OPEN SOURCE DIY Mic Project - ORS 87 - Stripped Down u87

[mic test]

I don't know my way around and wanted to ask if it's all right
I measured here

 

[Khron]

I don't know my way around and wanted to ask if it's all right
I measured here

... And that is exactly how you (most likely ) zapped your JFET.

The gate pin is "in the air", not connected to anything..? Any particular reason for that? The 1Gohm resistor that's supposed to be connected to it, is soldered on the board, i see.

Why would you have needed to measure the capsule bias voltage anyway, though?

 

[mic test]

You're right, I'll re-solder that
It has very little gain Apollo Twin 10dB full volume for headphones DT 770 and I sound minimal it sounds muffled and very quiet compared to a brown Phanthera Microphone In the UAD display I come when I speak normally not about -27 dB the brown phanther test is at -13dB

 

[Khron]

I'm not surprised, if the JFET gate is completely floating. Is that still the same JFET from those measurements in the photos?

 

[mic test]

I connected it it is the second circuit in the first was everything soldered on the PCB

 

[Khron]

I connected it it is the second circuit in the first was everything soldered on the PCB

STOP PROBING ANYTHING ON THE JFET GATE!

It might help if you also read and understood other people's advice too:

As a general rule, you should always measure from GROUND to the pin you're checking voltage on. 1) Because generally, voltages are referenced to ground, and 2) because then you won't be inadvertently introducing a higher voltage at that point through the meter.

If you measure JFET pin voltages relative to ground, nothing bad will happen. DO NOT measure JFET pins relative to B+.

 

[TIDStudios]

The components to 'fine tune':
View attachment 124050

@RuudNL So are you just swapping out the c104 47nf cap for a 100nf cap, and changed the 160nf to 180nf? Is that all?

 

[Voyager10]

As an aid to troubleshooting, here's some information about the ORS87-Plus testbed I built.

Schematic​

Here's the schematic with the component values used. The changes from the ORS87+ schematic were mostly to follow the original U87/U87A values more closely.



For testing, the U87's 'calibration input' arrangement was rigged up by replacing R15 with 560R + 6K8 in series, and the input wired to the midpoint. (I understand the V1.1 PCB has space for 2 resistors). I didn't implement a pad or bass-cut switch. Feedback capacitor C4 was 2.7pF, made by putting 5pF and 6pF caps in series.

To simulate a single cardioid K87-style capsule, there's a 47pF capacitor (Ccap) connected between the FD and BP inputs (the Neumann test procedure includes a fixture which does exactly this).

The BF256B JFET was chosen after a shootout (here) between various types: it's nearly the best by various measures, and cheap and readily available. Biasing the FET was done by adjusting trimmer R7 for minimum THD at the output, when fed with a 100mV, 1KHz sine wave at the calibration input. Values in blue are DC measurements at the bias point, and in red are the measured AC values.

The input stage looks like this:

Results​

Results are pretty close to the reported values for the U87A:

Measurement	Value
Overall gain, CAL to Vout, 1KHz (measured at 100mV in)	-1.6dB (0.83x)
JFET open-loop gain (1KHz)	+19dB (9.0x)
THD, 100mV RMS in, 1KHz	0.013%
Max input voltage. THD < 0.5%, 1KHz (RMS)	530mV
Drain current at optimum bias	0.38mA
Polarization voltage	45.0V

I also did a frequency response and distortion plot using REW. This is with a 100mV RMS input level.



We're getting a moderate 3-4dB dropoff at 15KHz, and a stronger rolloff after that. Obviously some of the distortion is coming from the transformer, which is relatively cheap compared to many alternatives.