how to isloate audio ground from the amplifier - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: March 22, 2018, 02:05:02 pm
content: hello friends,I'm working with a speaker project which also has a vu meter along with it. For the VU meter the audio input is fed from the amplifer output (from the left channel positive and ground). but while connecting the ground wire from amplifier output to the VU meter input, the speaker produces a distortion sound and I believe its because of lack of isolation between the audio signal from the amp and the VU meter circuit. So what should I do to isolate the ground?? I have already completed the project so im looking for smaller solutions to fit inside the enclosure. does a diode at the audio ground solve the problem or should I use an audio transformer??but I don't know how to use it. Could anyone help me out??. Thank you in advance.

author: Audioguru
author_rank: Super Contributor
timestamp: March 22, 2018, 02:33:43 pm
content: The output of your amplifier might be bridged. A bridged amplifier output has two amplifiers. Each amplifier feeds one speaker wire but the signals are with opposite phase so that the output voltage across the speaker is doubled which produces almost 4 times the output power. Then your connection of the VU meter ground is shorting one output of the amplifiers to ground.Connect the input ground wire of the VU meter to the amplifier's ground instead then it will show half the speaker's voltage.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 22, 2018, 04:13:47 pm
content: Thank you very much for your reply. I'm using a 25W+25W amplifier module for my project which is based on the class D chip TDA7492P. I have attached a pic of the board below. Could you explain a bit about where to find the amplifier ground??

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2018, 04:24:35 pm
content: The amplifier is definitely bridged.The schematic is probably similar to the one found on the data sheet and the ground connecting can be found, using the continuity test function on a multi-meter.http://www.st.com/content/ccc/resource/technical/document/datasheet/35/01/97/6b/20/ce/40/6f/CD00211320.pdf/files/CD00211320.pdf/jcr:content/translations/en.CD00211320.pdf

author: Audioguru
author_rank: Super Contributor
timestamp: March 22, 2018, 07:03:03 pm
content: Are you the guy who is feeding only one channel of the Bluetooth stereo to only one speaker?The TDA7492 in your photo is missing the heatsink shown on the product from AliExpress.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 22, 2018, 08:56:18 pm
content: Quote from: Hero999 on March 22, 2018, 04:24:35 pmThank you for your valuable reply. Actually im not that good in electronics..I learn it by doing these kind of experiments and thus my knowledge is limited. And while seeing an SMD PCB thing are too confusing for me. If you could give me some details about where to locate the ground, i mean the position or area on the board it would be a great help.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 22, 2018, 09:05:42 pm
content: Quote from: Audioguru on March 22, 2018, 07:03:03 pmNo sir I'm not that guy. I'm using both the speakers for both the channels. since the vu meter circuit which i have build have only one channel as input, I'm using an additional taping from the left channel for it.The board that you have shown is for 50w+50w i guess. Mine is a 25w+25w one and brought it from banggood. I think this 25w one dont comes with any heat sink in any online sites, anyhow i'm planing to fit a small heat sink for it in my project to be on the safe side

author: Buriedcode
author_rank: Super Contributor
timestamp: March 22, 2018, 10:16:31 pm
content: The modules ground is the bottom side of that DC barrel socket (from your picture). I cannot see a specific pad for ground on that board so it looks like you'll have to solder a wire to the DC socket.You mentioned an audio transformer which is one option, but I feel I am not qualified to recommend a particular part number.Another option, possibly overkill would be a differential amplifier using an opamp:https://www.electronics-tutorials.ws/opamp/opamp_5.htmlYou would still need to connect the amplifier boards ground (again that DC socket) to the ground of the VU meter. But it would mean the VU meter shows the full voltage swing.There is actually two differential amps on that PCB because the output from the bluetooth module is differential, which is then converted to single ended by the on board dual opamp (NE5532, one opamp per channel). I'm unsure why they have done this since the input to the TDA7492 is differential, so that part of the circuit only adds noise.

author: Audioguru
author_rank: Super Contributor
timestamp: March 22, 2018, 11:33:01 pm
content: Each speaker wire of the bridged amplifier has a DC voltage that is half the supply voltage. If your VU meter measures DC then a coupling capacitor must be used in series to block the DC but pass the AC to the VU meter.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 05:50:38 am
content: Quote from: Buriedcode on March 22, 2018, 10:16:31 pmThank you for your valuable suggestions. By the bottom side of the DC barrel socket did you mean the ground power supply to the board?? I'm also attaching a pic of the bottom side of the board with this post so that you can have a look.You mentioned about another option of using opams but I have a doubt. does the small opams could work at the output voltage and current levels of this amp??

