Distorted Sound From Wharfedale EVP-X12PM Monitor - Page 1
Forum section: electronics / repair

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 06, 2021, 04:44:08 pm
content: I am hoping someone can give me some ideas with this one.I have here a Wharfedale EVP-12PM Active Monitor and the sound coming out of it very distorted!!I have found the service manual on the internet and uploaded it to ElektroTanya here:https://elektrotanya.com/wharfedale_pro_evp-x12pm_sm.pdf/download.htmlSo far I think I have narrowed it down to the power amplifier. (Schematic on Page 7).Please see attached page 7 of the service manual with the schematic for my model outlined in redI have probed the signal from the input of the power amp (pin1 of the connector) and listened to it using my headphones. At this point the signal is not distorted and sounds perfect!I have checked the power supply voltages:I have confirmed that both the + and - 15V rails are stable (both reading 14.8V DC) on both the pre amp and power amp PCBs.And also the + and - voltages of the power amp (both at 66.2V DC) are nice and stable.And the MUTE pin of the power amp connector is steady at 13.8V.So I am pretty certain it is the power amp circuit at fault somewhere.Any tips of what I might look at next?Could anyone point out to me which type of power amplifier this is? e.g. (Class A/B).My knowledge of power amps is quite limited so any help is very much appreciated.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 06, 2021, 05:30:41 pm
content: 1. Are you sure it's not the speaker that is distorting?2. Check in circuit for shorts on all the output transistors, and measure all 6 white ceramic 0.22 ohm emitter resistors, they should all read the same.3. If not the above then suspect the JFets Q1 & Q2, You could cut the legs of these that attach either side of R111 to remove them from the circuit.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 06, 2021, 05:50:33 pm
content: The Mute pin should be at -12v, not positive as you state, so you should get the green power LED lighting.The mute on the gates of Q1 and Q2 should also be negative for these Fets not to be muting the signal.The anode of D11 is probably the easiest place to measure this. Should be around -12v.Edit: it's not the Anode, is it, its the "pointy end".Cathode.

author: bob91343
author_rank: Super Contributor
timestamp: October 07, 2021, 12:36:27 am
content: If you can put in a sine wave and look at the output with an oscilloscope, you may find the answer.Distortion takes many forms. There is clipping, crossover, nonlinear operation, etc. Once you identify the wave shape you will be halfway there.Also, rubbing of a dynamic speaker cone over its travel causes distortion.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 08, 2021, 03:24:39 pm
content: Quote from: Audiorepair on October 06, 2021, 05:30:41 pmGood idea, I just borrowed a speaker cabinet and connected it to the speaker output jack and when playing back both the internal and external speaker sound distorted.So at least I know the speaker and filter are ok. So now the fault is definitely somewhere on the power amp board.The distortion kinda sounds like clipping mixed with some very fuzzy noises.. sort of.. It is hard to describe.Quote from: Audiorepair on October 06, 2021, 05:50:33 pmYou are quite correct sir, twas just a typo. I just forgot the '-' on my original post!

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 08, 2021, 04:27:21 pm
content: Quote from: Audiorepair on October 06, 2021, 05:30:41 pmJust tested the four power transistors and also all the other transistors for good measure. There are no shorts across any of these which is a good sign.Also the four (not 6 on this model) emitter resistors all measure 0.22 ohm (+- 0.01 ohm). So these seem fine.Quote from: Audiorepair on October 06, 2021, 05:30:41 pmI can't cut them off as they are so crammed in I cannot get my wire cutters on them. I will have to de-solder them, (typically requiring all the transistors to be un-bolted from the heat-sink) so this is a later tonight job.However measuring them in circuit I noticed that on both Q1 and Q2 there is 40 ohms between D and S on both of them (in both polarities). Does this sound correct?

