Which oscilloscope probes for testing audio amp speaker outputs - Page 1
Forum section: electronics / beginners

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 06:28:36 pm
content: I have an integrated audio amp that produces 160 watts into 4 ohms per channel that I would like to test for "clipping point" with an oscilloscope.I was told that when connecting and testing speaker outputs from an integrated power amp that 100x or 1000x probes should be used.Assuming no use of an inline resistor from the amp speaker-level outputs, the output to the scope would be ~126volts.How can I know what value probe to get? I see three types available on Amazon: 100x, 1000x, and a switchable between 50x and 500x.Alternatively, I've seen high power inline 4 or 8 ohm resistors available that are used for this type of test. Could 10x probes be used if this type of resistor is used? With a 4 ohm inline resistor the speaker output would be around 25 volts. Which is the best approach, the alternate probe approach or the inline resistor approach, and why?I understand that since the amp has a 3-prong grounded power plug, I would need either an isolation transformer for the amp to plug into, or isolation probes to plug into the scope. The isolation transformer is much cheaper than isoloated probes. Which is the best approach and why?

author: Benta
author_rank: Super Contributor
timestamp: June 11, 2020, 06:37:38 pm
content: A standard x1/x10 probe is fully sufficient for this job. If the amp has a bridged output, you might need two probes. Amp earth and 'scope earth should be the same."I was told" makes my hair stand up.

author: TimFox
author_rank: Super Contributor
timestamp: June 11, 2020, 07:04:38 pm
content: Be sure to check: most amplifiers of that specification have "bridged" output, where neither connects to ground and it will be seriously unhappy if you ground one.Also, 160 W into 4 ohms is 72 V pk-pk, or 25 V rms. or 36 V pk-pk on each side of a bridged output. As Benta also stated, a x10 probe will get that down to 7.2 V (0r 3.6 V) pk-pk, which is quite reasonable for an oscilloscope input.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 07:31:20 pm
content: Quote from: TimFox on June 11, 2020, 07:04:38 pmSo you guys would recommend that I use the in-line high powered (e.g. >100 watt resistor) to reduce the voltage down to ~25, and using the 10x probe as opposed to higher attenuated probes without inline resistance?How about using the isolation transformer to plug the amp into as well to be safe?

author: Benta
author_rank: Super Contributor
timestamp: June 11, 2020, 07:43:03 pm
content: Quote from: gfmucci on June 11, 2020, 07:31:20 pmYou're overcomplicating this totally.If you want to test the amp under load, then use your 4-ohm power resistor as if it was the loudspeaker. I've no idea what you mean by "in-line".The output of your amp will be isolated from the mains by design (if it's not, commit it to the landfill immediately and sue the company that made it for x million dollars).Connect the probe point to one side of your resistor and the probe ground clip to the other side.Do the measurement.All the rest is "Weekly World News".

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 07:45:58 pm
content: Quote from: Benta on June 11, 2020, 06:37:38 pmThat's why I'm asking here. But then again, when I am told "X" here, I will have to still say "I was told." So I guess hairs will be in a perpetual state of standing up.

author: Benta
author_rank: Super Contributor
timestamp: June 11, 2020, 07:50:35 pm
content: Quote from: gfmucci on June 11, 2020, 07:45:58 pmFair point, my apologies. Just always start with common sense and KISS, then everything evolves nicely.

