Milivoltmeter for audio work - Page 1
Forum section: electronics / testgear

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 09, 2022, 10:44:16 pm
content: I was over the tapeheads website where they discussed milivoltmeter for measuring output from tape recorders. Most would say something like the Leader 185/186 etc.. is best but those are not in production any more. Besides I don't think those need to be analog with a galvanometer. A DMM doesn't seem to have good accuracy for the bandwidth from 20-20kHz. I have the Fluke 189 and 289 and I think they are OK but not quite good for those types of measurement.What would you guys think is a good choice besides buying from the old analog meter from ebay.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 09, 2022, 10:56:28 pm
content: I don't know the Leader meters, but I don't quite understand how a galvanometer could have better "accuracy" than a 50000 count meter, regardless of the bandwidth. The Fluke 189 has 100kHz of bandwidth and can show voltages with a minute fraction of a 100-division galvanometer.One aspect that I think may give this impression is the natural dampening of a galvanometer when shown variable voltages on its inputs - the typical 3 readings per second DMM tends to show a cacophony of numbers under these conditions. However, the bargraph can help show trends and variance.So, answering your question: I would experiment using the 189 under the same measurement conditions of an analog VOM to see if the information you get makes sense or is useable.(I haven't done measurements in cassete tapes in decades, so I might be missing something)

author: TimFox
author_rank: Super Contributor
timestamp: April 09, 2022, 10:56:36 pm
content: What bandwidth do you need for the audio voltage measurement?I'm not familiar with current production, but older analog and digital-readout units such as the-hp- 400GL (average responding, analog meter, 0.1 mV min full-scale, 4 MHz),-hp3400A (true rms, analog meter, 1 mV min full-scale. 10 MHz bandwidth),-hp3403C (true rms, digital display, 10 mV min full-scale, 100 MHz bandwidth),and Marconi 2610 (true rms, digital display, 2 mV min full-scale, 25 MHz bandwidth) are nice. I own one each of all but the 400GL.I don't know how old the units used by your tapeheads posters are, but the traditional AC millivoltmeters were the tube-based -hp- 400 series, not as wide a bandwidth as the later solid-state units such as the 400GL .

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 09, 2022, 11:09:23 pm
content: The meters you suggested are all out of production. As I stated in my post I want to find something current. There must be some made for those types of work today.

author: ci11
author_rank: Regular Contributor
timestamp: April 09, 2022, 11:23:50 pm
content: You may want to look at some "psophometers" Rohde & Schwarz and Siemens made them in Germany, and Boonton, UP and Ballantne made them in the US.These are very high precision millivolt meters that can read down to -100dbV. Their response can be dead flat between 15Hz to 100kHz or weighted through various filters, even custom external ones. This is great to dialing in tape head bias. They were used by analog telephone networks to measure noise floor levels to ensure standards compliance and of course, telco's had all the money to afford the very best. Stunning pieces of analog engineering that are no longer made.The ones I use are Rohde & Schwarz UPGRs. There is one on German eBay right now:https://www.ebay.de/itm/304428989805?hash=item46e161bd6d:g:4o8AAOSwPURiOfUFBTW - I use a 189 also and there is no way I would compare its bargraph - or any bargraph on any DMM - to how you can characterize AF signals.

author: nctnico
author_rank: Super Contributor
timestamp: April 09, 2022, 11:31:37 pm
content: Quote from: BeBuLamar on April 09, 2022, 11:09:23 pmThe Vici VC8145 bench DMM has a good accuracy over the audio frequency spectrum but you have to stay withing certain levels because of the true-rms converter it uses. ACV measurements are specified for 0.8% up to 20kHz while staying within 75% of the measurement range.As you already found out a lot of DMMs are optimised for 50Hz only. You need to check the specs carefully but there are DMMs out there that are also suitable for audio work.

author: Fungus
author_rank: Super Contributor
timestamp: April 10, 2022, 12:04:22 am
content: Quote from: rsjsouza on April 09, 2022, 10:56:28 pmThe 187's lowest range is 50mV so the 51 segment bargraph will struggle to show 1mV wiggles.One possibility is to use a $2 opamp to amplify the signal so the bargraph shows it better.

