Anyone want to help me debug this headphone amp?  - Page 1
Forum section: electronics / projects

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 05:07:42 am
content: Hey guys,Having some problems with this DIY headphone Amp project Iv been trying to build.I built 2, and they both did the exact same thing......worked fine for about 3/5 minutes, then BEEEEEEEEEEERRRERERWAWAWAWAWAHUMMMMMMEEERRRRRRRRRRRRRRRRRRWAWAWAWAWHUMMMMMAEEERRRRRRRRRRRRRRRRRR!!!!!!Now I'm just getting a not very nice Hum like DC on the output, but no source signal though.I checked the trouble troubleshooting pages for the project, and all my resistance measurements were bang on, but I went to check the voltage reading and Iv got some real issues! It says to check with the AC adaptor plugged in, but I only have the batteries and here are my strange readings with two fresh 9vs (Red is what I'm getting).Full project files attached, and any help would be really appreciated!

author: Jon86
author_rank: Frequent Contributor
timestamp: December 10, 2013, 05:14:33 am
content: Go and get an AC Adapter, surely you've got loads of them lying around?It'll narrow down the problem a lot more and make your life a lot easier.If I was you I'd just trace the entire thing through, following the schematic. Have a look at each point on a scope if you can, and just look very, very carefully at each section. If you've done the same thing twice you may just have made a simple little mistake, like putting a chip in the wrong way around or swapped the +-12v regulators.If you do find what the problem is, don't expect it to work. If you've ran it for a bit before it went wrong, you've probably killed something.Good luck

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 05:42:05 am
content: Quote from: Jon86 on December 10, 2013, 05:14:33 amUnfortunately I have a tone of AC to DC, but not even one AC to AC.Quote from: Jon86 on December 10, 2013, 05:14:33 amIt starts getting strange now.......I don't think anything has been fried, as I got the heat gun out to check for cold solder joints and this seems to reset the Amp to working for another 3/5 minutes, then same problem, and a dodgy Hum till I blast the board again with the heat gun (same for both boards)

author: Zad
author_rank: Super Contributor
timestamp: December 10, 2013, 06:15:53 am
content: You know the mantra. Check voltages. It may be oscillating, or your power regulation may be faulty due to a dry joint or short circuit. Take as much out of circuit as you can and build it back up step by step.Remove the amplifier ICs from their sockets, apply power and measure it again. See how it compares. See if you get +/- 9V on the inputs to the power MOSFETs. Insert IC2 - this IC regulates the power for the audio amplifiers, and makes sure that Gnd is half way between the + and -. Start from Pin2 of U2A, then the output (p1) and on to the gate of the MOSFET Q1. Similarly follow the path to Q2. Check that the voltages are approximately what you would expect.If this checks out, insert U1 (the pre-amp). Check voltages are ok. If you have a crystal earphone (or for that matter a small loudspeaker) try applying a small input test signal and check what you are getting out on pin 1 and pin 7. If this is fine, move on to insert U3 and then U4.

author: Jon86
author_rank: Frequent Contributor
timestamp: December 10, 2013, 07:11:13 am
content: If the heat gun does it, then I'd say dry solder joint / bad solder joint / broken part. An AC to DC adapter should work fine too, as long as you allow for the RMS to DC increase. So if it wants 10v AC, give it 14V DC.

author: miceuz
author_rank: Frequent Contributor
timestamp: December 10, 2013, 07:54:23 am
content: The power supply goes out of regulation. You are getting nothing on power rails. Does anything get hot on the board? Like 7X12 regulators? Are you sure you have inserted -12 regulator properly?Since you have all the opamps socketed, this is really easy way to test it - remove everything you can remove easily and test power rails. Another thing to try - isolate 7X12 section by removing D1, D5 and run from batteries.Doublecheck the diode orientation.

author: Tuomas
author_rank: Contributor
timestamp: December 10, 2013, 09:58:00 am
content: Quote from: Jon86 on December 10, 2013, 07:11:13 amFrom a quick glance the board seems to require AC, it has two half-wave rectifiers and caps, to generate positive and negative voltage. So a DC power supply would probably not work, it needs to be an AC supply.

