Hi!
I'm turning to you for help finding a faulty component causing a "crackling" noise in a Presonus Studio Channel (Microphone preamp).
I've recorded the noise (link at the bottom) and my question is if any of you more experienced guys -based on the sound in the link- can give a good guess on what type of component is likley to cause this type of noise, if that even possible?
What do you say? Does it point in any direction? Is it still likely to be a faulty cap? 🤔
I'm turning to you for help finding a faulty component causing a "crackling" noise in a Presonus Studio Channel (Microphone preamp).
I've recorded the noise (link at the bottom) and my question is if any of you more experienced guys -based on the sound in the link- can give a good guess on what type of component is likley to cause this type of noise, if that even possible?
- The noise emerges early in the circut (before the tube and input gain knob) and gets lounder/disappear when I turn the input pot.
- It is not affected by any microphone connected/disconnected to the preamp
- Sometimes it is audible directly on startup, other times it takes a few minutes before it appears.
- It is not affected when I wiggle any components.
- There are no visual indication of heat damage on any of the components, no bulging caps.
What do you say? Does it point in any direction? Is it still likely to be a faulty cap? 🤔
Welcome to diyAudio 🙂
Is the front end of this unit where the noise is occurring solid state?
My first thoughts are either a faulty semiconductor (transistor) or a faulty cap, typically something small (not an electrolytic) like a compressed disc ceramic or very occasionally a 'suflex' tubular polyester type. Resistors are not above suspicion either.
Perhaps very careful use of freezer spray applied a drop at a time on suspect parts might help. An air duster can used upside down works well as freezer... usual warning... cheapo air dusters are butane based so don't squirt on anything that could cause a problem in that regard.
Is the front end of this unit where the noise is occurring solid state?
My first thoughts are either a faulty semiconductor (transistor) or a faulty cap, typically something small (not an electrolytic) like a compressed disc ceramic or very occasionally a 'suflex' tubular polyester type. Resistors are not above suspicion either.
Perhaps very careful use of freezer spray applied a drop at a time on suspect parts might help. An air duster can used upside down works well as freezer... usual warning... cheapo air dusters are butane based so don't squirt on anything that could cause a problem in that regard.
It seems to me the circuit is oscillating or marginally stable, as it depends on volume pot. Perhaps the pote itself has cracked the carbon track. Also high value pots become noisy when DC current is allowed to flow in the wiper. You said a tube in the circuit. Perhaps the socket it dusty, rusty or corroded.
Thanks Mooly!
As my nickname suggests I'm totaly new to the world of electronics, but since I a while back was able to fix an old reverb unit by simply changing the caps, I was hoping this also would be fixable without any deeper understanding... 🙂
I guess the best way for me to answer your question regarding "solid state" is to attach a picture on the input section. The cap on the side I de-solderd to have a closer look on, since it had som mecanical damage on the plastic wrapping, but it did not seem to have leaked.
The "freeze approach" seem like an intresting idea 👍
Anyway, this is what this newbie is staring at. I recognize the electrolytic caps, the two transistors, two integrated circuts, some larger resistors and a bunch of very small ones. The inputs on the right are line and microphone, and the one to the left is an instrument-input,
As my nickname suggests I'm totaly new to the world of electronics, but since I a while back was able to fix an old reverb unit by simply changing the caps, I was hoping this also would be fixable without any deeper understanding... 🙂
I guess the best way for me to answer your question regarding "solid state" is to attach a picture on the input section. The cap on the side I de-solderd to have a closer look on, since it had som mecanical damage on the plastic wrapping, but it did not seem to have leaked.
The "freeze approach" seem like an intresting idea 👍
Anyway, this is what this newbie is staring at. I recognize the electrolytic caps, the two transistors, two integrated circuts, some larger resistors and a bunch of very small ones. The inputs on the right are line and microphone, and the one to the left is an instrument-input,
Sorry if I was unclear (english is not my first language), but the noise itself remains the same regardless if I turn the input knob, it just gets louder and beeing amplified along with any input signal. Also, I've tried to change the tube, but the result was exactly the same.
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CIrcuits with vacuum tubes are subject to carbon tracking of the dirt in the high voltage areas. This can cause popping in humid weather. Dust on resistors or capacitors with high voltage on them, dust between traces, dust between tube pins, dust between terminals. A deep clean may help, but once carbon is created, nothing dissolves it. Scraping may help.
No need for a vacuum tube in a preamp. Suggest you buy something with op amps. Avoid high gain (>30) and 4558's & 301's, they hiss.
No need for a vacuum tube in a preamp. Suggest you buy something with op amps. Avoid high gain (>30) and 4558's & 301's, they hiss.
Hi!
Well it's not likely that the tube (which do produce a very pleasent distortion to the signal) is at fault, since I've tried to replace it with the exact same result. A good cleaning would be a good idea though.
Tubes typically do not cause popping. All the components required to feed the tube provide short distances for current to arc over when dusty or contaminated by solder flux. 110 v up is not necessary for op amps or transistors.
2nd harmonic distortions in tubes are caused when they are run to red plating. Too much current causes red plates. Which typically does not occur in 12ax7 12au7 preamps. Red plating is designed into output stages of guitar amps. 6L6 & 6V6 tubes typically.
I own a 12AX7 preamp, a dynaco PAS2. It sounded very accurate until age & oxygen caused the paper caps to go off value. Changing to mylar caps made it sound weird. I've found the accuracy I need with 1/100 the power line draw with an op amp mixer. an upgraded Herald RA-88a. I expunged the hum & hiss from this bargain kit. Peavey PV-8 op amp mixers are good from the factory but lack the RIAA curve function for LP's.
