Thread: Capacitor dump from an amp...POP in the speakers, and a hum as well.

Part 2:
The system with the new amp has a hum in it, and on Friday morning I did a simple series of tests which identifies where the problem lies

• I have the two amps side by side (Focal Arche, and "XXXX"), and neither has a power cord connected to it.
• I only have the left channel monitor powered on, and with an XLR cable plugged into it.
• With the XLR cable also plugged into the XXXX amp, there is a hum.
• When I pull the XLR cable out of the XXXX (and not reconnected to anything except the monitor), the hum is identical – it does not even click or pop as I pull it out.
• When I plug the XLR cable into the Arche....the hums stops immediately (remembering that the Arche has no power cable connected). It makes no click or pop as I plug the cable into the Arche. It just goes silent - as it should.

Now that must tell us something...I'm just not sure what! (I think I rather like that emoji from StereoNet)

I consulted someone who has worked in electronics for decades, but is not necessarily good at explaining to someone of my "ground level at best" understanding. He said that the XXXX amp is lacking termination at the XLR output to the monitor except when I select Pre-Out on the amp, and he also mentioned "loop resistance" being missing. He was quite scathing at what he sees as beginner's mistakes.

So my question is, what does it mean to "terminate" a circuit,
what is "loop resistance" and what does it do, and
how easy, and how cheap would it be to design this amp to be like almost all other amps on the market, that don't produce a hum with a studio monitor? Less than a dollar for a resistor and perhaps a capacitor?

I used to be an electronics technician with an interest in audio back when everything was analogue & simpler than it is today. Someone who is still active in this field will have more knowledge that me.

Back when I built/fixed amps, some were single supply with amplifier DC output 1/2 supply voltage at idle. There is a capacitor between the amplifier output & speakers so DC voltage is not fed to speakers. Only music which is AC. Charging that capacitor is what normally causes the pop.

Better amps had a power supply with ground, plus volts & equivalent negative volts. At idle the DC output of the amp (with output transistors matched & biased correctly) was 0 volts. Speakers connected between ground & amp output. Cannot recall if this was via a capacitor but if it was there should not be a pop because no DC volts.

Hum is usually
- capacitors in power supply being faulty or too small resulting in ripple in DC power to amp)
- earthing. Earthing can be no earth or an earth loop.

hope this helps

Just thinking about hum & earths, most modern appliances are double insulated so no earth. Wonder if audio equipment is the same?

When everything was earthed via the power lead & hum was due to an earth loop, a quick (potentially dangerous) solution was to remove the earth pin from a double adaptor & plug the offending piece of equipment into this. Earth was then via audio cables only. Not mains cables & audio lead. Please don’t do this.

Many of the components these days go through a wall wart. Headphone amps like this one are essentially pre-amps with very low power output, although this one is 10w per channel which is pretty high. Its power supply is 24V 5A.

Originally Posted by FenceFurniture

Its power supply is 24V 5A.

Is that 24v DC or AC?
Just thinking out aloud, if DC could it be powered by +12v & -12v using an old computer power supply or similar. Maybe 2x 12v wall warts. Then connect speakers between the 0v/ground from the power supply & amplifier output. That way output capacitor does not have a DC charge or very little so hopefully no pop.
Would need to use the same 0v/ground as ground for input as well if output is feeding other active devices.

Thanks for your help Pete.

It's DC. 500 bucks worth of DC wall wart! They included it as part of the deal.

Breaking news though – they emailed me this morning to say send it back for checking and if faulty (it's not) they'll replace it, if not faulty they'll give me a full refund. I'll be pressing for a refund because nothing is going to stop the capcitor discharge when it goes to Standby. They have already told me that this is part of the circuit design to dump when powering down. They just didn't test it with active speakers before they released it, is my guess.

I built and repaired many amplifiers in years past, the “pop / crack” when amplifiers power up / down is a common problem with the output stage driving or being driven one direction or another before the entire circuit has stabilised. If it is not controlled within the circuitry an additional de-thump circuit can be added on the output to keep the signal disconnected for a few seconds until it is stabilised.
Unless you want to dig into the circuit to pick up a control signal for the de-thump add on its not so simple, take the sellers offer for refund then find a system that performs as you require.

Your hum issue will be an “earth loop”. Termination is making sure all cables are connected correctly and at the correct point.
With low level signal it is also necessary to ensure that only one earth point exists for the circuit, having the circuit earthed in multiple location can create an inductive loop that will pick up electro magnetic radiation. For audio amplifiers this is usually 50Hz mains radiation that most people know as hum.
To eliminate earth loops sometimes an interconnecting cable will require the earth lead or shield to only be terminated in one of the connectors on the end of the lead. Disconnecting earths should only be done on low signal cables where the circuit is understood and is already earthed via other connections.

Edit: Earth is a relative term and with double insulated equipment signal earth and mains earth may not be connected. Earth loops can occur in the low voltage signal side even if it is isolated from mains earth.