Using scope to measure amp volume bias - Page 1
Forum section: electronics / beginners

author: adauphin
author_rank: Unknown
timestamp: September 04, 2012, 04:03:36 am
content: This would be my first attempt at using my new found friend...Tek 465B. I want to get started with this scope to learn it and eventually master it so bear with me.Once I start using it, all the instruction will sink in and I will surely learn quick. What I want to perform is a measurement on both channels using a mono test tone, likely 1kHz, and see where my system is giving me a higher left channel bias.I can tell the left is louder but just slightly, enough to concern me. I used a sound meter and came up with about a 1-1.5db bias to the left. 1db with the CD and 1.5db with the turntable. I have moved about every cable around and still come up with a left bias. Likely a preamp but not sure.From a complete beginners standpoint, how would I connect the scope to the speaker leads or speaker terminals to perform this task?Having a dual trace I should be able to see both and compare values. I'm not sure on the max amount of power the scope can test and after looking at the manual and watching a few videos, I'm still uncertain.I have a grasp on most of the settings on the scope but would need assistance on selecting the proper functions. Again, after a few times I will need to ask less and less questions but for the first time, I will need as much help as possible. A simple button left depressed or not will obviously change things.This is for a home 2CH setup also.Thanks for any help you can provide.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 04, 2012, 08:38:22 am
content: You had me off on the wrong track with the term "bias",until read the rest of your posting."Bias" actually has a precise meaning when applied to amplifiers,so "difference" would be better.1.5dB difference in power amounts to a power ratio of 1:1.425,so that if.say,the lower channel was 10 watts output,the other would be 14.25 watts.If the "sound meter" is the sort of thing with a calibrated microphone on the front,used for sound pressure measurement,they are quite touchy as to positioning,so you may well have a discrepancy of that order.Have you tried swapping the speakers to the other channel,to see if the sound level difference swaps sides?As to looking at the outputs with an Oscilloscope,one thing you have to determine,is whether you can connect the earth clip of the 'scope to either of the speaker leads.Back in the valve (vacuum tube) days, the speaker transformer effectively isolated the speaker voice coil connections from any DC power supplies,but in some solid state amplifiers it does not,so you may possibly damage something.It would be wise to check this.If there is a problem,you could still check each channel separately by using the two channels in the "differential mode".(check your manual).If you use 10X probes,you shouldn't have anything to worry about as far as the voltage at various power levels is concerned.By the way,if you are suspicious of the preamps,why not test them first?

author: T4P
author_rank: Super Contributor
timestamp: September 04, 2012, 04:38:06 pm
content: Quote from: vk6zgo on September 04, 2012, 08:38:22 amI guess most bigger amps are mostly transformer isolated right? (30-100W)I have yet to see a 300W amp on a toroidal, those are SMPS for obvious reasons

author: David_AVD
author_rank: Super Contributor
timestamp: September 04, 2012, 09:06:44 pm
content: Some general info:There is no rule regarding the size (power) of the amplifier and whether it uses SMPS, toroidal transformer or whatever. Some 50W amps use SMPS and some 2000W amps use a toroidal.Most hi-fi amplifiers have the negative speaker output terminal at ground potential, but always wise to check with a meter if unsure.Feed a 100mV sine wave into the amplifier's AUX L & R inputs then start probing the L & R signal paths. Start with the record out, then progress to pre-out (if it has them) and then the speaker outputs.If the amplifier has pre-out and power-in RCA (phono) sockets with push-in steel links, remove the links, clean them up, spray a little contact cleaner on the sockets and put the links back. This can be a cause of intermittent / different channel levels.Check the obvious things like the balance control. Dirty switches and/or pots can also cause issues.Hope that helps.

author: T4P
author_rank: Super Contributor
timestamp: September 04, 2012, 09:54:46 pm
content: Yeah the 50W amps that are SMPS'd are usually cheap crap ... or rather good enough.Heck i'd rather everything be SMPS'd (Cheaper on wallets as they are lighter to ship)

author: vk6zgo
author_rank: Super Contributor
timestamp: September 05, 2012, 12:23:53 am
content: Quote from: T4P on September 04, 2012, 04:38:06 pmMy comment was nothing to do with mains isolation,but more to do with DC isolation.Some amps using BJTs back in the day did present DC on one leg of the speaker leads,with the other lead isolated.Others still, had no DC problems,but had one leg earthed,so caution with earth clips is a good idea.

author: adauphin
author_rank: Unknown
timestamp: September 05, 2012, 01:36:52 am
content: Well the amp in question is a B&K ST-202 and the pre is a Yamaha CX-1000. What would be the correct procedure to measure for DC current from the RCA outputs as well as the speaker output from the amp? I surely don't want to damage anything...originally I checked the speaker output from the amp measuring DC mV to ensure there wasn't significant offset.Just to ensure I measure correctly, how do you probe the connections correctly? I know this sounds very basic but I'm uncertain of this. Should I probe the center of the RCA and clip the outer shield, or the chassis? This is where I'm somewhat stuck and I don't see this in the manual as it's written for having more than basic knowledge of the scope.The only pulse generator I have currently is a Global Spec 4001, identical to this pic. Would this more ideal than a test CD and if not what would the ideal source tone be if I cannot generate a 100mv sine wave? I'm not sure if this generator producs a sine wave rather than a square wave.Thanks again, all help and criticism is appreciated.

author: David_AVD
author_rank: Super Contributor
timestamp: September 05, 2012, 03:25:40 am
content: There won't be any DC on the RCA connections. The RCA shield is normally connected to the amplifier chassis.You need a function generator, oscillator or test CD with sine wave output. A pulse generator is not really suitable for the task at hand.Have you confirmed if the difference in levels is due to the pre-amp or amplifier yet? That's your first step.