Scopes and Speakers - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: August 07, 2018, 11:21:22 am
content: Hi,I have an audio amplifier circuit working. I can scope it and see it following the input and can hear tones when I put a speaker on it. However, when I try to do the two together -- scope with a speaker -- my output is distorted.Is there something that I can add to my circuit for purely test purposes that will let me continue to view the output signal while looping in the speaker?Thanks,Carl

author: goldfinger
author_rank: Regular Contributor
timestamp: August 07, 2018, 11:28:17 am
content: Quote from: eev_carl on August 07, 2018, 11:21:22 amWhat type of audio amp circuit are you measuring? What is the input and output voltage your running, wattage of amp, power supply rail voltage. With little information you have provided it sounds like maybe your clipping the amplifier.. with some more info people will be able to offer you more informed adviceSent from my iPhone using Tapatalk

author: kg4arn
author_rank: Unknown
timestamp: August 07, 2018, 02:10:52 pm
content: Schematic?

author: eev_carl
author_rank: Regular Contributor
timestamp: August 07, 2018, 07:17:08 pm
content: I think this is a problem with my circuit, specifically an LM386 power stage. Hooking both the speaker and scope to the preamp output (a 741) doesn't show any of the distortion I was complaining about. I'll close this topic off and pull together the schematic and scope screenshots for a better post.

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2018, 07:57:38 pm
content: Speakers are notoriously nasty components, the voicecoil moves according to current through it, as well as it produces current when it moves. I would expect the waveform to look pretty nasty when driving a speaker in most cases.

author: Audioguru
author_rank: Super Contributor
timestamp: August 07, 2018, 11:30:31 pm
content: An LM386 is designed to properly drive a speaker if you build it on a pcb (a solderless breadboard will probably cause it to oscillate on the AM broadcast band) and use all the parts shown on its datasheet.A lousy old (50 years old) 741 opamp cannot drive a speaker.

author: james_s
author_rank: Super Contributor
timestamp: August 07, 2018, 11:42:08 pm
content: My interpretation is the 741 is the preamp, ie the signal source for the power amp, not driving the speaker directly.The important question here is does it *sound* good? If the audio sounds distorted then start troubleshooting, but if it looks ugly on the scope but sounds fine? Probably not an issue.

author: Dave
author_rank: Super Contributor
timestamp: August 08, 2018, 04:52:05 am
content: Quote from: james_s on August 07, 2018, 11:42:08 pmI wouldn't want my amplifier oscillating at high frequencies, even if it's out of the audio frequency range.Find the problem and eliminate it, ignoring it isn't doing you any favors.

author: GregDunn
author_rank: Frequent Contributor
timestamp: August 08, 2018, 07:43:45 am
content: Quote from: james_s on August 07, 2018, 07:57:38 pmIf the amplifier has enough output capability and acceptably low output impedance, it will sink any back EMF from the speaker and the waveform should be unaffected by the load. You generally only have problems like that driving speakers from something with an output transformer. With a LM386, though, the intrinsic distortion of the chip is already pretty high at power outputs of over a few hundred milliwatts, especially if the speaker impedance is 4 ohms or less - it just can't source enough current. As suggested earlier, this chip needs to be properly bypassed and built on a good layout or it will misbehave at ultrasonic frequencies too.

author: abraxa
author_rank: Frequent Contributor
timestamp: August 08, 2018, 06:20:09 pm
content: Could it be that you have your probe set to 50 ohms input impedance instead of 1M ohm?

author: james_s
author_rank: Super Contributor
timestamp: August 08, 2018, 08:22:20 pm
content: Quote from: Dave on August 08, 2018, 04:52:05 amSure but if it's oscillating at ultrasonic frequencies it's probably not going to sound good, those ultrasonic oscillations are going to result in audible distortion too. I guess it depends on how nasty the waveform looks, I have rather low expectations when it comes to something like the LM386, it's not exactly high end hi-fi.