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 06:18:28 pm
content: Quote from: Zad on December 10, 2013, 06:15:53 amTrouble is all the reference voltages I have are using the VAC power, not the batteries.......and I don't have a AC to AC wallwart.Quote from: Jon86 on December 10, 2013, 07:11:13 amI was inclined to think bad solder joint, but the fact iv built two with the exact same problem makes me think bad solder joint or defective part are quite slim, apposed to a orientation issues?Quote from: miceuz on December 10, 2013, 07:54:23 amThank you, I will try this, and report back!

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 08:10:54 pm
content: Here are the unsocketed voltage reading running of 2x9v batteriesThese are the reading when its "working" for a few minutesThese are the reading when not working

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 08:44:32 pm
content: Quote from: TheSolderShack on December 10, 2013, 08:25:52 pmGave then a good going over with alcohol and a nylon toothbrush last night, here is how they both look now.

author: Monkeh
author_rank: Super Contributor
timestamp: December 10, 2013, 09:43:42 pm
content: You mean these aren't the right type of irons?

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 10:16:57 pm
content: Quote from: TheSolderShack on December 10, 2013, 09:40:55 pmSeem to be OK with the resistor values, even checked against the suggested reading and am within range on both boards (Red is my readings)Quote from: TheSolderShack on December 10, 2013, 09:40:55 pmYep, checked type and orientation (1N5818 & 1N4002)Quote from: TheSolderShack on December 10, 2013, 09:40:55 pmIm used to soldering super sensitive germanium transistors, to if anything a little cautious with the soldering temps.Just don't know what to do now

author: fcb
author_rank: Super Contributor
timestamp: December 10, 2013, 10:55:14 pm
content: Looking at the pix with the voltages shown on it, i'm struck by the -3v on pin8 of the various opamps...Pull all the opamps out of the holders and debug the power supply section first. According to the schematic you should have +/- Vbatt-Vdiode (about 7.8v on rechargeables).Make sure your always measuring w.r.t. 0V in your system.Also, would help if you linked to the schematic!

author: redwire
author_rank: Newbie
timestamp: December 10, 2013, 11:00:55 pm
content: You need to back up and take measurements on the power supply Vregs U5, U6 and see if power is there.If not, the Vregs might be overloaded due to a short (they will get hot then) or the Vreg's have died.If power is ok to S1, then look at U2. It does a low-voltage disconnect of power and it might be malfunctioning.What did you substitute for Q1 and Q2? Does the LED (reference) light up?

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 11:35:07 pm
content: Got a little strangeness going on with the 12V regulators.....if I short out the heatsink/GND pin of either regulator to these 2 point I get audio out and not just hum. (found this out by holding the board with my thumb on the heatsinks and finger on one of the pads!)QuoteYou tell me what you want photos of, and I will send them!Will take voltage readings of U5/U6 BRB!!!

author: peps1
author_rank: Regular Contributor
timestamp: December 10, 2013, 11:57:05 pm
content: U5in=8.3Vgnd=8.3Vout=8.3VU6in= 8.3Vgnd= 8.3Vou=8.3VTHATS NOT RIGHT!I really cant get stable measurements off these thing now!U5in=0vgnd=0vVout=78mVU6in= 0Vgnd= 30mV (going up)ou=78mVAnd now back to 8v accross all pins!

author: peps1
author_rank: Regular Contributor
timestamp: December 11, 2013, 12:28:03 am
content: Schematic is in the OP with all the project files as a PDF.Desoldered U5 and U6 on both boards, cleaned and resoldered, but the same problem persists, and think the chances of a shot in the same place on two separate builds is slimchecked both boards on both sides with a Jewelers Loupe and can see any shorts.

author: Strada916
author_rank: Frequent Contributor
timestamp: December 11, 2013, 02:58:44 am
content: rule number 1 they shall check all voltages and follow traces and check at every point. whilst checking voltages also check the switch is not faulty. ie it changes state ok. also do not connect batteries if the AC is pluged in. check check check. It's the only way you will find the problem. look at the schematic and follow all voltage rails. do this with all ic out of circuit if possible.

author: peps1
author_rank: Regular Contributor
timestamp: December 11, 2013, 03:41:43 am
content: Quote from: Strada916 on December 11, 2013, 02:58:44 amThanks, I have checked voltages, and they are wrongTraces are all intact and have done a pad to pad continuity check on them all, switches are working fine, No AC supply to plug in with batteries.Just found a 0-9V 0-9V transformer in the parts bin, so will knock up a 18VDC supply and check the AC and DC power rails tomorrow.

