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Mottlefeeder

Combo lockdown project

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11 minutes ago, Stub Mandrel said:

I once made a little combo with four 4" speakers. Last year (or the year before...) I put a small port in it, which improved the bass response. On a hunch I removed most of the wadding inside and it improved the sound and volume - I think I had over damped it. It's worth experimenting, you can alwasy put it back in.

I've heard it said that you should snap your fingers in side the enclosure to check whether it sounds too dead. Whoever suggested that was not building cabs this small, so I think I'll leave the wadding in until I finish the rest, then experiment after that.

Moving on, my next problem is the power socket - 

Several years ago, I decided to standardise on Torberry connector for all my 12 v battery connections. Unfortunately, the panel-mount version leaves the plug protruding too far from the back panel, and likely to be broken, so I need to recess it somehow.

David

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You've swiped my battery charger!

Those are based on 'power pole' connectors - they are the dog's do-do's for high current 12V use, I use them for my astro kit. I made 3D printed panel holders that allow them to be recessed possibly a bit fragile for music kit without a redesign.

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5 hours ago, Stub Mandrel said:

You've swiped my battery charger!

Those are based on 'power pole' connectors - they are the dog's do-do's for high current 12V use, I use them for my astro kit. I made 3D printed panel holders that allow them to be recessed possibly a bit fragile for music kit without a redesign.

That battery charger is fine for larger batteries, but I think it only goes down as far as 7Ahr. I use a CTEK charger for my audio gear - 2.3Ahr or 7Ahr, and only take the caravan battery for all day busking sessions using larger amps.

Recessing the socket was easier than I thought it would be - I'm old school with regard to plastics, so it's back to model-makers plasticard for me. I bought a 2mm thick sheet to make the speaker cab vent. Hmmm, smell that solvent.

David 

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Another day of 3 steps forward and one back...

Sorting out the wiring between the amp recess and the cab, one of my cable clips sprang off and disappeared. And where did it land - inside the speaker cab, between the speaker chassis and the speaker cone! I couldn't shake it out so I've had the grill off, and the speaker out, and while I was in there, I took out the acoustic wadding, then put everything back together again. The battery holder is now securely blocking that hole so it can't happen again, and I'm back to wiring up the amp and preamp plate. I've less room than I thought I would have, so it's getting a bit messy.

David

 

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It works - in part...

Finished the wiring up, connected the electronics panel to the speaker cab, powered it up, started to turn up the volume and I heard bass. Turned it up a bit more and I got distortion and motor-bike-engine oscillation, so that's the first problem -  the preamp has way too much gain. Also, the gain pot crackles as it is turned, which suggests a dc component in the signal? I need to check that out.

Second problem occurs when I power off - I get a loud click/crunch sound from the speakers. It's not mechanical, so I need to work out which bit of the circuit is causing it.

On  a positive note, I'm playing a 5-string through a practice amp and it's not farting out on me.

Getting there.

David

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23 minutes ago, Mottlefeeder said:

It works - in part...

Finished the wiring up, connected the electronics panel to the speaker cab, powered it up, started to turn up the volume and I heard bass. Turned it up a bit more and I got distortion and motor-bike-engine oscillation, so that's the first problem -  the preamp has way too much gain. Also, the gain pot crackles as it is turned, which suggests a dc component in the signal? I need to check that out.

Second problem occurs when I power off - I get a loud click/crunch sound from the speakers. It's not mechanical, so I need to work out which bit of the circuit is causing it.

On  a positive note, I'm playing a 5-string through a practice amp and it's not farting out on me.

Getting there.

David

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It could be a number of things. The distortion could be due to toow much gain, it may also be the output coils saturating. The low cost Class D amps from Ebay/Aliexpress/Banggood etc often scrimp on comonents and that means capacitors and inductors are not up to scratch. Your preamp may also be oscillating and that could be the layout or the lack of an hf bypass  cap in the feedback of the opamp.

I assume you are using a single supply for the preamp? If the Opamp is a dual rail amp with the inputs biased to half rail (as with most onboard  preamps and stomp boxes) you may need more decoupling (smoothing on the lower part from memory), The draw on the negative rail on many opamps is higher than the positive rail and as the rails collapse you can get some really weird noises before the amp finally falls silent.

