fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 Oh boy. Failure to sort the problem. Now with Beymas fitted, still getting this low volume buzzing ( like a delicate farting i suppose ) nothing like what i thought a speaker farting would sound like Its from both drivers, and it happens at any volume, input gain at low medium or high, bass eq rolled off completely, varying the main volume , varying eq , changing cables. I'm stumped right now Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 Recording - just using open E. Bass EQ rolled off completely. Sounds like i;m really attacking the string but it was just a normal bedroom level pluck Voice 001.mp3 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 17, 2020 Share Posted April 17, 2020 23 minutes ago, fleabag said: Recording - just using open E. Bass EQ rolled off completely. Sounds like i;m really attacking the string but it was just a normal bedroom level pluck Voice 001.mp3 179.04 kB · 1 download Seems to get more noticeable as the volume reduces. Crossover distortion? You could invest in a cheap oscilloscope module. Mind you, it's an ashdown you haven't got the sub-octave turned up a bit without noticing have you? (BTDTGTTS) 1 Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 The crossover is disabled as the tweeter is just a hissy pile of poo. I dont use the sub-octave, so its permanently switched out ... same with the compressor. I dont know what the letters mean any other ideas Stub ? Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 Just as a guess, i reseated the drivers, equal torquing down , in case the basket was slightly warped due to uneven mounting, but that changed nothing. I'm beginning to wonder if its the amp Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 17, 2020 Share Posted April 17, 2020 1 hour ago, fleabag said: The crossover is disabled as the tweeter is just a hissy pile of poo. Crossover distortion is if the amplifier isn't set for enough bias current, so when the signal 'crosses over' from negative to positive there's a brief moment where the signal is cut off. It tends to be more noticeable with softer signals as it doesn't increase with volume. Basically the half of the amp that does the top of the signal is switching off before the half that does the bottom switches on. If yours in is a linear class B amplifier (rather than a class D) there's a biasing arrangement in there to prevent it happening. This can go wrong. Sometimes its as simple as adjusting a preset to get the right 'quiescent current' or it could be a diode or transistor needs replacing. The most common cause is when a transistor is used to set the bias current and kept at the same temperature as the power transistors by being held against the same heatsink. If it comes lose you get crossover distortion that gets worse as the amp warms up. Easy to spot if you have a scope (see picture above) but needs a bit of amp repair experience to fix. 1 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 17, 2020 Share Posted April 17, 2020 This ashdown amp uses TR5 on the heatsink (near bottom right - it even says 'on heatsink' on the circuit diagram!) to prevent crossover distortion. If that's your problem see if you can spot a small transistor that (should be) bonded to the heatsink between the big power transistors. If it looks like its come away a small bolb of heat-transfer compound and gently pushing it back in place could cure the problem. (Of course, I could be completely wrong and it's something else altogether). 1 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 17, 2020 Share Posted April 17, 2020 *blob, not bolb. Quote Link to comment Share on other sites More sharing options...
Mottlefeeder Posted April 17, 2020 Share Posted April 17, 2020 Starting from zero, you could have a faulty speaker, or just one that needs to be run it, so take each one out in turn and press the cone in with your fingers at the join of the cone and dome. Any scratchiness indicates the coil/coil former is touching the magnet, possibly indicating damage in transit. Secondly, leave it on the bench and play music through it - does it still sound distorted? If so, it indicates a possible faulty speaker. Finally, connect up an amplifier, and play a low tone through each speaker, ideally about 20-40 Hz, either from a tone generator, or an mp3 recorded from a tone generator and set on repeat. Keep the signal low enought that the cone moves a few millimeters, but not so loud that you get overtones. Leave it to run in for 12 hours and then try it back in the cab. If you run it in at 20 Hz, you will be able to sleep at night because you will not hear it unless you turn it up loud enough that it produces overtones. David 1 Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 Thanks stub - i'll pull the amp out tomorrow and check that pesky TR5 If it needs a bolb , i'll sort it Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 11 minutes ago, Mottlefeeder said: Starting from zero, you could have a faulty speaker, or just one that needs to be run it, so take each one out in turn and press the cone in with your fingers at the join of the cone and dome. Any scratchiness indicates the coil/coil former is touching the magnet, possibly indicating damage in transit. Secondly, leave it on the bench and play music through it - does it still sound distorted? If so, it indicates a possible faulty speaker. Finally, connect up an amplifier, and play a low tone through each speaker, ideally about 20-40 Hz, either from a tone generator, or an mp3 recorded from a tone generator and set on repeat. Keep the signal low enought that the cone moves a few millimeters, but not so loud that you get overtones. Leave it to run in for 12 hours and then try it back in the cab. If you run it in at 20 Hz, you will be able to sleep at night because you will not hear it unless you turn it up loud enough that it produces overtones. David I've done an equal pressure test of cones and they're fine. I dont have a spare amp, to try that tone test Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 17, 2020 Author Share Posted April 17, 2020 (edited) There doesnt seem to be any problem with TR5 that i can see Its quite a large photo, but it seems you have to click on it a few times to get a big picture Edited April 17, 2020 by fleabag Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 18, 2020 Share Posted April 18, 2020 (edited) 11 hours ago, fleabag said: There doesnt seem to be any problem with TR5 that i can see Its quite a large photo, but it seems you have to click on it a few times to get a big picture Looks like in your circuit it's TR10 that does the job (top centre). The heatsink compound on it does look a bit suspect. You could try gently poking it to see if it is in good contact with the heatsink. This is your amp: https://el34world.com/charts/Schematics/files/Ashdown/Ashdown_apc033xa_schematic.pdf Edited April 18, 2020 by Stub Mandrel 1 Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 18, 2020 Author Share Posted April 18, 2020 Yup, that white gunk looks messy - i'll give it a wiggle Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 18, 2020 Author Share Posted April 18, 2020 Decided to pull the whole PCB out and check all the solder joints but nothing seems amiss. Looking like a tech problem, and since i cant take it to my local tech, its back in the cupboard for this combo. 1 Quote Link to comment Share on other sites More sharing options...
