Small audio amplifier, popping noise when power is turned on/off

[Plecto]

Hi. I'm having an issue with this class A headphone amplifier that I just made. It's powered from a 15-0-15V 30VA center tapped transformer and when I cut the power from the primary side of it, I get a pop sound in my headset. I know that this isn't caused by the voltage dropping, because my 4700uF capacitors keeps the amp operating for several seconds. I assume that the pop is caused by some spike in voltage from the transformer or something? I'm afraid I don't know transformers well enough to understand this I'm also having another weird issue when turning the amplifier on/off, it causes strange things to happen with other equipment around it. If I connect my headset to a different amplifier and then turn the first amplifier on/off, I hear a weak pop which have to be caused by something happening on the mains Also, my USB sound card starts acting strangely when turning my amp on/off (spotify has just downright stopped working a couple of times). I can't really begin to understand how my tiny amplifier can affect the mains this way so I would like some tips

 

[WimRFP]

Just a question: are you using equipment with a three-pin plug (L, N, PE) on a wall socket without safety ground (protective earth)?

Equipment that needs PE have so-called Y-capacitors (capacitors between L-PE and N-PE) for interference suppression. The PE wire is mostly connected to the metal casing that may be your signal ground also. When you don't have a mains socket with PE, the case (and signal ground) may be at half the mains voltage due to the voltage division of the Y capacitors.

 

[Plecto]

I'm not sure what our type of mains voltage is called, but I know that there a three-phase system going through the street, two of the phases are in my socket, ground is the neutral between them so there's a voltage of 230/sqrt(3)=130V between each of the phases and ground/earth. The casing of the amplifier is connected to the signal ground (I was forced to due to the input/output connectors). PE is not connected to the case, I didn't really think of doing this as I didn't think it would have any effect other than for safety reasons. I'm curious to what's actually happening here though. I know that the voltage across an inductor is proportional to the change in current, can this be causing a huge voltage spike on the mains wire that's causing this to happen? Am I also right to say that these Y-capacitors will prevent this rapid change in current?

 

[WimRFP]

Sorry, I don't understand your description of what is in your socket (N + L, L + L, N+L+PE, etc). It looks like a grounding/EMC issue. You may measure both AC and DC between the grounds of several devices when they are not connected to eachother (but are on the mains).

You may also measure the voltage between equipment grounds/casings and the electrical installation's ground pin or PE wire in a socket (or metallic tubing if that is connected to PE also). Frequently ground issues lead to hum. With equipment I mean everything that you connect together.

If you disconnect a relative low power device, you don't get a large spike. Disconnecting a power hungry device causes a larger spike in the mains, but the arc between the switching contacts limits the spike. A device may have X-capacitors in it, these are larger then the Y-capacitors. When you switch-on a relative large capacitor at the mains peak voltage, you get a spike (ringing) due to the capacitor inrush current. When disconnecting, the capacitors don't give spikes.

Disconnecting a transformer may give a spike in the secondary circuit. Normally these spikes are absorbed by small foil or ceramic capacitors in parallel with the electrolytic smoothing capacitors. Standard 50 Hz smoothing electrolytics are not able to absorb/smooth a fast transient due to their construction.

 

[Plecto]

I have 2x1uF ceramic disc caps on the secondary, I hope that's enough. Just let me know if I'm getting this right. The reason equipment around the amplifier is acting funny is because the amp is making a large voltage spike on the mains? I don't quite get what's causing the pop though. If there is a large spike present on the primary side, this will raise the voltage on the secondary side, but that shouldn't increase or decrease the voltage on the output. I have an op-amp input stage with a class AB bjt output stage, perhaps I'm exceeding the rated voltage on the op-amp so that it can't regulate it's output properly thus causing a spike on the output?

 

[WimRFP]

Does it also pop when you have nothing connected to the input? Is there sufficient separation between the mains wiring and small signal wiring to avoid capacitive coupling?

Check if the popping reduces when you disconnect all LF input wiring from the amplifier's PCB. Can you (just for a test) increase the distance between your PCB and the transformer? Such checks can help you to figure out how the switching transient enters your amplifier.

Can you post a picture of the amplfiier and maybe a circuit diagram?

 

[Plecto]

Sorry for the late response. I see now that the issue of the amplifier messing around with the surround equipment (causing the keyboard to fail among other things) caused my sound card to cause these popping noises I was the most concerned about. Disconnecting the input just gives off a very short, but kind of loud pop when the amplifier is turned on or off, turning the volume all the way down doesn't make a difference so if the pop is caused by coupling to the input, I don't see how it can make it's way through the whole amplifier when the input is grounded. I'm just going to accept this popping noise for now (it's nothing to worry about I think), but the issue with the spike that's induced to the mains is an issue, I don't want to reboot my computer every time the amp is turned on or off to make my keyboard work Could you try and explain what I have to do exactly to avoid this issue?

 

[WimRFP]

We are still in guessing mode (as we as forum don't know how you connected everything), so I can't explain what is happening.

