Mains Spike when Switching On - UK Vintage Radio Repair and Restoration Discussion Forum

Nicknoxx · 29th Mar 2014, 2:33 pm

Hello,

This is my first post here so I hope I've got it in the right place. I've recently bought an old radiogram which wasn't working. I've put a small D class amplifier inside and connected it via phono leads to the house Hi-Fi. I wanted to keep the exterior the same so I've used the original switches, volume control and speakers.

The problem is, when I power on the unit, using the original push button switch on the front or at the mains, I get a loud squeak through the speakers even if the volume is turned right down.

Am I right in thinking that it would be possible to connect something across the mains, maybe a capacitor or varistor, to prevent this and if so, what sort of thing should I be looking for?

Many thanks for any help.

Nick Brooks

G8HQP Dave · 29th Mar 2014, 4:36 pm

Why do you think the mains is responsible for the noise? I would have blamed the Class D amplifier or its PSU.

ValvoStef · 30th Mar 2014, 10:26 am

Hello,

Was your class D amp working OK before you build it into the radio? Is the loud squeak permanent or just a short one after the power up?

emeritus · 30th Mar 2014, 12:21 pm

I seem to recall that in the 1970's there were problems with switch-on noise: the Patent Office's classification scheme for amplifiers had an entry for "anti-plop" circuits designed to prevent it. It wasn't my technical field, but I understood that the noise arose from establishing DC conditions in the output stages on switch-on and thus was unaffected by the volume control setting or the input signal level.

30th Mar 2014, 1:07 pm

Why the (powerful) amplifier, phono inputs don't need much. I guess that the HiFi is on before the radiogram? In that case you will get a turn on thump, the original would have turned on the output at the same time giving a while for it to give any output (transistor and valve) missing the thump.

Nicknoxx · 2nd Apr 2014, 9:41 am

Thanks for all the replies.

I've only ever used this amp with a 12v supply from a vehicle before, never with a psu so that could be the problem. The squeak is one loud short one on power up. It happens whether the is an input or not.

Could I fit a capacitor across the output of the psu to prevent this?

Thanks again all.

G8HQP Dave · 2nd Apr 2014, 9:14 pm

Before you fix a fault you first have to find the fault. My guess is that the amp is not stable at lower than usual supply voltages. Suddenly connecting it to a battery does not give it time to generate an oscillation, but a mains PSU may come up more slowly. Adding a capacitor across the supply could make things worse!

It may be that others have hit the same problem. Google may help. We can't do much with so little information - our crystal balls only work for vintage technology, but even then we usually need a manufacturer's name and model number.

cmjones01 · 3rd Apr 2014, 12:43 pm

Let's try and be clear about the setup you've got here: do I infer correctly that you're not using the original electronics of the radiogram, but you have your new class D amplifier with its input wired to the radiogram's volume control and its output wired to the radiogram's speaker?

First of all, have you disconnected the original wiring from the speaker to the radiogram chassis? If that's still connected, what the class D amplifier will do when connected to odd bits of radiogram power amplifier is anybody's guess.

As others have suggested, this may be a power supply startup problem. What kind of amplifier are you using? What does it specify for the power supply? What kind of power supply are you using? Can you try a different power supply, even temporarily?

If the noise is really a high-pitched squeak, it's not coming from the mains. Clicks, cracks and thumps can come from mains switches, and some thumps and pops are simply a normal part of amplifiers starting up, but a squeak means the amplifier is unhappy about something.

It could also be feedback: what is your wiring to the input and output of the amplifier like? Does moving the input and output wires further apart make any difference?

I hope this helps
Chris

dave cox · 3rd Apr 2014, 1:34 pm

You say you've used this amp with a battery, I assume there was no switch on noise in that scenario ? What sort of PSU are you using ?

One thing to try is to temporarily put a switch between the PSU and the amp and then turn on the PSU FIRST then turn the amp on by operating the second switch. Its also worth watching the speaker cone as you turn on to check for any subsonic instability.

dc

Nicknoxx · 3rd Apr 2014, 7:08 pm

Thanks again chaps.

Yes, all the original electronics have been removed but I'm using the original speakers.

The noise is a high pitched squeak

It appears that the amplifier has suppression on power on but this only works if the amp is powered before it's switched on (if you see what I mean).

The amp volume control is connected mechanically to the original volume control, not electrically.

I'm using the original power switch but using it to supply power to the PSU.

I'll try Dave's suggestion of putting a switch between the PSU and the amp as this will help isolate the problem.

