LM3915 Issues - Page 1
Forum section: electronics / projects

author: MarcusS
author_rank: Regular Contributor
timestamp: July 28, 2016, 09:12:49 am
content: Has anyone else noticed the datasheet for the LM3915 Audio Power Meter Schematic on page 26 is wrong.I'm trying to add a nice looking Vu meter to the output of an amplifier I have been designing and got myself caught up on connecting the speaker minus output of the amplifier to the ground of the vu meter circuit basically shorting out one side of the audio amplifier. The calculations for the voltage divider are simple enough as is calculating Vref but that all goes out the window if you cant connect speaker minus to pin 2 and pin 4 of the IC.I have had it working but incorrectly by connecting the audio input between pin 4 and 5 but the problem is pin 4 is no longer referenced to pin 2 of the IC. Has anyone built 2 of these for stereo run it from a single voltage supply and not shorted out the audio amplifier or created crosstalk between channels. I am keen to see if anyone has done anything fancy with resistors or decoupling capacitors to get around the issue.

author: rollatorwieltje
author_rank: Unknown
timestamp: July 28, 2016, 11:09:20 am
content: The TI datasheet? It's not wrong, it just assumes the power supply to your LM3915 is floating. You should power it from a separate power supply, i.e. another secondary winding on your transformer or an isolating DC-DC converter.

author: MarcusS
author_rank: Regular Contributor
timestamp: July 28, 2016, 12:53:22 pm
content: My lm3915 circuitry is actually being powered off a recom rp30-4812 which is a shielded isolated dc dc converter but the problem is the 12v minus out of the dc dc converter is shorting out speaker minus of the btl amplifier design as 12v minus out is still essentially ground in comparison to speaker minus which is the negative side of the amp output waveform.

author: Audioguru
author_rank: Super Contributor
timestamp: July 28, 2016, 03:23:11 pm
content: The ground of a BTL amplifier is the ground for its audio. The speaker does not connect to ground since both of its wires are fed signals with opposite phase.The LM3915 power meter circuit does not connect to the -12V, it connects to one of the wires feeding the speaker and the amplifier's ground. Ground is the 0V between the +12V and the -12V.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 09, 2016, 02:07:52 pm
content: I cant understand how you could connect it that way. The lm3915 is being powered off an isolated 12v dc dc converter it has a vout+ = 12v and a vout- = dc dc converter output ground. The dc dc converter ground has a significant voltage differential to the amplifier ground. I have tried connecting one side of the amplifier output to the input and connecting pins 4 and 2 to the vout- of the dc dc converter but it doesn't work correctly.I am happy to follow instructions if someone can tell me exactly how they would connect 2 lm3915s (1 for each channel) powered off a single TDK CCG30-4812s with the outputs of each channel of a TDA7498 BTL class d amp IC going to each lm3915.

author: Audioguru
author_rank: Super Contributor
timestamp: September 10, 2016, 01:10:13 am
content: Connect the input of the LM3915 circuitto only one BTL speaker wireand the amplifier's ground. Then re-calculate the value of R1 for half the output voltage swing. Then the LM3915 circuit can use the amplifier's positive and ground power supply.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 10, 2016, 02:05:12 am
content: Where do I connect the Vout- of the dc dc converter too then. It doesn't make sense. The datasheet says to connect pins 2 and 4 together which is connected to ground and feed the input into pin 5. I cant connect the vout- of the dc dc converter to amplifier supply ground.

author: void_error
author_rank: Frequent Contributor
timestamp: September 10, 2016, 09:12:07 am
content: Why do you need an isolated DC-DC converter in the first place?

author: MarcusS
author_rank: Regular Contributor
timestamp: September 10, 2016, 09:21:25 am
content: There are 2 reasons. I have the converter on the board for other things and secondly the LM3915 will not handle the 36V that powers the amp.

author: Audioguru
author_rank: Super Contributor
timestamp: September 10, 2016, 04:04:35 pm
content: Reduce the amplifier supply voltage or add a new positive one so that the LM3915 is powered from about +9V then it will not get too hot when powering a bar of 10 LEDs.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 10, 2016, 09:24:51 pm
content: I have a positive supply voltage for the lm3915 it is called a 12v output dc dc converter. It takes the 36V input that powers the amplifier TDA7498E and reduces it down to 12V which I need for some other things and happens to be the ideal voltage for a 10V logarithmic display using an LM3915.I cant lower the amplifier supply voltage, it is a 160Wx2 BTL amplifier and it nominally requires 36V. I don't have a problem powering 10 leds and the lm3915 from the dc dc converter I have a problem with ground references.I take it that I can only try to find a non isolating dc dc converter module so that the 12v ground from the dc dc converter is the same as amplifier ground. At the moment, the dc dc converter output ground is 18V above the amplifier ground.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 10, 2016, 11:35:19 pm
content: Can't exactly find any non isolated dc dc converters that meet me requirement. Don't really want to have to put on the board a non isolated and an isolated dc dc converter. Has anyone actually powered an lm3915 from an isolated dc dc converter and knows what needs to be done.