If you just want to reduce the noise production from the switch, you may connect a 1 nF to 10nF X2 safety capacitor across the contacts of the switch. If you use a switch with two contacts (so you switch L en N), you need to put a capacitor across each contact.

If this helps, you may add a low value series resistor (22 Ohm range) to reduce the discharge current through the switch and capacitor. Note that now the switch is no longer fully open as there is some bypass current via the capacitor, so don't forget to unplug.

 

[Plecto]

I connected a 0.33uF cap in parallel with a resistor and the primary of the transformer, this fixed every single problem. No pop when it's turned off, no messing around with my keyboard and a low volume bass heavy pop when the amp is turned on. I used this cap and resistor from a computer PSU so I didn't give it much though, I assume that the ones that made the PSU knows what they are doing I've read a little bit about snubber circuits and I see caps in series with resistors and I see caps in parallel with resistors, but I don't know when to choose one instead of the other. I want to know if I know the theory behind this. The cap will have a high impedance at 50hz, but a very low impedance when the transformer starts making a voltage spike. The low impedance path caused by the cap will prevent the voltage rising too much so it should work to just have a cap and no resistor, shouldn't it? The reason for having the resistor there is probably to have a place for the power to actually dissipate rather than just going back and forth between the cap and the transformer, but why is it a problem that it does that for a little while?

 

[WimRFP]

Without resistor, you can have a large current peak when turning on (when the mains voltage is at its peak value). This also introduces a transient. To reduce the peak current, a RC series circuit is used.

You are right that series resistance is also used to avoid ringing (the back and forth energy storage effect) in circuits where the transient source itself has low internal dissipation (at the ringing frequency). Instead of RC networks also varistors and other TVS devices are used.

I would recommend you to reduce the capacitance to the point where you see an increase of switching noise. Make sure it is an X-rated capacitor. (safety reasons).

 

[Plecto]

Alright, so we are back to a pop noise. I made a new amplifier with pretty much the same design except for a slightly higher gain and the pop sound when the amplifier is turned on is quite loud if high sensitivity headphones are connected. There is no pop when the amplifier is turned off as the capacitor still keeps the amp going. There is no pop if the amplifier is turned back on quickly enough so the capacitors aren't fully discharged. This points away from the transformer being the issue though, doesn't it? Output offset voltage is only about 10mV so I don't believe that to be an issue. Could this have something to do with the inherent characteristics of the NE5532 op-amp when it's turned on? I don't see where this pop comes from or what would produce it

 

[kam1787]

It is common for 'pops' to occur - transients occur with the initial current in-rush, and with the transformer [on/off]. There are circuits which delay the turn-on time [to the op-amp]

 

[Plecto]

It is common for 'pops' to occur - transients occur with the initial current in-rush, and with the transformer [on/off]. There are circuits which delay the turn-on time [to the op-amp]

Alright, but what is happening exactly? I did some thinking and I thought of two possibilities. At the momen the amp is turned on, the input bias current at the non-inverting input starts flowing, but because of the input capacitor, the bias current won't lead to a voltage drop so the voltage starts at 0V and then builds up to TYP 2mV and MAX 8mV (depending on the input bias current). This will lead to the output going from a maximum of -96mV to 0V during start up, not sure if this is enough to cause such a loud pop though Another possibility is the rapid rising of supply voltage and the PSSR of the op-amp. I'm not quite sure how this works, but if I remember correctly, any ripple on the supply will be present on the input of the op-amp, but reduced by the number of dB spesified by the PSSR, is this correct? I'm not sure how to fix this though, perhaps adding some 10Ohm resistors in series with my 4700uF decoupling capacitors will help? This will increase the time i takes to charge them, but also decrease the maximum supply voltage (which isn't that much of an issue in this design). Reducing capacitor value to 2200uF or perhaps 1000uF might not make a difference, I'm not sure? It will make the caps charge faster so that might lead to a shorter, but more powerfull pop?

 

[Plecto]

So I did some testing. I added 4Ohm series resistors to the secondary of the transformer to increase the charge time of the capacitors, this didn't reduce the pop in any significant way. I'm not sure how to approach this. The longer it takes for the capacitors to charge, the lower the frequency of the pop, right? But the dv/dt of the caps will always be really high the very moment the amp is turned on regardless of series resistance, so I'm not sure if it's possible to get the pop sound below audible frequency or even if this is the right approach of doing so. Another approach would be to use really fat traces and lower capacitor values to make the caps charge faster so that the pop will be in the inaudible ultrasound spectrum

 

[Audioguru]

NO SCHEMATIC!
Since your amp uses an output capacitor (or two of them?) then the supply must not have dual-polarity (which does not need an output capacitor to charge).
Your output capacitor charges very quickly to make a POP but it is simple to design the circuit so it charges fairly slowly with no POP.

A very fast charging time sounds like an explosion instead of a slower POP or CLICK sound.

Please post the schematic.

 

[Plecto]

Disregard the values shown (I have to run in three minutes so I don't have time to change them right now).

The amplifier is supplied by a dual sided transformer so I don't have anything in the output path (sloppy of me not to mention this). The pop sound is low in frequency, like a thump.

