Proper Way to Measure Guitar Amp Output - Page 1
Forum section: electronics / projects

author: lofe
author_rank: Contributor
timestamp: February 23, 2024, 05:42:42 am
content: Here is a video showing the hum:https://youtu.be/zVp_CdsV8ZoI have a Marshall JCM2000 DSL50 tube amp (schematic here:https://www.drtube.com/marshall-jcm2000-dsl/). I bought this used and never owned another tube amp before. I noticed there's a hum when the standby switch is turned on from the cabinet speaker (a 1x12 inch 8 ohm Mesa Boogie). The hum is not super loud, but because I have a small space, it's next to me and the hum is noticeable and a bit annoying when I'm not playing. So I wanted to find out where the hum is coming from. This turned out to be quite hard with oscilloscope and multimeters. I'm a noob in serious electrical stuff. I have a Siglent SDS1104X-E scope, and the SDM3065X DMM.My measurement setup is using a dummy load with 8 ohm connected through a 1/4 TS audio cable, plugged into the amp's output. Connected the scope ground to the ground side of the wire, and tip to the tip side of the dummy load.The DMM measures the output as 0.5mVAC when the standby switch is off, and 4.5 mVAC when the switch is on (and the hum is audible). Using the scope, I get really weird output, the output waveforms are different between these two scenarios, but the peak-to-peak range are similar and RMS voltage stays around 10 mVAC. I'm pretty sure I probably did something wrong with the scope measurement. Anyone has any suggestions?Thanks!

author: moffy
author_rank: Super Contributor
timestamp: February 23, 2024, 06:51:14 am
content: The oscilloscope is connected to the mains and hopefully mains gnd whereas the DMM is probably a floating input independent of gnd, that might explain the difference you are seeing.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 23, 2024, 07:28:30 am
content: Just to clarify: is the hum coming from the speaker, or from the amp when you replaced the speaker with the dummy load?

author: cncjerry
author_rank: Unknown
timestamp: February 23, 2024, 09:29:48 am
content: Does your amp have a three prong plug? If so, try lifting the ground but be careful. See if you can fin an extension cord you can plug the amp in without the ground. You could have a ground loop. Does the hum go away if you unplug the guitar?

author: CaptDon
author_rank: Super Contributor
timestamp: February 23, 2024, 02:18:17 pm
content: Depending on where you have the gain settings guitar amps are insanely noisy even with the guitar input cord unplugged!! Many amps had an A.C. mains 'polarity' switch where one polarity would hum like crazy and the other polarity was 'usable' but still noisy. I can say as a professional player it changes with every venue! We run into ground loop hum all the time!! The old SCR dimmer packs were a royal pain in the ass at half brightness!! They got into everything!!! As another poster mentioned, try lifting the third pin mains ground. I actually have an isolation transformer in my gig gear for the really bad venues!! B.T.W., Marshall as popular as they are isn't exactly top of the line pro audio gear!!! My personal opinion is they suck but every guitar player has owned one, except me, I just fix them for friends and have a deep disrespect for Marshall gear. My bigger kit is a SuperNova 2X12 Combo amp made by TelRay Concordia the makers of Fender amps. It is basically a solid state Fender twin twelve highly modded by me. It came from the factory with an internal 'Morley Oil Can' for the 'Rotating Sound' feature but the Morley got stolen out of the amp!! My tube amp is a Peavey Classic with 4 X 6L6 / 5881 finals. Effects pedals make it scream and the tube distortion is so mellow. My baby gig amp is a Rocktron, I think R20 is the model which I totally modded including a complete separate power amp with a hot speaker. The original 'in chassis' power amp now drives my talk box. Depending on the age of your Marshall it may be time for new filter capacitors or a different amp.

author: themadhippy
author_rank: Super Contributor
timestamp: February 23, 2024, 03:21:26 pm
content: Quoteworst advice ever,are you aware the number of muso and crew who have been seriously injured or killed by doing that.

author: Randy222
author_rank: Frequent Contributor
timestamp: February 23, 2024, 03:56:42 pm
content: That Marshall is an amp box? Or is it just a pre-amp? I ask because there's two items listed, a Tube "amp" and a speaker box that looks like it too is an amp.What does the Standby switch do? And when you use this switch the hum noise comes from which item?