author: Gyro
author_rank: Super Contributor
timestamp: June 11, 2020, 07:52:53 pm
content: 'Integrated amplifier' sounds consumer. If so, be aware that it probably won't stand running to the clipping point for any significant period of time, especially if you're driving both channels.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2020, 07:58:04 pm
content: There are some points that aren't clear here.First, why do you think your amp will put out 126 volts? That seems unlikely. Make/model?Second, there's absolutely no point in testing an amplifier without a load and I'm not sure what you mean by 'inline resistor'. Typically you use a load resistor similar to the speaker impedance (8 or 4 ohms typically) in place of the speaker. You could just use a speaker but it would get very loud and their impedance is more complex and variable. A good first project would be to make a 4/8 ohm load bank. You need 8 50W 8R power resistors from All Electronics (a couple of bucks each) and a chunk of aluminum heat sink.Third, probes are just not an issue. The standard 1X/10X probes you get with any cheap scope will suffice and in the vast majority of cases, no probes at all would work just fine. The scope has an input impedance of 1M (a million ohms) and many will read +/-40 volts p (80 Vp-p) directly--enough to likely spot your clipping.As for grounding, don't rely on an isolation transformer unless you clearly know what you are doing. An isolation transformer with 3 pronged plugs will still pass thru the ground, which yields you exactly nothing in this case except maybe some blown fuses. What you need to do is verify that you have a common ground (aka not bridged) amplifier where the negative speaker terminals are tied to ground. You very likely do have this type of amp, bridged isn't that common. If not, you have additional complex issues before you scope anything. In case it hasn't been mentioned yet, PLEASE watch Dave Jones' video "How not to blow up your oscilloscope" until you understand it thoroughly.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2020, 08:02:21 pm
content: Quote from: Benta on June 11, 2020, 07:43:03 pmBad advice. On many common amps, there's a 50% probability you just fried the outputs, blew a fuse or melted your probe ground. My hairs are standing up.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 08:08:55 pm
content: Quote from: Benta on June 11, 2020, 07:43:03 pm"In-line" as in "in-line resistor." Not in parallel. Here is a website that uses the term "inline" to describe a different type of resistor:https://www.jbugs.com/product/9384.html?msclkid=678421e1c7781afdf96a7f0349f65f8d&utm_source=bing&utm_medium=cpc&utm_campaign=**Dynamic%20Search%20Ads&utm_term=jbugs&utm_content=Dynamic%20Website

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2020, 08:19:50 pm
content: Quote from: gfmucci on June 11, 2020, 08:08:55 pmThe terms used in that link only make any sense in the specific case of using a 6 volt original Volkswagen wiper motor on a 12 volt system. Ironically, that resistor would work as your speaker load, but will overheat quickly, thus my suggestion use an array of 8R models that look similar. There's any number of ways to configure this.https://www.allelectronics.com/category/530350/resistors/power-resistors-25-100-watts/1.htmlHowever, there's no 'inline' or 'series' or 'parallel' testing an amp--you have a resistor with two terminals and an amp with two terminals. I suppose you could say connecting the scope across the resistor is 'parallel', but that's not how I would typically explain it.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 08:39:16 pm
content: Quote from: bdunham7 on June 11, 2020, 07:58:04 pmOutlaw RR2150. Because when I put 160 watts and .01 ohms resistance into an ohms/volts/watts calculator I get 126 volts; that's without any "inline resistor" defined as resistor placed in series in the line leading from the amps speaker output to the scope's probe. Of course, with the 4 ohm load resistor, the volts come down to ~25.QuoteI got that piece of advice from Sourav Gupta, Iquesters Solutions LLP, Senior Electronics Design Engineer. He's the one who suggested no voltage attenuation before the probes, and using a 100x or 1,000x probe.QuoteOr these:https://www.amazon.com/KKmoon-Aluminum-Resistance-Wirewound-Resistor/dp/B082V2N5SR/ref=sr_1_3?dchild=1&keywords=200+watt+8+ohm+resistors&qid=1591907136&sr=8-3QuoteGood to know that. Thanks.QuoteI don't.QuoteI got this idea from viewing Dave's "Don't Blow Up Yourself, Your Scope, Your House, and Your Neighborhood" youtube. That led me to this isolation transformer, with two-prong plug:https://www.amazon.com/dp/B00B886DGS/?coliid=I2TN4DZI425KYH&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_it_imQuoteThis reminds me of an old Mary Chapin Carpenter song, "I take my chances." No, no, no. Not with a $700 amp. I don't know how to verify that.QuoteI did. And I'll probably watch it a couple more times before I do anything.By the way. The same "blowing up stuff" can occur with power from a USB cable. Which brings me to this next question: Will this device work to isolate USB ground, and if not, what will?https://www.amazon.com/dp/B07GB4CQQ3/?coliid=I3DMV60RNKQJY1&colid=3B04RKX85L5W0&psc=1&ref_=lv_ov_lig_dp_it

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 08:42:43 pm
content: Quote from: Gyro on June 11, 2020, 07:52:53 pmI kinda think it will:https://outlawaudio.com/shop/index.php?id_product=24&rewrite=rr2160-stereo-receiver&controller=productThat's my same amp with same power specs, but with a couple fewer bells and whistles.

author: themadhippy
author_rank: Super Contributor
timestamp: June 11, 2020, 08:50:55 pm
content: Quotemight not say bridged at the speaker terminals but bridging 2 channel ic amps is fairly common

