Recommendations for inexpensive DMM with dBm readout at audio bandwidth - Page 1
Forum section: electronics / testgear

author: bottlegardener
author_rank: Contributor
timestamp: February 27, 2021, 06:06:46 pm
content: Hi all, sorry if this has been discussed to death but I couldn't find an answer in the search.Is there a cheap DMM on the market with decent bandwidth and a dBm (at 600 ohm) readout?My 10-or-so-years-old Agilent u1251b died after my cat coughed a hairball directly on the dial. I disassembled and cleaned it, and that revived the display and rotary switch but the tactile switches still don't work and it reads nothing from the probes in any mode. I suspect it must be a component level issue. Keysight quoted me $500 (!) which is well beyond my current budget.The Agilent meter was always a bit overkill for my needs (analogue audio maintenance). Just wondering if there's a budget meter that out there with >20kHz BW? Or is my best bet finding an ancient Fluke 8060A?Thanks in advance for any help.

author: Grandchuck
author_rank: Super Contributor
timestamp: February 27, 2021, 06:23:18 pm
content: I wonder if something like this would serve?https://www.harborfreight.com/search?q=5%20in%201%20dmm

author: Grandchuck
author_rank: Super Contributor
timestamp: February 27, 2021, 06:26:56 pm
content: Oh, looking closely, perhaps that dB function is only for a built-in mic.

author: bd139
author_rank: Super Contributor
timestamp: February 27, 2021, 06:36:58 pm
content: Brymen BM789 reads dBm and up to 100KHz.I am considering one but still have an 8060A anyway.Data sheet here:https://static.eleshop.nl/mage/media/downloads/BM780_datasheet.pdf

author: Zucca
author_rank: Unknown
timestamp: February 27, 2021, 06:55:56 pm
content: Reagarding bench DMM Keithley 2015, used units are cheap and has audio jazz in it.

author: gnuarm
author_rank: Super Contributor
timestamp: February 27, 2021, 07:59:49 pm
content: Quote from: Grandchuck on February 27, 2021, 06:23:18 pmI wonder why they though a rounded bottom was good on a device that has prop legs? That thing has got to be tipsy.

author: Electro Fan
author_rank: Super Contributor
timestamp: February 27, 2021, 09:11:05 pm
content: I have a Radio Shack 22-812 that supports dBm at 600 ohms. I think the meter has long been discontinued but you can still find them, sometimes maybe even new in the box, on eBay - maybe for ~$20-$45. I have found it to be a good meter for basic low DC voltage work and some AC work. It also does mA and uA, C/F temp, hFE, HZ/Duty Cycle/Width, Rel, and more, plus it happens to have a RS232 interface. It has off settings on both ends of the dial, and the first choice after off on the left is DC voltage. Some buyer at Radio Shack back in the day knew what they wanted.http://www.repeater-builder.com/radio-shack/22-812/22-812-operating-manual-en.pdfJust recently, after looking for a long time for a DMM with a variable ohm setting for dBm, I bought a Brymen 789. The Brymen 789 is an excellent meter.Going a little off topic I'd say if someone wanted to buy just two meters (say to measure voltage and current at the same time) and wanted one serious meter to handle almost anything and another to make less rigorous but still reasonably accurate measurements, the two meters that would offer the most value together might be a Brymen 789 and an Aneng 8008.Edit: one small note on the Radio Shack 22-812 is that I don't think it is a true RMS DMM. The manual says:"AC VOLTS (Maximum Measurement: 750Vrms at 50/60 Hz, Average Responds, RMS Calibrated, AC Coupled)" so I think it does AC averaging (although it says 'RMS calibrated') and I think I noticed the difference this makes the other day when the AC I was measuring at about 18 Volts wasn't quite matching another meter that is true RMS. Also, even though it says it will handle 750V RMS I am not using it for high AC voltage; at anywhere approaching 750V I'd use a Fluke, Brymen, or Keysight. But overall except for missing the TRMS the Raio Shack 22-812 is a pretty nice meter for the price, and if you just need something that does dBm at 600 ohms this would be a viable solution. I think the RS 22-812 might have been a back in the day Aneng 8008, and maybe with a few extra features.

author: Fungus
author_rank: Super Contributor
timestamp: February 27, 2021, 09:27:56 pm
content: Quote from: bottlegardener on February 27, 2021, 06:06:46 pmYes.Anything else is going to cost $150+ USD

author: Electro Fan
author_rank: Super Contributor
timestamp: February 27, 2021, 09:47:40 pm
content: https://www.ebay.com/itm/NOS-Radio-Shack-Multimeter-22-812-46-Range-PC-Interface-Leads-Manual-Gray/224304142013?epid=1000878779&hash=item34399196bd:g:lgwAAOSw~apf8qup

author: TimFox
author_rank: Super Contributor
timestamp: February 27, 2021, 10:08:14 pm
content: Note that any reasonable voltmeter with a dBm readout at a user-set impedance level does not add that resistance to load the input, and the dBmpowervalue will only be true if the voltmeter is connected across a physical load resistance. However, a similar unit, "dBu", is only avoltagemeasurement with the reference level 0 dBu = 0.775 V = (600/1000)1/2, and is independent of the actual load resistance. This unit is common in audio applications, due to the historical use of 600 ohms in professional equipment.