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 06:00:08 am
content: Quote from: Audioguru on March 22, 2018, 11:33:01 pmMy Vu meter is based on LM3915N IC. The circuit which I'm using has already a diode at its positive audio input side to consider only the positive cycle of the audio. The circuit works fine since I have already used for an another project (with a another amplifier based on LA4440 IC ), but I don't know how to make it compatible to this class D amplifier IC board i.e the noise by connection the ground wire

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 01:39:41 pm
content: Quote from: Buriedcode on March 22, 2018, 10:16:31 pmI have removed the DC socket for the project and I have attached the pic of the board. You said that the ground will be below the DC socket and I have marked it on the board. Is this the amplifier ground??

author: Audioguru
author_rank: Super Contributor
timestamp: March 23, 2018, 01:48:54 pm
content: The LM3915 does not need a diode in series with or to ground at its positive input since it measures only positive voltages. It does nothing with negative input voltages. Adding a series diode causes its measurements to be 0.4V to 0.8V too low depending on the diode's current and its temperature. The datasheet shows peak detector circuits and active rectifier circuits to feed the input of the LM3915 that do not cause its measurements to be too low. The peak detector holds the peak voltage long enough for our slow vision to see it.The LA4440 amplifier is an ordinary stereo amplifierwith one wire of each speaker grounded (its outputs are not bridged)so your LM3915 is measuring only the live output signal. Its datasheet shows that both of its amplifiers can be connected to make a single bridged amplifier, that is like both amplifiers in your TDA7492 amplifier.Your TDA7492 amplifier has two bridged amplifiers. You connect the input of your VU meter circuit through a series coupling capacitor to only one of its speaker wires and the ground wire of your VU meter to the ground of the amplifier.Shorting one of its speaker wires to ground will destroy the TDA7294 amplifier.Then your VU meter measure only half of the speaker voltage.Use a multimeter set for OHMS to measure the ground on the pcb with the ground on the DC power jack.

author: Audioguru
author_rank: Super Contributor
timestamp: March 23, 2018, 01:58:00 pm
content: The instructions that came with the TDA7492 amplifier should show the positive and negative (ground) connections of its DC power jack. We haven't seen the instructions and we have not measured the jack so we don't know.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 03:57:36 pm
content: Quote from: Audioguru on March 23, 2018, 01:48:54 pmThank you very much sir for you help. I'm still having some doubts regarding it. I know its very annoying to ask these sort of simple things but for me its the only hope sir. I still dont get it on how to measure the ground on the pcb with the ground on the DC power jack. All the copper pads are coated with insulation right. I have an exposed power ground on the dc socket pin but dont see the ground of the amplifier. (by ground you mean the ground of the bridge amplifier right not the left and right ground right??)

author: Audioguru
author_rank: Super Contributor
timestamp: March 23, 2018, 04:54:00 pm
content: On a bridged amplifier no speaker wire is grounded, each speaker wire is powered from its own amplifier, then the speaker voltage is doubled and the speaker power is almost 4 times higher. If you ground a speaker wire then you cause distortion and probably will blow up the amplifier.The amplifier has a ground which is the 0V or negative of its DC power supply. Therefore one or more terminals on its DC power jack is a ground connection but you must measure it to see which terminal.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 23, 2018, 05:43:05 pm
content: You should read up on the terms "bridged" and "differential". I used the latter term, but in the context of speaker amps its the same thing as bridged (albeit probably less specific).What has been said a few times here is - each connection to the speaker is driven by an amplifier, neither of these connections are at ground. You are still thinking in terms of "single ended" amplifiers, where one side of the speaker is connected to ground, whilst the other side is driven by an amplifier. In that case, your VU meter would have no problems, but your amplifier board - and many modern amplifiers - drive speakers differentially.So, if you use a scope and measure each speaker terminal with reference to ground (the DC jack) you will see a signal. One will be the opposite of the other - as one side of the speaker is driven high (with respect to ground) the other side will be driven low. So if an amplifier is powered by 12V, it can create ~12V across the speaker in one direction,but also in the other direction. Like a H-Bridge! Check out the datasheet on page 6:http://www.st.com/resource/en/datasheet/cd00205863.pdfSo what this means is... you have two signals for each speaker, neither of which are ground. You can either just look at one of these signals, which means your VU meter will see a signal sitting at half the power supply voltage, and only give half the indicated voltage (you can use a series capacitor to block DC so it only shows the signal and not the DC offset) ... OR... combine the two differential signals into one, which is why I mentioned a differential amplifier. This creates a single signal, referenced to ground, that will work with your VU meter. It doesn't have to be accurate, so even an LM358 would do.I'll post an edited picture of your boar din a minute to confirm the ground connection.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 23, 2018, 06:00:37 pm
content: SO yes, the area you indicated is "ground" (0V reference). I have also attempted (badly) to show the signals on the speaker outputs. Neither of them goes negative, but the signals are centered on half the supply voltage, and are opposites - the speakers on care about the difference between these signals. Connecting your VU meter to one of these terminals, and our actual ground (as indicated on the diagram) would mean your VU meter would show half voltage at all times. Using a DC blocking capacitor would mean it only see's AC, but as each output is only half the signal it would show half the voltage.Try it yourself - connect your VU meters ground to the amplifier boards ground, and the "signal" to one of the speaker outputs - any of the 4 outputs will do. And see what it shows.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 06:52:48 pm
content: Quote from: Audioguru on March 23, 2018, 04:54:00 pmThank you again for your reply. Ok sir now things are getting cleared about the ground connections.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 07:15:29 pm
content: Quote from: Buriedcode on March 23, 2018, 05:43:05 pmThank you again for your kind reply. could you help me with the type and value of the capacitor that can be used for the dc blocking. I'm planing to first use this amplifier ground and if its not that good then i'll move on to the differential amplifier option.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 07:17:59 pm
content: Quote from: Buriedcode on March 23, 2018, 06:00:37 pmI just used a multi meter ohm function and found that the dc jack ground and the left channel ground is having a resistance of 350 ohm.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 23, 2018, 07:20:10 pm
content: Ill connect the ground to the vu meter let you guys know about it.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 23, 2018, 07:38:22 pm
content: Quote from: Adhith on March 23, 2018, 07:17:59 pmThe left channel has no ground. Nor does the right channel. No speaker is connected to ground. Both terminals of the speaker are driven by push-pull outputs. In this way the reason they label them "+" and "-" is for phasing. If you connect speakers backwards you will still hear music it will just be out of phase with the other channel, it has nothing to do with ground reference.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2018, 04:44:48 pm
content: Quote from: Adhith on March 23, 2018, 07:20:10 pmYes, that will work.As this is a class D amplifier, you might find the existing filter is inadequate at removing all the high frequency content, so the VU meter might indicate there's a signal, even though there isn't one. Another RC low pass filter might be required, before the VU meter.

