A 'better' VU meter driver. - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: August 22, 2023, 10:31:44 pm
content: I don't like how every VU meter circuit has a resistor draining the capacitor, the decay doesn't look good to me.So I designed this circuit to discharge the cap linearly, so instead of the VU meter needle dropping; Quickly, slower, even slower, slowlllly setlling at -whatever db.Instead, it goes; dropping steadily, still dropping at the same speed, still.I also made it so it works on both waves of the signal, this way in an LED VU meter, you don't have to problem that when the signal is low enough frequency, the LED's jump between one and another constantly. this happens at like 15hz so it's not a big thing, but still. And the linear discharge also helps with this because you don't have to discharge it as quickly to compensate for the slower discharge over time.The emitter follower before the buffer op amp is because the input bias current of the op amp will discharge the capacitor not linearly. If you have a bigger capacitor like 22uF and decrease the 100k resistor in the current sink circuit, the problem becomes less aparent and you may delete the emitter follower.Or you can chain 2 emitter followers together to get high enough gain (otherwise the same problem happens, only even more problematic with this), And then you don't need the buffer. And at this point if you increase the capacitor to like 470uF or something high like that, you increase the attack time too because the op amp has finite current drive capability.The current is controlled by the 100k resistor on the emitter+base connection in the current sink circuit, less resistance = more current, faster decay.Ofc you can omit the other phase circuitry and save on op amps.Below the circuit I drew a single phase variety. You want at least ~700mV at the current sink to keep it stable, The chained emitter followers will drop the voltage around 1.2V and since the circuit keeps the output at 0V, the voltage at the current sink will be 1.2V.If however you're using a single emitter follower into a buffer or whatever, you may run into problems with the current sink dropping in current. If this happens you can put a diode after the buffer or before the buffer to add another 0.6V drop.Also you don't need a rail to rail op amp, because of all the diode drops the output of the op amp will be higher than 0V.I did make a version of this circuit with an ancient LM381, adapting an existing VU meter circuit. It was a bit tricky because one input of that op amp is internally biased.To make it work I shorted the DIFF negative input to ground, and instead connected the feedback loop to the Single Ended negative input. (Since internally this is connected to a 10k to ground, I didn't need an R2, I just made R1 correspondingly higher value)Then I connected a 250k resistor from the positive input to ground. Internally the positive input base is biased with a 250k resistor to 1.2V, So if I connect a 250k to ground it will be biased to 0.6V instead. And that will account for the 0.6V drop from the positive input base, to the negative S.E input which is the emitter of the same transistor.Surprisingly the offset voltage was tiny with that ancient op amp. only 50mV with 5x gain. maybe I got lucky with the silicon lottery or maybe they actually have that small of an offset voltage.But yeah you want an op amp with low offset voltage. The output voltage at 0 signal will be offsetVoltage*GainThe output resistor you'll choose to limit current. Since the output voltage can swing to whatever the op amp can swing to, minus the 2-3 diode drops. 10V across a small resistance VU meter wont be healthy for it.Also, you can limit the output voltage by making the reverse diode a zener instead, The one that goes directly from the output of the op amp to the negative input. The output will then be limited to the zener voltage, minus the 1.2 to 1.8V from the Vf drops.It really does look better imo than the regular resistor dropper.TimFox can modify the circuit to be real VU Meter ballistic spec compliant if he wants to.[ Attachment Invalid Or Does Not Exist ]You can also make it overshoot like an analog VU meter if you want, by adding a ~10uF capacitor from the negative input to ground.Keep in mind that the capacitor effect will be dependent on the impedance of that point, for the top circuit it will be R1 || R2, or 1/(1/R1 + 1/R2). So if you instead had higher impedance at that point, you'd choose a lower value capacitor.

author: TimFox
author_rank: Super Contributor
timestamp: August 22, 2023, 11:05:59 pm
content: If I needed one, I would adopt the 1972 design from Wireless World, where I first encountered the problem in college.Seehttps://www.casa.co.nz/electrics/meters/Peak-Program-Meter-Schematic_5p.pdf

author: Benta
author_rank: Super Contributor
timestamp: August 22, 2023, 11:19:28 pm
content: Quote from: TimFox on August 22, 2023, 11:05:59 pmI'm flabbergasted. That's exactly the one I was thinking of when replying in the other VU-meter thread.

author: TimFox
author_rank: Super Contributor
timestamp: August 22, 2023, 11:21:32 pm
content: I hadn't remembered the exact name of the BBC design, which I got from your post.Unfortunately, my cache ofWireless Worlds from that era disappeared sometime in the last 50 years, but I googled the term "Peak Program Meter" to find that article.