author: Audioguru
author_rank: Super Contributor
timestamp: September 11, 2016, 12:17:46 am
content: Here is as tutorial for a DC to DC buck voltage converter. It uses PWM to reduce the voltage and the input ground, circuit ground and output ground are all the same and can be the same as the amplifier and LM3915 grounds.https://www.maximintegrated.com/en/app-notes/index.mvp/id/2031If the supply for the LEDs is +12V and the LEDs use 2V then the output transistors in the LM3915 have 10V across them. If the LED current is 20mA and all 10 LEDs are lit then the poor LM3915 must dissipate 200mA x 10V= 2W but its maximum allowed dissipation is only 1.3W when it is very hot. So you need fewer LEDs lighted or less current or less voltage.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 01:01:51 am
content: It doesn't matter about the maxim application note. I have not built a buck converter for this application as I need EMC compliance.My DC DC converter modules do not have input and output ground connected together. As I said before I have a TDK CCG30-4812s.I have calculated and measured my LED current as 7mA each my vref is made of 3.9k and 22k which if you use the formula in the lm3915 datasheet gives an led current of 7.5mA. This also aligns with what I am measuring. That combined with one 82 ohm resistor in series with the common of all leds gives only 200mW dissipated in the LM3915. So 12V is not and never has been an issue.I don't have an issue calculating the values for an LM3915. My issue the whole time has been how to handle completely isolated supply voltages in the system that DO NOT share a common ground. If I had a common ground I wouldn't even be asking the question.The problem I have just discovered is that the 2 LM3915s must not share the same ground i.e. the same output gnd of the dc dc converter. I have just proven that what I was seeing is that the leds are responding to left minus right and that is showing on both channels of lm3915 even if only one channel is driven.

author: void_error
author_rank: Frequent Contributor
timestamp: September 11, 2016, 02:14:04 am
content: Hmm... since it's a BTL amplifier with a single supply then the L+/L- & R+/R- speaker outputs will idle at roughly half of the supply voltage, and with an input signal present they outputs will be 180 deg out of phase - L- is L+ inverted.You only care about the the L+ and R+ for the amplitude.Looking at the LM3915 datasheet I can see the input is internally clamped to the negative supply which means it'll act almost like a half wave rectifier.If you AC couple the input of the LM3915 you can tie the negative output of your DC-DC converter to the ground of the BTL amp but since you said you can't do that this is not a solution.What you can do is design (or just buy it, but they're expensive) an isolation amplifier using optocouplers, a google search will give you an idea how, and reference their inputs to the amp's ground. That way you isolate the input of the LM3915.

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 02:29:00 am
content: I think this is quite a tricky problem.Is the 36 V supply quite stable? It's not a class D is it?This is what an hours thinking came up with, I've not done the values, or checked the impedance needed for driving Ref Lo, etc. etc... Just 1 channel shown.

author: void_error
author_rank: Frequent Contributor
timestamp: September 11, 2016, 02:53:57 am
content: Quote from: StillTrying on September 11, 2016, 02:29:00 amIt's a TDA7498, which is a class-D amp.His issue is that he can't reference the LM3915 ground to the amp's ground, that way it would have been a lot easier.

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 03:43:12 am
content: Quote from: void_error on September 11, 2016, 02:53:57 amI think he can as long as he can take the 0dB being the mid ~18V point.Here's an attempt at a class D version, but only peak detects on 1/2 of the waveform.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 04:03:01 am
content: I was just mucking around with some diodes myself before I saw the posts. At the moment I am contemplating adding a second isolated 12v dc dc converter for the other channel or option b although still haven't gotten it confirmed in my head if I put diodes between V- of both lm3915 ic's so that they cant backfeed through each other.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 04:06:31 am
content: If only 1 channel is driven the leds are correct in terms of magnitude but the issue is the leds light up on the other channel. If I drive both channels it seems like the leds only show up when there is a difference between the channels.If I use a signal generator on both channels with the same frequency and magnitude no leds light up at all on either channel.

