DMM with THD capabilities - Page 1
Forum section: electronics / testgear

author: pope
author_rank: Frequent Contributor
timestamp: January 03, 2023, 10:11:01 am
content: Hi,I'm into audio electronics and I was wondering whether there are any DMM out there that can also measure THD. AFAIK, keithley 2015 and 2016 can do THD but can you recommend some newer and hopefully cheaper meters?Having said that, I do have indeed an audio interface and some software to do THD (or THD+N) measurements but I would like to be able to measure the THD on different stages of a circuit and the audio interface isn't always the best solution due to higher voltages. Also, although I have a brymen 869s meter, GAS is calling for a benchtop meter

author: Kleinstein
author_rank: Super Contributor
timestamp: January 03, 2023, 12:41:06 pm
content: THD is not a usual DMM feature. This is more something for a DSO with build in generator. However these are suitable only for rather high THD (e.g. > 0.5%).An audio interface / sound card is more like the suitable instrument. For the higher voltages (power amplifier out) it may need a suitable probe / divider and protection.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 03, 2023, 12:51:51 pm
content: The Keithley Series 2000 is also THD capable!"The 2000 THD series of Keithley Instruments are "Total Harmonic Distortion" analyzers, which are used for audio analysis and distortion measurements. The frequency range starts from 20Hz to 20kHz and a built-in low distortion sine wave Generator provide the adjustable signal for testing amplifier stages or speakers for distortion. Thus, for example in the production of mobile telephones, the microphone, the amplifier and the speaker part may be tested with this single device. The full digital multimeter part was taken from the model 2000 and allows measurements of 100 nV to 1000V."

author: Berni
author_rank: Super Contributor
timestamp: January 03, 2023, 12:53:07 pm
content: Even on that Keithley 2015 the THD feature doesn't go all that low, so likely not very useful for modern audio stuff with very little distortion. (Unless you are working on old tube amps?)You can do a similar thing using an oscilloscope and FFT, but most scopes don't have the dynamic range required to make useful measurements on very low distortion stuff.Unfortunately only dedicated signal analyzers or sound cards have the dynamic range to pull this off. You can still create a buffer circuit with input dividers and protection, so you adapt the dangerous high level signals into the input of one of these.

author: AVGresponding
author_rank: Super Contributor
timestamp: January 03, 2023, 01:07:36 pm
content: Quote from: HighVoltage on January 03, 2023, 12:51:51 pmYou're misunderstanding what the manual is saying. By "2000 THD series", they mean any 20xx THD meter, which is two. The 2015 THD and the 2016 THD. The 2000 is just a DMM, and does not have the internal sine generator and THD measurement circuitry. The THD meters are a LOT heavier because they have a LOT more internally going on.Yes, I am speaking from personal knowledge; I have a K2000 and a K2015THD, the THD meter weighs about half as much again as the plain DMM.The OP doesn't mention a budget, and idk what the market is like in Greece, but I was patient and paid ~£400 for my K2015THD, with a fairly recent cal.

author: pope
author_rank: Frequent Contributor
timestamp: January 03, 2023, 01:41:09 pm
content: Thank you gents.I do have a DSO with FFT capabilities but as already has been mentioned, it's not possible to measure low THD. By "low" I mean anything from 0.1% all the way down to 0.001% roughly.Budget is around €500 (give or take) but it has to be new or at least with an invoice.Location is not a big problem nowadays so anything within the EU should be fine.I'm not in a huge rush and the main reason I asked here is to see if there's anything commercially available or I'm just wasting my time.I'm also very keen on available diy projects.Thanks again for all your help

author: HighVoltage
author_rank: Super Contributor
timestamp: January 03, 2023, 03:02:30 pm
content: Quote from: AVGresponding on January 03, 2023, 01:07:36 pmThanks for the explanation!I do not have a Keithley THD meter. They used to be cheap! Not anymore.For experimenting, it would be nice to test a K2015