author: fcb
author_rank: Super Contributor
timestamp: December 11, 2013, 11:31:25 am
content: Debug the + and - rails separately. Take out the opamps first.

author: Strada916
author_rank: Frequent Contributor
timestamp: December 14, 2013, 11:32:39 am
content: check battery terminals are connected.connect one lead of dmm and then the other end at a point down steam. Then wiggle terminal.

author: Strada916
author_rank: Frequent Contributor
timestamp: December 14, 2013, 12:15:59 pm
content: What are the voltages at D2 and D6 A and K sides.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 14, 2013, 12:36:55 pm
content: Quote from: peps1 on December 10, 2013, 11:57:05 pmU5 GND is supposed to be what it says, GrouND! So whatever you use to put the negative terminal of your meter on it is not connected to U5 GND, while it should. Work from there...

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 14, 2013, 02:57:57 pm
content: Firstly, please make sure your multimeter isn't nearly flat, this can screw with measurements.These measurements are using batteries.Remove D1 + D5, then check voltage between pin 8 and 4 on U2, look for 16v~if you don't have 16v~, and you have significantly less, check voltage across R2 and across R1 if it is greater than say... 0.8v, your diodes D6 and D5 are in backwards.next, if you have 16v there, check between pins 8 and 4 of U1/3, you should also have approximately 16v~if you don't have 16v~ there, check the voltage across C8 and C9, ideally it should be the same as the voltage across your batteries. since you don't see 16v across the rails, you will see one or both caps lower than the batteries voltage.on the rail with the caps lower voltage, check the voltage across the gate and source of the fet, you should be looking for +-4v at least

author: peps1
author_rank: Regular Contributor
timestamp: December 17, 2013, 01:15:42 am
content: Quote from: peter.mitchell on December 14, 2013, 02:57:57 pmPut in fresh batterys just incaseQuote from: peter.mitchell on December 14, 2013, 02:57:57 pm= 17.57 with diodes outQuote from: peter.mitchell on December 14, 2013, 02:57:57 pmQuote from: peter.mitchell on December 14, 2013, 02:57:57 pm= 6.22vQuote from: peter.mitchell on December 14, 2013, 02:57:57 pmC8= -2.3v C9= 9.12Quote from: peter.mitchell on December 14, 2013, 02:57:57 pm10N20C D-G = 15.24v11P06 D-G = 0v

author: mrkev
author_rank: Regular Contributor
timestamp: December 17, 2013, 02:53:22 am
content: I really don't get that arangement of U2. Am I the only one?Is it there just to swich off the battery when it's too low? I see two comparators and I'm not even sure what will the output state be if the voltage at R5 R9 divider gets close to voltage at that diode (since switching on and off can IMHO make it very slowely oscillate).

author: peps1
author_rank: Regular Contributor
timestamp: December 17, 2013, 04:52:40 am
content: Could battery choise be a issue here, been using Greencell mercury free Zinc Chloride batteries.........But, have had one of the amps hooked up to my bench power supply for 30 mins? with no issue

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 17, 2013, 01:27:30 pm
content: Quote from: peps1 on December 17, 2013, 04:52:40 amNo they're fine.Quote from: mrkev on December 17, 2013, 02:53:22 amYeah, pretty much. Low power cut off, low voltage the led turns off too. It may oscillate a little bit, but not too much (maybe a few cycles).All i can think of is check R5 and R9's resistance. Everything else looks fine as far as i can see.

author: peps1
author_rank: Regular Contributor
timestamp: December 17, 2013, 05:45:37 pm
content: I dont get it, trouble free working for hours using bench psu on the DC rail, 3 mins using batteries instead and the problem source again......on 2 separate builds!

author: ivan747
author_rank: Super Contributor
timestamp: December 17, 2013, 06:08:28 pm
content: I wonder, could you upload an audio clip of how the problem sounds?