author: bdunham7
author_rank: Super Contributor
timestamp: April 10, 2022, 12:12:15 am
content: Quote from: BeBuLamar on April 09, 2022, 10:44:16 pmThe issue isn't accuracy or bandwidth--the F189 and F289 have that covered quite well--it is sensitivity. A 6.5-digit DMM with a 100mVAC range will have a resolution of 100nV, but will be too noisy and inaccurate below 1mV. A meter like the old HP403B or the Leaders will have a 1mV full scale range much better suited for such readings.As far as modern equivalents to the old analog AC voltmeters, AFAIK it is slim pickings. There's the Tonghui TH1912, a rather strange 38,000 count meter that I've never personally seen, but it does have a 3.8mVAC range. I don't know of any better-class vendors with an equivalent product, although there are some Aliexpress no-name versions out there. Another alternative would be a low-noise low-bandwidth preamp, but really your best bet is probably to do what most other people do and get one of those old HP or Leader AC voltmeters. Other than not being current--meaning you have to restore and maintain one yourself--there isn't much advantage to more modern designs if you could even find them.

author: rsjsouza
author_rank: Super Contributor
timestamp: April 10, 2022, 02:47:38 am
content: Quote from: BeBuLamar on April 09, 2022, 11:09:23 pmThe closest I know is the Sanwa EM7000, which has FET inputs and claims to measure from 40 to 1MHz (in low voltage). Its biggest drawback is the absence of a mV scale, though.Quote from: bdunham7 on April 10, 2022, 12:12:15 amThanks for the clarification, although I still struggle to see how the range from 100nV through 1mV (a whole 10000 counts within the full 1M counts of the 6-1/2 DMM) would be any noisier or less accurate than a galvanometer at 1mV FS with 10µV of resolution. At those levels, noise due to probing or other sources would be a problem with both instruments anyways. Oh well, perhaps I need to crack open one of my remaining cassete players and probe away to see for myself the inadequacy of my DMMs...

author: bdunham7
author_rank: Super Contributor
timestamp: April 10, 2022, 03:55:31 am
content: Quote from: rsjsouza on April 10, 2022, 02:47:38 amWell, a good 6.5 digit DMM might give a decent attempt at it, but they typically are not specified for at least the bottom 1% of the range, if not more. So below 1mV you are on your own and unless you know your model, you don't know if the low voltages will be lost in the noise of residual counts or zeroed out by the firmware. Keysight's TrueVolt meters might be better at low-voltage AC, but even they are not specified below 1mV. The old analog models seem to be quite accurate when I test them using divided voltages--typically within a needles-width of being spot on or perhaps 1-2%.If you are measuring noise, you may want a TRMS model like the HP3400A. I had one but I let it get away and now they go for big bucks.

author: ehobby
author_rank: Contributor
timestamp: April 10, 2022, 04:06:45 am
content: I have several HP meters that would work well (mostly analog 3400s, 410C, 400FL, 3456A, etc.) but I splurged a few years ago and bought a GW Instek GVT-427B. It is a bit of a specialty meter as it is a two needle (two channel) meter. It covers 10Hz to 1Mhz and has scales from 300uV (actually 350uv) to 100V (-90dBm to +43dBm) full scale. I can easily read down to 5uV.It is great for audio work especially stereo audio. You can put a single audio tone into both channels of an amplifier and read the resulting signal levels at a similar point on both channels further into the amplifier to see if the signals are tracking/amplifying at the same levels. As you change the input frequency or amplitude of the input signal you can see how each channel tracks by watching the two needles move. In theory they should track each other relatively closely. If there is a frequency/filter or bias problem you will see the needles track differently. Though you can get actual measurements it is often more handy to just watch the needles move in relation to each other. You can see the two signals on two separate voltmeters but it may be very difficult or really slow you down to try to keep track of two different needles on two separate meters.Another good use is to be able to put one channel on the input of a circuit and the other on the output, then adjust the voltage scales to get the needles in a good position near each other (or exactly on top of each other if the amp has a gain of, for example, 10 or 100). As you change the input voltage and frequency the needles should track the same up and down the scale. If not, the amp stage is non-linear or is doing some frequency filtering. This is great for both troubleshooting or working on a new design.There are a lot of interesting things you can do with a two channel dual needle voltmeter you probably would not try or easily do with a single channel voltmeter, especially a DMM. It does work well as a regular single channel voltmeter, or you could look at it like it as two single channel voltmeters (the channels are independent). Another consideration is if you are looking at a signal with moderate movement it is easy to track on the analog meter vs. spinning digits on a DMM. You can look at two signal points on a scope but if the changes are at a lower level vs. the size of the signal it can be very hard to read the change between traces, whereas the needle(s) movement (convergence or divergence) is easy to see.I suggest you look into one of these meters as your primary audio related AC voltmeter. My primary meters are Fluke DMMs for most of the things I do. However for audio and analog device work, an analog meter is usually more useful. For stereo work the GTV-427B is what I use.

