NEED HELP!!! Aurycle a460 Build, input gain issue

[Roachbeatz]

I recently did a DIY u87 type build to the Aurycle a460. I swapped out almost all of the components with parts I  purchased from mouser and a used an Advanced Audio AK67. After finishing the build and plugging it in, I got no sound at all from the capsule. I removed the wire from the backplate and screwed it into a different hole on the capsule. After powering it back on, I started to get sound...but here's the thing...I have to max out my gain on my preamp in order to hear an audible voice through it. If I tap hard on the grill I don't hear any "thumping" noise at all 90% of the time. But if I keep tapping, the mic will clip and pop then all sound drops out and comes back. Also, if I slightly brush my finger along the red wire connected to the capsule, I can hear it loud and clear. Also, the 10dB pad seems to be doing nothing at all in respect to volume. I am pretty lost as this is my first build and I am not very savvy yet with all of the terms used with microphone building. I do have a trim pot in R6 at the moment as I was going to bias the fet but wasn't able to make it that far yet. Below is a copy of original BOM and attached is the BOM I used instead; also the schematics. If anyone can please help point me in the right direction as to what to do it would be EXTREMELY appreciated! Thanks.

R1 Resistor 1/4W 6.8K
R2 Resistor 1/4W 560
R3 Resistor 1/4W 1M
R4 Resistor 1KM
R5 Resistor 1KM
R6 Resistor 1/4W 2K-3.9k
R7 Resistor 1/4W 1.8K
R8 Resistor 1/4W 20K
R9 Resistor 1/4W 180K
R10 Resistor 1/4W 7.5K
R11 Resistor 1/4W 2.2K
R12 Resistor 1/4W 2.2K
R13 Resistor 1/4W 47 R14 Resistor 1/4W 47
R15 Resistor 1/4W 150K
R16 Resistor 1/4W 150K
R17 Resistor 1/4W 330K
R18 Resistor 1/4W 4.7K
C1 Capacitor CBB 0.01μ
C2 Capacitor NPO 1000P
C3 Capacitor NPO 220P
C4 Capacitor CA42 22μF/16V
C5 Capacitor 0.47μ
C6 Capacitor CBB 0.047μ
C7 Capacitor CBB 6800P
C8 Capacitor CA42 4.7μF/35V
C9
C10 Capacitor 330P
C11 Capacitor 330P C12 Capacitor CA42 6.8μF/40V
C13 Capacitor CA42 2.2μF/50V
C14 Capacitor CA42 2.2μF/50V
CF Capacitor CC1-150p
D1 FET 2SK170
D2 Transistor 1015
L1 Inductor 47μH
L2 Inductor 47μH

 

[Roachbeatz]

Update: So I removed the pot on R6 and added in a 2.2k resistor. Everything is working correctly now. But I am still having the gain issue. My gain knob goes up to 60 and I have to use every bit of "60" in order to get a good signal from the mic. All of my other mics get a great signal at about 15-18 on the gain knob.  Anyone know why or where to start in regards to why I am getting such horrible input gain?

Is it possibly the transformer? I kept the stock in  (1.8:1). Should I be using the 2SK170 FET instead? I understand this is a Medium Gain fet.

Right now I have a high gain 2N3819 in with the stock 1:8:1. Should this FET be used in conjunction with a T13 transformer instead?

Sorry I'm a complete noob at all of this. Just need some answers to help get me rolling please and thanks!

 

[Khron]

DC voltage readings?

 

[Roachbeatz]

I'll have to look up where I take the readings at. When I said I was a complete noob at this I was serious. Possibly you can assist me, based on the schematic I attached, which contact points would I place the multimeter probes onto for the readings you request?  Also, what all readings would you need, such as supply, drain, etc.?

 

[Khron]

Let's try to take them in order of priority; all voltages in reference to ground:

- common point of R11 and R12
- C12
- R8 / C5 / JFET drain
- C14
- C13

 

[Roachbeatz]

Let's try to take them in order of priority; all voltages in reference to ground:

- common point of R11 and R12
- C12
- R8 / C5 / JFET drain
- C14
- C13

When you say in reference to ground, does this mean black probe on ground of circuit board, and red probe on one of the contacts of the component? Also, I take these readings with 48v on I assume. Does gain have any significance here, or is line input fine?

 

[Roachbeatz]

EDIT!:::Not sure if this matters but for R10 I used a 6.8k 1% resistor. I was told to use this value instead of 7.5k .... "With 6.8K feeding the output transistor there will be another db or two of output level before clipping." -Dave (Advanced Audio)

 

[Roachbeatz]

Also, when testing with the multimeter, what setting should I have the meter on?