author: Messtechniker
author_rank: Frequent Contributor
timestamp: January 03, 2023, 03:02:42 pm
content: Quote from: pope on January 03, 2023, 01:41:09 pmThe Bob Cordell "Distortion magnifier" may be of interest to you:https://www.cordellaudio.com/instrumentation/distortion_magnifier.shtml

author: pope
author_rank: Frequent Contributor
timestamp: January 03, 2023, 03:28:09 pm
content: Quote from: Messtechniker on January 03, 2023, 03:02:42 pmThank you. This looks very interesting indeed!Unfortunately I can't find many info and I don't feel like paying $30-40 for the Linear Audio paperback. Do you have one yourself and if so, how do you like it?

author: colorado.rob
author_rank: Frequent Contributor
timestamp: January 03, 2023, 03:45:43 pm
content: A CMU200 with the CMU-B41 audio option can do THD measurements.

author: pope
author_rank: Frequent Contributor
timestamp: January 03, 2023, 03:53:15 pm
content: Quote from: colorado.rob on January 03, 2023, 03:45:43 pmThank you but this is well beyond my budget and my needs.

author: Berni
author_rank: Super Contributor
timestamp: January 03, 2023, 04:38:36 pm
content: 0.001% is very low THD. The only cheep way of getting there is a high quality USB sound card and then analyzing it using software.

author: JohanH
author_rank: Frequent Contributor
timestamp: January 03, 2023, 04:44:01 pm
content: Check the diyaudio forum, soundcards seems the way to gohttps://www.diyaudio.com/community/threads/howto-distortion-measurements-with-rew.338511/Disclaimer, I don't know anything about the software referred to, I imagine there are other alternatives, too.

author: switchabl
author_rank: Frequent Contributor
timestamp: January 03, 2023, 04:47:06 pm
content: The QuantAsylum QA403 audio analyzer might fit the bill. 0.001% is well within its capabilities and it has input ranges up to 40 Vrms. But availability has been a problem, it is unclear when they will be back in stock.On the budget end, you could look at the E1DA Cosmos ADC. It seems to have very respectable performance but there is no built-in generator, range switching (up to 43 Vrms) is fully manual, input impedance is somewhat low and you need to bring your own software.If you are working with high power amplifiers (or want to make do with an audio interface), you will need external attenuators. This article may be of interest if you want to build one:https://audioxpress.com/files/attachment/2677

author: tooki
author_rank: Super Contributor
timestamp: January 03, 2023, 05:14:40 pm
content: Quote from: HighVoltage on January 03, 2023, 12:51:51 pmNo, they’re not saying the entire 2000 series is THD-capable. They’re saying the THD modelswithinthe 2000 series are THD-capable.Quote from: AVGresponding on January 03, 2023, 01:07:36 pmActually, it was four models: 2015, 2015-P, 2016, and 2016-P. The 2016 and 2016-P can output higher voltages on the test signal output, while the P versions can also “identify peak spectral components”. As best I can tell, the 2016 and 2016-P are discontinued. I’ve attached an old datasheet that covers them all. The current datasheet and website list only the 2015 and 2015-P.(FYI, “THD” isn’t actually part of the model numbers, as there are no 2015/2016 models without THD functionality. The front panel text “2015 THD Multimeter” doesn’t mean it’s the “2015 THD” + “multimeter”, it means it’s the “2015” + “THD multimeter”. In fact, the -P models say e.g. “2015-P Audio Analyzing DMM”.)Quote from: AVGresponding on January 03, 2023, 01:07:36 pmI paid about $400 for my 2015, too. It’s hands-down my favorite meter of all the ones I own (K2015, K197, K197A, Fluke 87V, Keysight U1252B, plus some cheapies).