author: Audioguru
author_rank: Super Contributor
timestamp: March 24, 2018, 05:08:12 pm
content: Quote from: Hero999 on March 24, 2018, 04:44:48 pmGood point. We cannot hear ultrasonic switching of the class-D but the VU meter might show it.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 25, 2018, 04:19:59 pm
content: Quote from: Buriedcode on March 23, 2018, 07:38:22 pmsorry sir my mistake. i meant the wire other than the +Ve output of the amplifier. SO what should I call that terminal. the lower side terminal will do??

author: Adhith
author_rank: Frequent Contributor
timestamp: March 25, 2018, 04:27:03 pm
content: Quote from: Hero999 on March 24, 2018, 04:44:48 pmthank you again. could you suggest me a RC low pass filter for it, the values of R, C??. If i'm using it then there is no need of an additional DC blocking capacitor right??I have attached the schematics of my VU meter circuit and the circuit do have a 2.2mf electrolytic capacitor at the input and I was under the assumption that its a filter for the input. Is this the case or I'm wrong??

author: Adhith
author_rank: Frequent Contributor
timestamp: March 25, 2018, 04:55:08 pm
content: I just fed the ground near the DC jack and the +ve audio output of the left channel to the VU meter input (without any RC filter), but the led was not even lighting up. There is no problem with the VU meter circuit since I have tested it with the previously mentioned LA4440 amplifier board and it works fine. So what would be the reason that the board is not working with my class D amp board?? While measuring the voltage between this +ve left channel output and the ground near the DC jack it is found that it is around a constant of 6.4V. its not changing a little according to sound or increasing or decreasing with the volume or even when there is no audio playing. The voltage appears at these terminals as soon as the amp is powered, afterwards its moreover a constant. So why is it so??. does this shows the ground of the amp is some other terminal??

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2018, 07:45:52 pm
content: Quote from: Adhith on March 25, 2018, 04:55:08 pmThat's to be expected. The output of the amplifier will be biased at half the supply voltage, hence the 6.4V. I don't know why the VU meter isn't responding at all. It could be the RF parts of the signal.The VU meter needs to be AC coupled to the amplifier. Try adding this circuit between the amplifier and VU meter. It both blocks DC and greatly attenuates the ultrasonic class D switching frequency. If it doesn't work, then you might need to build another op-amp based amplifier, connected to the input of the class D amplifier and drive the VU meter from that.