author: mrkev
author_rank: Regular Contributor
timestamp: December 17, 2013, 07:16:40 pm
content: Quote from: peps1 on December 17, 2013, 05:45:37 pmBtw what kind of diode are you using as D7? And can you measure voltage across R9 and D7 (idealy with and without the problem)

author: mrkev
author_rank: Regular Contributor
timestamp: December 17, 2013, 07:44:55 pm
content: Quote from: peter.mitchell on December 17, 2013, 01:27:30 pmWell i didn't mean that "I don't get it" as I don't understand it, I just REALLY don't like that design. When just one of your bateries is dead, this thing will not switch off until the sum of both voltages is OK.Btw, LED won't turn off, its connected directly to input...

author: peps1
author_rank: Regular Contributor
timestamp: December 17, 2013, 08:46:46 pm
content: Quote from: ivan747 on December 17, 2013, 06:08:28 pmhttps://soundcloud.com/peps1-1/1-1https://soundcloud.com/peps1-1/o2-amp

author: Joeee
author_rank: Newbie
timestamp: December 20, 2013, 09:09:05 am
content: Hi there,i've also build this amp.Mine worked out fine from the first time i turned it on. I can offer you a high res. picture of my pcb (top/btm). Or even measurements on my working unit. If you want these i can make them this evening.For Measurements i would like you to mark connection points in the pcb so that we are shure we are talking about the same things.As for christmas i'm not at home so i can't help you with that, but i'm sure we get yours to work!Greetings

author: peps1
author_rank: Regular Contributor
timestamp: December 20, 2013, 10:21:56 am
content: Thanks Joeee!Just out of interest, what type of batteries are you using in your one?I don't understand why, but I've got a overwhelming feeling my problems stem from these heavy duty Zinc Chloride batteries I'm using!Have a set of rechargeable in the post to test this hypothesis, but would be interesting to hear what you are using?

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 20, 2013, 11:32:45 am
content: Quote from: peps1 on December 17, 2013, 08:46:46 pmPersonally, the first one sounds like the battery monitor oscillating, and the second one sounds like one of the rails dropped out.Personally, i'd just bridge the D-S of both fets and forget about it.

author: peps1
author_rank: Regular Contributor
timestamp: December 20, 2013, 11:57:23 am
content: QuoteWhat are the cons in doing this?

author: Joeee
author_rank: Newbie
timestamp: December 20, 2013, 12:19:28 pm
content: Of course mine runs of rechargeables.As soon as you plug in the mains power supply it starts charging them!( from nwavguy "The O2 works pretty much like you’d expect. The batteries are charging any time it’s plugged into AC power--even while using it. It barely gets warm on AC power and not at all on battery. There’s an audible “click” at turn on (most noticeable with sensitive IEMs) and a soft “thump” on power down."With the rechargeable Batterys mine works when plugged into the wall supply and without the wall suply until the batteries are empty. For measuring i would assume to have fully charged 9V batteries and no wall connection.I can' remember exactly but it think when building it, it didn't work without batteries but the wall connection.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 20, 2013, 01:16:30 pm
content: Quote from: peps1 on December 20, 2013, 11:57:23 amIt just means it won't auto turn off when the battery gets low. But you probably won't want to use it when it gets low anyway because lack of output swing.Also, i mean you take the fet out and put a wire from the drain to the source on the pcb (i think you understand this, but you never know). if you take out the accompanying comparator, you could get a tiny bit longer battery life too.

author: Joeee
author_rank: Newbie
timestamp: December 20, 2013, 05:51:49 pm
content: Here are the pictures oft my working build. O0Measuring has to wait a bit - my akkus have to recharge.

author: mrkev
author_rank: Regular Contributor
timestamp: December 21, 2013, 01:02:10 am
content: I don't know if you noticed my question, so i wrote it again: what kind of diode are you using as D7? And can you measure voltage across R9 and D7 (idealy with and without the problem)?

author: kc9qvl
author_rank: Regular Contributor
timestamp: December 21, 2013, 10:01:51 pm
content: Recheck the required value for R8Bom says 270k.Your picture shows 245 KIt looks like Q1 is shutting off on battery power.Edit also the value of R5 also supposed to be 270 K per the Bom

author: Joeee
author_rank: Newbie
timestamp: January 10, 2014, 08:49:02 am
content: Hi peps1,have you solved your problems?

author: peps1
author_rank: Regular Contributor
timestamp: January 10, 2014, 10:43:54 am
content: Quote from: Joeee on January 10, 2014, 08:49:02 amSwitched from Zinc Chloride to Ni MH batteries and have not had any problems in the short time Iv been testing with the new batteries.Anyone able to explain that?