author: tautech
author_rank: Super Contributor
timestamp: February 27, 2021, 11:45:59 pm
content: Quote from: bottlegardener on February 27, 2021, 06:06:46 pmSDM3045X for $ 389 can do what you need.

author: nctnico
author_rank: Super Contributor
timestamp: February 28, 2021, 01:01:44 am
content: Quote from: Fungus on February 27, 2021, 09:27:56 pmVici VC8145 can do true RMS up to 20kHz and can show dBm for a whole flurry of impedances. There are limits to the amplitude though to reach maximum accuracy.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: February 28, 2021, 02:37:45 am
content: Not sure what your needs are, but many people interested in audio (myself included) use vintage analog audio millivoltmeters for audio-frequency AC measurements. With the burden of bandwidth off your chest, you have many considerably cheaper but reasonably high quality DMMs to choose from.An interesting option you might want to consider is a vintage HP400E (or EL - both capable of dBm readout) coupled with a cheap (or not so cheap) DMM of your choice. Not only these HPs have an impressive bandwidth, unmatched by any DMM (10 Hz to 10 MHz), but they also sport a DC output (scaled to 0-1 V for any input range). They can act as wide band amplifiers and AC to DC converters. You can connect any DMM to the DC output and get a highly accurate digital reading, if you wish.I paid $14 plus shipping on mine on eBay, a couple of years ago. With a little patience, you can get one under $50 shipped.In the example below, I fed a (challenging) signal of 1 mVrms, 100 KHz (from my Siglent AWG) to a HP400E (which displays it correctly as 1 mV on its analog scale), a BM689S (which cannot even measure it) and a Fluke 189 (which measures 0.919 mV) – both in AC mV mode. The meter on the right is a Fluke 87V in DC mode (impersonating here a cheap DC voltmeter connected to the DC output of HP), which measures 0.9971 mV (factoring in the scaling). The same signal measured with my DMM6500 is 0.9986 mV, which is probably the most accurate value I can get by measuring directly the AC signal. Note that the digital reading of the DC output of the HP400E is the closest to that. I could measure with no issues a signal of 3 MHz as well, just for the sake of it.I have a HP400E, which has a linear mV scale on top and a logarithmic dB scale underneath. There is a HP400EL version as well, with a linear dB scale on top and a logarithmic mV scale underneath (if scaling directly the dB to 0-1V is more important to you).Just a thought.

author: Electro Fan
author_rank: Super Contributor
timestamp: February 28, 2021, 03:30:25 am
content: Quote from: Caliaxy on February 28, 2021, 02:37:45 amThat is all very cool. Thanks for posting.

author: bottlegardener
author_rank: Contributor
timestamp: March 01, 2021, 06:08:34 am
content: Thanks for all the great suggestions folks!The older HP AC voltmeter sounds like a perfect fit for most of my uses. I wasn't aware of their bandwidth performance.

author: bd139
author_rank: Super Contributor
timestamp: March 01, 2021, 08:00:05 am
content: 400E I picked up here with internal photos:https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg2340645/#msg2340645The only problem with them is they are average responding. The 3400A is a better if you want “true RMS”Make sure you get one with an IEC connector for mains. The older ones have a completely unobtainable weird one.

author: bottlegardener
author_rank: Contributor
timestamp: March 06, 2021, 05:54:27 pm
content: As an update, I was able to find an HP 400EL as part of a bundle with a RCA WA-44c audio band oscillator and a Mercury Model 1700 VTVM for $60CAD locally.The 400EL seems to be working well with good accuracy. I think its meter movement will be much easier to read than my u1251b for things I need to do frequently like adjusting tape tape machine bias, azimuth and such. Its limitations (average responding but with compensated RMS scale; distortion error) won't be too much of an issue to me. I also have a basic Fluke 114 and an old HP 1222A scope to cover my other bases.Thanks again everyone for the thoughtful suggestions and tips.

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2021, 06:08:55 pm
content: I find moving-pointer meters easier to read when adjusting for maximum (e.g., tape bias) or minimum (e.g., bridge null), compared with digital meters.If you are measuring a sine wave with low (few percent at most) THD, the difference between TRMS and average-responding, rms calibrated is negligible.TRMS can be useful for the residual output of a THD meter (only harmonics of the fundamental) or for measuring random noise.

author: bottlegardener
author_rank: Contributor
timestamp: March 06, 2021, 08:09:08 pm
content: That's a great point about it being useful for nulling. Perhaps I should build a little 2-into-one summing jig with a polarity flip on one side for fine azimuth adjustments.Do you reckon something like this (plus a polarity switch on one side) with the output end's shield floating would work well with two balanced audio signals?:

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2021, 10:02:50 pm
content: That should work, assuming the balanced source is happy with a 1,000 ohm load. If you need better matching than 1% on the resistors, you could either measure them with a 3.5 or more digit ohmmeter (absolute accuracy not important, only resolution) and trim them as required with small series resistors, or sort them into matched pairs. However, the 400EL input is single-ended (low is grounded).