Mottlefeeder

Depending on what it is you are trying to adjust out, a neck shim might allow you to back off the truss rod tension David

Odd - it has an electronic crossover, but only one speaker according to the website, and no mention of battery operation, which would help it compete against the likes of the Rolland Bass cube. David

Thanks @basstone, but I've already done my modification - fixed gain of 10dB and a switched passive attenuator of 10 dB between the instrument and the input capacitor. It has the advantage of dropping the input impedance for high gain active basses. The gain switch click is now a lot quieter, as is the power on/off click, so I think I'll stop there. So, what have I achieved? The positives - It's small, battery powered, loud for its size and plays a low B without farting. The amplifier is rated at 90w x 2 into 4 ohms x 2 with a 14.4 volt battery, but it is not clear whether its power supply compensates for lower battery voltages, so it could be down around 100W or worse, but it's still pretty loud. At a recent band rehearsal in someone's garden it was still going after 2 1/2 hrs, but that was at a relatively low volume. The negatives - Twin speakers means two ferrite magnets, and a lead acid battery, so it weighs 9.7kg, which is a little heavier than I had hoped. Secondly, small speakers are less efficient, so it uses four times the power to generate the same volume as my neo Eminence Basslite 1 x 10 cab. Having said that, where I need a small loud combo, this will do the job. A few pictures and a couple of sound clips to finish off. For the sound clips, instrument EQ was flat, HPF was at 30Hz, and for the Yamaha, pickup blend was 75% neck, 25% bridge. The recorder is a Tascam guitar trainer which I assume has reasonable microphones on it. Ibanez acoustic.mp3 Yamaha solid body.mp3

I think the main advantage to a manufacturer is that all their fretboards are cut the same so they save on a bulk order. They can then put in frets or lines. David

Finally built the new circuit with the switched gain on veroboard. I didn't finish working out the layout before a trip to see family in the North, and I didn't finish the build before a trip to see family in the South. Add in some self isolation between visits, just to be on the safe side, and that's a month gone. The circuit has enough gain, which was the reason for the redesign, but the gain switch causes a loud 'crack' when operated, so I'm not out of the woods yet. I'm now looking at a fixed gain preamp, with a passive attenuator on the input. More rework... Definitely reached the point where I wish I had not started this. David

Back in the 70's, Barkley James Harvest had a hit with 'Mocking bird'. When I saw them at the Hammersmith Odeon, they had a neon tube prop that they only used once, for that song, on their 5th encore - they planned 5 encores? David

I've had mixed results depending on the design of the cab. Some need to be tilted back, with support under the front, but others need support at the back if they are tilted back far enough to be useful. I also had one that was back- supported, and worked fine on the flat, but tipped upright again when used on a raked stage. It's worth checking if you haven't already done so. Staying with your design, if you have access to an angle grinder, (or a hacksaw?), you can remove part of the hinge casing to ensure that the handle travels past 90 degrees. As an alternative, you could screw 'captive nuts' https://www.screwfix.com/p/insert-nuts-type-d-m6-x-20mm-50-pack/61859 into the speaker cab base and bolt on a bracket when needed. David

Sanded the panel down, back to bare metal in places, then re-primed and re-top-coated it. Printed off another set of transfers and managed to apply them to the right parts of the panel, As they started to dry, they started to peel off - it seems that they don't stick to a matt paint finish. Dug out my time-expired dry transfers and applied a simplified set of control legends. They appear to be staying on, and now have two coats of lacquer to help them. Assuming no more problems here, I can start to reassemble it tomorrow. David

A short break and back to work. The back panel has been cleaned up, painted, and transfers applied. Unfortunately I followed the supplied instructions, which used a china mug as the recipient, and this resulted in too high a temperature for the transfers and paint I had used. Putting the transfer back under water did not make it come off (which bodes well for the future) but I now have to scrape off the bubbled transfer, probably respray the panel, and start again. David

A little more progress. The original plan was: input buffer -> HPF -> variable gain stage to provide a volume control, -> amplifier. That was simplified to: fixed gain buffer -> conventional volume control -> fixed gain (x10) -> amplifier, as in the previous post. Having discovered that my earth referencing was all wrong, and fixing that, I then decided to try and resurrect most of the original three stage design - buffer -> hpf -> conventional volume control -> boost -> amplifier. I discovered that all the extra capacitors I had in circuit meant that i could not fit that circuit onto the area of stripboard that I had, so I tried putting the gain in the buffer stage and removing the boost stage: buffer (x10) -> HPF -> conventional volume control -> amplifier. Unfortunately, the gain at which my passive bass can drive the amp to full power gives me clipping in the preamp with my active basses, so I do need some form of adjustable gain. The compromise is a Hi-Lo gain switch in the buffer stage. The current circuit looks like this. David

Finally, I've found the source of my problems (with this preamp). The pcb that I have been adapting came from a previous project that used positive and negative supply rails and the input and output were referenced to the zero volt line. In this battery system I've used a positive rail and a zero volt earth reference, so the input and output were referenced to the zero volt rail, while the bias resistors, feedback loops and volume control were still referenced to the mid-point rail - diagram (A). At low gains, it worked, but as I increased the gain, the circuit became progressively more unstable. I've now wired it so that the input, output and volume control are referenced to the zero volt rail, the bias resistors are referenced to the mid-point rail and the feedback loops are referenced to the zero volt rail by blocking capacitors - diagram (B). It now works as it should, with minimal hiss, no instability, and minimal switch-off click. David