author: Audioguru
author_rank: Super Contributor
timestamp: March 25, 2018, 11:00:38 pm
content: Your VU meter circuit is completely wrong:1) The LEDs are backwards with the anodes at ground and the cathodes driven to +12V.2) The input diode passes DC to the input of the LM3915 and ruins the accuracy of the lower LED turn on voltages. The diode and the 2.2uF capacitor it charges form a very poor peak detector thatshortsthe amplifier output badly each time the input goes 0.7V or more positive that could blow up the amplifier and/or the diode.The datasheet shows a simple opamp peak detector circuit that I have used and it works perfectly. It is accurate and has an input capacitor to block the 6.4VDC from the amplifier output. The opamp needs a dual polarity supply or an opamp that has inputs that work all the way down to ground like one of the two opamps in an LM358.You cannot add a series capacitor to your horrible diode circuit.Since the amplifier output is +6.4V then it must have a +12.8V supply. Then if the horrible unregulated pot in your circuit is set so that the 10th LED lights when the input peak is 10V, the series diode's voltage drop prevents the 1st and 2nd LEDs from lighting when they should. A peak detector with an opamp will allow all LEDs to light when they should.In your circuit, the +6.4VDC will cause the 8th LED to light in the DOT mode or cause LEDs 1 to 8 to light in the BAR mode with no signal.The horrible pot in your circuit is its unregulated reference voltage but the LM3915 alredy has a regulated reference voltage at pin 7 that is set with two resistors on pin8.You show 2N3906 transistors with a maximum allowed current of only 200mA driving LED strips that might draw 500mA each?You have each output of the LM3915 driving 1k resistors in series with the base of the transistors. Then when all LM3915 output are driving these resistor the LM3915 will be too hot and will fail. The datasheet recommends reducing the output currents so the heating is not too high. How much current does each LED strip need?Here is my peak detector circuit:

author: Adhith
author_rank: Frequent Contributor
timestamp: March 26, 2018, 06:03:22 am
content: Quote from: Hero999 on March 25, 2018, 07:45:52 pmThank you very much again. Ok let me add the circuit before the amp and check if its working or not. I let you know as soon as I have done it.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 26, 2018, 06:42:07 am
content: Quote from: Audioguru on March 25, 2018, 11:00:38 pmThank you very much Sir for your detailed suggestions. I now understand that I should have taken much more seriously in understanding the faults in the circuit. the circuit was found from the internet and I had build another project based on it and it works, so i didn't go in depth in understanding the concept. I now totally know that I shouldn't have gone blind over the circuit.1) yes the leds are connected backwards but I have connected connected correctly in my project, sorry about that mistake2) I'll remove the diode and capacitor from the circuit3) In my setup i"m using two voltage input for the VU meter board. the voltage fed to the LEDs during the transistor switching is given by the +ve output of a WiFi led controller. The voltages to the pin 3 and and the input to the 47K potentiometer is fed by a 12V buck/boost converter.I'm already using these boards in the same project for other functions,so just used these to use on VU meter board also for a steady supply.4)I'm using 12 V RGB led strips for the 10 outputs of the VU meter . the strips are have three individual leds per section. I'm using a continuous 4 sections for each of the outputs of the VU meter board, so that makes 4x3=12 individual leds in each row of the 10 output sections of the VU meter output line.5) So is it over 200mA current limit?? but I wont be using it in too brightness.6) The LM358 is a single supply op amp right?? is it capable of working in the output ranges of the of the amplifier?? in the datasheet its written that it could withstand upto 32V7) Yes i remember some heating issues with the IC in my previous circuit based on the same IC and also after few months 3 strips were not working, but back then I thought it was normal for these ICs to heats a little on continuous use and was under the assumption that it might be the loose wiring to the leds that makes it not to light up. Now I totally understand the case. So what should be done to reduce the heating??

author: Adhith
author_rank: Frequent Contributor
timestamp: March 26, 2018, 07:22:17 am
content: Are the grounds the same in the circuit??

author: Adhith
author_rank: Frequent Contributor
timestamp: March 26, 2018, 08:21:33 am
content: just powered a similar RGB led strips and used multi meter to find the current draw. For the white light at maximum brightness it shows a total draw of 860 mA for the strip that contain 38 sections( each containing 3 leds). So 860/38 give around 23mA. I'm using a continuous 4 section of these strips in each output lines of the VU meter. So 23x4=92mA will be the total current draw through each transistors in the VU meter right??. So there is no problem of overloading the transistors right??