 

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2 hours ago, Chienmortbb said:

The draw on the negative rail on many opamps is higher than the positive rail and as the rails collapse you can get some really weird noises before the amp finally falls silent.

This, or anything else that causes the supply rail voltages to 'collapse' at different rates.

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6 hours ago, Chienmortbb said:

It could be a number of things. The distortion could be due to toow much gain, it may also be the output coils saturating. The low cost Class D amps from Ebay/Aliexpress/Banggood etc often scrimp on comonents and that means capacitors and inductors are not up to scratch. Your preamp may also be oscillating and that could be the layout or the lack of an hf bypass  cap in the feedback of the opamp.

I assume you are using a single supply for the preamp? If the Opamp is a dual rail amp with the inputs biased to half rail (as with most onboard  preamps and stomp boxes) you may need more decoupling (smoothing on the lower part from memory), The draw on the negative rail on many opamps is higher than the positive rail and as the rails collapse you can get some really weird noises before the amp finally falls silent.

 

I'm assuming the preamp is oscillating, although it has a capacitor across the gain pot in the feedback loop. All connections to pot terminals are shielded, with the shield insulated at one end, and connected to the earthing star point at the other. The pot casing probably isn't earthed, so that is something to look into. However, I am hoping that when I get the gain structure right, the problem will go away.

The amp module is a Chinese import to Amazon UK , and is disappointingly noisy, so it may be going back. Disconnecting it from the preamp gets rid of the shut-down noise, so the problem is definitely to do with the preamp.

The preamp is powered from the 12-13 volt battery, with a TLE2426 rail splitter to provide the midpoint. There is one 100uF capacitor upstream of the rail splitter, 2 x 0.1uF from the mid-rail to the supply and ground rails, downstream of it and also as close to the dual op-amp chip as possible, and a further 0.1uF between supply and ground as close the the second op-amp chip as possible. The circuit diagram above, showing the resistive splitter was an over simplification. The only other capacitor is 10uF on the output, as a DC blocker, so as the supply voltage collapses to zero, it may be discharging into the amp module input?

Given that the splitter is designed to source/sink 40mA in order to hold the rail centred, and the circuit does not have asymmetric smoothing capacitors, would you still be looking at more decoupling on the lower side of the mid rail, or should I be looking at a bigger reservoir capacitor on the preamp so that the amp dies first?

I'm at the limit of my electronics knowledge here, so I may be talking rubbish...

David

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'Motor boating' is usually caused by power supply issues, probably the power amp causing the preamp supply voltage to drop creating a slow feedback loop and low frequency oscillation that depends on volume.

See if you can isolate the preamp power supply better, perhaps add a sizeable electrolytic capacitor near the preamp and large inductor in the power amp supply. Ideally wire them 'star' style rather than daisy chaining them.

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I seem to have walked into a dead-end here, so I'm having a rethink.

The metalwork fits the existing amplifier module, but that appears to have quality issues, so I'm returning it and I'm loathe to replace it like for like. As @Chienmortbbcommented, the alternatives may also have quality issues, and it would cost me Import duties and processing charges to find out.

The next option is to use a physically larger, and now obsolete class D module that I already have, and start again with the metal bashing. The first problem with that solution is that I then do not have that amp module available for its previous role. The second problem is that if/when that module dies, I'm back to square one.

Option 3 is to fit the combo cab with a speakon socket and a power/charge socket (the battery is inside the cab), and to mount an amp module on a plate/power bulge on the back. This keeps the obsolete amp accessible to swap between old and new projects, and allows me to swap in a newer and/or more powerful amp if and when required.

Option 4 is to go upmarket and buy an Ice or Hypex or equivalent module and a power supply to run it from 12 V. However, for the cost of doing that, I can get a car stereo Class D amplifier that is designed to do the job, and built to be robust enough for a car environment, and I can sell it on when I stop using it. 

At the moment I'm going with option 3.

David

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14 hours ago, Mottlefeeder said:

I seem to have walked into a dead-end here, so I'm having a rethink.

The metalwork fits the existing amplifier module, but that appears to have quality issues, so I'm returning it and I'm loathe to replace it like for like. As @Chienmortbbcommented, the alternatives may also have quality issues, and it would cost me Import duties and processing charges to find out.