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 04:19:03 am
content: Quote from: MarcusS on September 11, 2016, 04:03:01 amAs each channel has an independent signal I don't think there's any way to join their 0V/grounds, 2 independent floating supplies are needed. Even if you go that way, you're still only going to be looking at peaks of one polarity aren't you, Is looking at the peaks of just one polarity good enough?Quote from: MarcusS on September 11, 2016, 04:06:31 amWe could do with a schematic.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 04:38:16 am
content: Yeah I only need to see one side of the waveform. Ill put up the schematics.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 04:49:38 am
content: Amplifier Schematic.

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 07:47:51 am
content: I stillthinkthis is still the way to go, noisolatedsupply needed. I made it into a sim working version 250kHz ?, and fiddled with R1,2,3,4 to make Ref_Hi - Ref_Lo = 1.2V.LT worky version included if NE1 wants to improve!

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 11, 2016, 07:54:48 am
content: The -12v from the LM3195s' DC DC converter is floating with respect to the audio - there is no other audio reference to the LM3915s (as far as I can see).A suggestion would be to connect the LM3915 -12v via a pair of resistors to 1/2 of the 36v supply. It would need a large cap so the two audio inputs don't pull it around DC wise.In other words, one (say) 3k3 resistor from +36v to -12 (from DC DC converter) and the other from -12 to ground. Then a large cap from the -12 to ground - say 1,000uF (needs experimentation at low volumes).Another idea would be to take the LM3915 input from the audio input (wiper of volume control) and then connect -12 to ground. That relies on the audio input being AC coupled as it comes in (no DC offset on the input).

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 09:00:31 am
content: Still Trying are you saying to ditch the isolated dc dc converter all together. Chris Mr, the -12V does already exactly sit at 18V and never budges with respect to the 36V without any capacitors. The issue I had was crosstalk between the channels from an lm3915 point of view due to sharing the same dc dc converter. I could take the signal from the input side of the amp but I do want it to be a calibrated db gain, THD and rms power meter.

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 11, 2016, 09:12:13 am
content: Yes, the -12 will 'sit' at 18v but only because of the two sets of input resistors to the LM3915 (R3, R4, R5, R6). The 1nf capacitor on the DC DC converter (C13) provides a little smoothing but not much. If you take a scope from the ground to the -12v to the LM3915 whilst there is the same audio present on both channels what do you get? It looks like you would get the audio on the -12v (filtered a bit by the 1nf (C13) and the resistors (R3, R4, R5, R6)).Are those polarised capacitors (C13 & C54)?To put it another way - whatforcesthe LM3915 -12 to be 18v?

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 09:46:05 am
content: No they aren't polarized capacitors they are just monolithic ceramics. I had desoldered them earlier in the day to see if it would change anything. Not sure why the -12 sits at 18V with respect to the 36 it just does and doesn't seem to budge.

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 11, 2016, 09:51:56 am
content: .. and with a scope from GND to -12 and the same audio on both channels (full output) - what do you see?A 'work around' would be to put a big (1,000uF) cap in place of C13; check polarityTrouble with that being that with low frequency audio going in one channel it would be 'seen' on the other if there were no input (depends on value of large capacitor).

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 10:37:31 am
content: Would a second isolated dc dc converter for the other channel lm3915 be the most technically correct solution and the wiring kept the same as the schematic the only difference being that each v- and each v+ for each lm3915 come from different dc dc converters.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 10:39:41 am
content: I did try something similar to your work around already but with 1uF and I got all leds lighting up solid.

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 11, 2016, 11:07:20 am
content: Without a solid reference for the LM3915 -ve rail, no amount of isolated DC converters are going to help.Assuming you don't have an oscilloscope, try connecting a 2K2 resistor between -12 and GND and then measure the voltage across it - over!

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 11:47:28 am
content: I checked on the scope and yes the 12- from the dc dc converter is moving around in response to the audio signal compared to the 36V gnd. However it does look like the leds are doing what I need them to do with the exception that it lights up leds on both channels.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 11:52:22 am
content: Actually the leds don't really work properly. I need 5V RMS for the first led to come on and then the last one comes on at 20V RMS.