author: Audioguru
author_rank: Super Contributor
timestamp: March 26, 2018, 09:48:13 pm
content: You are powering the LEDs from +12V so the base current of the transistors is (12V - 0.7V)/1k= 11.3mA. If all 10 transistors are turned on then the total output current from the LM3915 is 113mA and since it is powered from +12V then its heating is 113mA x 12V= 1.36W plus it operating power= more than its absolute maximum allowed heating. It is designed to drive one ordinary 3V LED from each output. Then its LED supply voltage can be 5V then if the LEDs use 10mA each the total heating is (5V - 2V) x 10mA x 10= 0.3W which makes it just a little warm. You can double the LED current to 20mA each then it gets only a little warmer but far from its maximum.Of course in the peak detector circuit I showed all grounds are connected together and to the ground of the amplifier power supply. The opamp in my peak detector cancels the forward voltage drop of the rectifier transistor so that the lowest LEDs on the LM3915 light when they should light. The transistor allows the peak holding capacitor to charge very quickly without loading down the opamp.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 27, 2018, 05:07:52 am
content: Quote from: Audioguru on March 26, 2018, 09:48:13 pmSo inorder to double the LED current to 20mA each i should chage the base resistors right?? from the calculations (12V-0.7V)/20mA = 565 ohms. so I should change from the 1K resistor to 565 ohm resistor right??OK now things are clear with the peak detector circuit. but just one thing, It is fed from the amplifier output right?? not input right??

author: Audioguru
author_rank: Super Contributor
timestamp: March 27, 2018, 03:55:24 pm
content: Quote from: Adhith on March 27, 2018, 05:07:52 amNo. The LED strips have resistors inside that set the current to 23mA each but you have four sets of LEDs then their current is 92mA. The transistor base current must be 92mA/10= 9.2mA. But then the LM3915 must produce total output current of 92mA to drive the transistor bases and it overheats. In my VU meter I use a huge resistor in series with the LEDs (no transistors) to share the heat with the LM3915. you can use darlington transistors instead of transistors since their base current can be 25 times less.QuoteFeed its input from one speaker wire and ground.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 27, 2018, 04:50:37 pm
content: Quote from: Audioguru on March 27, 2018, 03:55:24 pmSo considering the heating issues as a major drawback I'll go with changing the transistors. One of my major concern on moving to darlingtion is the size, like I said before the vu meter was the final thing in my project thus I cant think of having larger area for the PCB. So could you suggest some commonly used darlington transistors with the same dimensions as that of the 2n3906 transistor ??

author: Audioguru
author_rank: Super Contributor
timestamp: March 27, 2018, 05:40:48 pm
content: Quote from: Adhith on March 27, 2018, 04:50:37 pmI looked in Google for "PNP darlington in TO-92 case" and found the MPSA64. Look at its datasheet to see how much base current for a reasonable saturation voltage loss.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 27, 2018, 07:03:51 pm
content: Quote from: Audioguru on March 27, 2018, 05:40:48 pmThank you very much sir for your sincere help. Could you please help me with these last things1) I looked over the datasheet and got a little confused with the beta value. in the graphs two beta values are shown 100 and 1000. we usually take the lowest value right?? to avoid complications, i'm not sure of it.2)so for considering the calculation for base resistor i just need some help too. The 10 led outputs of the LM3915N is having a 5v output right??( in the datasheet under output voltage its written 5v)3)while calculating the voltage drop we usually take drop across the resistor and a 0.7v at the pn junction right?? since this is a darlington I should take its double right?? coz of two junctions.below is my small calculation for the base resistor, it may be a total blunder sometimes but it would be a great help if you could read it and point out the mistakesSo assuming the beta value as 100, rounding off the maximum current for each strip to be around 150mA and base voltage as 5v and taking two junctions drop as 1.4V(0.7x2)So for an output current of 150 mA, base current = 150/beta value= 150/100 = 1.5mAbase current = (5v-0.7v-0.7v)/Rb = 1.5mASo Rb = (5v-0.7v-0.7v)/1.5mA = 2400 ohms(2.4k)so base resistor = 2.4k ohms