Running it is not the problem. I'm trying to get rid of the loud click when you shut it down. David

Thanks for that. So far, the resistor/capacitor combination is working for me. If it works less well when I add another op-amp in circuit then I'll start looking at more complex solutions. The "plate amp" needs 200mV for full output, so about 600mv pk-pk. Assuming the preamp output voltage is supply rail - 1.5V, I could manage with a preamp supply of 4.5 to 5 V, assuming that the figures I have been given refer to sine wave measurements. However, my gut feel is that more headroom is better. David

Some progress to report - I've breadboarded a non-inverting, unity gain buffer followed by a conventional volume control followed by a gain of 10 non-inverting stage, powered from a 12 V battery via a 240 Ohm resistor (dropping about 2 volts) and a 470mmF smoothing capacitor. It is stable, loud, and has a subdued click when switching off, and I can live with that if I make no further progress. Thanks to @Stub Mandrel for pointing me in the right direction I tried to use an LM317 to regulate the supply, but the audio circuit started motorboating - whether that was because I adjusted the voltage while it was live, I don't know. I'm also a bit short on headroom if you assume the minimum battery voltage to be 10.5 v and the regulator drops 2.5 volts. The next test is to see if I can add a variable HPF before the volume control, without it becoming unstable. David

You are right - there isn't a difference. The diagram I have shows the contact numbering going down on one pole and up on the other, like an integrated circuit. That numbering does give you a difference, but it looks like it is wrong. One lives and learns. David

I disagree. The supplier's connection table in my second post above shows that when the switch is in the central position, both poles connect to the same end, but with the switch not central, the two poles switch to opposite ends. If you turn the switch through 180 degrees your centre position will connect to a different set of contacts, so orientation is important. David

I can't tell from the photo whether it is notch up or notch down, but I also can't confirm that any two switch manufacturers do it the same way, so the only way to check it would be with a multimeter, or a battery, bulb and length of wire. You need to check whether 'switch in central position' connects the centre and upper contacts or centre and lower contacts in the original switches and then check that you have done the same. David

The switches are not symetrical in their operation - Toggle position vs contact position: P1 • 2-3, 5-6: ON P2 • 2-1, 5-6: ON P3 • 2-1, 5-4: ON If you have the switch wired upside down, then it will not switch the way the original did. That might be worth checking. David

I think that would make sense if the single coils need to be phase reversed to give you a scooped mids sound. David

We are on the same page, but your method would prevent a high output bass from causing clipping within the preamp. Thanks for that. David

It seemed like a good idea, but without a 'scope to find out what is going on, I'm minded to simplify rather than tweak. The pots are quite a distance from the pcb, and my use of screened cables to/from the volume and the variable hpf pots may be providing the capacitance giving the instability. It will be easier to stay conventional, scrap the variable hpf, mount the pcb on the input jack socket and take its output to a volume control then to the amp - the signal path stays on the pcb. David

A bass on its own does not deliver enough signal to drive the amp to clipping levels, so a unity follower would raise the input impedance to suit the passive bass, but not allow me to use the full capabilities of the amp. I think it would have to be non-inverting, with gain, and fed from a stabilised supply, feeding into a conventional volume control. David

Did a little bit of investigating, and the conclusions are not good. I had hoped to use an inverting op-amp configuration with a pot in the feedback loop to provide variable gain / volume, but it was unstable, as well as noisy on power down. I've disconnected the pot, and substituted a trimmer on the pcb, and a conventional volume control between the preamp and the main amp. It's still unstable and noisy on power-down, so the problem is elsewhere, possibly the 100 mm lengths of screened cable between the pcb and the HPF dual pot. So, I'm looking at a strip out and rebuild to get a stable preamp. I've stripped it out, and substituted a pedal eq unit instead. No instability, no hum, no noise on power down. In summary, I've now got a fixed 50Hz 12dB/Octave HPF in circuit within the amplifier module, and I'm wondering what I gain by rebuilding the preamp. Without a preamp, my active basses will drive the amp to something less than full power, and have their own eq, so my work around is to omit the preamp pedal for quieter gigs, and put it in circuit when needed for the volume boost, or the passive bass, or for rooms with odd acoustics. End of the road? David

Making slow progress, but getting there. 1. A plywood base plate 2. Amp and sockets fitted 3. Plate fitted to speaker cab. I still have to check whether the preamp clicks during switch-off with this amp, and fix it if it does. David

The specification for a speaker usually defines the range by the frequencies at which the volume has dropped to 50%. Going slightly below the specified low limit will result in even lower volumes, until you reach the point where the speaker+box no longer behave as designed, and the speaker movement becomes uncontrolled. This could cause speaker damage. Hypothetically, using a sub to reproduce the fundamental will put you in the audio spectrum used by the kick drum, and the result will be indistinct and muddy. David