author: Benta
author_rank: Super Contributor
timestamp: August 22, 2023, 11:54:33 pm
content: If you want to peruse old issues of Wireless World or other magazines, this page is fantastic:https://worldradiohistory.com/index.htm

author: jonpaul
author_rank: Super Contributor
timestamp: August 23, 2023, 01:18:15 am
content: A VU meter is designed for analog meter movements and the attack, decay time constants are defined by the broadcast and pro audio industry via NAB, SMPTE and AES.You can change any of the parametes but the resut will not be a tru VU indication but really a volume level you define.See the many references on origial VU meter definitisnJon

author: TimFox
author_rank: Super Contributor
timestamp: August 23, 2023, 03:30:54 am
content: Quote from: ELS122 on August 23, 2023, 02:41:24 amQuit whining. My posts have been relevant to your threads, and the articles I cite can be useful to anyone who cares to read them.ThatWireless Worldarticle contains a detailed analysis of what is useful in such a meter, besides the complete wiring diagram.jonpaul agreed with me about VU metersYou specifically said "TimFox can modify the circuit to be real VU Meter ballistic spec compliant if he wants to." and I replied what I would want to do for that purpose.

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 23, 2023, 03:31:13 am
content: Quote from: ELS122 on August 23, 2023, 02:41:24 amYou really are Faringdon's audio related alter ego, aren't you?

author: David Aurora
author_rank: Frequent Contributor
timestamp: August 23, 2023, 06:02:14 am
content: Quote from: ELS122 on August 23, 2023, 04:30:33 am

author: mikerj
author_rank: Super Contributor
timestamp: August 23, 2023, 08:09:19 am
content: Quote from: ELS122 on August 23, 2023, 04:30:33 amHaving a bad day by any chance?

author: gbaddeley
author_rank: Regular Contributor
timestamp: August 23, 2023, 08:55:37 am
content: Interesting design. VU has a very specific meaning. I assume you just want to monitor the audio signal level, without being compliant to the VU standard.Have you tried a much lower value cap that will self discharge, without the CCS load? Eg. 10 - 100 nF. However the decay would be exponential, not linear.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 23, 2023, 07:55:34 pm
content: LM381… wow, that brings back some remembrances.Back in the 1970’s when Disco music was all the rage, I built a RIAA preamp using this IC. Pretty good performance at the time.To keep this thread on-topic, I also built a 12 segment VU, using a Japanese IC, whose part number I can’t remember. Half-wave silicon diode rectifier provided terrible response characteristics, but hey! It looked cool to a teenager learning electronics.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2023, 08:56:42 pm
content: LM381, LM382 and the demon LM379S were playing Fleetwood Mac Rumours and Bachman–Turner Overdrive, when I used themedit: forgot SupertrampI have not yet seen a decent VU meter drive circuit and it's on my list to make one for some huge 4" Simpson meters from the 1950's given to me. The thing is fast, msec response times, very light pointer fast than the human eye.Old ballistics such as PPM are from an era of mag tape and speech intelligibility and relevance today I'm not sure.One problem is a single supply design needs at least 2-3 op-amp stages with a 1/2Vcc node.The meter coil when moving has generator-action so you need to both source and sink current to have good control. Nobody addresses this.Most designs have huge uF caps for peak detect and as if an op-amp can charge that quickly without going into current-limit. Neve used a beefy emitter follower.The obsolete TA7318 is for audio power meters and has the 4th root response, chinese kits still offer it.Here are some schematics for the discussion.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 23, 2023, 10:05:44 pm
content: For me, and that is only me, the best VU meter is a hybrid: a conventional analog meter with an LED instantaneous peak detector.In a mixing console I saw a particularly clever implementation of the LED peak detector: using a bi-color LED, it would show yellow for a mild overdrive, and red for a hard overdrive.EDIT: Floobydust, you forgot the BeeGees.

author: floobydust
author_rank: Super Contributor
timestamp: August 23, 2023, 11:40:02 pm
content: I checked the LM382 date code 736. Bee Gee's Saturday Night Fever was released end of 1977 so it wasn't a happening yet I think.P.S. This VU meter has a 3k9 on the back in series, and I measure 500uA FSD and it seems to have an internal bridge rectifier. That's probably what stopped me, most meters I have have got have diodes inside. It also uses two #51 lamps for lighting.

author: TimFox
author_rank: Super Contributor
timestamp: August 24, 2023, 02:15:53 am
content: Real VU meters during that era had internal rectifiers and ballistics tuned to obtain the standard response.They said "VU" in big letters.The hi-fi industry copied the "VU" label, but did not care about the standards.

author: floobydust
author_rank: Super Contributor
timestamp: August 24, 2023, 03:22:21 am
content: Yes- an important point is the type of VU meter you have, in designing a driver circuit. Three types I know of, all aftermarket ones I have lying around:Kyoritsu KM-66 has inside: copper oxide bridge rectifier+resistor+cal trimpotBach-Simpson in pic has inside: bridge rectifier +external 3k9 resistorMicronta has nothing I believe it's just the coil. I'd have to check.Most have a rectifier inside which thwarts any driver circuit increasing the damping factor I think.I didn't feel comfy taking the meters apart and removing the extra circuitry.