Analog VU Meter & Volume Pot - Page 1
Forum section: electronics / beginners

author: vidarr
author_rank: Frequent Contributor
timestamp: August 29, 2019, 07:39:16 pm
content: Hi guys,I want to add an analog VU meter to a simple amp circuit I built as a learning exercise. I've been researching online, but I cannot find what I think I am looking for. Also, I've been researching where to put the volume potentiometer. To me, it seems like the more knowledgeable people are saying to put the pot at the beginning of the circuit after the input. But, there are a whole lot of other people saying they have better sounding audio putting it at the end of the circuit, right before the speakers. There seems to be no clear answer.Can someone please guide me in the right direction? I attached the circuit I am using. It is just the circuit provided in the datasheet for the amp chip (TDA1517).Thank You

author: MasterT
author_rank: Frequent Contributor
timestamp: August 29, 2019, 09:19:44 pm
content: It looks like VU out of magnetic tape recorder, so try this name +schematic as google search pattern.Volume control pot is always at the input of power amplifier. Speakers want to see low output impedance of the PA, setting (high power) pot would introduce substantial resistance in series, and it's not acceptable.

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2019, 09:27:56 pm
content: First, confirm if the VU meter has internal diodes/bridge rectifier or not.I can't quite see in your pics if there are diodes inside, or if the meter's coil connects to the terminals straight away. You can also (on diode-test) use a DMM to see if the meter moves pegs with either polarity applied. Kyoritsu usually has diodes built-in their VU meters.Best to drive it off the loudspeaker outputs. For monitoring line-level, you would need to add a meter driver IC like TA7318P and there are many cheap kits on eBay or Aliexpress that have it.edit: Note TA7318P is for audiopowermeters, the IC has built-in 1/4 root math to convert voltage to power.Majority of the TA7318P chinese boards are fakes with LM358 DIP-8, not the TA7318P SIP-9.

author: Benta
author_rank: Super Contributor
timestamp: August 29, 2019, 09:39:47 pm
content: Volume control at the input of the amp, no discussion.Classic analog VU-meters often had germanium diodes inside to provide a somewhat logarithmic response.

author: bob91343
author_rank: Super Contributor
timestamp: August 29, 2019, 09:45:39 pm
content: A VU meter has some special characteristics and specifications. The FCC defined it, decades ago, in terms of its ballistics so that it would come closest to an actual representation of radio station modulation.Most VU meters, at least those that meet the standard, include rectifiers and should be placed in series with a 3.9k resistor (if I remember correctly) and the combination connected across a 600 Ohm line.I assume you aren't doing this, and as a result should probably not call it a VU meter.So then what you need to do is find a place in your circuit that has the ability to drive the meter. Then adjust the parameters associated with the meter so that 0 VU represents maximum acceptable signal (ignoring headroom). A good idea is to include an adjustment so that it can be calibrated occasionally.

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2019, 10:09:18 pm
content: The volume pot is best located at the input of the TDA1517 amplifier.But for the VU meter, you want low impedance drive for good ballistics, so the output of the power amplifier is best. No cost what so ever having the meter located after the amp anyway, not worth arguing.OP's potentiometer and source is likely 10k or greater, and putting a VU meter there is a non-linear load and will make distortion. It's not a simple resistive load, there is inductance, generator action and diodes so I would never put a VU meter directly on line level signals.

author: vidarr
author_rank: Frequent Contributor
timestamp: August 30, 2019, 01:15:08 pm
content: Here is a better picture of the VU meter. There are only two leads that go into the meter. I don't know what that component inside the meter is but it has four leads. So, one exterior lead goes to the far left of the component and the other exterior lead goes to the far right. Inside the meter, from that component that I don't know, one lead goes to the top of the meter and the other goes to the bottom of the meter.I tried looking this meter up online, but I found nothing from its numbers.Thanks everyone.

author: vidarr
author_rank: Frequent Contributor
timestamp: August 30, 2019, 07:33:59 pm
content: If I put this meter parallel to a speaker, could this cause damage to the meter?Also, with a stereo amplifier that has these kinds of meters, it would have one meter right before the Right Channel Speaker, and another meter right before the Left Channel Speaker? I only have the single meter, so I should pick either the Left, or Right, channel?Thank you

author: floobydust
author_rank: Super Contributor
timestamp: August 30, 2019, 09:54:02 pm
content: You must add a resistor/potentiometer to use the VU meteror else it would get damaged. The potentiometer could need to be 1k or 10k ohms. The black blob is a bridge rectifier, I forgot if it's germanium or silicon.That edgewise VU meter is made by Kyoritsu, high quality Japan 1987 and rare. Last seen in Sony portable reel-reel tape recorders like TC-521. The meter part number would be like EW-xx, EW-20 or EW-16 are the DC panel meters.If you want to use one meter for stereo, I would just use one channel or add another potentiometer to mix the two channels feeding the VU meter.edit: tried to draw a clearer schematic for OP

author: vidarr
author_rank: Frequent Contributor
timestamp: August 30, 2019, 10:54:51 pm
content: Since this is a project just for learning's sake, what way would be the easiest to incorporate the VU meter? I don't fully understand the schematic you attached. I understand the VU meter and the resistor, but I don't see where the VU meter is inserted into the circuit. That is my primary question.THANK YOU!

author: bob91343
author_rank: Super Contributor
timestamp: August 31, 2019, 12:23:22 am
content: The problem with connecting the meter to the amplifier output is that its reading changes when you adjust the system volume. If you are willing to accept that, simply turn up the volume to a point you like and attach the meter to the amplifier output via a series resistor. You adjust the resistor to get the reading you like.If you decide to make the sound louder, the meter will peg and there is risk of damage to it.The internal rectifier would likely be copper oxide if it's an old meter; that was the standard for many years.You can get fancy and calibrate the meter in terms of amplifier power output. You drive it with a sine wave while measuring amplifier output and select a resistor to give the reading you choose. Then do it again for other power levels and finally put all the various resistors on a switch and mark it with the power needed for a 0 reading at each setting.

