Mottlefeeder

Four jack sockets, three switches, three opamps, two pots and one 9V battery in a box not much bigger than a tv remote. It's a: Hi-Z buffer with switched gain + 18dB/octave fixed frequency HPF + headphone driver + passive headphone monitor mixer with volume limiter, and I don't think all that would fit in the title box. The circuit evolved as I built it, so I have not got an accurate copy yet. If there's any interest, I can sort one out. David

OK, I've trawled through several months of Bill's contributions here and I can't find what I thought I read. Adding a second speaker increases the sensitivity of the system by 6 dB, by halving the impedance the amp sees, resulting in the delivery of four times more power. I can't find any comment on gains by running two speakers in close proximity. In my experience, adding a second identical cab gives me deeper, more effortless bass, but I can't find any science to back that up, (and we know how important it is to follow the science don't we). The alternative is to suggest that running a 1*12 louder would sound just as good as running a 4*12 which used the same drive units, and porting and volume per speaker. David

One option that might be worth considering for monitoring is to fit a Y cable to the mixer main-output and feed both the PA amps and a small personal mixer to which you can add your own instrument and/or mic feed. In our band, I take an attenuated DI feed from the full-range speaker, and a DI feed from my bass amp. I mix them to get the sound I want at the volume I want. DI box ~£30 (if needed) and 2-3 channel mixer ~£40 - for just a couple of IEMs it's a cheap option, less so when they all want one. David

I can't see anything in the original post that limits us to putting all the speaker boxes in parallel, so, if you connect each pair of 8 ohm speakers in series, and connect the two series circuits in parallel you will get a four-speaker, 8 ohm system. According to Bill Fitzmaurice, adding a second speaker adds 6dB to your volume, partly due to getting more power by the halving of the impedance and partly due to the influence of each speaker on the other. With the speaker system described, you will not get the 3dB increase in volume due to dropping the impedance, however I think you will get a 9db increase in volume (8 times as loud?) by adding in three extra cabs. And your amp will run as cool as it did with one cab. David

I'll start by saying that I'm biased - we busk under a gazeebo in the rain, so using powered speakers is not an option for us. Your existing mixer and a separate power amplifier (or two) would give you more versatility and an easier swap out of failed parts. Powered speakers need two cables each, but passive speakers need one cable each and all of your mains needs are at the back of the stage. Also, powered speakers will have smaller speaker enclosures due to the built-in amp, so they use more power to generate the same amount of bass. Passive speakers and power amps are readily available second hand; powered speakers less so. David

Looking at your stretched waveform, and comparing it with Stub Mandrel's, I'm not seeing any cross-over distortion, so I'm starting to think about other things it might be. Since the amp has FX out and in jack sockets, have you got anything that will allow you to listen to the preamp output (FX send) or play into the Amp input (FX return)? If you turn down the volume of an active bass, switch off the amp power, connect the bass to the FX return socket, power up and then slowly raise the volume on the bass you should get enough signal to tell whether you still have a problem. If so, then you know it is not the preamp. If you have a mixer or another amp of any description, you can plug your FX send into it and check whether the distortion is being produced in the preamp. Totally off the wall, you could send Ashdown a PM, point them at this thread, and ask them what might be causing the problem, and whether it is DIY fixable. When I had a problem, they satisfied themselves that I was competent, and then sent me a replacement PCB "If it fixes the problem, send the faulty one back, and pay us: or if it doesn't, send the whole amp and new PCB back and pay us to fix it for you." David

Keep zooming until you have only 2-3 cycles in view. Then increase the gain if you need to. I cannot see any crossover distortion on this trace. You've tried this amp with two cabs, and got the same result - just to rule out other possibilities, have you tried it with another bass, just in case it is a faulty string, or a string rattling in the nut or a fading battery or something similar? David

It's doing it with two cabs...