author: david77
author_rank: Frequent Contributor
timestamp: April 10, 2022, 07:51:33 am
content: Any half decent DMM will do for most of the things you might want to do on audio stuff. You mentioned Tapehead.net, so I assume you want to align a cassette or R2R deck. This is one of the areas where I still prefer the analog meter. It's absolutely possible to set up a recorder using a DMM but it somehow feels more natural to watch a needle while trying to align the heads or eq.I would actually try to get hold of one of the old used meters that have been mentioned. I usually use a HP 400F or a nice old tube voltmeter that I restored. There also where many clones of the HP400 series available until relatively recently.

author: hpw
author_rank: Frequent Contributor
timestamp: April 10, 2022, 08:19:47 am
content: If you may get an old HP 400F as lowest -80dB range as 0.1mVIMHO in today's measurements, a spectrum analyzer using a GOOD sound card & attenuation if required, gets you more about noise (hum, hiss, ..) and signals analyzing.. just carefully on tube gear

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 10, 2022, 09:28:16 am
content: Quote from: Fungus on April 10, 2022, 12:04:22 amThere is no need to measure that low. The reading is generally higher than 100mV.

author: Fungus
author_rank: Super Contributor
timestamp: April 10, 2022, 09:39:32 am
content: Quote from: BeBuLamar on April 10, 2022, 09:28:16 amWhat exactly is the problem with the DMM? They have the bandwidth, the resolution, a bar graph... what's wrong with what they show?If the signal is 100mV then maybe an oscilloscope is the best tool. A cheap DSO will show far more than any meter.

author: thared33
author_rank: Contributor
timestamp: April 10, 2022, 12:11:59 pm
content: I've heard many audio engineers say they use the Fluke 8060A for this very purpose, but it's long out of production. It's what I use and it's very accurate.

author: rob77
author_rank: Super Contributor
timestamp: April 10, 2022, 12:29:03 pm
content: just for the sake of curiosity what is exactly the measurement in question ?

author: rsjsouza
author_rank: Super Contributor
timestamp: April 10, 2022, 01:33:15 pm
content: Quote from: Fungus on April 10, 2022, 09:39:32 amIndeed. If that is the case, any decent modern equipment will do. Heck, evem the aforementioned 8060A has enough resolution and sensitivity.

author: BravoV
author_rank: Super Contributor
timestamp: April 10, 2022, 02:56:47 pm
content: Quote from: Fungus on April 10, 2022, 09:39:32 amYep, but looking at the Op's response, looks like the Op prefers the old style analog meter with wiggly needle.To Op, if you still insist on analog meter, GW-Instek still produce them, its still listed in their web site.Something looks like this below, but newer version called GVT-417B (with "B" suffix), or try search used one like below older model.Click to enlarge ...

author: bdunham7
author_rank: Super Contributor
timestamp: April 10, 2022, 03:03:54 pm
content: Quote from: BeBuLamar on April 10, 2022, 09:28:16 amThen your F289 will handle that just fine. It may seem 'twitchier' than an analog meter, especially measuring noise--but it's not wrong about that. The analog meters have a deceptive calmness that some prefer.