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 08:53:34 pm
content: Quote from: bdunham7 on June 11, 2020, 08:19:50 pmI used that resistor example noting it was not the same type I would use, but to emphasize that the use of the word "inline" is not uncommon. It's used to describe resistors, fuses and other components. Here's one for fuses:https://www.amazon.com/Inline-Circuit-Breaker-Protection-02-100A/dp/B07628SSWB/ref=sr_1_2_sspa?dchild=1&keywords=inline+fuse+holder&qid=1591908358&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEySjFHMDMxRFFQRlNHJmVuY3J5cHRlZElkPUEwNzY1ODE4MzQ2OUFRM1FNWVZNTCZlbmNyeXB0ZWRBZElkPUEwMjQ2NDUwMjhDQUZXRkw1VUlCVSZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=This might be a case of using the word "soda" or "pop" in different regions to mean the same thing, although the "soda" guy might not know what "pop" means.

author: TimFox
author_rank: Super Contributor
timestamp: June 11, 2020, 09:19:32 pm
content: "Outlaw RR2150. Because when I put 160 watts and .01 ohms resistance into an ohms/volts/watts calculator I get 126 volts; that's without any "inline resistor" defined as resistor placed in series in the line leading from the amps speaker output to the scope's probe. Of course, with the 4 ohm load resistor, the volts come down to ~25."You do not understand the power specification for the amplifier. If the specification is valid (and there is a long history of manufacturers lying and exaggerating the power), it means that the amplifier will deliver 160 W mean power into a 4 ohm load. That usually means that it will only deliver 80 W mean power into an 8 ohm load (same voltage, lower current). These amplifiers are virtually always "constant voltage" sources, so that the outputvoltageis a weak function of the load resistance, over a reasonable range of voltage (not a dead short. Therefore, the amplifier willtryto put 25 V rms across your dead short, but the current limit (or smoke) will kick in first at the huge current of 25 V into an almost dead short. Also, if you were to dissipate 160 W in a 0.01 ohm resistor, that would be only 1.265 V rms, not 126. You should always check the order of magnitude before trusting a detailed computation.

author: bdunham7
author_rank: Super Contributor
timestamp: June 11, 2020, 09:39:34 pm
content: Quote from: gfmucci on June 11, 2020, 08:39:16 pmI'd suggest you draw a quick schematic of how you intend to connect things and post the picture because there's a lot of room for miscommunication here. There's no point in a huge semantic debate, but 'inline' will refer to a specific application. If someone is selling you an 'inline' anything they are selling it for a particular purpose. In any case, a diagram will make things much clearer. And I would suggest a 5-amp fast-blow inline fuse on your speaker output(s) (on the red side) while you are doing this testing.QuoteThat transformer is not powerful enough to run your amp under full power conditions. Also, if you have a two-prong cord, the internals are already isolated from the mains plug entirely (but not from the case).I believe your amp is not bridged. You will likely have continuity between the two black speaker outputs, but the best way to not blow things up is to start by taking the ground clip off of your scope probe. Then you can run the amp under power and probe both sides of the outputs, only connecting the ground clip when you are satisfied that there is no signal on the ground side.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 09:47:44 pm
content: Here are the amp specs: 110 watts per channel, 20 Hz - 20kHz, 0.05% THD, both channels driven into 8 ohms 165 watts per channel, 20 Hz - 20kHz, 0.05% THD, both channels driven into 4 ohms.The amp has a 3-prong plug. Power consumption: 400 watts at full load.So, something more like this would work?https://www.amazon.com/Valuepro-GRP-1200-Transformer-Isolation-Benchtop/dp/B00R5CDG8O/ref=pd_aw_sbs_328_2/141-0682910-2872241?_encoding=UTF8&pd_rd_i=B00R5CDG8O&pd_rd_r=8ed81c5d-5c5a-4d7f-87c1-4847de52c860&pd_rd_w=GyOpI&pd_rd_wg=rAg4o&pf_rd_p=c016812c-44e7-4d82-86d0-7b21dc29f2de&pf_rd_r=ZK0PT5ASQXYKW3W4FCXE&psc=1&refRID=ZK0PT5ASQXYKW3W4FCXE

author: TimFox
author_rank: Super Contributor
timestamp: June 11, 2020, 09:54:37 pm
content: Those specs make sense, note that the power into 8 ohms is less than 4 ohms. As written, the spec is not the clipping level, but the power at a reasonable distortion value, and says nothing about shorted load. The power at 4 ohms is less than twice that into 8 ohms (at stated distortion) due to the higher current required.Again, I would not assume that the output is non-bridged, since the wrong choice here might destroy the amp. Bridging is used in solid-state amplifiers to reduce the required DC voltage internally, since it is easier with transistors to run higher current, rather than higher voltage.

