TN-73 VU meter Driver leakage current problem - Page 1
Forum section: electronics / projects

author: Niklas B.
author_rank: Contributor
timestamp: June 26, 2020, 12:49:08 pm
content: I wanted to build an analog VU meter to put on my stereo setup (and to maybe sell some on ebay). So i looked for driver circuits, because i have about 0 experiance with driving such devices, and foundhttp://objectivesounds.co.uk/articles/driving-vu-and-other-ac-meters/and based my design around Figure. 6My eagle designBut the problem is that the 500uA meter its designed for goes instantly out of range.The reason is the leakage current in stand by is about 2mA. The only thing I can do about that is to increase R6 to some rediculus value of about 500K 1Gwhere I can get it dow to about 1-10uA but completely removes thw ability to make adjustments with the trimer.I have put hours of testing inot this and can not for the life of me figure out what is wrong, is it the design, one of the components or even the PCB? I don´t know, please help I´m going crazy looking at thisAlso if someone has some time and is in the mood for looking over a design by someone who is at the moment only a hobbyist but is planing to make it his profession here is the full drawing and layout(R7,9,10,11,12 are unpopulated, shorted or populated depending on my input signal)

author: Messtechniker
author_rank: Frequent Contributor
timestamp: June 26, 2020, 01:04:31 pm
content: Been using the attached circuit for years and built quite a few. Maybe you can take some ideas - especially as to the precision rectifier (blue ICs) - from it.

author: Niklas B.
author_rank: Contributor
timestamp: June 26, 2020, 01:21:31 pm
content: thanksI´ll look into that

author: TimFox
author_rank: Super Contributor
timestamp: June 27, 2020, 06:52:58 pm
content: In your first drawing, with the TL072 op amp, what is the DC voltage on the non-inverting input with respect to your ground symbol? A positive voltage there will give a positive voltage at the TL072 output, which will flow through the diodes, the meter, and down the variable resistor to ground. With 3k total resistance, your 2 mA "leakage" current corresponds to 6 V at the input. Typically, when driving a floating ammeter from a diode bridge, one should capacitor couple the amplifier output to the bridge (there are several ways). Seehttp://www.mcmlv.org/Archive/TestEquipment/HP400.pdf, the manual for the -hp- 400 series of vacuum-tube AC voltmeters, where the meter is fed from a circuit with two diodes and two capacitors in the feedback loop equivalent to yours.

author: Niklas B.
author_rank: Contributor
timestamp: June 27, 2020, 07:06:30 pm
content: so if I understood corectly i schould do something like this

author: TimFox
author_rank: Super Contributor
timestamp: June 27, 2020, 08:35:39 pm
content: You still need a resistor to provide DC feedback to keep the op amp output near its center.Again, what is the DC voltage on your non-inverting input? If you have a single power supply, referenced to ground, then you have to(1) bias the non-inverting input properly(2) pay attention to the common-mode input range of the op ampWith a bipolar power supply, it is easier to achieve (1) and (2), but you still need a DC feedback path.

author: Niklas B.
author_rank: Contributor
timestamp: June 27, 2020, 09:02:26 pm
content: The DC voltage is, with R6 + 13 = 12K, 8.89V and with R6 + 13 open it is at about 1.6VThe powersupply is (as seen on the schematic) a simple 7809 liniar voltage regulator so its just hooked up to +9V and 0V

author: floobydust
author_rank: Super Contributor
timestamp: June 27, 2020, 11:37:32 pm
content: I don't see it working with a single supply and theTL072.It is not a rail-rail input op-amp, it does not work (8.3.3) with common-mode input below 4V and phase-inverts below 1V. You would have to add input DC offset like the low cost chinese VU meter driver kits. I think the LM358 might be a better choice because it can operate down to 0V input. Even then I wonder about ignoring -ve AC input voltage.The obsolete TA7318P was for VU meter drivers, it has non-linear "1/4 root compression" or logarithmic amp I call it.Newer kits are still called "TA7317P" but fake because they use the LM358 and linear transfer function. If you don't see a SIP package IC, it's not a TA7318P.The circuits herehttp://www.objectivesounds.co.uk/articles/driving-vu-and-other-ac-meters/assume dual rails.

author: Niklas B.
author_rank: Contributor
timestamp: June 27, 2020, 11:48:01 pm
content: So what do you think would be a better choice1. to get some TA7318P because i can find some for about 10€ a pice or2. try the lm358 or3. put a +4V -1V powersupply on the current design (I have no idea how to acutally do that without a second seperate power input

author: floobydust
author_rank: Super Contributor
timestamp: June 28, 2020, 01:56:15 am
content: There are varying level of quality VU meter driver circuits out there. I have not used the TN-73, only big old meters.So I would change over to LM358, add 1/2Vcc bias to the op-amp input to keep it single supply. Something like this sch I think could work. I did not do detailed math for LF rolloff.The TA7318P is hard to find and its only advantage is the logarithmic curve for amplifier audio power (not VU) meter scale.

author: Niklas B.
author_rank: Contributor
timestamp: June 28, 2020, 08:57:46 am
content: Thank you so much I have been going crazy over hereI´m going to test that setup asap and will be sharing the results here sometime in the near future

author: Niklas B.
author_rank: Contributor
timestamp: July 24, 2020, 05:30:47 pm
content: After intensive testing i have concluded that this circuit does not workAs long as no load (VU Meter) is conected on the output everything worky fine and i have an output signal at least when i´m measuring from GND to one of the four output pointsbut as soon as annything is conected to the output the signal disapears even my 1Moszilloscope inputMy theroy is that the input somehoe can´t deliver enouth power so the voltage breaks downI don´t know if this circuit requires grermanium diodes or another opamp but if annyone knows please tell me I´m at a compleat loss here what to do

author: floobydust
author_rank: Super Contributor
timestamp: July 24, 2020, 11:28:53 pm
content: I'm not clear what problem you are having. Which op-amp (TL072 or LM358) are you using, power is single 9V?Can you show a picture? The circuit I posted earlier works when I tried it. It's not the best, could use some polish.

author: Niklas B.
author_rank: Contributor
timestamp: July 25, 2020, 08:18:01 pm
content: this is 1:1 the circuitusing the LM358ADRG4 and 1N4148 diodesand a single +9V powersupply

author: floobydust
author_rank: Super Contributor
timestamp: July 25, 2020, 08:29:05 pm
content: I think the diodes are wrong in your drawing. "AC" to the op-amp output and (-) feedback.On the bench I added 220pF from pin 1-2, 7-6 for stability, the LM358 acted funny at no signal.

author: Niklas B.
author_rank: Contributor
timestamp: July 25, 2020, 09:31:36 pm
content: i actually got the dam diods wrong... thats a first for me

author: Niklas B.
author_rank: Contributor
timestamp: July 26, 2020, 02:59:42 pm
content: yeah so after reversing 4 of the diodes everythin is working corectlythanks for the help