Forum Replies Created
-
Posts
-
Attaching 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.
-BrachI’m sorry to hear you are having trouble. I have never heard of that particular issue before. Do you hear anything through your headphones when you plug them in to the output jack (keep the volume low when testing this)? Let me know what you find out.
-BrachGiovanni,
Sorry for the late response.
In reference the “quick start guide”…First of all make sure the depth, attack, and release knobs is all the way to the left. Plug in headphones or computer speakers and then test it again. If you still can’t hear any sound, then double check your solder joints in the audio section of the circuit board…which is all the components connected to the LM396 amplifier chip. Make sure everything is connected properly and nothing is connected that shouldn’t be.
If you still can’t figure it out, let me know and we’ll start looking at the voltages around the audio section to see if everything is functioning properly.
Good luck.
-BrachGreat! I’m glad to hear that it’s up an running. Yes, the socket should be tight enough to make good contact with the tube pins…I’ve also learned that lesson the hard way!
Enjoy the amp!
-brachIt looks like you fixed that issue. What does TP9 read now?…i’m assuming that it’s around 70V?
Are all the other test points reading correctly?
Just for good measure you may want to reflow the other joints around the signal path and tighten the other tube pins. To keep from having to pull the board out of the chassis just solder as many components as possible on the component side of the board and see if that fixes the signal path problem.
Do you, by chance, happen to have access to an oscilloscope or other signal tracing tool? If not, no problem…just thought i’d ask.…and yes, R7 and R14 would measure about 750R in the circuit because they are nearly in parallel. One side of both resistors are on the same node. R7 goes to ground and R14 goes to ground through the secondary of the output transformer which has a negligible dc resistance.
-brachLet’s get this fixed…
According to your voltage chart (by-the-way, thanks for that…it’s very helpful) R8 only has 1 volt dropped across it, which tells me that there is very little current flowing into pin 4, which is why TP7 only has 32mV across it. The problem seems to be with either pin 4 or 7 on the tube. The fact that you are getting no sound at all leads me to think there is an open somewhere in your circuit (probably at pin 4 on the tube). Resolder pins 4 and 7 on the tube to make sure they are solid, as well as R8 and R7. Also check to make sure the tube socket itself is making good contact with the tube pins. If you pried the tube socket contacts open too far then try to pinch them closed a bit more. When you get TP7 and TP9 fixed then the voltage on TP5 will probably normalize. Make sure you drain the caps before soldering pin 4! Keep me posted on what you find.
Good luck!
-brach