author: Audioguru
author_rank: Super Contributor
timestamp: March 27, 2018, 08:15:44 pm
content: Beta is current gain and is used when a transistor or darlingtonis an amplifierwith plenty of collector to emitter voltage,not when it is a saturated switch. The datasheet says that its maximum saturation voltage loss is 1.5V when its collector current is 100mA and its base current is 0.1mA but the saturation voltage loss is less when the base current is higher, use 1mA. If the saturation voltage loss is 1.5V then the LED string gets only 10.5V when the supply is 12V and it might not light. The graphs on a datasheet are for a "typical" device that you cannot buy, use minimum or maximum printed spec's.The outputs of the LM3915 are collectors of transistors that go near 0V when activated. They have no voltage, your darlingtons provide the voltage. Their output current is adjustable but the calculations for the output current affects the reference voltage. Your supply is 12V and the darlington base-emitter voltage is about 0.8V and the output of the LM3915 is about 0.1V so for 1mA darlington base current the resistance should be (12V - 0.8V - 0.1V)/1mA= 10.2k, use 10k. The darlingtons will also need a resistor from the base to emitter to turn them off, use 10k.The first page of the datasheet shows the reference pin7 set to about 10V with a 1.28k resistor and a 8.06k resistor. Use 1k and 6.2k instead to make a regulated reference of about 9.5V.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 27, 2018, 09:34:30 pm
content: Quote from: Audioguru on March 27, 2018, 08:15:44 pmThank you again sir for your kind help. So things are clear with the base resistor.1)In the circuit which I'm using the pin 7 is only grounded through a 1K resistor. it doesent have a second resistor and its connection to the pin8 instead, pin 8 is directly grounded. Seems like it doesn't create any voltage reference i guess ?? how bad is this to the performance of the VU meter??2) you said about that the darlington also need a resistor from base to emitter to turn them off right?? in the circuit which I have attached there is already two resistors from the transistor. one from the base and another from the emitter and its other ends are connected to the VU meter output pins. So the resistor from base to emitter for turn off is included in this right?? So replacing the two 1k with 10K will do right??3)I have pointed out the corrections in the circuit which is attached below. could you please have a look and see its correct? the IN4148 diode and the 2.2mf capacitor at the audio input will be removed in actual circuit

author: Audioguru
author_rank: Super Contributor
timestamp: March 27, 2018, 10:27:59 pm
content: You have the regulated reference voltage not used then the input voltage that turns on each LED depends on the regulation of the power supply feeding the pot.The current of each output can be as high as 10 x (1.25V/1k)= 12.5mA.The transistor base current is 11.2mA and the darlington base current is 1.07mA.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 28, 2018, 03:52:51 am
content: Quote from: Audioguru on March 27, 2018, 10:27:59 pm1)I'm using a 12v regulated supply from the buck/boost converted to feed the pots, anyhow I'll use the reference voltage of 9.5v with the resistors like you have suggested to be on the safe side.2)also the 100K and 220 ohm resistors connected to the pin 5 of the LM3915N is not needed right?? since we are feeding the pin 5 from the peak detectorI'll make the circuit today itself and let you know about the results. Thank you once again

author: Adhith
author_rank: Frequent Contributor
timestamp: March 28, 2018, 12:27:55 pm
content: unfortunately the MPSA64 transistor was not locally available. So I had given an order for it and it would take around 5 days to reach me. So I let you know after i get it and finish buildinding my circuit.

author: Audioguru
author_rank: Super Contributor
timestamp: March 28, 2018, 02:36:44 pm
content: I live in North America so I buy Canadian and American things here, not in India. Can't you look for Indian things and buy them there?

author: Adhith
author_rank: Frequent Contributor
timestamp: March 28, 2018, 04:59:53 pm
content: Quote from: Audioguru on March 28, 2018, 02:36:44 pmyes sir, I normally do buy things that are commonly avaliable here. The place where i'm coming from have very limited options in electronics sir. when I went to look for darlington today I understood that the shopkeepers haven't seen those type of transistors, even the buck boost modules was available only since last year. Things are tough here and we dont even have hobby shops, radioshack and things like that. just a few small electronic shops thats all .So the case is that if i look for an alternate component also it wont be available here sometimes. I have a good contact with a shop, where they take orders from shenzhen, china so they are usually helping me out to find the unavailable components with very cheap price due to my interest in electronics. I dont mind struggling, i just somehow need to learn electronics ,thats whysir, what about the 100k and 200k resistors at the pin 5 of the VU meter that I mentioned in my previous post?? i dont need those right?? just feed pin 5 dirlectly from the peak detector output right??

author: Audioguru
author_rank: Super Contributor
timestamp: March 28, 2018, 10:43:27 pm
content: Disconnect everything from the pin 5 input of the LM3915 and connect it to the output of my peak detector circuit.Many cheap electronic parts from China are fakes or manufacturer's rejects. There are many videos on You Tube showing garbage put inside cheap fake lithium batteries from China.I buy genuine North American or European parts that are inexpensive and in stock from a reputable local parts supplier.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 30, 2018, 11:23:44 am
content: Quote from: Audioguru on March 28, 2018, 10:43:27 pmOk sir I'll disconnect everything from pin 5 and connect to the output of the peak detector.yes I admit that there are lots of fakes here and is a serious problem. Most of the time the shops sells two variants of IC , a cheap variant and other costs a couple of bucks extra. I always buy the good one but I know that extra money is not a surety for the original product but it seems to have a good casing when compared with the lower cost variant and thus move on with it. But the batteries sold here especially 18650s have lot of fakes with different labelled capacities than the company actually produces.In my previous post I mentioned that I got a 6.3V between the left channel positive output and the ground near the DC jack just after switching on the amp, what is this voltage actually??. I know that we are taping from a bridged amplifier so half of supply voltage (12v/2 = 6v) is seen,but it could be seen even without a music playing so makes me little confusing that I'm i using the wrong terminals or not. Also this peak detector could block this signal right??

