Odd "charging" noise in guitar preamp - diyAudio
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Old 6th January 2013, 07:29 PM   #1
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Default Odd "charging" noise in guitar preamp

I am starting this thread to discuss a problem that occurred after constructing the guitar preamp shown in the schematic attached.

The preamp is fed into a transistor power amp - a JLH 30 watt design. It works pretty well, and is designed to have sufficient headroom for input voltages up to 1 Vpp.

The strange thing that happens (I posted it on another thread, but could not provide sufficient information) is that when turning on the amp, I can hear a subtle "charging" noise of several kHz, that lasts for some seconds and then goes. I say "charging", because it is clearly pretty much like such a sound.

I assume that an oscillation happens before the power supply is stabilised. I have made the following tests:
  • applied a 2.2R resistor in series with a 2.2uF tantalum cap across the +30V terminals - nothing happened
  • turned all controls down - no change in the noise
  • discarded the 220u cap - the noise became louder but shorter
  • removed both the 220u and the LM317 supply, feeding the preamp through a 1k resistor connecting on the 1000u cap - same results as before
  • short-circuited the power amp's input - no such noise could be heard, which persuaded me it came from the preamp
  • simply removed the LF353 IC from its socket, without making other changes - no noise
The last test has led me to believe there is an oscillation around the presence circuit, happening on turn on. I also noticed that the noise continues until the 15V supply rail has reached 2-3 volts, and then goes away. Given the fact that the R-C constant in that point is quite large, it goes without saying that the +15V supply rail rises really slowly and I was able to notice that.

Does anyone have a similar experience or solution in mind? Thanks in advance!
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Last edited by audiostrat; 6th January 2013 at 07:49 PM.
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Old 6th January 2013, 08:03 PM   #2
benb is offline benb  United States
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I don't have much of a clue what the problem is, but after a little thought, a simple way to make sure the +15V voltage goes up proportionally with the +30V voltage is remove the 22u at the +30V output and put it across the top 100k divider resistor, and put an identical value across the bottom 100k resistor (of course removing the 220uF there). If you need about 22u across the regulator output, use two 47u's across the divider resistors.
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Old 6th January 2013, 09:02 PM   #3
djoffe is offline djoffe  United States
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the inputs to the opamps are outisde of the common mode range for a long time during turn-on. For the opamp to do what you expect, its inputs (both TL072 and LF353), per the data sheet, must be at least 3 volts above the negative rail.

The previous poster's suggestion would make that happen more quickly.

In the worst case, you might need some kind of mute circuit.

There are also some other ways to make the "+15V" rise more quickly.


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Old 6th January 2013, 09:40 PM   #4
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Quote:
Originally Posted by djoffe View Post
the inputs to the opamps are outisde of the common mode range for a long time during turn-on. For the opamp to do what you expect, its inputs (both TL072 and LF353), per the data sheet, must be at least 3 volts above the negative rail.

The previous poster's suggestion would make that happen more quickly.

In the worst case, you might need some kind of mute circuit.

There are also some other ways to make the "+15V" rise more quickly.


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Update My Dynaco
Well, thanks for both answers! It seems to be a logical explanation that the behaviour becomes unpredictable until the 3V are reached, but I tried the instant rise by removing the 220u, and the situation changed, but not the way I wanted (see my first post). I will give it a try and have a look at the sites you posted too, when I find time.
Still it is weird, but the parallel rise in the supply rails should be given a chance!
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Old 6th January 2013, 09:45 PM   #5
djoffe is offline djoffe  United States
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another more complicated way would be to have separate positive and negative regulators, then the "15V" point which is your supply midpoint would just be the center tap of the transformer.
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Old 6th January 2013, 09:50 PM   #6
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Quote:
Originally Posted by djoffe View Post
another more complicated way would be to have separate positive and negative regulators, then the "15V" point which is your supply midpoint would just be the center tap of the transformer.
Of course! However, the goal of this design was to build something out of existing parts (bulky transformer with no centre tap), that would be extremely simple and small.
In this manner, I am well aware of the fact that this design is not even close to a thoroughly planned hi-fi scheme - but it works and is very cheap.
Well, it works with this little exception.

Last edited by audiostrat; 6th January 2013 at 09:53 PM.
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Old 6th January 2013, 10:12 PM   #7
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Quote:
Originally Posted by audiostrat View Post

The strange thing that happens (I posted it on another thread, but could not provide sufficient information) is that when turning on the amp, I can hear a subtle "charging" noise of several kHz, that lasts for some seconds and then goes. I say "charging", because it is clearly pretty much like such a sound.

I assume that an oscillation happens before the power supply is stabilised.!
Does it really matter if you just want a cheap solution ?

Its not as if you are selling it as pro equipment.
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Old 6th January 2013, 10:22 PM   #8
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Quote:
Originally Posted by nigelwright7557 View Post
Does it really matter if you just want a cheap solution ?

Its not as if you are selling it as pro equipment.
Well, you are right in a way. But after all, I don't think it is right to know that your circuit tends to oscillate even in such a way. Because that could be bad for the components used - if I don't know the cause, how can I be sure that it won't stress my speaker, or burn any transistors? It could also be oscillating in much higher frequencies that are not audible.
So, why preserve a possibility of damage? Even if the equipment is not that serious.
Plus - and that could demystify my approach - it is my first amp build, and I want to do it the right way from all aspects in order to lay the foundation for future proper designs and gain precious experience.
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Old 6th January 2013, 10:29 PM   #9
jaycee is offline jaycee  United Kingdom
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Why on earth this wasn't designed on a conventional symmetric +/- 12v supply i have no idea. That would completely eliminate the problem.
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Old 6th January 2013, 10:43 PM   #10
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Originally Posted by audiostrat View Post
So, why preserve a possibility of damage? Even if the equipment is not that serious.

.
It wont damage the amplifier.
Unless you have put less wattage speakers than the amplifier it wont damage your speakers either.
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