author: fake-name
author_rank: Regular Contributor
timestamp: January 10, 2014, 01:20:01 pm
content: Quote from: peps1 on January 10, 2014, 10:43:54 amYour Zinc Chloride (commonly called zinc carbon) batteries probably have much higher internal series resistance.The quiescent current of the amp across the ESR is drawing the power down below the cut-out threshold, the amp is turning off, the voltage recovers, amp turns back on, lather, rinse, repeat.Zinc carbon batteries are complete shit, they're not worth using anywhere. Alkaline or rechargeable are the only batteries worth bothering with.---If you're feeling curious, measure the quiescent current of the amp, and the esr of the batteries (and maybe some proper alkaline for comparison).

author: Strada916
author_rank: Frequent Contributor
timestamp: January 10, 2014, 01:25:38 pm
content: I'm sure myself and others mentioned that most 9v batteries have next to no grunt. Only good for the lightest power requirements. Anyway lesson learned I guess.

author: peps1
author_rank: Regular Contributor
timestamp: January 10, 2014, 01:27:36 pm
content: Quote from: Strada916 on January 10, 2014, 01:25:38 pmWhat lesson?

author: c4757p
author_rank: Super Contributor
timestamp: January 10, 2014, 01:47:24 pm
content: Quote from: peps1 on January 10, 2014, 01:27:36 pmSuspect the battery?

author: Strada916
author_rank: Frequent Contributor
timestamp: January 10, 2014, 04:01:41 pm
content: All I'm saying is don't trust 9v batteries is all.

author: peter.mitchell
author_rank: Super Contributor
timestamp: January 10, 2014, 06:05:49 pm
content: Quote from: c4757p on January 10, 2014, 01:47:24 pmHe did, I told him it was probably fine.I shouldn't have assumed;I assumed the difference between the batteries would make little difference in this application, due to the relatively low current draw and the OP saying they were new, but as we see, "if you assume, it makes an ass out of u and me" (but in this case, mainly me).I expected around 50mA load, but probably lower, maybe 35mA draw from the batteries at idle, wouldn't be very much more.I'm honestly somewhat surprised the batteries can't hack it. Though, they probably can, it seems the low voltage cutoff is a bit high, around about 13v (quick in the head calcs), which is around 6.5V per battery...That said, the difference in current delivery between chemistries for 9V batteries is pretty dramatic, you'd be pushing your luck to get more than 250mA out of a fresh Zinc Chloride/Carbon, about 1A from a fresh Lithium and around 3A from a fresh alkaline. I'm not sure about the current delivery of the rechargeable 9V cells but i think they would be fairly decent. As for capacity, at nominal discharge rate (1/20C), it is about 400mAh for Zinc Chloride/Carbon, 1200mAh for lithium and 600mAh for alkaline, though, note, the capacity drops significantly as current is increased for a number of reasons.I don't like a couple of things about the power design that would alleviate the issue a bit;Not much bulk storage when using a battery supply. Add caps on battery terminals, lower peak currents on battery means they last longer, also less prone to LVCO oscillation, potentially better audio performance(doubt it).Low voltage threshold based off LED Vf. Use something else, Vf changes with temperature and current, it also means that there are other modifications needed if you can't find the LED or for aesthetic reasons a different one needs to be fitted.Low voltage threshold set too high. Set it higher, or adjustable (pot or provide alternate resistor values), when used with non rechargeable batteries, you lose a lot of capacity, most have a end of cycle voltage of 4.8v~2x Diodes in power path dropping 0.30v each. Nuff said.Uses 9v batteries. I feel in this application they're really not appropriate; sure they save design time but they're expensive, pretty low absolute capacity but also not very dense, they're not as common as other batteries (AAs ect).If you use both batteries and AC, but not rechargeable batteries, you have to remember to disconnect the batteries before plugging it in. Battery/AC select switch would be nice, possibly Bat/AC/AC+Charge for bonus points, switch trickery might even let you move the AC filter caps onto the batteries in relation to the first point.The audio design is fine though, I don't feel it is very "special", almost all of the circuit decisions seem logical/default/obvious, however I won't dismiss the time spent optimizing component choices, you really can't do much better without spending a significant amount more.