author: pope
author_rank: Frequent Contributor
timestamp: January 03, 2023, 05:26:21 pm
content: The QuantAsylum QA403 (or rather the QA404) is in my list but I'm not sure yet. BTW, the 404 won't have isolation...There's also the autoranger by linear audio but as I said I anted to ask if a commercial DMM with THD exists but it looks like the answer is no. t least for the specific requirments.I currently use an audio interface with REW as well as some other pieces of software and I'm totally fine with the results but I as hopping for a standalone option that can also withstand higher voltages.BTW, I don't do power amps. I'm mostly into pro audio gear.

author: elecdonia
author_rank: Frequent Contributor
timestamp: January 03, 2023, 08:18:24 pm
content: My opinion here is based on my 40+ year career repairing, restoring, and designing home, pro-sound, and musician audio equipment.Formosttroubleshooting, repair, and restoration of audio gear, a distortion test rig with residual THD of .01% will suffice. By “residual distortion” I am referring todirectlymeasuring the output of the audio oscillator with the THD analyzer. The most sensitive range on the THD analyzer should be .1% THD full scale or better. Most units with .1% full-scale will have residual distortion about 10 times lower, .01-.015% THD is typical.I haven’t used a Keithley 2015THD myself. It has a good reputation as a self-contained THD measurement system.Other complete “one-piece” audio oscillator/THD analyzer systems to consider:Hewlett-Packard 8903A.There is also the 8903E model which omits the internal audio oscillator. The HP 8903 series is legendary among 1980-vintage audio test gear. I intend to get one myself if I find one at the right price.Sound Technology 1701A distortion measurement system.Another legendary THD analysis system from the 1980 time frame. I own one. It is my primary THD measurement system. It’s residual THD is .0015%. That’s about as good as it gets for a pure analog measurement system.Audio Precision System One.This is the first among a large family of high-performancedigitalaudio analyzers. These work with a PC or laptop. They are capable of measuring THD and much, much more.If your budget is limited it is still possible to find older (1970’s vintage) pre-owned separate THD analyzers and audio oscillators with adequate performance. Brands/model numbers to look for include:THD analyzer:HP 333A, HP 334A, HP 339A. I own a HP 334A. It was my primary THD analyzer before I got the Sound Technology 1701AAudio oscillator:I have a Krohn-Hite 4300A ultra-low distortion audio oscillator. Another good Krohn-Hite model is the 4400A.

author: jasonRF
author_rank: Regular Contributor
timestamp: January 03, 2023, 08:21:02 pm
content: Quote from: JohanH on January 03, 2023, 04:44:01 pmI have used REW, just as shown in that thread on diyaudio. In the first few pages you will see examples from $120 (US) and $45 (US) audio interfaces and see how well they do. Even the cheap audio interface will be better at THD measurements than the overwhelming majority of scopes.I would not expect the soundcard/REW approach to be as good as you get with audio analyzers, but it is within the budget of even casual hobbyists.Jason

author: bson
author_rank: Unknown
timestamp: January 03, 2023, 08:44:26 pm
content: If you insert a 1kHz notch filter and apply a 1kHz test tone, then the DMM set to ACV RMS will measure THD+N RMS. Divide by the test tone RMS level and you have it as a fraction.

author: pope
author_rank: Frequent Contributor
timestamp: January 04, 2023, 08:55:16 am
content: Many good recommendations. I need to do my homework! Thank you

author: Brianf
author_rank: Regular Contributor
timestamp: January 04, 2023, 09:42:58 am
content: To protect your soundcard,http://pmillett.com/ATEST.htm

author: Kleinstein
author_rank: Super Contributor
timestamp: January 04, 2023, 10:37:17 am
content: Quote from: bson on January 03, 2023, 08:44:26 pmThe method with the notch is the standard method. However the notch is usually not perfect and one would still use a scope (e.g. with FFT function) to look at the residual signal.One can use the scope also to trim the notch or generator frequency if needed.The notch extends the test range to lower distortions - unless the notch itself adds distortion.Using RMS for THD+N only works if the notch is near perfect, which is rare (higher order). One still needs the very good generator.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: January 04, 2023, 11:25:27 am
content: Look for an AWA F242A Noise And Distortion Meter.