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 05:30:37 pm
content: The voltages on all 4 speaker connections move independently you can't common any of them, so I don't think there's any way you can do it that way without having 2 floating supplies. But...Quote from: MarcusS on September 11, 2016, 11:52:22 amThink of it this way, I think it's accurate enough if you want to go in this direction, if you want to measure at the speaker connections then I don't think you have a choice.At 0dB no-output the amp holdsboth endsof the speaker at the mid voltage point, - both ends at 18V on 36V supply, so that there's 0Vacrossthe speaker.Your 20V RMS is peaks of +/-28V, so..To get this peak of 28V across the speaker connections the amp raises one speaker connection 14Vabovethe mid voltage point, and the other speaker connection 14Vbelowthe mid voltage point, giving 28V across the speaker. As the voltage on each side of the speaker is an exact +/- mirror image of the other side, you only have to look at one of them referenced to 0V.Looking at just this one side of the speaker, at 0dB output the speaker is held at the mid point = 18V , when the 28V peak comes along this one speaker side raises to 18V + 14V = 32V.This gives you the 2 Vref levels needed for the LM3915, Vref LO = 18V and Vref High = 32V, referenced to 0V.In practice you'd potential divider this voltage down, div by 10 gives Vref Low = 1.8V and Vref High = 3.2V referenced to 0V.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 11, 2016, 09:58:12 pm
content: Does pin 4 R Low need to connect to pin 2 on the lm3915? If it doesnt then couldnt we put spk + and 36v ground between 5 and 4 with the appropriate divider and power the 3915 between pin3 and pin 2 with the dc dc converter.

author: StillTrying
author_rank: Super Contributor
timestamp: September 11, 2016, 11:26:08 pm
content: Quote from: MarcusS on September 11, 2016, 09:58:12 pmYes, if you want to ref everything to GND, the TDA7498's gnd pin4, the LM3915's gnd pin2, and the DC-DC converters -ve can be connected together.The LM3915's Ref Low doesn't connect to this gnd because you're now detecting one side of the speaker's voltages, 18V = 0dB, 30V = 30dB. If you supply this 18-30V to the LM3915's Sig In via a div 10 potential divider the Vrefs you need are then Vref Lo = 1.8V and Vref High = 3.0V, they're handy values of Vrefs because the difference between them is 1.2V, which is the native voltage of the LM3915's voltage reference = no voltage adjusting resistors needed.You'll still need 1 resistor from Vref High(3.0V) to gnd to set the LEDs current.Is that the direction you're trying? I'll update the sim with these values soon.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 12:11:06 am
content: I cant connect the Vout - of the DC DC converter to 36V gnd. But I take it from your response that I need to connect pins 2 and 4 together on the LM3915 for the IC to work.

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2016, 01:32:01 am
content: I cant connect the Vout - of the DC DC converter to 36V gnd.Oh dear! Why not when it's isolated.But I take it from your response that I need to connect pins 2 and 4 together on the LM3915 for the IC to work.No pin 2 is it's power gnd and pin 4 is Vref Low.I've worked out some values(or rather LT has) as below, I've had a play with it and will add description as needed, can't you try something like that?R5, R6 and C4 produce Vref Low from the speaker's mid-voltage,Greenon the plot.The LM3915's internal 1.28V ref produces Vref High = (VrefLow + 1.28V).Redon the plot.The Sig In inBlueon the plot shows a 18W peak 500Hz signal with its +ve peaks crossing the vref Low value.Edit:I wound up the500Hz signaluntil it's peaks just reached theVref HighLED #10 level, Watts in the 4R speaker were 250 peak, 175 RMS, that's not far off.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 01:52:22 am
content: When i connect vout - of the dc dc converter to 36v gnd all leds go hard on. In your diagram is 0v the vout - of the dc dc converter and 12v = vout+ from the dc dc converter.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 01:56:48 am
content: Also, if pin 2 and 4 dont need to be connected then i could connect pin4 to 36v gnd pin 5 to the output of the amplifier and then the dc dc converter connects between pin 3 and pin 2. Would that work also.

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2016, 02:06:00 am
content: Quote from: MarcusS on September 12, 2016, 01:52:22 amYou must have something else connected somewhere such as the Vrefs.Careful you don't blow the LM3915, 3V to 25V max.QuoteYes, the 0V and 12V power across the Lm3915 is supplied by the DC-DC converter.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 02:08:14 am
content: The aim of the vu meter is for the last led to indicate clipping (22v rms = 80w rms) and then each led lower is 3db less. I.e. 40,20,10,5,2.5,1.25,0.6,0.3,0.1.

