Combo lockdown project

[Mottlefeeder]

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

[Chienmortbb]

8 hours ago, Mottlefeeder said:

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

If you have little headroom as far as supply voltage you need an LDO (low drop out regulator). I use the Linear/Analogue LT1085. It is a bit overspec’d for your use but it would work well on your application. 

[Mottlefeeder]

12 hours ago, Chienmortbb said:

If you have little headroom as far as supply voltage you need an LDO (low drop out regulator). I use the Linear/Analogue LT1085. It is a bit overspec’d for your use but it would work well on your application. 

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

[Chienmortbb]

52 minutes ago, Mottlefeeder said:

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

If you are running from a battery then the voltage should be quite clean. Can you run the preamp directly from that?

[Mottlefeeder]

5 hours ago, Chienmortbb said:

If you are running from a battery then the voltage should be quite clean. Can you run the preamp directly from that?

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

David

[Stub Mandrel]

3 hours ago, Mottlefeeder said:

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

David

You could add a diode in series with your dropper resistor.

I suspect your partial 'cure' is stopping the preamp output dropping rapidly to zero before the power amp has shut down.

 

[Chienmortbb]

Does it happen with the  preamp disconnected? If so I agree with @Stub Mandrel also put a capacitor from +V to 0V just after thr diode, or if one is alrready there, increase it. Power down noises of many types are caused by the removal of power to an opamp.

Edited April 1, 2021 by Chienmortbb

[Mottlefeeder]

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

[Mottlefeeder]

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

[Mottlefeeder]

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

 

Edited May 24, 2021 by Mottlefeeder
Spelling

[Mottlefeeder]

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

[Mottlefeeder]

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

[basstone]

On 28/06/2021 at 17:13, Mottlefeeder said:

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

If your circuit is the one above with the switched resistor in the op amp feedback then the sound you are hearing is because of a small dc offset in a very sensitive part of the circuit. You could quickly try switching in a resistor in the other part of the feedback ie the grounded part. Try maybe 2k plus 2k switched. Better yet swap round the resistors and low cut capacitor so resistors are connected directly to ground. Hope that makes sense and is worth a try before the passive attenuators option. Good luck

[Mottlefeeder]

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

Edited June 30, 2021 by Mottlefeeder

[Mottlefeeder]

A quick update - 

Checking out mains power supply options, I noticed that I was getting a high-pitched buzz through the speaker. I rejigged the earth connections to get as close to a single earth point as I could, but then noticed that the noise varied as I moved the bass around, so it was probably radiated interference. With a bit more testing I discovered that the amplifier case isn't connected to earth. 

Connecting it to the earth star point cured the problem completely.

 

David

[Stub Mandrel]

6 hours ago, Mottlefeeder said:

With a bit more testing I discovered that the amplifier case isn't connected to earth. 

 

Slapped wrist! 🙂

[Mottlefeeder]

In my defence your honour, I was just following orders.

 

The supplied instructions referred to fixing the module using the supplied screws or cable ties, and made no mention of the need to earth the case, which has no discernable earth terminal. 

 

Having avoided cheap far-eastern modules because they have a reputation for being noisy, I assumed that a major brand would have incorporated whatever was necessary into the module. Having said that, to be fair to them, a bass pickup is a lot more sensitive than anything likely to be fitted in a car, so it is probably fit for purpose in their eyes.

 

One learns (or is reminded of things one has forgotten).

 

David

[Woodwind]

Just a quick congratulations on this project.

 

A year on how are you finding it now?

 

[Mottlefeeder]

On 03/12/2022 at 03:32, Woodwind said:

Just a quick congratulations on this project.

 

A year on how are you finding it now?

 

Thanks for asking.

I'm still very happy with it from a portability and sound point of view, but the guts hanging off the back is a bit unsightly, so I may rebuild it into a slightly larger case in order to tidy that up - but keeping the same enclosure size and tuning.

I took it down to the SW bass bash and it was liked by those who tried it, so I'm also happy that my enthusiasm for it is justified.

As far as volume is concerned, I've been asked to turn it down on a couple of occasions outdoors, so it is plenty loud enough to keep up with acoustic guitars, vocals and electronic drums going through the PA at appropriate volumes for a street market. A 7AHr 12v battery lasts about 4 hours, so taking a spare battery means I'm set up for the day.

David