I have a Mackie DL1608 digital mixer with analog preamps with a weird problem.
The problem lies within the preamp circuit.
The hiss is common to all channels at very high gain, close to max all of the sudden a hiss appears, even without anything connected to the preamp.
Could this be mitigated or eleminated?
But one of the channels has a different problem,
With a mic connected if I increase the gain to the noticeable hiss zone and grab or excite (talk) into the mic, something that will light the signal led, it produces a kind of humm effect that latches and does not go away unless I reduce the gain out of the hiss zone.
I will post the schematic for the preamp
Any ideas?
I have swapped op amps U21 and U22 on channel B-1 with other channels and also electrolytic capacitors C142 and C230 but the problem did not go away or migrate.
Thanks for any help!
The problem lies within the preamp circuit.
The hiss is common to all channels at very high gain, close to max all of the sudden a hiss appears, even without anything connected to the preamp.
Could this be mitigated or eleminated?
But one of the channels has a different problem,
With a mic connected if I increase the gain to the noticeable hiss zone and grab or excite (talk) into the mic, something that will light the signal led, it produces a kind of humm effect that latches and does not go away unless I reduce the gain out of the hiss zone.
I will post the schematic for the preamp
Any ideas?
I have swapped op amps U21 and U22 on channel B-1 with other channels and also electrolytic capacitors C142 and C230 but the problem did not go away or migrate.
Thanks for any help!
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Update!
The sound is not a hum it sounds like switching noise.
I removed u24 ic, q37 and q38 which are responsible for lighting the signal led d46 and the noise goes away, still don’t know what could be causing it.
With these parts installed from a good channel the noise is still there
The noise only occurs when the signal led is on!
The sound is not a hum it sounds like switching noise.
I removed u24 ic, q37 and q38 which are responsible for lighting the signal led d46 and the noise goes away, still don’t know what could be causing it.
With these parts installed from a good channel the noise is still there
The noise only occurs when the signal led is on!
Unfortunately I don’t own an osciloscope
@rayma , there are only two electrolytic caps and I swapped them with a good channel.
Tomorrow I do more tests!
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Did you just get this mxer or is it something you have had for a while?
It's not likely that the gains would ever need to be at the max position, in fact a mic gain of 30ish dB is usually plenty, if the markings on the controls are accurate that is about 1/2 of the total available. 60dB of gain is huge, it would bring the noise floor right up to audible levels. Have you plugged in a mic to test? There may be nothing wrong with the mixer.
It's not likely that the gains would ever need to be at the max position, in fact a mic gain of 30ish dB is usually plenty, if the markings on the controls are accurate that is about 1/2 of the total available. 60dB of gain is huge, it would bring the noise floor right up to audible levels. Have you plugged in a mic to test? There may be nothing wrong with the mixer.
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I’ve had the mixer for many years! I indeed do not use the gain up so high but last gig, the “latching led signal noise” migrated to a lower gain setting and was quite noticeable during the set, so much that I had to gate it off.
It also occurs only on channel 1
At first I thought it was a cable problem, but it is not. The noise, not the hiss, only occurs with a plugged in mic, and when there is something to excite it enough to light the signal led
It also occurs only on channel 1
At first I thought it was a cable problem, but it is not. The noise, not the hiss, only occurs with a plugged in mic, and when there is something to excite it enough to light the signal led
The schematic shows that there are many different internal ground nets. Could something have gone wrong with the grounding, causing excessive crosstalk from the LED driver to the signal path? Increased ground resistance due to a dirty connector, for example?
In old-fashioned big console mixers, the design was usually so modular that you could easily swap entire channels. If that should also be the case in your mixer, you could take out the module for channel 1 and swap it with some other channel to see if the problem moves with it or stays in channel 1.
OK that is all good info. If I inderstand correctly the noise only occurs once an input signal is high enough and that results in the signal led staying on? I'd suspect this (LED stays on) is simply a result of the noise signal being present and constant which means there is probably nothing wrong with that part of the circuit.
A switching noise would originate in the power supply, there would be filters for that but a failing capacitor could leak some onto the ground which then finds it's way into the audio circuitry. Do you have a schematic for the power supply?
Power supply is an external 12V brick! I will try another one.
The noise is present every time the led flashes but only latches at high gain settings and only after I “excite” the mic by talking or tapping it.
Only one channel does this
Could humidity around the feedback resistors and capacitors cause this issue? I live in the middle of the Atlantic in a very humid place
Attached is the preamp section image and a link to the full service manual
DL1608 service full
The noise is present every time the led flashes but only latches at high gain settings and only after I “excite” the mic by talking or tapping it.
Only one channel does this
Could humidity around the feedback resistors and capacitors cause this issue? I live in the middle of the Atlantic in a very humid place
Attached is the preamp section image and a link to the full service manual
DL1608 service full
Attachments
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Check the +/-15V supplies and ground in that channel are correct (and have low-ohm connection to the rest of the circuit), and test the transistors that drive the LEDs for that channel. Look very carefully at any solder joints for dry joints or hairline cracks, especially on the potentiometer legs which will see mechanical stress in use.
I fear there are many ways it could fail and it may take some hunting down. For instance it could be a fault at the other end of the ribbon connectors too...
I fear there are many ways it could fail and it may take some hunting down. For instance it could be a fault at the other end of the ribbon connectors too...