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2016, 02:19:03 am
content: Quote from: MarcusS on September 12, 2016, 02:08:14 am22V RMS across a 4R speaker is 121W RMS. Have I got the wrong speaker R value?OK I'll use 6R.22V RMS is 30.8V peak, but Vref High in the sim is 34V(after the X11 potential divider) so I'll try to tweak it down a bit.LTspice is quite easy to use when someone is providing the schematic,if you decide to have a try of it.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 02:23:10 am
content: 6 ohm speaker system i have designed to pair with this amp.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 02:49:12 am
content: I will give it a go. Im trying to get my head around why your circuit has a voltage divider for div low instead of that being connected to 36v gnd. The other voltage divider makes sense although my calcs give me 3.3k and 27k.

author: Audioguru
author_rank: Super Contributor
timestamp: September 12, 2016, 02:56:16 am
content: TDK company does not make a CCG30-4812s voltage converter. A voltage converter will have the +36V and ground input and have a +12V and ground output. The grounds are connected together and there is no -12V that is not needed.Of course the outputs of the BTL amplifier are at about half the positive supply voltage but the input of the LM3915 must be averaged at ground so simply use a coupling capacitor from one output of the amplifier and feed it to the input attenuator of the LM3915.Your LM3915 has no pin numbers.Why does your circuit have a "Vref Lo"?? Pin 4 of the Lm3915 should be connected to the circuit ground.Why is your Vref so low? Make it about +10V (and adjust the input attenuator to match) then input offset voltages will not mess up the outputs.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 03:03:46 am
content: Tdk does make a ccg30-48-12s because they are soldered and working on my board in front of me.

author: Audioguru
author_rank: Super Contributor
timestamp: September 12, 2016, 03:09:00 am
content: Quote from: MarcusS on September 11, 2016, 11:52:22 amYou do not understand that the LM3915 has a logarithmic response. The datasheet shows that if the 10th LED lights with an input of +10V then the 1st LED lights with an input of +0.447V. Each output is active with 0.707 times (-3dB) less level than the next output. Then with an input of 20V RMS to light the 10th LED, the 1st LED lights with an input of 0.894V RMS.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 03:14:09 am
content: You must also understand that most dc dc converters are isolated and thus the output gnd is not connected to input ground. If it was it would be non isolated and this thread wouldnt have started. You can search on farnell or digikey to see that there arent many non isolated converters and for high end audio you dont want other voltage sources being able to put noise onto the main amplifier gnd anyway.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 03:17:26 am
content: I didnt mean i want 5v rms for the first led to turn on. That is how it is actually behaving right now with my ground reference issue. I want 80w rms into 6 ohms for led 10 and each led below that to be half power -3dB. This is how an lm3915 is intended to be used in my understanding.

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2016, 03:23:57 am
content: Quote from: MarcusS on September 12, 2016, 02:49:12 amYep, you could produce Vref Low from the 36V supply. But producing both Vref Lowandthe Sig_In from the same place(through their same proportion potential dividers) should mean that if the mid 18V is off a bit, it effects both of them - so theVref Lowvoltage level should still run right through the very middle of thesignal'swaveform even at low volume levels. That was the idea anyway.If you're looking at the last circuit I posted, R7 28K represents theinternalresistor string inside the LM3915, I added it because of the small effects it has on the 2 potential dividers.Changed speaker to 6RHad to change the potential dividers R2 from 3K9 to 4K7, and R6 from 390R to 470R.Plot looks good to me!Blueis the 500Hz (peak 31V) reachingVref High,Yellowshows the peak Watts in the 6R = 112W = 79 W RMS.Shirley that's close enough!I've not worked out the 3dB representations, I'm leaving that to you.

author: Audioguru
author_rank: Super Contributor
timestamp: September 12, 2016, 03:24:05 am
content: Quote from: MarcusS on September 12, 2016, 03:03:46 amTwice before you said it was a TDK CCG30-4812s which is not the TDK CCG30-48-12s that it is. The TDKCCG30-48-12s has a lot of output ripple and its output ground is isolated from its input ground but you do not need the grounds to be isolated.

author: Audioguru
author_rank: Super Contributor
timestamp: September 12, 2016, 03:34:35 am
content: Why don't you guys understand that the audio is AC, not DC so simply use a coupling capacitor to block the 18VDC but pass the audio.