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2019, 05:26:14 am
content: Quote from: vidarr on August 30, 2019, 10:54:51 pmI update the schematic, with a line around the VU meter and it's internal rectifier. Is this understandable?

author: vidarr
author_rank: Frequent Contributor
timestamp: August 31, 2019, 02:21:02 pm
content: Yeah, that is 100% clear now.But, if I put the VU meter after the input, before the amp chip, would the circuit look similar? I would just change the resistor?"The problem with connecting the meter to the amplifier output is that its reading changes when you adjust the system volume. If you are willing to accept that, simply turn up the volume to a point you like and attach the meter to the amplifier output via a series resistor. You adjust the resistor to get the reading you like.If you decide to make the sound louder, the meter will peg and there is risk of damage to it."Where do VU meters typically go to "avoid" this?Thank You

author: Audioguru again
author_rank: Regular Contributor
timestamp: August 31, 2019, 05:18:30 pm
content: Your signal source might not have enough power to drive the VU meter then it will be distorted or at a low level.

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2019, 06:16:12 pm
content: What did you want the meter to show? It's like an AC voltmeter.Historically, people used them to prevent overload for the system. Going over +3dB or pegging the meter could mean things are distorting and clipping.It takes energy to move the meter's pointer, it does not move for free.If you are taking headphone output from something like a cellphone or mp3 player, it has enough power to drive a meter and you could connect the meter there with the same circuit.But a weaker line-level signal (PC soundcard, tape deck etc.), or the VU meter being AFTER the volume control, you would need to add an op-amp buffer to drive it, so it does not load down the signal and cause distortion.Found some info here:http://www.objectivesounds.co.uk/articles/driving-vu-and-other-ac-meters

author: vidarr
author_rank: Frequent Contributor
timestamp: August 31, 2019, 07:14:28 pm
content: The audio source will be a mobile phone or tablet most of the time. If the sound is decent, I may use it at one of my computers. It is mostly for watching news, not movies or music, so the sound doesn't have to be amazing.The meter really is just for fun because I have it. I just want to learn how it works and how to create a circuit for it. I think now, with your guys' help, I am starting to get a good idea. I read things online, but sometimes asking a simple question and getting a simple specific answer helps a lot. I looked to a lot of schematics with VU meters and see how they work, kind of. But I had a lot of "why" questions that I never find answers to.Thank You

author: bob91343
author_rank: Super Contributor
timestamp: August 31, 2019, 09:30:45 pm
content: vidarr, I respect your attitude. If you want to learn, it's best to get into it with your hands and make stuff happen. Lessons learned this way are far more durable than conversation.So if you risk blowing the meter, you have learned something. It's just a motor but only goes about a quarter turn before hitting the stop. And on power removal, the spring brings it back to starting position. The secret is the spring; that allows the motor to turn just so far before the spring holds it back.Since the motor is very sensitive, it's also delicate. Too much current can overheat the fine wire in the armature, and cause mechanical shock when it hits the stop.Since it only responds to dc, you have to rectify the signal if you want it to respond to ac. Actually, most rectifiers also pass dc so you kind of have both worlds when you use one.

author: vidarr
author_rank: Frequent Contributor
timestamp: September 01, 2019, 01:02:41 pm
content: I already burned out so many things putting circuits together wrong, including two (and a half) power supplies and three transformers (and many other random small parts). I really like this VU meter, so it will suck throwing it in the trash. I am still soldering the rest of the circuit and building the aluminum and wood housing. I have another week or so before the VU meter part of the project starts. I read the article floobydust gave the link for. He provides a number of circuit options and then at the end says none of them are good except maybe one. If I had a cheapo meter to mess with, or a simulated meter online...Thank You

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 01, 2019, 01:44:34 pm
content: Quote from: vidarr on August 31, 2019, 02:21:02 pmWe use VU meters (or the modern equivalent) while mixing sound for "production" of a finished product.The finished product may be a recording (cylinder, disk, cassette, CD, MP3, etc.)Or it may be a live program (broadcast, network, online, streaming, etc.)But in any case there are certain constraints and standards for audio levels.If the audio levels are too low, then the Signal-to-Noise Ratio (SNR) suffers and the result is noisy.But if the audio levels are too high, then they distort and clip from running into the limits of the system.So the VU meter was developed to monitor audio levels to keep them in the "sweet spot".Not too low "down in the mud" of the noise-floor, and not to high, distorted and clipping.In the digital era, it is even more critical to avoid too high = clipping.At the destination end of the system (the audience), the audio signal is re-constituted.At some point, the audio signal is at the same ratio as when it was created/transmitted.But then the end-user has a "volume control" where they can select some portion of the original level.If you can connect your VU meter BEFORE the volume control it will represent the level of the original program.But if you put it AFTER the volume control, it will only represent your prefered listening level.Now, that may be what you want, but only you can decide how to connect/use the meter.

author: Audioguru again
author_rank: Regular Contributor
timestamp: September 01, 2019, 08:41:05 pm
content: I use a peak detector circuit to correctly show the actual maximum levels of my LM3915 logarithmic 10-LEDs sound level meter that shows sound levels since it has a microphone and preamp for it.I added a circuit to boost the sensitivity 10 times when levels are low.Unfortunately the LM3915 is not made anymore.

author: vidarr
author_rank: Frequent Contributor
timestamp: September 01, 2019, 09:43:38 pm
content: Quote from: Richard Crowley on September 01, 2019, 01:44:34 pmGot It!Thank you.