author: ehobby
author_rank: Contributor
timestamp: April 10, 2022, 10:27:01 pm
content: Another option is the Heathkit IM-25. I have restored two of these, better than the original accuracy, and have one on the bench. It has a big meter so a bit easier to read, covers the 10hz to 100khz range and can read down to 1mv. AC accuracy (~4% including reading the needle position and panel meter accuracy) is not as good as for some of the HP gear but for audio, it really does not matter that much when working with audio. Really.When I think about fixing audio devices some of the most important things to check for are linearity, tracking between channels, distortion and hum/noise. Hum and noise are easy to see on a scope. Gross distortion can be seen on a scope. Small amounts of distortion can be measures on a THD or IMD analyzer (or even a PC sound based solution). Linearity and tracking are easy to see on an analog dual needle ACVM, a little harder on a scope or single ACVM. A DMM will also work but best if the signal is fairly static. I used a Fluke 8060A through the 80s for audio level work. It is a really great DMM, holds its accuracy well and fairly cheap on epay. My bench Fluke 45 is what I use in place of the 8060A for some measurements where I need an actual accurate TRMS measurement for.If you understand what you are measuring, it is easy to pick the right tool for the job. Just make sure you have the right tools. If you are only going to do one or two audio device repairs, it is probably ok to just use a good DMM and lots of fairly static measurements, along with your scope. If you are really interested in audio, audio repairs and perhaps audio design, get some real analog audio service/test equipment and make things easy on yourself right from the start.

author: Fungus
author_rank: Super Contributor
timestamp: April 11, 2022, 05:49:26 am
content: Quote from: ehobby on April 10, 2022, 10:27:01 pmOn a modern DSO? You can display the difference between two signals using the math channel. It can probably display phase difference between them numerically, too.

author: SmallCog
author_rank: Frequent Contributor
timestamp: April 11, 2022, 07:25:25 am
content: Quote from: BravoV on April 10, 2022, 02:56:47 pmAt work we had a draconian rule come down on us from above that we could have no electrically powered equipment >x years old.Health and safety rifa madness! Anyway a very nice HP mV meter went in the e-waste bin and was replaced with one of these GW Instek’sI’ve never used it but I know it took months to arrive, we honestly thought they’d scammed us.Entirely possible nobody has actually used it… but Health and Safety paid to replace all our old stuff.

author: J-R
author_rank: Super Contributor
timestamp: April 11, 2022, 08:57:00 pm
content: Experimented with some 0-20kHz signals and various DMMs and generally speaking I was able to get reasonable results down to about 0.2mVAC. Any DMM with a reasonably fast analog bar graph seemed to be an option if you were wanting to monitor an audio signal.But I am curious what the specific process will be with the tape head measurements the OP is referring to?

author: jsantoro
author_rank: Regular Contributor
timestamp: April 11, 2022, 10:55:46 pm
content: As David77 said, an old HP 400F or Leader 181/186 would be my choice over a new Instek or that Tronson brand dual needle HP/Leader knockoff. I use 2 Leaders daily and would not give them up for anything. I check cal yearly on all my stuff and these 2 have not budged in years. When you need to null/peak a signal, count DBs etc. analog is the way to go and easier on the eyes. If you are working on analog tape and will be doing it a lot this is what I would recommend. Not You couldn't use a DMM to do it but it should have readout in DB like the 8060 and read AC flat up to whatever frequencies you are working with .

author: Fungus
author_rank: Super Contributor
timestamp: April 12, 2022, 01:12:24 am
content: Quote from: J-R on April 11, 2022, 08:57:00 pmMaybe tape head alignment...?

author: Fungus
author_rank: Super Contributor
timestamp: April 12, 2022, 01:18:13 am
content: Quote from: jsantoro on April 11, 2022, 10:55:46 pmHis Fluke 189 and 289 have dB readout up to 100kHz.(as do many Brymens, etc.)The 8060A is a fine multimeter in many ways but it doesn't have a bar graph.

author: rob77
author_rank: Super Contributor
timestamp: April 12, 2022, 05:11:46 am
content: Quote from: Fungus on April 12, 2022, 01:12:24 ammost probably it is, but head alignment is frequency agnostic.. you can align the heads while playing a single tone. so i don't understand the fuss around the bandwidth of the meter. virtually any modern dmm has the bandwidth necessary to measure AC in the needed frequency range. ddm and/or scope is the only tools one need for such a task.