In Audacity, the stretch facility is on the toolbar above 10-11 seconds of the recording. The magnifier with the '+' sign will stretch the whole recording, and the buttons to its right will expand a defined section of the recording. David

In Audacity, towards the right of the tool bar, there are tools to stretch out the recording in time. Using those, you can expand the view until you can see individual cycles. I can see no sign of crossover distortion. You do have some warbling harmonics - the lighter coloured parts of the trace - but I have no idea if that is normal for a plucked string. David

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

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

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 been pleasantly surprised by the Behringer Xenyx 302 USB mixer. For £40-45 you get one mic/line it with 2-band EQ and a stereo line in with 2-band EQ. The audio levels are right for my MP3 player and my laptop, so I can choose my line input with one button press, and drop off as much bass as I want so I can play along. It will record from mixer to Audacity, but I have not yet found a way of playing back an Audacity track and recording a new Audacity track at the same time - it's not something I need to do. David

If you want to cool your hot drink, do you blow or suck? A fan in suction pulls air from anywhere on the suction side. A fan blowing directes the air in a particular direction. If you want something cooled, the fan should blow onto it. David

The amplifier will deliver up to 500w into 4 ohms, so a 4 ohm cab should be rated for that, or used with care if not rated for full power. The amp will drive a single 8 ohm cab at about 250W, or drive two 8 ohm cabs at 500W. The output into a single 8 ohm cab will probably be slightly more than half power due to the power supply not having to work so hard. David

I wonder if that would help me learn double bass? David

Fair enough - my load-in is usually anything but smooth so it colours my view of the world. David

I've used Snark clip-on tuners on a 5-string Yamaha and found that at certain points on the headstock, they do not register the low-B, or take a couple of seconds before they decide to display any note. As others have said, they can be confused by harmonics, so pluck close to fret 12 or tune to the fret 12 harmonic. David

I disagree - looking at the size of the wheels, that is unlikely to cope with a dirt-track car park, rough concrete flooring, kebstones etc. You need something with soft tyres on 100mm (or bigger) wheels. David

I use these or similar - https://www.amazon.co.uk/100x-Professional-Reusable-Cable-Ties/dp/B073WVS45S/ref=sr_1_5?crid=10K0S00X4GH6O&keywords=reusable+cable+ties+200mm&qid=1582219594&s=diy&sprefix=Reusable+Cable+Ties%2Cdiy%2C271&sr=1-5 - plastic cable ties that can be undone and cost a few pence each. They don't stick to each other and at the end of a gig, the number of cable ties attached to the back of my rig tells me how many cables I need to find and pack. David

I'm sure I read - but I can't find it now - that you can tune the damping in a cab just by snapping your fingers in it. If it sounds reverberant, it needs more damping, and if it sounds dead, it needs less damping - the optimum is somewhere between those two. Can any speaker builders confirm that the principle is right? David

No problems with the bridge, but some observations about the string clamp - It is designed with two clamps per string, the one parallel to the fretboard is like a string tree with some clamping action, but the main clamp is on the end of the clamp block, where you bend the string through 90 degrees, and thread it through a hole and tighten a screw onto it. This is not the thin part of the string that you wind around a tuning post, this is the string at full thickness - and some strings don't like being bent through 90 degrees. After losing a couple of strings, I made a string clamp block and trimmed the original string retainer so that the new string retainer sat in the right place to hold the strings. I've added some pictures now I've find them. David Original Modified - Strings with shallow break angle, clamp screws on underside (the pink on second shot is a reflection of my hand; the metal is black)

yes that's the one - it was strung BEAD then, but is now back to EADG.

I built a kit which used an 'Overlord of Bass' bridge/tuner block and it was fine. Several people played it at the 2018 North-West bass bash and no-one thought it bad. It's pictured at the bottom of this page - Andyjr1515 has also used one on at least one build, and not warned people off. There is a half-way house between the Chinese Steinberger clones and the Hipshot or ABM products - Nova, made in Brazil, which you can see via facebook. It is now also being cloned by Chinese suppliers selling on Amazon. The original got good reviews, I haave not checked out the clones. David