author: StillTrying
author_rank: Super Contributor
timestamp: September 12, 2016, 03:58:51 am
content: Quote from: Audioguru on September 12, 2016, 02:56:16 amWhy must? But, I can see that connecting VrefLow to gnd and AC coupling Sig_In could a good idea.QuoteAnd.QuoteCould be good ideas, but after astatus bartaskbarcrash followed by LT crashing I no longer have any .asc version!!

author: void_error
author_rank: Frequent Contributor
timestamp: September 12, 2016, 04:38:11 am
content: Quote from: MarcusS on September 12, 2016, 03:17:26 amPlease be aware there is no such thing as W RMS, only V RMS and I RMS.The LM3915 lights the LEDs up depending on voltage, not power, as the only input value is voltage.Quote from: Audioguru on September 12, 2016, 03:34:35 amI did previously suggest thathere.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 01:23:35 pm
content: Turns out what was suggested doesn't work seems like the internal impedance between pin 4 and pin 2 is enough to cause all leds to turn on solid.

author: void_error
author_rank: Frequent Contributor
timestamp: September 12, 2016, 01:59:20 pm
content: Pins 4 & 2 are meant to be tied together unless you're using a single supply with your incoming signal DC coupled and referenced to 1/2 of your supply voltage but that seems not to be the case here.Leaving pin 4 floating is not a good idea because the internal resistive divider won't work.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 02:08:38 pm
content: Any more suggestions that might work. I was kinda hoping that it compared pin 5 to pin 4 internally with the comparators to determine the scale.

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 12, 2016, 03:59:12 pm
content: My suggestion would be that you read the data sheet and ask questions about things you don't understand.You say "I was kinda hoping" - what's the point in Kinda hopingyou need logic, reason and a rigorous approach to tackle these things rather than guesswork.The LM3915 needs pin 4 and 5 to be the signal input to generate the LED output. See data sheet page 2 where there is a box that is labeled "signal source". You have already put your scope on pin 4 (-12 on your circuit) and find that it is not as you previously described as "doesn't budge" but has audio on it - no wonder the display doesn't work. All you have to do is hang two scope probes - one on pin 4 and one on pin 5 (the difference between the two is the signal source) - forget about what the LM3915 displays; the difference between those two points is what you are interested in. Only when those two points have what you expect should you move on to look at what the LM3915 is displaying.QuoteSee data sheet page 3 which tells you what the range of RLO can be (otherwise known as pin 4). See data sheet page 7 for an internal schematic of the LM3915 that shows pin 4 is the bottom of a resistor ladder.To put it another way, going from injecting a signal to the input to looking at LEDs as the output is too coarse for testing. You need to know what to expect at every stage and check that is the case until finally looking at the LEDs.Clearly, pin 4 (-12 on your circuit) should not budge at all as it is one of the inputs tobothLM3915s. Stick the 1,000uF on I suggested earlier and then measure the two inputs as described.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 04:21:16 pm
content: I have studied the datasheet in great depth the block diagram says you can compare pin 4 to pin 5. That is not correct it is only correct if pin 2 is connected to pin 4. In my instance where i have an audio ground that is different to 12v ground what can i do. I did try 1000uf and it does not work. What i tried this evening was to have pin 2 not connected to pin 4. I had amplifier in on 5 and amplifier gnd on 4 and then vu meter powered from dc dc converter on pins 2 and 3. Cant see on the block diagram where you cant do that and very confused why it refers to the comparayors as floating on page 1.

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 12, 2016, 04:25:33 pm
content: Where does it say in the data sheet "you can compare pin 4 to pin 5" - which page and which paragraph?

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 04:42:18 pm
content: It doesnt say in text the block diagram shows pin 5 goes to a buffer which connects to 1 side of a comparator and pin 4 connects to the other side of the comparator

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 04:43:58 pm
content: All i want is someone to correctly tell me how to add an lm3915 to each output of the amplifier schematics i posted as a power indicator