Mottlefeeder Posted April 18, 2020 Share Posted April 18, 2020 I've had two amps with distortion caused by the speaker relay - often the relay used was designed for automotive use (electric windows and the like) so that it can cope with the inductive load of the speaker. if the contacts get tarnished, it can create a bad connection. If you still have the pcb out, you could try shorting out the speaker switching contacts and then powering up and connecting the speaker after a few seconds. David Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 18, 2020 Author Share Posted April 18, 2020 Unfortunately i've reached the end of my lectronic talents, which is poor to start with, so that went over my head Quote Link to comment Share on other sites More sharing options...
Mottlefeeder Posted April 18, 2020 Share Posted April 18, 2020 On your pcb picture, right hand side, about half way down - a transparent plastic box with contacts in it. Its job is to disconnect the speaker during start-up, shutdown and faults. Relays to pass small signals tend to have gold plated contacts, and relays for electric motors tend to use tungsten contacts. Sparks can erode gold very quickly, but tungsten is tougher and survives that kind of treatment. Unfortunately, tungsten contacts depend on sparking to clean the surfaces, so if they only get low level signals, they can tarnish and cause distortion. Amplifier builders really need small signal integrity to provide high quality sound, and spark survivability for when they disconnect speakers, but you can't have both so they went for tungsten - also it's cheaper. If you are not familiar with relays, you may struggle to identify which pair of contacts to short out, so it's probably best left alone. David 1 Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 18, 2020 Author Share Posted April 18, 2020 Wise words The cupboard is the only place for the combo Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 18, 2020 Author Share Posted April 18, 2020 One last test before storage - as the amp was detached from the combo, i hooked it up to my 1x15 and got the same buzzing / light farting, so i'm pretty sure its the amp. At least i know that much 1 Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 19, 2020 Share Posted April 19, 2020 Here's an idea. Stick a microphone into your computer and use this to grab what the waveform looks like. Use a good, clean signal like a E or A with the treble rolled off and finger-plucked at the middle to get as clean a signal as possible. Then share a screen grab and we can see if the distortion is visible (it might not be). Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 19, 2020 Author Share Posted April 19, 2020 How to attach a mic to a laptop - i suppose it would have to be a USB mic ? I dont have a mic and not sure how you record a waveform Quote Link to comment Share on other sites More sharing options...
Stub Mandrel Posted April 19, 2020 Share Posted April 19, 2020 1 hour ago, fleabag said: How to attach a mic to a laptop - i suppose it would have to be a USB mic ? I dont have a mic and not sure how you record a waveform Does it have a built in mic? Try going that website and see what happens. you just play into the mic and click the box to 'freeze' the waveform and take a screen grab. Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 19, 2020 Author Share Posted April 19, 2020 Googled my lenovo lappie and it has built in mic, but never used it so i can only assume it works. Which website are you thinking - i didnt see any mention of that Quote Link to comment Share on other sites More sharing options...
fleabag Posted April 19, 2020 Author Share Posted April 19, 2020 (edited) Just tested the mic and its working fine. Problem is, googling how to record sound hasnt revealed any answers. I spoke into the mic, and can hear that on headphones, but whether it was recording or not, i have no idea. Do i need external software recording proggy ? Edited April 19, 2020 by fleabag Quote Link to comment Share on other sites More sharing options...
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