Guitar amplifier makes a loud pop noise (power spike?) when turning on - Page 1
Forum section: electronics / repair

author: d3vCr0w
author_rank: Contributor
timestamp: October 05, 2020, 05:43:50 pm
content: Hi people, I got a guitar amplifier (Marshall Valvestate 8100) a couple of weeks ago and it works but makes a loud pop noise when I turn it on, is it normal?Here is a videohttps://youtu.be/FT8pAYjkWKYThanks by the way.

author: Kekke466
author_rank: Newbie
timestamp: October 05, 2020, 06:03:07 pm
content: Hello, some popping is to be expected in most guitar amplifiers. Yours does seem a bit extreme. In some situations it can be solved with a series resistor and capacitor on the standby switch. (I don't think yours has a standby switch). In some amplifiers they solve it with relays to prevent popping.In your amp, every time you switch it on you get an arc, if the internal contacts of the switch are burned in (because of arcing), the resistance of the switch contacts keeps going up. Try measuring the resistance of the switch, if it is quite high. One thing you could do is replace the on-off switch. You won't get rid of the pop but the buzzing sound coming together with the pop might stop.

author: d3vCr0w
author_rank: Contributor
timestamp: October 05, 2020, 07:06:53 pm
content: Alright, I'll take a look to the on-off switch once I get home, thanks for the tip. And, could the power spike be present because of the main power capacitors not working properly? As far as I understand after the power transformer and the diode rectifier there are two big capacitors which 'filter' and 'smooth' the energy going to the amp.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: October 05, 2020, 07:52:17 pm
content: You'd have a considerable hum, if the smoothing caps didn't do their job, so noas far as I can see from the schematic, there is no speaker bypass relay, so the pop when powering up is inevitable. If it is more intense than it should be, hmm maybe, maybe not...

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 05, 2020, 08:16:41 pm
content: That turn on pop is pretty bad IMHO, but the design has nothing to prevent it.You might want to take a look at the +/- 15 volt supply caps too.These rails (unusually) supply the bias voltage for the power amp.In fact, why not just replace the two main PSU caps and the two 15 volt rail caps anyway, we know they fail or get sick.Not a big or difficult or expensive job really, and pretty good insurance if you intend to keep the amp.Use good quality 105 degree rated caps, they only cost a tiny bit more.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 06, 2020, 11:20:42 am
content: By the way, the Youtube video shows a "pop" at switch on, but the bigger problem is the half a second of nasty 120Hz ripple afterwards.This is very likely indicating power supply caps not charging smoothly/fast enough IMHO.

author: Circlotron
author_rank: Super Contributor
timestamp: October 06, 2020, 11:54:32 am
content: My guess is only one of the two big power supply capacitors has gone bad so the pos and neg power supply rail voltages don't rise symmetrically.

author: Renate
author_rank: Super Contributor
timestamp: October 06, 2020, 03:44:27 pm
content: I agree with everybody else, look to the big caps (C61, C62).I'd double check the +15 V, -15 V to make sure that you didn't blow a zener open (ZD1, ZD2).Here's the schematic:https://www.thetubestore.com/lib/thetubestore/schematics/Marshall/Marshall-Valvestate80-80W-8080-8100-8412-Schematic.pdfThere is no muting or anything.The accepted standard for guitar amps for "whumps" is pretty low.Maybe people are into the power when they turn it on and off?I got sick of the power off whump on my little Fender Acoustasonic 40 so I added a µC supervision muting.https://www.eevblog.com/forum/repair/roland-cube-60d-humming/msg2735676/#msg2735676

author: megaoetzwuchst
author_rank: Newbie
timestamp: September 29, 2024, 11:22:13 am
content: Hi, did you finally find out what the issue was - did replacing the power amp caps fix it ? I've got a similar switch-on sound with an Valvestate 8280 amp, it's really loud and basically blocks me from switching it on in a setting where I only want to use it quietly (e.g. at night!). no idea if that is expected.