author: Audioguru
author_rank: Super Contributor
timestamp: March 30, 2018, 11:58:32 am
content: All audio amplifiers that use a single positive power supply voltage bias the input so that the output is at half the supply voltage so that it can swing equally up to near the supply voltage and down to near 0V.The peak detector has an input coupling capacitor that passes the audio but blocks the DC.Audio amplifiers that use a positive and negative supply bias the input so that the output is at 0V and can swing equally up to near the positive supply voltage and down to near the negative supply voltage.An ordinary amplifier with a single positive supply has DC on its output that causes trouble if it feeds a speaker so it has a coupling capacitor to feed audio to the speaker but block the DC.A bridged amplifier has two amplifiers, each one feeds one speaker wire. Since both amplifiers in your bridged amplifier have a single positive supply then both have their outputs at half the supply voltage so a coupling capacitor is not needed to feed the speaker.

author: Adhith
author_rank: Frequent Contributor
timestamp: March 30, 2018, 07:07:09 pm
content: OK understood Sir. I'll let you know once I'm finished with the circuit

author: Adhith
author_rank: Frequent Contributor
timestamp: May 18, 2018, 03:29:51 pm
content: First I apologize for the long delay for the reply. I was preparing for exams and stuffs and thus couldn't focus on my hobby project. I have now build the op amp based filter and changed to the transistor to the darlington. But the circuit is not working at all. All wires were double checked still not working. I taped a wire from pin 5 and connected to the the audio then its working and i think the problem is with the filter circuit. I'm attaching the new circuit that i used. What could be the reason?? the diodes at the end is for a different sort of function to turn on the LEDs and there is is no problem with that functionSince the LM358 has two op amps and I need only one for my purpose, i could totally leave the pins for the other opamp right?? In the second pic of the opamp block diagram I leaved the pins 1, 2 and 3 and used the other pins of the second opamp

author: Audioguru
author_rank: Super Contributor
timestamp: May 18, 2018, 11:23:45 pm
content: The circuit does not have a filter. Maybe you mean that the "peak detector circuit" is not working? Then a part is defective or its pins are connected backwards.Since you have two opamps but one is not used then it should be disabled. With its pins disconnected then it has a voltage gain of hundreds of thousands and probably oscillates at a high frequency causing interference to the opamp you are using. To disable an LM358 opamp make its voltage gain= 1 by connecting its (-) input to its output and ground the (+) input.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 19, 2018, 09:50:33 am
content: Yeah sorry ...I meant the peak detector .Its not working either. Used another LM358 by setting the unused op amp to gain 1 and still nothing. My soldering is proper and neat so I think the possibility of shorting the traces won't be a reason . My concern is towards a fault in the initial circuit layout that I have designed. I have attached a screenshot of my design with this. The layout is bit congested and may be annoying but it would be great if you could help with it. I'm also attaching the actual pic of my circuit also1) the green trace between the components indicates the orientation in which the components are arranged and does not indicate a bottom copper connection trace.2) the long green trace connecting the two +12v pad indicates that the pads are connected externally with a wire.3) The pins 1,2 and 3 of the LM358 are cutoff and thus it don't have a through hole on the PCB board. This is done to provide a way for the other traces to run through its bottom side.The output pins side works fine since I have tested it separately, so I think the things at the left side could be taken as the area of analysis.

author: Audioguru
author_rank: Super Contributor
timestamp: May 19, 2018, 04:07:15 pm
content: Your circuit board appears to be connected correctly but I do not see how the 47k pot is connected.Use a digital multimeter set to measure DC volts:1) With no input signal then pin 5 of the LM3915 should be close to 0V.2) With an audio input signal of a few volts then pin 5 of the LM3915 should show the positive peak voltages of the signal.EDIT:It looks like the 120k resistor is connected to pin 1 and pin 2 of the opamp that are shorted together, and both the 120k and 100k input resistors are connected to pin 3 of the opamp that is shorted to ground.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 19, 2018, 06:35:44 pm
content: The pad named"47K pot" is connected to the middle pin of the pot. The other pins of the pot is not shown in the diagram but is connected in the right way.No no, like I have said in my previous message, the legs of the pins 1, 2, 3 are cut from the IC. there is no through hole made for it in the PCB board. I have attached an edited pic to explain it. Since the resistor is close to the IC it creates a confusion and I admit it.I checked the circuit with the multi meter and found out a serious mistakeWhile taking a Voltage reference from pin 5 and ground I found that a voltage of around 12V (DC) is seen as soon as the the power supply to the VU meter is given without playing any audio. So its clear that the 12V power supply that i used for the pin 3 is some how reaching pin 5. But I still cant figure out that How this is happening. Could you help me out??