 

[Khron]

1) Yes.
2) Of course you'll need the phantom power on; otherwise, how will the circuit be powered, and what voltage readings can you get?
3) What "line input"???

When you say in reference to ground, does this mean black probe on ground of circuit board, and red probe on one of the contacts of the component? Also, I take these readings with 48v on I assume. Does gain have any significance here, or is line input fine?

Well, we're looking for (DC) voltage readings, so... volts then?

Also, when testing with the multimeter, what setting should I have the meter on?

 

[Roachbeatz]

1) Yes.
2) Of course you'll need the phantom power on; otherwise, how will the circuit be powered, and what voltage readings can you get?
3) What "line input"???

Well, we're looking for (DC) voltage readings, so... volts then?

Sorry if my responses seem like I have no clue what is going on...I kind of dont lol. I'm literally learning as I go. What I meant about the 48v was since I'm feeding the 48v from my interface, does the gain knob on the interface play any role here? Like does it matter if the gain is turned all the way down to 0 on the interface while taking the measurements? Also in respect to DC voltage, I was referring to what value on the multimeter dial should I use to collect these readings? 200m, 2, 20, 200, 1000?

 

[Khron]

No, it does not. And since the circuit is supplied with said 48v, that's pretty much the highest voltage one would expect to see, so... Using the 200mV range wouldn't make all that much sense, would it?

Sorry if my responses seem like I have no clue what is going on...I kind of dont lol. I'm literally learning as I go. What I meant about the 48v was since I'm feeding the 48v from my interface, does the gain knob on the interface play any role here? Like does it matter if the gain is turned all the way down to 0 on the interface while taking the measurements? Also in respect to DC voltage, I was referring to what value on the multimeter dial should I use to collect these readings? 200m, 2, 20, 200, 1000?

 

[Roachbeatz]

Understandable, again, I'm still learning. I wasn't sure exactly how the DC voltage worked as I've never used it. I've only used the 1.5v and 9v option for batteries and ohms for resistance. I suppose now that I look at my question, it seems a bit redundant.

 

[Roachbeatz]

Let's try to take them in order of priority; all voltages in reference to ground:

- common point of R11 and R12
- C12
- R8 / C5 / JFET drain
- C14
- C13

- common point of R11 and R12 >>>>>>32.2v
- C12                                                            >>>>>>17.8v
- R8 / C5 / JFET drain                        >>>>>>3.2v
- C14                                                          >>>>>>31.7v
- C13                                                          >>>>>>31.2v

 

[Roachbeatz]

To my understanding, the FET drain should be more around 10v...correct?

I turned my 5k pot up all the way and retested the voltage on the FET drain and I am now getting 4.5v...I have some 10k pots on the way and should be here Thursday

EDIT* > I tried putting a 20k resistor in place of the 5k pot...this gave me a drain voltage of 9.8v

 

[Khron]

These readings are with the 2N3819 installed? Might be worth trying a 2SK170 as well, if you have one (or several).

It doesn't have that much to do with the "gain" of the JFETs, as much as with the bias conditions. Not to mention, the parameter spread of JFETs is all over the place, to begin with (really wide tolerances).

Point being, the bias (read: component values that set it) that would be "ok" for a 2SK170 could be way off for a 2N3819 (can't be bothered to do the math or run any sims right now, though).

 

[Roachbeatz]

These readings are with the 2N3819 installed? Might be worth trying a 2SK170 as well, if you have one (or several).

It doesn't have that much to do with the "gain" of the JFETs, as much as with the bias conditions. Not to mention, the parameter spread of JFETs is all over the place, to begin with (really wide tolerances).

Point being, the bias (read: component values that set it) that would be "ok" for a 2SK170 could be way off for a 2N3819 (can't be bothered to do the math or run any sims right now, though).

Yes that is with the 2n3819. I do have a 2sk170.....I also have a J309. I've read that the J309 gives a clearer transient response in the high end.  Which would you recommend between the 2?

 

[Roachbeatz]

Replaced the 2n3819 with the 2sk170...my gain is now normal!

My next question, how do I hook up the oscilloscope for biasing?

 

[Khron]

There you go, then. Who would've figured, two different parts behave differently?

You'll also need a signal source (desolder the capsule from the gate coupling cap, and couple the signal source there).

The scope probe goes to either end of the DC blocking cap, ahead of the transformer.

 

[gyraf]

as also mentioned in this thread - https://groupdiy.com/index.php?topic=75443 - the world is currently flooded with fake transistors posing as jfets. Your 2N3819 is most probably such one.


/Jakob E.

 

[Khron]

https://groupdiy.com/index.php?topic=61057.msg956855#msg956855 :