author: CaptDon
author_rank: Super Contributor
timestamp: February 23, 2024, 07:55:17 pm
content: A couple of things, as if I haven't already rambled enough....Lifting the ground was for testing only to see the effect on the hum. Indeed, I was laid out playing barefoot and holding a properly grounded amp and guitar when my mouth came into contact with the chrome metal ball of an SM58 on a sound system that not only wasn't grounded but was chassis hot to 120vac!!! As for Dummy load vs. speaker, your dummy load is 8 ohms purely resistive and a speaker is 8 ohms impedance and in reality is 50 or more at certain resonances of the speaker and cabinet. The speaker is about 5.5 ohms resistive so you can't compare noise measurements. Also, putting the scope on an isolation transformer may be useful when making millivolt measurements since a lot of weird influences are visible when you get down into the nitty gritty. B.T.W., my opinion about standby switches, they were never incorporated to save power or tube life! Most musicians use them like a mute switch during breaks to eliminate all of the hiss and hum always present on stage....AND.....For the dumb ass who forgets to turn the volume down on the acoustic guitar and goes out for a smoke during break and suddenly the acoustic starts howling at about 120Db since the amp wasn't muted/standby either!!!! Instant way to piss off a sound man!!Cheers mate!!

author: lofe
author_rank: Contributor
timestamp: February 24, 2024, 06:18:37 am
content: Thanks for the response. I recorded a video to show the hum. It comes out from the speaker. All volumes and gain have been turned to minimum setting. The input also does not affect this hum.https://youtu.be/zVp_CdsV8Zo

author: lofe
author_rank: Contributor
timestamp: February 24, 2024, 06:21:54 am
content: Quote from: dobsonr741 on February 23, 2024, 07:28:30 amNot from the amp. The amp itself make some minor hum noise from the transformer, but it's reasonably quiet.

author: lofe
author_rank: Contributor
timestamp: February 24, 2024, 06:37:23 am
content: Quote from: CaptDon on February 23, 2024, 02:18:17 pmI had a small practice amp and amp modeling on computer, so I don't know what's "reasonable" level of noise for tube amps. In Guitar Center, I listened to some tube amps and they seem reaonably quiet. I'm also trying to take this opportunity to learn some electronics and find the cause of the hum.

author: themadhippy
author_rank: Super Contributor
timestamp: February 24, 2024, 02:26:38 pm
content: sounds about right for a marshall

author: lofe
author_rank: Contributor
timestamp: February 24, 2024, 03:02:15 pm
content: Quote from: themadhippy on February 24, 2024, 02:26:38 pmThat's fair. Maybe this level of hum is expected for a Marshall amp. It's still an interesting problem or observation when I tried to measure the noise. I suspect that the scope probe is changing the behavior of the output signals due to reactance or some electrical interactions and the hum output level is very small (in the single mVAC digit range). I am just trying to understand and learn.

author: lofe
author_rank: Contributor
timestamp: February 25, 2024, 05:48:18 am
content: I think I have found the problem/solution.Set upIt appears that I need to measure the audio output in "float" differential mode using two probes and channels (CH1 and CH3). CH1 on the active side of the dummy load (8 ohm 100W resister), CH3 on ground side of the resister. The probe's ground wires are connected to the ground. Math function (CH1 - CH3) is applied.MeasurementStandby switch in off position (with little to no noise), both channels show the same signals. The math output is about 1 mV.Standby switch in on position, with audible noise, CH3 shows the ground signal, but CH1 shows a different signal. The math function output signal is about 5.77 mV.See the attached images for the differences.

author: themadhippy
author_rank: Super Contributor
timestamp: February 25, 2024, 01:08:38 pm
content: if your measuring the output of course your going to see a difference with standby on or off.As an experiment try playing your guitar through the amp first with standby on,them with it off.

author: CaptDon
author_rank: Super Contributor
timestamp: February 25, 2024, 05:07:03 pm
content: I am surprised to see this kind of differential between the amplifier chassis (which should be grounded through the third prong) and the ground potential of your scope (which should also be grounded through the third prong) if both are being supplied from the same receptacle. This is why the Unit Under Test (U.U.T.) is usually powered through an isolation transformer to eliminate the apparent ground loop current you seem to be seeing. Many guitar cabinets seem to have a natural resonance around 120Hz which happens to be the frequency of hum from a full wave 60Hz bridge rectifier and that makes any residual hum seem bigger in S.P.L. than what the signal down in the multi-millivolt range would lead you to expect. If the hum is really objectionable you will need to measure the ripple voltage on the power supplies and judge if it is time to replace the filter capacitors. One of the things I hated about Marshall tube amps was the 450 volt rated filter caps which have around 425 volts when operating but have 540 volts until the tubes warm up and begin to draw current. This leads to shorted blown filter caps, shorted bridge rectifiers and idiots who put huge over-rated fuses in trying to get the amp to work again and burn up the power transformer.

author: Randy222
author_rank: Frequent Contributor
timestamp: February 26, 2024, 08:49:36 pm
content: Quote from: CaptDon on February 25, 2024, 05:07:03 pmBest to check if the mains GND wire is actually grounded in the loop (mains gnd all the way back around to scope gnd). My DHO scope uses a wart dc-dc supply via usb-c, but the scope has a earth gnd port in rear. The rear port is zero ohms to the BNC input gnd.