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Make sure that the feedback wire is connected to the correct lug of the output jack. If it’s connected to the ground lug instead of the positive lug of the output jack then it would exhibit symptoms like those. Let me know what you find.
Good luck,
-BrachTroy,
I’m sorry the display is giving you trouble.
I agree with you that it does seem like there is no issue with the display itself. My guess is that some pins are not making good contact with the header pins. I often have to bend the display leads pretty far to get them working properly. Are any segments working on all three digits, or are only pieces of 1 or 2 digits working? Also, are the same segments not working on each digit? If we know which segment or digit is not working then we can tell which pins are not making contact.
Let me know.
Good luck!
-BrachThat’s really cool! Please keep us informed with how it works in your installation.
Good luck.
-BrachThat supports my theory. By setting the bias that high, you are increasing the load on the SMPS, allowing it to stabilize. You now have 1mA going through each FET, which is fine, as long as you are ok with the sensitivity of the Cortado at that bias setting. If you need to adjust the bias back down (for sensitivity reasons) then just do as i mentioned in the previous post and plug in a different condenser mic into another channel on the converter to add a higher load to the phantom power SMPS at the same time the Cortado is plugged in to the first channel.
Let me know if you have any more questions.
-BrachI’ve never heard of that problem before, but I don’t think the higher voltage in itself would cause an oscillation. But to answer your question, there is not an easy way to reduce the voltage in the Cortado. Your can try to tinker with the zobel network (R7 and C3) to see if you can nullify the oscillation, but even then, I doubt that’s going to have any affect on it because I don’t think it’s coming from instability in the XLR cable. My thoughts are that you are somehow hearing a lower harmonic from the switch mode power supply of your converter. SMPS’s can be pretty finicky if they don’t get the proper load they want to see…in which case they can go into “burst mode” and become extra noisy….and Cortado’s do present a very low load. Plug in another condenser mic to a different channel on the converter at the same time to give the 48V rail a bit more of a load. The Mackie has a linear power supply, so it’s much more stable.
Good luck.
-Brach…I just got back in town, sorry that I wasn’t able to respond earlier.
That’s great to hear that’s it’s working now! I was thinking there had to be more variables to this issue than I knew about because these Cortado circuits aren’t that complicated. Good job getting it figured out. I’m glad to hear that you are pleased with your Cortado and Espresso!
-BrachAttaching a different piezo disc was going to be my next suggestion, but since you already tried that and it’s still distorted (good job troubleshooting, by the way), then lets start looking at the circuit. As you’ve heard, FETs are all over the map when it comes to consistency in gain. If it is the case that you happen to have a very high gain FET then you can compensate for it by raising the value of R8…try around 2-4k; then check your bias to make sure it’s not too low. Good luck.
That’s good, this confirms that there is most likely something wrong with your piezo disc. I’m assuming that the disc is shielded, is this correct?
Please make sure that the shielding is not connected to any part of the brass or ceramic part of the disc. Also, it’s important to make sure that the PZ1 and PZ2 wires are making proper contact with the 2 parts on the disc. When you measure the resistance between the red and white wires, the reading on your meter should continue to rise until it is too high to measure. This should let you know if the red and white wires are connected properly to the disc.Check these voltages to make sure they are in the same ballpark…This still seems like a piezo disc issue, but we shouldn’t rule out other components. Let me know what you find.
Are you getting 46V on the circuit board at the point where the XLR jack cable attaches to the board? You should be getting 46V on both the red and white wires (pins 2 and 3) from ground (pin 1).
Nick,
It seems like you have 2 unrelated issues. The first is that your Cortado will not bias low enough. This is because the internal gain of some of the FETs is high enough that they need a little more than the 5K of the trim pot to get the bias down to .075V. The .1V that you are able to bias to is perfectly fine. The main issue in this circuit is that the FETs are matched to get the lowest common mode rejection.
The second issue is causing the scratchy bursts of sound. This is usually caused by low phantom power (use your meter to make sure you’re getting 48v from your mixer) or the one side of the balanced configuration of this circuit becoming shorted somewhere. Make sure no part of the piezo disc is shorted to the shielding…continuity test between the 3 wires coming from the piezo disc cable. Also check the XLR jack and make sure nothing is wired backwards or shorted. While you’re at it, double check all your solder joints; you might want to re-flow them if they look suspicious.
Let me know if you have any more questions.
Good luck.
-BrachEven if the solder joints on those parts i mentioned in my previous post look fine, resolder them anyway. Don’t add anymore solder just heat them up until they flow onto the pad. We call it “reflowing” the solder joints. Sometimes it’s easier to reflow all of them than try to trace out the single bad joint. Also make sure that all of those components are in the correct place and nothing is switched. Since you said the volume control has no effect on the sound then it must be in the signal path after pot VR9. Look very closely at all the solder joints around the audio amp ic U3, make sure nothing is bridged…use a magnifying glass if you need to. Good luck. Let me know what you find.
-BrachThat is very peculiar. I’ve never heard a sound that could be described as a rattle coming through the headphone jack. Does it sound more like a mechanical rattling noise or an electrical oscillation? An oscillation could sound like anything from a
“thump thump” to a high pitched whine.
Since the sound is coming from the headphone jack, I’m suspecting the circuit around the LM386 audio amplifier chip could be the issue. Please double check (and even re-solder) the pads in the audio path…R14-16, R19-20, R22, C8-11, C13, C3-4, U3, and of course make sure the speaker and the headphone jack are properly soldered.
Good luck.
-BrachMatthew,
I don’t remember experiencing this particular wooshing sound before.
I know you have experience building circuits, but i have to say this…please make sure all the wires (xlr and piezo) are hooked up to the correct place (don’t forget to check in the xlr jack). Also check with your multimeter to make sure you are getting a very high resistance (almost infinite) between the piezo disc wires…I’ve built hundreds of these things and I still make wiring mistakes sometimes.
It sounds like you know what you are doing on the pcb side of things, which makes me think this issue might be related to the piezo disc. If you unsolder the piezo disc wires from the pcb and plug the XLR in to the preamp, do you still get that sound? If you don’t, i would suggest trying to hook up a different “test” piezo disc (without even shielding it) just to see if you get more expected results.
If you still get the noise when the disc is unsoldered then there’s a good chance that it’s a component failure. It’s pretty unlikely to have a component failure, but still possible.
Let me know what you find.
-Brach