Another cause of popping is bad solder joints. Push on things with a wood stick before you spend $$ on freeze spray. Where the stick causes popping, there is the problem.
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What you have in that area of the circuit is as solid state as it gets 😉 no thermionic valves.
I think is your best way into the problem. Ideally you should try and trace this using a very sensitive oscilloscope to look at where the noise is being introduced. You would also need a circuit diagram to help understand how it is all configured... so I'm guessing we have neither of those 🙂
I would try the freezer. Learn to dispense it one single drop at a time from the straw on the can and then drip it onto likely suspects. If you just blast a large area, you chill everything and have no way of knowing what the problem might/might not be. You also get condensation... to dispense carefully on one part at a time.
That central chip is one I can't identify. Is it a noise gate of some sort perhaps. That might be worth trying first with freezer. Then try the four transistors (two are tiny SMD or surface mount devices) and also the two SMD diodes next to them.
ElectricNoob,
I had a similar problem about 25 years ago with a Valley People TransAmp mic pre card. It was either a transistor or a cap, I don't remember which, but it was not the gain pot. As I remember, the problem went away on its own - I don't remember having anything replaced. That actually kind of bothers me!
If the issue is important to your work or fun, I suggest that you call Presonus. They likely will have someone who can help.
I had a similar problem about 25 years ago with a Valley People TransAmp mic pre card. It was either a transistor or a cap, I don't remember which, but it was not the gain pot. As I remember, the problem went away on its own - I don't remember having anything replaced. That actually kind of bothers me!
If the issue is important to your work or fun, I suggest that you call Presonus. They likely will have someone who can help.
First, I'm very thankful to you all for taking the time sharing your thoughts, even though I with my very limited knowledge sometimes have a struggle keeping up with. 🙂
Schmatics seem to be hard to find and I don't have an oscilloscope, but several wood sticks to poke around with!
A final thought; the components near the two inputs to the right seem to be similar parallell structures -one for each input(?).
If so, since the noise is not affected even if I switch to input 3 (left side), would that indicate that the problem is located closer to the amp section in the middle, or is that a "way to mechanic" way of thinking?
Schmatics seem to be hard to find and I don't have an oscilloscope, but several wood sticks to poke around with!
A final thought; the components near the two inputs to the right seem to be similar parallell structures -one for each input(?).
If so, since the noise is not affected even if I switch to input 3 (left side), would that indicate that the problem is located closer to the amp section in the middle, or is that a "way to mechanic" way of thinking?
I'm no expert at all on stage and Instrument type equipment but as I think this is described as a mono preamp it will be what is called 'balanced circuitry' where the inputs consist of a - (minus, a + (plus) and ground rather than the more common unbalanced two conductor set up of typical home audio gear.
Nothing wrong with that but I suspect this type of noise isn't the 'bad connection' type of noise. It sounds more 'faulty component' type noise to me.
I honestly don't know. A lot depends on what that centre chip is and is doing. Does for example that chip take both inputs (left and right sides of the board) and mix them, possible also with a noise gating type of function. I really don't know.
With a scope (and circuit) we could identify better what is going on. The TL082 is a common dual opamp with its outputs on pins 1 and 7. Is it possible the noise could be being introduced in that section of the circuit and mixed in with the right-hand mic/line inputs.
Its all an unknown and why I think you stand a fair chance of finding some clues with freezer.
That is a classic symptom of something that just might respond to freezing.
Maybe some of the others have some other ideas 🙂 I'll look in again tomorrow.
Then, Freeze spray it is (even though it is december and I live in sweden)🙂.
I'll do my best and let you know!
Many bad components like resistors, capacitors transistors diodes, have a bad weld inside. These can also respond to pushing on the body or leads with a stick. Very cheap test. A chopstick or a ballpoint pen with the metal point & ink pulled out will work.
Besides the $20 a can of spray costs here, I have to pay $10 freight from N. Carolina or Minnesota. Plus a handling fee if my order is <$50.
I do lots of repairs without a scope. Used scopes are chock full of worn out parts that fail 20 minutes after you take it home. I use an analog voltmeter on AC scale with one probe blocked by a capacitor to prevent readings on DC voltage. I use a .047 UF capacitor and two alligator clip leads. Analog meter with a 2 vac scale, the pointer will kick when something pops. Use on 20 v scale if you might touch the power supplies (+- 15 v usually). Start at the output and work back towards the input. That will give you an idea which part of the circuit is causing the popping. DVM don't respond quick enough to see pops, and most inexpensive ones lie on AC voltage that is not 50 or 60 hz. (power line frequency). DVM are great on DC analysis. My Simpson 266xlpm analog meter is 36 years old, no electrolytic caps to go bad in it.
Besides the $20 a can of spray costs here, I have to pay $10 freight from N. Carolina or Minnesota. Plus a handling fee if my order is <$50.
I do lots of repairs without a scope. Used scopes are chock full of worn out parts that fail 20 minutes after you take it home. I use an analog voltmeter on AC scale with one probe blocked by a capacitor to prevent readings on DC voltage. I use a .047 UF capacitor and two alligator clip leads. Analog meter with a 2 vac scale, the pointer will kick when something pops. Use on 20 v scale if you might touch the power supplies (+- 15 v usually). Start at the output and work back towards the input. That will give you an idea which part of the circuit is causing the popping. DVM don't respond quick enough to see pops, and most inexpensive ones lie on AC voltage that is not 50 or 60 hz. (power line frequency). DVM are great on DC analysis. My Simpson 266xlpm analog meter is 36 years old, no electrolytic caps to go bad in it.
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