author: Gyro
author_rank: Super Contributor
timestamp: June 11, 2020, 09:56:26 pm
content: Looking at the manual, it doesn't look like running a full power test would be a good idea - It makes reference to not using two pairs of 6R speakers (ie. 3R load with both switched in) because it will fry the mains transformer (or words to that effect).If that is an issue that the manufacturer feels worth mentioning in the manual for practical use (ie. listening), then a sustained full power test just doesn't sound worth the risk. If it sounds undistorted at the highest listening level that you are comfortable with, then there's no point is stressing it for the hell of it. Repair might be different.Industrial and Pro audio amps tend to be designed to take sustained maximum output at minimum load impedance, consumer stuff always has thermal limitations.P.S. You can forget about the isolation transformer - looking at the rear panel picture, the IEC connector doesn't appear to have a ground pin (unless it's a photoshop glitch).

author: gfmucci
author_rank: Regular Contributor
timestamp: June 11, 2020, 10:03:55 pm
content: I don't think I want to test that amp any more. I'll stick with low voltage devices for testing.

author: MosherIV
author_rank: Super Contributor
timestamp: June 12, 2020, 07:41:31 am
content: Hi, if the original point was to test for 'clipping point', you do not actually need a scope.Your ears will tell you !The human ear (brain) is very good at at detecting something wrong with sound.The sound will start to sound distorted.However, it is very bad at determining sound level.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 12, 2020, 09:56:25 am
content: Quote from: MosherIV on June 12, 2020, 07:41:31 amI never get the beast to the clipping point. Too many neighbors would call the cops. Oh, wait, the liberal mayors disbanded the PD's.Seriously, I was wanting to perform this test out of a sense of wanting to learn how to test this sort of equipment properly. From what folks have been saying, I'd need to spend several hundred dollars on isolation transformers or isolated probes (pricey) and inline resistors (admittedly cheap) so as not to risk blowing up my amp or scope. So I've abandoned the fling until I learn more.

author: Gyro
author_rank: Super Contributor
timestamp: June 12, 2020, 10:19:47 am
content: I hope we (and I) didn't put you off too much. My comments about consumer don't detract from the quality of the unit for listening pleasure, even at ear-splitting volumes!I think many of us have done (or know someone who has done) something silly when testing an amp and regretted it! A bit of disappointment now is fairly insignificant compared to the potential for despair and beating yourself up if you kill the thing - especially if the manufacturer refuses to repair it under warranty (fried mains transformer for instance) due to what they deem to be 'abuse'. It's the sort of test that some HiFi mag reviewer might do once on a manufacturer demo unit, but not worth risking your own.

author: exe
author_rank: Unknown
timestamp: June 12, 2020, 10:46:27 am
content: Quote from: MosherIV on June 12, 2020, 07:41:31 amEarplugs.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 12, 2020, 12:25:48 pm
content: You need to get a couple of these:A 10X probe should be fine. For many scopes, the attenuator will get down to 50V/div with a 10X probe. If the output is too high you may need to adjust the trace vertical position to see the clipping point. Also, there really isn't any diagnostic benefit to "seeing" where your amp clips. Further, at the clipping point you are running a very high DC current through the output transistors and they will heat up fast. Be very careful.