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 08, 2021, 06:38:44 pm
content: Ha, the reason I suggested cutting the Jfet legs is that to unsolder them requires taking everything off the heatsink.Messy, time consuming, and prone to damaging stuff as you will no doubt find out.And testing a power amp with the output transistors in free air will make those transistors very hot very quickly even with no load, and liable to thermal runaway and magic smoke.So you are going to have to bolt everything back on the heatsink again after desoldering the Fets.I don't think you can get any kind of reliable info from measuring the Jftets in circuit, other than a short.You haven't given any indication of what this distortion actually is.I'm guessing you don't have a scope.Edit: Ah, sorry, I missed a post back there where you did describe the distortion.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 08, 2021, 08:12:11 pm
content: Quote from: Audiorepair on October 08, 2021, 06:38:44 pmYes but that's part of the fun is it not!I will be able to do it just fine though.I will take off those two JFETs you suggested and then put it back together and test if the distortion is gone.Quote from: Audiorepair on October 08, 2021, 06:38:44 pmCan you give me any pointers regarding testing the JFETs? I have never actually tested one before.I only have a multimeter at the moment. Perhaps I should invest one of those little ATMega 328 component testers.Quote from: Audiorepair on October 08, 2021, 06:38:44 pmWould a recording of the audio be useful in any way? I can always record it on my phone and link it here.I don't have access to a working scope at the moment. I used to use the one at my old job, and my HeathKit scope I have inherited has not been turned on since 1981 to my knowledge. Something I will have to have a look at soon I reckon.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 08, 2021, 09:02:32 pm
content: I don't think there is much point testing a Jfet.What seems fine at low voltage is not what might happen in circuit.Easiest way to test it is to take it out of circuit and thus let the circuit test it.Or just replace it, they are very cheap.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 09, 2021, 01:03:53 pm
content: Well before removing those JFETs I decided to do some more probing using my headphones.To do this probing I am connecting the input of a portable 9V battery guitar amp to the points on the board using my multimeter probe and a crocodile lead so I can hear the signal from the probe, and I am using headphones from the guitar amp.I tested one side, then the other side of R11 (where the two suspect JFETs are connected) and the signal is not distorted at these two points (see attached image).So I would think this rules out the JFETs would you agree?

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 09, 2021, 01:40:09 pm
content: Some strange behaviour just now.While I was probing I disconnected the speaker from the output so I could hear my headphones without a big 12" speaker in my face.I connected it back up and tried running it again and the distortion was gone for about 20 seconds!Then it started to hum and the hum started getting louder and the distortion faded back in.Very odd. Perhaps a clue?

author: SilverSolder
author_rank: Super Contributor
timestamp: October 09, 2021, 04:02:48 pm
content: Sounds like a bad capacitor somewhere? Have you looked at the ripple & voltages on the supplies?

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 09, 2021, 05:07:13 pm
content: Quote from: SilverSolder on October 09, 2021, 04:02:48 pmYes possibly! Perhaps power supply capacitor?I will measure the ac on the different rails to check the ripple.Edit:Just taken some readings from the power supply using my multimeter.These are the DC readings from my power supply rails:+70V: +66.8V DC-70V: -66.8V DC+15V: +14.8V DC-15V: -14.8V DC-12V: -13.8V DCThese are the AC readings from my power supply rails:+70V: 0.06V AC-70V: 0.05V AC+15V: 0.06V AC-15V: 0.06V AC-12V: 0.04V ACSo the ripple looks quite low to me.I also noticed that if I turn up the volume on the monitor really loud there is a very quiet but prominent 100Hz (to my ears) hum.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 09, 2021, 06:32:08 pm
content: You didn't measure the AC on the mute circuit.Are C6/C7 OK?

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 09, 2021, 07:14:59 pm
content: Quote from: Audiorepair on October 09, 2021, 06:32:08 pmYes the -12V is the mute circuit. Perhaps I should have stated it more clearly.I will check those two capacitors with my ESR meter before removing the JFETS.I have confirmed that after leaving the monitor unplugged for 15 mins or more to allow the capacitors to discharge, I can repeat the process every time:I turn it on and the sound is not distorted for about 20 seconds then the distortion fades in.

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2021, 07:21:38 pm
content: That evanescent behavior may be due to an oscillation that starts after something warms up. In analog amplifiers, a really good oscillation (even at supersonic frequencies) will totally mess up the performance in the audio range and DC.