The next option is to use a physically larger, and now obsolete class D module that I already have, and start again with the metal bashing. The first problem with that solution is that I then do not have that amp module available for its previous role. The second problem is that if/when that module dies, I'm back to square one.

Option 3 is to fit the combo cab with a speakon socket and a power/charge socket (the battery is inside the cab), and to mount an amp module on a plate/power bulge on the back. This keeps the obsolete amp accessible to swap between old and new projects, and allows me to swap in a newer and/or more powerful amp if and when required.

Option 4 is to go upmarket and buy an Ice or Hypex or equivalent module and a power supply to run it from 12 V. However, for the cost of doing that, I can get a car stereo Class D amplifier that is designed to do the job, and built to be robust enough for a car environment, and I can sell it on when I stop using it. 

At the moment I'm going with option 3.

David

I've just rebuilt a small guitar combo as a bass combo and it's great but needs more power and ability to run off a battery would be great. So, I'm going through the same thought process, fortunately with @Chienmortbb on tap for advice. He's kind of talked me out of buying the module you used. I've been eyeing up some of the class D car amps as a ready built, robust alternative. I'm following your build with a lot of interest. However I have built an extension speaker to match the combo and of course you have to try it so I plugged it into my 'proper' bass amp. I'm now wondering whether just buying a TC BAM wouldn't be an easier and affordable option with a cheap inverter if ever I do need to be portable. However that wouldn't be very satisfying so I'm still thinking of your option 3 the car amplifier.

The only suggestion I can make is that I did consider using one or two 18v Li ion batteries as power supply, re purposed from my power tools. they are lighter than lead acid, you have a range of voltages available and if you could form sockets would be removable for charging. A +/-18V supply would let you get decent power into 8 or 4 ohms out of a bridged amp and you'd probably get an hour's playing or more out of them.

Anyway well done with the progress so far.

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2 hours ago, Phil Starr said:

I've just rebuilt a small guitar combo as a bass combo and it's great but needs more power and ability to run off a battery would be great. So, I'm going through the same thought process, fortunately with @Chienmortbb on tap for advice. He's kind of talked me out of buying the module you used. I've been eyeing up some of the class D car amps as a ready built, robust alternative. I'm following your build with a lot of interest. However I have built an extension speaker to match the combo and of course you have to try it so I plugged it into my 'proper' bass amp. I'm now wondering whether just buying a TC BAM wouldn't be an easier and affordable option with a cheap inverter if ever I do need to be portable. However that wouldn't be very satisfying so I'm still thinking of your option 3 the car amplifier.

The only suggestion I can make is that I did consider using one or two 18v Li ion batteries as power supply, re purposed from my power tools. they are lighter than lead acid, you have a range of voltages available and if you could form sockets would be removable for charging. A +/-18V supply would let you get decent power into 8 or 4 ohms out of a bridged amp and you'd probably get an hour's playing or more out of them.

Anyway well done with the progress so far.

The problem with a mains amp and an invertor is the standing current of the power supplies. My 150 W and 300 W invertors have 0.5-1.0A quiescent current, and my Ashdown MyBass takes another 1.0-1.5 A (from the battery via the invertor). That's OK if we are busking all day and using a 110AHr leisure battery, but it's a third of my built-in battery capacity gone before I've amplified anything. 

Looking at car amplifiers, something like a Kenwood KAC-M1814 (two pairs of bridgeable amplifiers) would give 2 x 45 W @ 8 ohms / 90 W @ 4 ohms which would put me up in PJB territory for a twin speaker rig, and only have one set of PSU losses. My existing Kenwood class AB car amplifier - 2 x 60 W @ 4 ohms - has a quiescent current of 300mA, but takes more than class D when amplifying. Hopefully the class D unit will give the best of both worlds.

I've considered lithium batteries, but the cost of new chargers, and a leisure battery capable of lasting all day, is more than I can justify. Also, the rest of band seem to have standardised on lead acid batteries and invertors, so there is merit in staying compatible with them.

David

Edited by Mottlefeeder
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Hi David, looks like you have considered everything. I have lots of 18v battery powered tools and associated chargers already so it probably makes more sense for me to think that way. One of the boards I was looking at needed a 36v split supply rail too so it seemed appealing and worth mentioning. 

