with here, so let's start with C4 and biasing:
Open-loop gain
Using the J113 FET, the chart below show the gain for a variety of drain currents (Id) and choices for C4:
View attachment 135028
The drain current is set by adjusting trimmer R7. It also sets the drain voltage Vd of the FET. We have Id = (V_D1 - Vd) / 57K, where V_D1 is the Zener voltage. In my build the zener measured as 33.6V, so we got the following:
Vd (V) | Id (mA) |
---|
22.2 | 0.20 |
19.4 | 0.25 |
16.5 | 0.30 |
13.7 | 0.35 |
10.8 | 0.40 |
8.0 | 0.45 |
5.1 | 0.50 |
Distortion
Two charts here. The first shows distortion varying with Id, for a fixed input level (100mV RMS at 1KHz).
View attachment 135029
The second shows distortion varying with input signal level, when Id is tuned for lowest distortion at 100mV.
View attachment 135030
Opinions
- Running without C4 isn't really an option, THD is very high without it, and it's asking a lot of the frequency-shaping network to provide corrective negative feedback. The network doesn't provide much feedback except at frequency extremes. I'll measure this later.
- The gain appears to be almost entirely dependent on the capacitances in the circuit, rather than the FET transconductance (which varies with Id, but is independent of C4).
- C4 = 10pF is probably OK for a 4:1 output transformer, and 5pF for a 7:1 or higher transformer, assuming you want around unity gain from capsule to mic output.
- Biasing is important but not super-sensitive - there's not a huge gain or THD variation over the 0.3mA to 0.4mA range, at least with this FET.
I've got some measurements with different FETs coming, we'll see what changes.