author: Chris Mr
author_rank: Regular Contributor
timestamp: September 12, 2016, 04:56:16 pm
content: Let's take the block diagram apart:There is a resistor network which is connected to 10 comparators - whose outputs drive LEDs. Both ends of that resistor network are available on pins 4 & 6BUT(time for first caveat) pin 6 must be either the same or higher voltage than pin 4 (data sheet page 4) and neither of them can go higher than +12v compared to pin 2.The impedance of the resistor network is (6.63 + 4.69 + 3.31 + 2.34 + 1.66 + 1.17 + 0.83 + 0.59 + 0.41 + 1) K. Notice that the lowest step is 1K and the next up is 0.41K.The other connection to the comparators, the connection that's common to all comparators, is the signal input - pin 5 - which is buffered.So, what we know is that, regardless of which bits you use internally, the comparators have the resistor network on one side and the signal input on the other. The resistor network needs a voltage across it so that each comparator has a different input which they compare to pin 5 and output accordingly.You need a voltage across the resistor network and guess what, those nice TI people provided one - but you don't have to use it. The voltage reference police won't come around and put you in jail if you don't. The reference is a 1.28v (nominal) between pins 7 & 8 and the current you take out of it is directly related to the LED brightness - here you have to comply as that is in LM3915 law. Notice that they tell you in the data sheet that the reference terminal, pin 8, takes 70uA in case you don't connect it to ground.It also says that pin 2 is the ground pin.They are right, the resistor network is floating as long as it is within the caveat that the low end must be the same or lower than the high end and both must be between pin 2 and should never be more than 12v.So when you put pin 2 at amplifier ground you went outside those requirements. Expect to have damaged the device - but there is always luck

author: macboy
author_rank: Super Contributor
timestamp: September 12, 2016, 05:13:36 pm
content: If you want the output of the LM3915 to look good, you need to precede it with a rectification and peak hold circuit with a slow(ish) decay. The decay should be set to decay from full scale (10th LED lit) to minimum (no LED lit) in about 0.2 to 0.5 second, but you can adjust this to your preference. The LM3915 is very fast, so if you don't use a peak hold circuit, the LEDs will show the instantaneous level of the positive half cycle of the waveform. This gives very uneven brightness and usually makes it very difficult to tell when the highest LED ever lights because it only lights very briefly at the highest peaks of the waveform. Read the section "Tips on recitifier circuits". There is a striking and obvious visual difference between a directly connected LM3915 and one using a rectifier/peak hold.If you are adding an op-amp based peak detector/rectifier circuit anyway, then it is easy to configure it to have a differential input for direct connection to any BTL amplifier such as yours.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 12, 2016, 09:15:36 pm
content: I didnt put pin 2 at amplifier gnd i put pin 2 at 12v gnd. It was pin 4 that was connected to amplifier gnd and pin 5 to the positive spk out.

author: StillTrying
author_rank: Super Contributor
timestamp: September 13, 2016, 02:00:33 am
content: Right.After the crash I rebuilt my rubbish simulation .asc from the .gif posted here, although my LM3915 was just an empty box, I hoped the voltages supplied to it would give at least some sort of result. (I don't know where the OP is up to with his real-life version).So I found a LM3915 model, connected it and ran it, and what happened, - well with increasing input the LEDs came on in order! - although the Refs would need tweaking. It's impossible to plot these 10 LEDs switching at 750Hz all on the one plot - so I did just that.I can see that the LED currents are switching on in order (by the colours) with increasing input signal amplitude even if no one else can!I won't be simulationingany further with this rubbish version but if anyone wants the .asc to play with I'll add it here.

author: StillTrying
author_rank: Super Contributor
timestamp: September 13, 2016, 02:58:24 am
content: Quote from: MarcusS on September 12, 2016, 09:15:36 pmYou confuse me with all these gnds, there's only one.It might be time to test your LM3915, connecting it's VrefLO to it's gnd, it's VrefHI to it's 12V supply, and a Pot across it's 12V with the pots wiper going to it's Sig_In would do.Meanwhile, I have another (better) go at getting the 18V speaker output safely into the LM3915's input.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 13, 2016, 11:29:41 am
content: Think I have found the solution. But I need to do some serious testing on both channels to make sure it is all well behaved. It involves decoupling the audio into the vu meter via a polyester film capacitor calculated high enough in value to not affect the frequency response of the vu meter in the audio band and then by doing so the dc offset is removed allowing me to take the floating output gnd of the dc dc converter and link that to the main amp ground thus having a stable reference.Tying the dc dc converter output gnd to main amp ground did not work without the right capacitance as either the audio signal was high passed into the vu meter and thus not responding to audio frequencies and if no capacitor is used at all the dc offset (that is present when the output of the dc dc converter and amplifier are connected) makes all the leds turn on solid.The capacitor value is critical and obviously related to the impedance of the voltage divider between pin 5 and 4 to determine the correct value.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 13, 2016, 11:31:42 am
content: Thinking about it now when I first posted the question the answer was in the question when I asked if anyone had done anything with decoupling capacitors. Turns out that its on the input of the vu meter that needs it, not decoupling the dc dc converter gnd to the amplifier ground.