author: Vicus
author_rank: Regular Contributor
timestamp: October 09, 2021, 07:32:20 pm
content: The scope will tell the true. So easy and fast.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 09, 2021, 07:38:59 pm
content: Quote from: paul_g_787 on October 09, 2021, 07:14:59 pmThe mute -12v goes to R7 and C7 , and then to the mute fets which you still haven't disabled to rule them out.R7 is 1 Meg. There is plenty of scope here for failing components to AM modulate the signal via the mute fets.Hence the hum.Take out the fets, and then progress from there, having at the very least removed the mute circuit as a possible candidate.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 09, 2021, 07:43:59 pm
content: Quote from: Vicus on October 09, 2021, 07:32:20 pmWell actually it won't.Audio amplifiers have feedback, so any fault anywhere will turn up everywhere, you will not find the point where it all goes wrong with a scope.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 09, 2021, 08:25:22 pm
content: Quote from: Audiorepair on October 09, 2021, 07:43:59 pmInteresting point you say that actually. It has made me think; what are the chances that perhaps the output section is struggling to provide enough current and then the feedback is trying to compensate buy upping the voltage therefore causing distortion?Just an idea. Will try the capacitors and JFETs in the meantime.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 09, 2021, 08:52:20 pm
content: Amplifiers do not usually fail in a distort mode.They either blow up or go DC.I think it would be better to look for reasons external to the amplifier itself why this is not working.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 09, 2021, 10:40:05 pm
content: Have you looked at the DC level of the amp output? Maybe it isn't zero...

author: Vicus
author_rank: Regular Contributor
timestamp: October 10, 2021, 09:31:50 am
content: Quote from: Audiorepair on October 09, 2021, 07:43:59 pmIn this case I think it will be pretty clear.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 10, 2021, 12:26:02 pm
content: Quote from: SilverSolder on October 09, 2021, 10:40:05 pmYes first thing I checked was DC across the load. There is no DC at all.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 10, 2021, 01:17:23 pm
content: Just tested the voltage on D11 cathode and it is -11.6VSo I just removed Q1 and Q2, C6 and C7.The distortion fault remains with these components out of circuit and so does the hum.Tested C6 and C7 and they are both spot on 9.8µF and working fine.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 10, 2021, 02:47:23 pm
content: If the amp circuitry appears to be working ok, I'd inspect the mechanical components: switches, relays, pots, connectors that are in the sound path. I have seen (heard!) amplifiers distort due to contacts or pots getting oxidized... Try bypassing each contact, or just clean them all with DeOxit...

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 10, 2021, 05:13:57 pm
content: Quote from: SilverSolder on October 10, 2021, 02:47:23 pmNo, at this point it does not appear to be working ok as much (as I would like it to).To recap: the fault is as follows:I have a hum(sounds like 100Hz) that is present with or without an input signal to the XLR and also present with the volume turned all the way down. However this hum gets louder when I turn the volume up.The output signal is audible but very distorted. The distortion is only present after approx 20 seconds of power up and kinda 'fades in'.To summarise: so far I have tried:Connect an external speaker cabinet with and without the internal speaker connected. The problem is there also ruling out the internal drivers and crossover filter.I bypassed the speaker connector by soldering a speaker lead to the board and the distortion was still there so it is not the output connector.Checked for DC on the output. There is none.Probed and listened the signal from between pre-amp to the power amp. It is not distorted and there is no hum; so this rules out the pre-amp, input socket, EQ faders and volume pot.Measured the power rails using a multimeter in DC and AC mode. They all read the correct voltages with no obvious rippleRemoved the JFETS Q1 and Q2 to disable the mute circuit. This had no effect on the fault.Removed and tested the capacitors C6 and C7 out of circuit and they are fine.So I am pretty certain the fault is either with the power amp. It could also possibly be the +/-70V supply not getting through to somewhere perhaps?In the past when I have had humming or distortion it has almost always been DC or a shorted semiconductor e.g diode or transistor (usually accompanied by magic smoke), however it does not look like it in this case.Currently I am a little stuck with this one, but as I said I am new to the workings of power amps so I appreciate all the help I can get.We all have to start somewhere right!

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 10, 2021, 07:44:06 pm
content: I was thinking about this today and then realised that the power supply is an SMPS, so the humming can't be 100 Hz ripple you might get from a transformer based supply.The only place 100 Hz ripple can occur is the high voltage capacitors that smooth the rectified mains voltage before the SMPS circuitry.So failing caps otherwise wouldn't have 100 Hz ripple on them, they would just have low DC and/or noise.So maybe there is some kind of grounding issue instead at 50 Hz.