Cheers

Nick

29th Mar 2014, 2:33 pm	#1
Nicknoxx Retired Dormant Member Join Date: Mar 2014 Location: Hereford Posts: 3	Mains Spike when Switching On Hello, This is my first post here so I hope I've got it in the right place. I've recently bought an old radiogram which wasn't working. I've put a small D class amplifier inside and connected it via phono leads to the house Hi-Fi. I wanted to keep the exterior the same so I've used the original switches, volume control and speakers. The problem is, when I power on the unit, using the original push button switch on the front or at the mains, I get a loud squeak through the speakers even if the volume is turned right down. Am I right in thinking that it would be possible to connect something across the mains, maybe a capacitor or varistor, to prevent this and if so, what sort of thing should I be looking for? Many thanks for any help. Nick Brooks

29th Mar 2014, 4:36 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Mains Spike when Switching On Why do you think the mains is responsible for the noise? I would have blamed the Class D amplifier or its PSU.

30th Mar 2014, 10:26 am	#3
ValvoStef Hexode Join Date: Mar 2014 Location: Fareham, Hampshire, UK. Posts: 482	Re: Mains Spike when Switching On Hello, Was your class D amp working OK before you build it into the radio? Is the loud squeak permanent or just a short one after the power up? __________________ Failing to prepare is preparing to fail ...

30th Mar 2014, 12:21 pm	#4
emeritus Dekatron Join Date: Jan 2012 Location: Brentwood, Essex, UK. Posts: 5,339	Re: Mains Spike when Switching On I seem to recall that in the 1970's there were problems with switch-on noise: the Patent Office's classification scheme for amplifiers had an entry for "anti-plop" circuits designed to prevent it. It wasn't my technical field, but I understood that the noise arose from establishing DC conditions in the output stages on switch-on and thus was unaffected by the volume control setting or the input signal level.

30th Mar 2014, 1:07 pm	#5
Guest Guest Posts: n/a	Re: Mains Spike when Switching On Why the (powerful) amplifier, phono inputs don't need much. I guess that the HiFi is on before the radiogram? In that case you will get a turn on thump, the original would have turned on the output at the same time giving a while for it to give any output (transistor and valve) missing the thump.

2nd Apr 2014, 9:41 am	#6
Nicknoxx Retired Dormant Member Join Date: Mar 2014 Location: Hereford Posts: 3	Re: Mains Spike when Switching On Thanks for all the replies. I've only ever used this amp with a 12v supply from a vehicle before, never with a psu so that could be the problem. The squeak is one loud short one on power up. It happens whether the is an input or not. Could I fit a capacitor across the output of the psu to prevent this? Thanks again all.

2nd Apr 2014, 9:14 pm	#7
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Mains Spike when Switching On Before you fix a fault you first have to find the fault. My guess is that the amp is not stable at lower than usual supply voltages. Suddenly connecting it to a battery does not give it time to generate an oscillation, but a mains PSU may come up more slowly. Adding a capacitor across the supply could make things worse! It may be that others have hit the same problem. Google may help. We can't do much with so little information - our crystal balls only work for vintage technology, but even then we usually need a manufacturer's name and model number.

3rd Apr 2014, 12:43 pm	#8
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,677	Re: Mains Spike when Switching On Let's try and be clear about the setup you've got here: do I infer correctly that you're not using the original electronics of the radiogram, but you have your new class D amplifier with its input wired to the radiogram's volume control and its output wired to the radiogram's speaker? First of all, have you disconnected the original wiring from the speaker to the radiogram chassis? If that's still connected, what the class D amplifier will do when connected to odd bits of radiogram power amplifier is anybody's guess. As others have suggested, this may be a power supply startup problem. What kind of amplifier are you using? What does it specify for the power supply? What kind of power supply are you using? Can you try a different power supply, even temporarily? If the noise is really a high-pitched squeak, it's not coming from the mains. Clicks, cracks and thumps can come from mains switches, and some thumps and pops are simply a normal part of amplifiers starting up, but a squeak means the amplifier is unhappy about something. It could also be feedback: what is your wiring to the input and output of the amplifier like? Does moving the input and output wires further apart make any difference? I hope this helps Chris __________________ What's going on in the workshop? http://martin-jones.com/

3rd Apr 2014, 1:34 pm	#9
dave cox Nonode Join Date: Jan 2009 Location: Bristol, UK. Posts: 2,062	Re: Mains Spike when Switching On You say you've used this amp with a battery, I assume there was no switch on noise in that scenario ? What sort of PSU are you using ? One thing to try is to temporarily put a switch between the PSU and the amp and then turn on the PSU FIRST then turn the amp on by operating the second switch. Its also worth watching the speaker cone as you turn on to check for any subsonic instability. dc

3rd Apr 2014, 7:08 pm	#10
Nicknoxx Retired Dormant Member Join Date: Mar 2014 Location: Hereford Posts: 3	Re: Mains Spike when Switching On Thanks again chaps. Yes, all the original electronics have been removed but I'm using the original speakers. The noise is a high pitched squeak It appears that the amplifier has suppression on power on but this only works if the amp is powered before it's switched on (if you see what I mean). The amp volume control is connected mechanically to the original volume control, not electrically. I'm using the original power switch but using it to supply power to the PSU. I'll try Dave's suggestion of putting a switch between the PSU and the amp as this will help isolate the problem. Cheers Nick