author: Audioguru
author_rank: Super Contributor
timestamp: May 19, 2018, 09:54:17 pm
content: With a +12V supply, the maximum output of an LM358 opamp is about +10.8V. With a base voltage as high as +10.8V the maximum output voltage of the 2N3904 transistor is about +10.2V but yours is +12V.Remove the 2N3904 transistor to see if the pin 5 voltage on the LM3915 drops to almost 0V. If its voltage does not drop then maybe the 330k resistor to ground at pin 5 is broken.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 20, 2018, 09:33:34 am
content: Quote from: Audioguru on May 19, 2018, 09:54:17 pmI didnt removed the transistor but instead removed from the LM358 IC (removing the IC would do right?? or should I remove transistor??) from the IC stand and then powered the circuit. It is found that still there is around 12v (actually 11.3- 11.5v), So its clear that the IC is not giving any output to the transistor to create the 12v.On further analysis its found that the pin 3 (where the 12v is applied) and the pin 5(audio input) has a resistance of around 330k and i think its the problem right. These pins should not be connected by any resistance right??to be precise the emitter and collector legs is having around 330K resistance

author: Audioguru
author_rank: Super Contributor
timestamp: May 20, 2018, 03:18:34 pm
content: With the LM358 removed then the base of the 2N3904 transistor will be 0V by R11 and its emitter will be 0V by R13. with its emitter at 0V then pin 5 input of the LM3915 will also be 0V. But if the base of the 2N3904 is shorted to +12.0V then its emitter will be about +11.4V.Find the short circuit which is a shorted 2N3904 or shorted wiring.Is the transistor an NPN 2N3904?? If it is a PNP 2N3906 then its opposite junctions will cause your problem.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 20, 2018, 06:49:45 pm
content: Quote from: Audioguru on May 20, 2018, 03:18:34 pmOk understood sir. Thank you very much for your help. Like I said before the resistance between the emitter and collector is showing around 335K ohm. actually the resistance should be zero right since there is no connection between the emitter and collector. There is no visible shortening of the legs or traces in the PCB though. I'll double check everything again and let you knowYes its a NPN 2N3904 transistor.

author: Audioguru
author_rank: Super Contributor
timestamp: May 20, 2018, 09:19:17 pm
content: When you measure the resistance of the transistor in the circuit then you are measuring the resistance of the circuit, not the transistor. Since the emitter of the transistor has a resistor of 330k to ground then the transistor would measure 3.3k or less for it to make it emitter near +12V.Disconnect one end of the 100 ohm resistor to see if the pin 5 of the LM3915 or the transistor is shorted to +12V.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 21, 2018, 05:11:51 pm
content: Quote from: Audioguru on May 20, 2018, 09:19:17 pmDone it but there is no shorting. and the traces are perfectSeems like I founded out the problemSince there is no way of shorting I concluded that it may be the transistor and removed it. To my surprise I founded out that the transistor is not manufactured in the actual standard. During the diode test it is found that the base and collector are switched between each other.I never had encountered this type of problem and totally feel stupid. So this is the problem right?? I'm very sorry to waste your time upon this ridiculous thing.

author: Audioguru
author_rank: Super Contributor
timestamp: May 21, 2018, 05:46:48 pm
content: That is what happens when you buy a cheap fake Chinese transistor from ebay, Amazon or Ali.Oriental little transistors like a 2SC945 have the collector pin in the middle. A 2N3904 transistor is American.

author: Adhith
author_rank: Frequent Contributor
timestamp: May 21, 2018, 06:10:12 pm
content: Quote from: Audioguru on May 21, 2018, 05:46:48 pmYup I understood Sir. The transistor was bought from a local supply. I have bought a lot of 2N3904 from the same shop but never had any problem with it. previously you have also warned me about the fake LI ion batteries from china in this same discussion itself. I have taken it seriously from that moment onwards . But I never thought that the fake ones will change the standard pin configuration itself, that's totally unfair. The thing is that I didn't knew it was fake. I'll be more careful next time

author: Adhith
author_rank: Frequent Contributor
timestamp: May 23, 2018, 06:43:41 pm
content: The transistor has been replaced and the circuit works great. It has good response and the LM3915 barely heats up. I sincerely thank each and every person who helped me with this discussion. Special Thanks to Audioguru for supporting me till the end.

author: Audioguru
author_rank: Super Contributor
timestamp: May 23, 2018, 10:40:06 pm
content: I am glad you got it working well.Mine has a microphone and it shows sounds in my home. I leave it in the BAR mode all the time and it is fairly bright.