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About Mottlefeeder

  • Birthday January 31

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  1. 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
  2. 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
  3. 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
  4. I have used this supplier for several cab grills and have been very happy with the results (he also offers a powder coating service) - http://www.speakergrills.co.uk David
  5. 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
  6. 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
  7. If you have the option of buying a pre-coated grill, I would seriously consider it. When you spray a grill, you spray from the front, and 70 % of your paint goes through to the newspaper behind. Then you spray at an angle so you don't miss the edges of the holes, and 70% of your paint goes through the holes... I reckon I spent £10-15 on paint for the last grill I painted. David
  8. 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
  9. 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
  10. 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
  11. Three steps forward, one step back... I was so engrossed in feeding battery and speaker cable through a small hole from the cab to the electronics that I forgot the speakers should be in parallel, and I wired them in series. Having fixed the speakers (8 screws) and the grill (6 screws), it all has to come off again to be rewired. On the bright side, I was able to use a 'normal 'amplifier to drive 10Hz though the 8 ohm load to run in the speakers. I've also fitted some sound absorbing polyester wadding in the front half of the speaker i.e. between the baffle and the brace, and also on the back wall. I can't see much reflection coming back from anywhere else. David
  12. A bit more progress - the garage is about 3-4 degrees C at the moment, so metal bashing/drilling/filing occurs in short sessions. David
  13. It makes perfect sense, but I hadn't thought it through other than for the bass frequencies. Thanks David
  14. The longer term plan was to use an FDeck clone HPF giving a fixed 12dB/Octave at 35 Hz and a further 12dB/Octave filter, variable between 35 Hz and 140 Hz. Some form of limiter/soft clipping was also envisaged, although I have yet to hear a simple compressor circuit that I could live with. I also had plans for some fancy power switching to allow one socket on the back to be used for power-in or battery-charge, or not in circuit when the internal battery was in use. I've decided to start with some simpler options, and swap out bits if I need to: first on the list was the HPF. The first Win ISD graph shows the response with no filter (red), 35Hz 12 dB/Octave filter (Blue) and 40Hz 12 dB/octave filter (Green). As you would expect, neither filter has much effect on the response of this speaker enclosure, with its relatively high tuning. Having said that, the battery sits within the speaker enclosure, and I had concerns that the enclosure response would vary depending on whether you fit no / a small / a large battery. In reality, the difference between the larger battery and no battery increases the volume by 7% which reduces the tuning by 2Hz, which results in response changes of less than 1dB, so it is not an issue. The second WinISD graph shows the cone movement at maximum power, using the same colours, and this shows that an HPF is definitely having an effect. The horizontal red line is the limit for controlled cone movement, not the limit for coil damage, so I probably don't need an HPF for speaker protection, but using an HPF should give me better cone control and a tighter sound. An added advantage is that by suppressing the lowest frequencies, the battery will last longer. I already had a preamp from a previous combo amp design, which included a variable HPF, so I used that as the basis for this interim preamp. It contains three op-amps, and I configured them as a Hi-Z input buffer, a variable HPF, and a variable gain stage to provide both volume control and compensation for differing outputs from basses. The last image shows the 'finished' (cobbled together) preamp and a basic schematic. David
  15. Hopefully - upgrading components on double sided PCBs was not in the original project spec. David
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