VU meter driver circuit modification - Page 1
Forum section: electronics / beginners

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 26, 2020, 12:44:43 pm
content: Hi all,I came across a very nice video on YT where a simple vu meter driver circuit is presented.My meters are very similar in regards to their full scale current (around 0.5mA or 500uA) but I want them to be accurate regarding their actual VU indication (e.g 1.23 Vrms to be reading 0 VU)As it's shown in the video the 0VU mark on these maters is around 240-250uA and the maximum reading (around 5VU) is about 500uA which should be roughly equal to 2.2Vrms input signal.What I want is my input signal to be displayed correctly on the meters as per this table:http://www.cranesong.com/Volts%20to%20dBu%20to%20VU%20Comparison.pdfOn the video circuit, the signal needs to be around 200mV for the meter to reach full scale and since 200mV is about 10 times less than my input signal will be I thought the circuit would work as is if I messed around with the second opamp gain.I changed it to attenuate the previous signal (the original video has 1+2 = 3 gain so I initially tried to make this about 10 times less so I put a resistor from the positive terminal of the second opamp to v_gnd to form a voltage divider of the input signal and made the negative feedback path resistors equal.I used 1k to form a 1/10 signal attenuation network before the signal hits the non-inverting opamp.But the circuit did not work as expected (I was getting way too much current on the output)I then thought that VCC could be higher if I needed to since my meter's bulbs need about 10V to light up so I upped the VCC to 10V.Again the needle goes higher with less signal.I'm confused to where the actual gain control on this circuit could be.Is it the second stage?Or should I do something before the signal even hits the peak detector?Since I will be using a single supply I can't think of any way to do that without clipping the opamp.Can anyone help?Can I use this circuit to display true VU values?Can I also use it with say a single +12 supply instead of 6V?I attached the circuit in question and the changes I perfomed.

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2020, 11:03:36 pm
content: Was the other side of the 1k resistor you added to attenuate the signal connected to ground or virtual ground?The schematics show the input signal and meter referenced to virtual ground so the resistor you added needs to be referenced to virtual ground also. It is easy to be confused about this because the output from the peak detector is pulled to ground which is actually the negative supply.I would have moved the meter to the position of R7 with anti-parallel diodes around it because I think it is easier to drive that way with a controlled current instead of voltage This has the disadvantage however of not grounding one side of the meter.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 26, 2020, 11:55:51 pm
content: The 1k goes to virtual ground I drew like the schematic symbol of vgnd.I'll try your suggestion and post back!

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 27, 2020, 02:59:34 pm
content: Is there another circuit that's easy to calibrate and works well with 500uA range meters?I can probably make the psu bipolar if needed.The other thing I needed to implement is a peak led indicator at around 2VU or 1.55Vrms.I thought about using a similar circuit to work as a rectifier and comparator but this only works with a bipolar supply.I can't seem to find a simple peak level (or volume clip) indicator that works on a single supply.Ideally I'd like to tailor the original circuit above to my needs and just add a simple led to light up when Vin is > 1.6Vrms.

author: David Hess
author_rank: Super Contributor
timestamp: October 29, 2020, 02:52:47 am
content: Quote from: belzrebuth on October 27, 2020, 02:59:34 pmIf the meter is placed within the feedback loop in place of the feedback resistor, then it is driven with a current. Now a diode can be added in series with the meter so it only responds to positive or negative currents and the forward voltage drop is irrelevant because it only sees currents. An anti-parallel diode added across the feedback loop keeps the operational amplifier out of saturation.Peak detection would normally be done with a separate stage.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 29, 2020, 07:44:22 am
content: I replaced my initial 1k with 6.1k (and it now works a lot better) it seems my calculations were wrong because I've put the (-) of my signal to actual ground instead of virtual ground.With the signal referenced to virtual ground I seem to need about 0.4 of the original signal.The meter now kind of tracks but it's not really accurate as per the VU scale but 0VU is about 1.22Vrms.I like how it responds to audio signals.I'm curious whether I actually need to connect the ground of my signal to virtual ground or the GND of the opamp = ground of the power supply.V_GND sits at about VCC/2 and if I use a grounded supply couldn't it cause problems if my incoming signal is ground referenced as well?Using a floating supply makes sense but when I finish this I'm going to have the RCAs for the input connected in parallel to another set of RCAs so I can put this in series with a mixer or whatever so I need to know it's okay for the sleeves of the RCAs of both input and output to connect together.Essentially with a passive passthru of my signal the sleeves of all the RCAs will be at the virtual ground's potential.Is this going to cause any problems if I only use an isolated power supply?On the original schematic it's not really clear where the (-) of the signal is going. It seems that only the (+) of the signal is actually connected to the vu driver circuit.Now, about the peak led circuit, I just messed around in Multisim and made the circuit below that seems to respond when breadboarded but it doesn't seem correct because I can't very accurately set the threshold for the led to light up.The led is very close to its turn on voltage at a broad range of the input signal (100mV to 1600mV I think)Simulation is worse with the output not able to go past 1.7V no matter what value of trimmer I'm using.On the breadboard I can make the led light at about 1Vrms of input signal with the values shown on the schematic below.My only problem is that I can't seem to properly trim it to light up at say 1600mV RMS.I suspect the input signal for the peak led stage should be coming from my main VU driver ciircuit with an adjustable gain stage or a comparator of some sort so maybe my led circuit is completely wrong.

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 29, 2020, 11:28:38 am
content: The circuit for the LED peak was not so smart so I added a comparator stage and a switch for +4dBu/-10dBV operation.The attached circuit works nicely for my purpose.

author: floobydust
author_rank: Super Contributor
timestamp: October 29, 2020, 08:52:39 pm
content: There are around three different types of VU meters - raw meter movement, built-in low Vf bridge rectifier (copper oxide or Ge), and the 3k9 standard resistor. Some of these parts are inside the meter, some are not. If the meter always moves up when connected to a multimeter on ohms (not peg -ve) then there is a bridge rectifer inside which is the norm for vintage VU meters.For your original circuit I am not a fan because it's 1/2wave rectified, has two big fat 10uF capacitors which makes the ballistics sluggish and a bit of work for the op-amp.A bit more more here:https://www.eevblog.com/forum/projects/tn-73-vu-meter-driver-leakage-current-problem/msg3108856/#msg3108856https://www.eevblog.com/forum/beginners/analog-vu-meter-volume-pot/msg2651670/#msg2651670

author: belzrebuth
author_rank: Frequent Contributor
timestamp: October 29, 2020, 09:20:45 pm
content: I agree, a full wave rectifier makes more sense and it's actually simpler.I'll try a full wave rectifier circuit as well before committing to a PCB.I wonder if it actually tracks better as well since it's only DC coming out so it's easier to scale/tame.Thanks for the link I should have researched more..