 

[Audioguru]

Your circuit works from a dual-polarity supply so there are no capacitors to charge. I do not see any output capacitors that might not be needed.

Your resistor values are very low so the coupling capacitor values are very high and are not even high enough for passing low audio frequencies.
If you use 47k for R15 and R16 then the coupling capacitors can be 0.33uF (330nF film type) and they will pass 10Hz and higher. For low input offset voltage then R6 and R16 should also be 47K then the feedback resistor values also must be 47 times higher. The values of C7 and C8 can be reduced.

Perhaps you are using non-polarized electrolytic capacitors that "store" (dielectric absorption) a charge? That would cause a thump. Film capacitors do not do this.

Your schematic shows a +/- 5V supply but since you are using a 15V-0-15V transformer the DC supply will be about +/- 20V.
The opamp does not have enough output current to drive the power transistors to a high level.

 

[Plecto]

Your circuit works from a dual-polarity supply so there are no capacitors to charge. I do not see any output capacitors that might not be needed.

Your resistor values are very low so the coupling capacitor values are very high and are not even high enough for passing low audio frequencies.
If you use 47k for R15 and R16 then the coupling capacitors can be 0.33uF (330nF film type) and they will pass 10Hz and higher. For low input offset voltage then R6 and R16 should also be 47K then the feedback resistor values also must be 47 times higher. The values of C7 and C8 can be reduced.

Perhaps you are using non-polarized electrolytic capacitors that "store" (dielectric absorption) a charge? That would cause a thump. Film capacitors do not do this.

Your schematic shows a +/- 5V supply but since you are using a 15V-0-15V transformer the DC supply will be about +/- 20V.
The opamp does not have enough output current to drive the power transistors to a high level.

The values in th schematic are wrong. Im on a smartphone right now, but Ill give the full parts list later. R15 and R16 are 9.1k. The feedback resistors are 120k and 10k so their parallel connection is close to 9.1k. I know the gain is quite high, but I need to be able to reach an output of +-16-17V with a phone or laptop soundcard as a source

Im not sure what you mean about those capacitors though. The 4700uF are alluminum electrolytcs (cheat China shit) while the caps connected in parallel with them are 470nF ceramic SMD caps. These SMD caps arent that close to the ic which isnt ideal, but Ive never seen the amp oscillate yet.

 

[Audioguru]

I was talking about the COUPLING capacitors C1 and C4 that MUST NOT be polarized electrolytic type. Also the FILTER caps C7 and C8.

Since you have 9.1k for R15 and R16 then the values of the C1 and C4 coupling capacitors must be 1.7uf for -3dB at 10Hz (flat down to about 50Hz) which is a pretty big and expensive film capacitor. That is why I recommended 47k ohms and 330nF.

I know about the main DC filter capacitors but 4700uF is not marked on your schematic.

How on earth will the opamps produce enough output current to drive the power transistors?? Oh, your speakers have an impedance of 100 ohms or more?
EDIT: I know. The part numbers for the output transistors are also wrong. You are actually using darlington transistors with a high current gain to drive low impedance speakers.

 

[Plecto]

I can give the full parts list now.

R22, R23, R26, R27 - 0 Ohm jumpers
R1, R2, R8, R9 - 6900Ohm
R17, R18, R19, R20 - 300Ohm
R15, R16 - 9100Ohm
R12, R21 - 10kOhm
R3, R4, R7, R10 - 1Ohm
R5, R13, R24, R25 - 120kOhm
R6, R14 - 10kOhm
C1, C4 - 3.3uF ceramic SMD
C6, C3 - 470nF ceramic SMD
C7, C8 - 2.7nF ceramic SMD
C2, C9 - 4700uF alluminum electrolytic
T1, T5 - MJE3055
T2, T4 - MJE2955
IC1A - NE5532

I was talking about the COUPLING capacitors C1 and C4 that MUST NOT be polarized electrolytic type. Also the FILTER caps C7 and C8.

Since you have 9.1k for R15 and R16 then the values of the C1 and C4 coupling capacitors must be 1.7uf for -3dB at 10Hz (flat down to about 50Hz) which is a pretty big and expensive film capacitor. That is why I recommended 47k ohms and 330nF.

I know about the main DC filter capacitors but 4700uF is not marked on your schematic.

How on earth will the opamps produce enough output current to drive the power transistors?? Oh, your speakers have an impedance of 100 ohms or more?
EDIT: I know. The part numbers for the output transistors are also wrong. You are actually using darlington transistors with a high current gain to drive low impedance speakers.

C1 and C4 are 3.3uF ceramic capacitor, and no, they weren't particulary expensive (I even have some 4.7uF). Because of the high input bias current of the NE5532 and the need for a bass-boost functionality in the feedback loop, getting the offset voltage to a low level was difficult if R15 and R16 had any higher resistance value. I their for chose higher values for C1 and C4 instead.

This is a headphone amplifier, it will never have to output +-18V into 8Ohms. The amplifier will never need to output a high wattage

But what about the popping sound though? That's the issue I want to fix