author: TimFox
author_rank: Super Contributor
timestamp: October 10, 2021, 07:46:33 pm
content: Can you tell if the “hum” is 100 or 50 Hz? This is important to tracing the source. An oscilloscope is required.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 10, 2021, 08:38:05 pm
content: Quote from: Audiorepair on October 10, 2021, 07:44:06 pmYes that is possible. I have a signal generator app on my phone. I can play 50Hz then 100Hz through the monitor and see which matches the hum to see which it is.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 10, 2021, 08:44:22 pm
content: I managed to find the schematic for the PSU. Uploaded to ElektroTanya here:https://elektrotanya.com/wharfedale_evp_p400_power_supply_for_evp-x12p_12pm_15p_15pm_18pb_15pb_215p.pdf/download.html

author: Audiorepair
author_rank: Frequent Contributor
timestamp: October 10, 2021, 08:50:44 pm
content: Once you get used to identifying 50Hz hum vs 100 Hz ripple it becomes very obvious.50Hz hum tends to be more or less a sine wave at 50Hz, one frequency.100Hz ripple is more like a triangle wave, and thus contains harmonics that go way beyond 100Hz you can hear.They sound very different, it is not just a doubling of frequency.I dare say you could Google for examples of this.Edit: Ripple is sawtooth, not triangle.

author: David_AVD
author_rank: Super Contributor
timestamp: October 10, 2021, 08:57:29 pm
content: I've had weird sounding stuff go on when the main input filter cap on an SMPS went pretty much open circuit. I was surprised that it was still working at all.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 11, 2021, 02:12:48 pm
content: OK I solved the hum issue! It was a grounding problem.I listened to a 50Hz sine wave on my phone and the sound is the same as from the monitor. After a little poking about I discovered an unpopulated GND pad on the power amp's PCB which had nothing connected to it from new by the looks of it. So it is relying on the wire going back via the power supply to the pre-amp for the GND connection. Hence picking up hum along the way.I attached a wire from the GND pad to the chassis and hum is gone! Dead silent now even when I turn the monitor up to 100% (with no input).However the distortion fault still remains.One step at a time I guess.I only just saw your earlier @Audiorepair about checking the voltage at the gates of Q1 and Q2 (measured at D11 cathode). So I just did this and it is as follows:When the monitor is powered on the voltage is at 0V but slowly climbs to -12V. Is this correct? I assume it must be, seeing as with Q1 and Q2 removed the behaviour of the fault was the same.The audio signal comes in when the gates of the JFETs reach 4.0V and the signal is immediately distorted but the distortion does not get worse from this point.Now if I turn the monitor down really low (approx 2-5% volume) and stick my head right up in the speaker cone I notice there is no distortion (that I can hear at least).

author: SilverSolder
author_rank: Super Contributor
timestamp: October 11, 2021, 04:19:54 pm
content: Do you have both monitors (i.e. do you have a working one)?Then you can compare and contrast voltages between the two...

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 11, 2021, 04:45:02 pm
content: Sorted!!!! This was a pain to find though.After reading a thread on another site regarding a similar issue, I decided to measure the temperatures using my multimeter of each transistor.I discovered that Q18 and Q21 were both heating up very hot compared to Q20 and Q23 and so were their corresponding emitter resistors R13 and R25. They were reaching over 60°C after only 2 mins of use at low volume!So I checked them for shorts again and no shorts, then I checked the emitter resistors again, both in tolerance.After checking them and their connections a few times I stumbled upon the fault. A bad solder joint one both emitter legs. However it was not as simple as it seemed.After a bit more testing I found that the connection between Q18 and R13, and also Q21 and R25 would raise to 300Ω and 425Ω when they got hot, then go back down to 0Ω when cooled back to room temp (which is why I didn't find it before).The solder was a bit dull but their was no obvious cracks. Regardless, I re-flowed the solder joints on the emitter legs of Q18 and Q21 and tested again and my fault was gone!!!This would explain why the distortion kinda 'faded in', as the joint got hot due to poor connection the heat must have been making the gap in the connection expand and the resistance increase. Perhaps the bad joint was caused by stress on the legs of the transistors? Just guessing at this though.My theory is that this issue was effectively increasing the emitter resistors on these transistors, but by different amounts therefore upsetting the bias and causing the distortion. Would anyone agree with this?So as a precaution I wicked away all the solder from every leg of every transistor bolted to the heat-sink and re-soldered all those joints while everything was bolted in place so there is no stress on any legs. Then cleaned up all the flux and put it all back together again. It has been running for over an hour now with no problem and I am very happy it is fixed!This was the hardest to find bad joint I have ever had (as it would come and go with the temperature)!Thank you all who helped, especially to Audiorepair who has now provided me a bit more understanding of how the mute circuit functions.Onto the next project now........