Running on batteries is a secondary consideration for me too, it's the small and portable aspect of the project that appeals.

It's a real shame that amplifier didn't work out. That TDA3116 chip looks interesting and there are higher powered ones too but if the implementation on the boards is poor as you and john have found then they aren't worth touching.

Anyway keep on going this looks like a great project.

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Still thinking about my options, given that no amplifier other than the reject is small enough to fit the electronics bay, and I'm now thinking of just putting the preamp in the electronics bay, and making 'plate amplifiers' to bolt on the back of the cab. That would give me the versatility of connecting up an amp appropriate to the gig, but still give me the HPF I need to protect the speakers, Also, the rework on the electronics bay metalwork would only have to be done once.

David

 

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I could have posted this as a NAD, but it's probably not of interest to most of the members.

I've now taken delivery of a Kenwood KAC-M1814. It is designed as a booster for poor car stereo systems, so it delivers 45 W x 4 into 4 ohms (or limited in some way to 45W x 4 into 2 ohms) / or bridged pairs to give 90W x 2 into 4 ohms. The first picture is the new amp compared with the 2-channel class AB amplifier that still gets dragged out occasionally. That one delivers 75W into 8 ohms or 150W into 4 ohms. That rig, with its 7AHr battery will run for about an hour, and I carry it as a back-up when we do generator-powered gigs etc.

The KAC-M1814 has good and bad points:

+  it is small and powerful
+  it has a built-in variable HPF going down to 50Hz
+  it has a built-in low battery voltage cut-off
+  it has been designed and tested to operate in hostile environments
+  it is sold by a reputable company

-  the quiescent current is 800mA. so even at low volumes, it will only run for just under an hour or so on my smaller battery
-  the power output into 4 ohms x 2 is good, but into more conventional 8 ohm speakers it is only 45w per speaker when bridged.
-  mounted on the back of the speaker cab, it is functional rather than elegant.

Just for comparison:
Ashdown MyBass + mains invertor: Quiescent current 1.7A  / average current 2A  / peak current  n/a
Kenwood class AB: Quiescent current 440mA  / average current 2.2A  / peak current 5A
Kenwood KAC-M1814: Quiescent current 800mA  / average current 1.2A  / peak current 2A

So, with an amp in a metal box, and a requirement for a larger battery (the speaker cab is designed for up to 7AHr), my small rig is coming in at 9.6Kg, and should run for about 4 hours on the built-in battery. For an all-day busk, I would probably use the more efficient Basslite 10 inch speakers, and a large leisure battery.

Back on track.

David

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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

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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

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14 hours ago, Mottlefeeder said:

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

I would power a preamp with a separate, linear, power regulator - just based around a 7809 or 7812 to give it the cleanest possible power source.

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The gain pot in the feedback path can work but you really need a fully active volume control which (from memory) needs two opamps. A full explanation can be seen here. The acive volume control was designed by Peter Baxandall who also designed the bass/tredle circuits use almost everywhere.

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25 minutes ago, Chienmortbb said:

The gain pot in the feedback path can work but you really need a fully active volume control which (from memory) needs two opamps. A full explanation can be seen here. The acive volume control was designed by Peter Baxandall who also designed the bass/tredle circuits use almost everywhere.

I would just put a unity follower before the volume control.

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5 hours ago, Stub Mandrel said:

I would just put a unity follower before the volume control.

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

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6 hours ago, Chienmortbb said:

The gain pot in the feedback path can work but you really need a fully active volume control which (from memory) needs two opamps. A full explanation can be seen here. The acive volume control was designed by Peter Baxandall who also designed the bass/tredle circuits use almost everywhere.

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

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18 hours ago, Mottlefeeder said:

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

I had prepared a longer answer but decided it was teaching granny to suck eggs.

In it I proposed a unity follower to give a nice hi-Z input stage, then a conventional volume control, followed by inverting amp with a gain of about 10 or so all fed by linear regulated supply derived from eth main one 🙂

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1 hour ago, Stub Mandrel said:

I had prepared a longer answer but decided it was teaching granny to suck eggs.

In it I proposed a unity follower to give a nice hi-Z input stage, then a conventional volume control, followed by inverting amp with a gain of about 10 or so all fed by linear regulated supply derived from eth main one 🙂

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

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