author: Fungus
author_rank: Super Contributor
timestamp: April 12, 2022, 01:52:12 pm
content: Quote from: rob77 on April 12, 2022, 05:11:46 amBack in the day we used to just listen to it and tweak the screw.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2022, 02:09:21 pm
content: Basically I use them to do level adjustments for the tape recorder. There are quite a good number of adjustments to be made. I do use my Fluke 189 and 289 but I just kind of wondering why those guys at tapeheads forum insist on an analog AC milivoltmeter. And if they are really needed some manufacturers should be making new ones but I really don't see new ones made. Buying used one how do you know if they are in calibration.

author: Fungus
author_rank: Super Contributor
timestamp: April 12, 2022, 02:13:48 pm
content: Quote from: BeBuLamar on April 12, 2022, 02:09:21 pmAnalog readings are warmer than digital readings.

author: david77
author_rank: Frequent Contributor
timestamp: April 12, 2022, 02:16:38 pm
content: Basically they're not needed anymore. It's nice to have one but not necessary. Your Flukes will give you good results.I think most of the guys still playing around with magnetic tape are middle aged or outright old farts who are set in their way and use what they always used when the technology was current. I would include myself in that subset of people.Most of the cheaper DMM's in the early 80's where indeed not suitable for AC 20-150kHz. As I said today most half decent DMM's will easily work fine up to ~200kHz or so.

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2022, 02:49:45 pm
content: I am 67 and I do own a good number of cassette tape decks.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 12, 2022, 03:19:56 pm
content: Quote from: david77 on April 12, 2022, 02:16:38 pmA tricky point with many of the analog RMS chips is that the bandwidth goes down with a reduces amplitude. So they may go to 200 kHz at full scale, but at 1% of FS the BW may not reach 10 kHz. So a small signal at the higher frequency is still not easy.

author: david77
author_rank: Frequent Contributor
timestamp: April 12, 2022, 03:20:41 pm
content: No offence intended. I'm 40 and I own a good number of cassette decks. professional reel to reels and VHS recorders. So I do understand your predicament.

author: david77
author_rank: Frequent Contributor
timestamp: April 12, 2022, 03:31:19 pm
content: Quote from: Kleinstein on April 12, 2022, 03:19:56 pmTrue. The 189 claims 1,5% accuracy at 10Hz - 20kHz in the 3V range. Easily good enough for aligning a tape deck and undoubtedly better than is easily achievable with an analog meter. Granted, the 8% accuracy at 20-100kHz sounds bad but in practice the only measurment in a tape deck where that would matter is measuring the AC bias. Something that is not usually done while aligning the deck.

author: TimFox
author_rank: Super Contributor
timestamp: April 12, 2022, 03:43:27 pm
content: Quote from: Fungus on April 12, 2022, 02:13:48 pmWhen adjusting something to obtain a peak reading (such as head azimuth), an analog display is easier to follow than a digital display. (This is also true when finding a null reading, as in a bridge.)

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 12, 2022, 04:48:32 pm
content: For peak reading I use an oscilloscope.

author: TimFox
author_rank: Super Contributor
timestamp: April 12, 2022, 04:52:49 pm
content: I also use an analog CRO to find nulls on admittance bridges. Instantaneous response.However, when tuning or tweaking something for maximum voltage, the usual moving-needle meter can move quickly enough and can give better resolution near the maximum than on an oscilloscope.

author: 2N3055
author_rank: Super Contributor
timestamp: April 12, 2022, 05:01:18 pm
content: Quote from: david77 on April 12, 2022, 03:31:19 pmWhat he meant is that RMS conversion chip used in meters (mostly) have different bandwidth at 10%, 50% and 100% of the scale at the same range. That is common to all log/antilog RMS converters. S/D RMS converters (like LTC1968, former Linear, now also part of Analog Devices) have linear frequency response independent of signal level. But unfortunately they are not used often...

author: david77
author_rank: Frequent Contributor
timestamp: April 12, 2022, 06:07:13 pm
content: Ah I see. I did not know that.