author: void_error
author_rank: Frequent Contributor
timestamp: September 13, 2016, 12:51:39 pm
content: Finally.Quote from: MarcusS on September 13, 2016, 11:29:41 amThat's called AC coupling the LM3915 input. It's exactly what everyone's been telling you. And yes, reference it to the power amp's ground.Also, take into account whatmacboy suggestedif you want it to look nice.You might have an even better/cheaper option: buffer the signal from your amp's input and feed it into the LM3915 via an AC coupling cap, and optionally use the peak/hold circuit in the datasheet. The result will be identical as the amp has a fixed voltage gain over the audio range.

author: StillTrying
author_rank: Super Contributor
timestamp: September 13, 2016, 10:02:13 pm
content: Quote from: MarcusS on September 13, 2016, 11:29:41 amYou might have to put the ACcouplingcapacitor at the top of your voltage divider so that it's not effected by the horrible change in impedance of pin 5, - about 200M for +ve voltages and 20K for -ve.I'll look back in 3 pages time to see where you've got up to.

author: MarcusS
author_rank: Regular Contributor
timestamp: September 14, 2016, 11:24:05 am
content: It was the Ac decoupling cap at the top of the divider that fixed it that allowed me to tie the dc dc converter output ground to main amplifier ground. Something I didn't think I could do due to it misbehaving. My original tests had the decoupling occurring across the dc dc converter rather than the input.

author: StillTrying
author_rank: Super Contributor
timestamp: September 15, 2016, 03:40:41 pm
content: Now that the multiple gnds problem is solved.With the high signal voltage level on the speaker outputs (+/- 14V peaks), and the LM3915's very high input impedance, it should be possible to produce a simple peak/envelope detector out of a few passive components.Here's using both speaker outputs so that both +ve and -ve peaks are detected, I don't know if the component values are very good - I've only simulated it with the 500Hz speaker signal.I think this has quite a good response for such a simple arrangement myself.

author: macboy
author_rank: Super Contributor
timestamp: September 15, 2016, 04:00:50 pm
content: Quote from: StillTrying on September 15, 2016, 03:40:41 pmThislooksgood in simulation, but won't work well due to the voltage drop of the diode. You will require considerably higher output level from your amplifier to light the first LED than normally necessary. This is why a precision rectifier is normally used for these high dynamic range type of signals.

author: StillTrying
author_rank: Super Contributor
timestamp: September 15, 2016, 06:12:33 pm
content: Quote from: macboy on September 15, 2016, 04:00:50 pmI'm surprised that you think the voltage drop, resistance, and capacitance of a diode is not modeled.I'll add it's output voltage at 5 Watt and at 80 Watt to my previous postin a bit.

author: macboy
author_rank: Super Contributor
timestamp: September 15, 2016, 07:14:14 pm
content: Quote from: StillTrying on September 15, 2016, 06:12:33 pmI'm sure that at 5 W the output looks fine too. However, the LM3915 LEDs are separated by 3 dB steps. There are 9 steps or 27 dB betwen the lowest and highest LED. If you tune the meter to light the highest LED at 80 W, then the lowest one will light at -27 dB from 80 W, which is 0.16 W. What does the output look like at 0.16 Watt @ 6 ohm ?

author: StillTrying
author_rank: Super Contributor
timestamp: September 15, 2016, 08:30:32 pm
content: I was thinking that the OP was only interested down to 5W, and 80W at the top end, but I think you could be right - better resolution needed at the low end. I'm only used to the linear 3914's and these don't have this problem.Peak Watts in 6R vs. Volts to the lm3915180mW = 0.065V360mW = 0.170720mW = 0.3401.44W = 0.5802.88W = 0.9505.84W = 1.4911.5W = 2.2423.0 = 3.3246.0 = 4.8292.0W = 7.00

author: Audioguru
author_rank: Super Contributor
timestamp: September 16, 2016, 03:46:56 am
content: I am glad that you finally understand to use a coupling capacitor to block DC but pass the AC.In the datasheet of the LM3915 there are good peak detectors shown. They charge the timing capacitor very quickly from a low source resistance so that short duration peaks can be seen. They discharge the capacitor slowly (about 30ms) so that our slow vision can see them.Your circuit has the capacitor charging fairly slowly through high value resistors and has it discharging very quickly since the capacitor value is too low and the 270k resistor to ground on the input of the LM3915 should be 1M as shown in the datasheet.