author: TimFox
author_rank: Super Contributor
timestamp: June 12, 2020, 01:30:11 pm
content: After taking all the safety precautions suggested above, if you really want to see the clipping in action you could use a higher resistance load (16 to 25 ohms, maybe) so that you do not run excessive current at high output. Vacuum-tube amps were unhappy with high load resistance, due to the output transformer and resulting high voltage swing on the primary, but transistor amps are less sensitive to that.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 12, 2020, 01:55:56 pm
content: Back in the days of tube audio amplifiers, we tested them for output power by replacing the speaker with a resistive loadThis load was made up of a higher value resistor & a lower value one "in series".In use, we measured across the lower valued resistor, to minimise any loading upon the value of the overall resistance of the load.We usually used an analog VOM to read the RMS voltage to determine the power output, which had a much lower input impedance than things like DMMs, VTVMs & Oscilloscopes.Because we were reading a fraction of the voltage across the total load, we had to use a "correction factor".This was easily determined, as we knew the ratio of the lower resistance to the overall resistance (plain old voltage divider theory).With the calculated total voltage, we used"V^2/R"to calculate the power output.We did hang an oscilloscope across the same points on the load, to determine the clipping point.Another thing we did was, using a Noise & Distortion meter bridged across the same points measure those parameters at more normal operation levels.The VOM, like a DMM, didn't have one lead grounded, neither did the N & D set, but a 'scope or VTVM did.Tube amplifiers used output transformers, the speaker terminals were not connected to ground, so there was no problem.Even with the early solid state amps, where the output had one side grounded, as long as you checked that, you were "good to go".Amplifiers with bridged output stages are another story---- connect either speaker output terminal to ground, & you can damage your amplifier.There are two workarounds:-(1)The "proper" way--------buy a differential probe.These are not as horrendously expensive as they once were, but the cost is not inconsiderable, either.(2) Using two channels of your 'scope, hang the probes off both ends of your load resistor, (with both ground clips to a convenient ground) & using, for instance, the "channel 1 plus channel 2" setting of your 'scope, you can see the voltage across the load resistor.This is commonly known as the "pseudo-differential" connection, & is usually frowned on by the purists in this forum , but for tests of the level you are attempting, will give satisfactory results.

author: gfmucci
author_rank: Regular Contributor
timestamp: June 12, 2020, 02:21:22 pm
content: Most differential probes appear to be high voltage probes, like this Micsig...https://www.amazon.com/Micsig-DP10013-Differential-Attenuation-Tektronix/dp/B074K4XPW3/ref=mp_s_a_1_1_sspa?dchild=1&keywords=differential+oscilloscope+probes&qid=1591971403&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExN0k4NTFTMVdHQTZaJmVuY3J5cHRlZElkPUExMDE5MjY5MlpQNVVYSEdYS1JZTiZlbmNyeXB0ZWRBZElkPUEwMTEwODgyM1E0Sk9GWllONUZJWCZ3aWRnZXROYW1lPXNwX3Bob25lX3NlYXJjaF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVlIs this the type you would use? This particular one is a 50x/500x as I mentioned near the beginning of the thread.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 23, 2022, 05:19:53 pm
content: A bit strange topic. TS doesn't know how to use an oscilloscope and even how to calculate a voltage, but he wants to check clipping?Hm.

author: EPAIII
author_rank: Super Contributor
timestamp: September 24, 2022, 03:00:34 am
content: The idea of attaching the ground clip of a probe to a point in a circuit that you are not DEAD CERTAIN is actually grounded is, to my way of thinking, just asking for trouble. That ground lead on the probe really is connected to the AC power's safety ground in most scopes I have ever used. It is a hard ground. And if it is attached to a point in the circuit which is not at ground potential, then CURRENT will probably flow. And it will be flowing through a very low resistance to that real ground. I SMELL SMOKE!Most professional equipment and many high end consumer circuits will have ground test points scattered around the PC boards for just this purpose. When higher frequency signals (MHz and higher) are involved, using these ground test points is the only way to see an nu-distorted signal on the scope's display.What I do is attach the ground clip of the probe to a KNOWN ground point in the circuit that is as close to the signal I am going to look at as possible. That is the ONLY point to ever attach it to. Actually, with audio frequencies which are fairly low, most of the time you can leave it unattached and just let that AC safety ground connection through the scope's power cord provide the return path for the signal. But connecting it to a ground point in the circuit is a better habit to be in.Then you can use the probe (10X or 1X) to look at each side of the load, one at a time. If you do not get a clear picture of what is happening that way, then use TWO channels/probes, one on each side of the load and set the scope to A-B mode to see the difference signal. I have used this technique (two probes and A-B mode) many times to see what a balanced (+/-) signal actually looks like.Virtually every scope has at least two channels and an input switch that allows you to combine them with a sum or a difference circuit.

author: magic
author_rank: Super Contributor
timestamp: September 24, 2022, 05:15:48 pm
content: Just FYI, this thread is 2 years old and has been necro'd by a spammer.