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Basschat easy-build lockdown cab project


stevie

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On 05/03/2021 at 16:50, stevie said:

Here's a diagram of the crossover. It doesn't get any simpler than this. I've assembled the circuit using a (chocolate block) terminal strip. So - no soldering needed. I'll post a frequency response of the finished system and of the assembled crossover later today.

The Pulse 10 is wired directly to the input and this circuit connects to the HF unit.

 

xover.jpg

 

Seems my build is being hamstrung by a mis-labelled 4.7k ohm resistor and the horn is not getting any signal at all! That will teach me to build something without measuring the devices...

 

before I buy the correct 4.7ohm item, is there an ideal wattage? is 5w large enough? 

 

Cheers

 

  

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6 hours ago, EchiDna said:

 

Seems my build is being hamstrung by a mis-labelled 4.7k ohm resistor and the horn is not getting any signal at all! That will teach me to build something without measuring the devices...

 

before I buy the correct 4.7ohm item, is there an ideal wattage? is 5w large enough? 

 

Cheers

 

  

10 watts will be plenty, multiply the (~25W) RMS power rating of the compression driver by~ 1/3 (the ratio of 4.7 ohms / 8 ohms) and you will see a maximum power dissipation of about 8.25W, which is pretty much the absolute worst case that the resistor will see. In actuality, given the crossover frequency and the power spectrum density for bass guitar, it will be about 1/2 of this value.  I have used this formula and approach in commercial speakers for decades without any problems.

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While it's true that the resistor in the crossover isn't going to see much more than 8W, I'm a great believer in overspecifying resistors in power speakers, having seen too many burnt-out crossover boards in the past, where the resistors have overheated and taken everything else with them. The best place to get these in the UK is Blue Aran, where a 4.7 ohm 20W resistor costs 79p.

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1 hour ago, stevie said:

While it's true that the resistor in the crossover isn't going to see much more than 8W, I'm a great believer in overspecifying resistors in power speakers, having seen too many burnt-out crossover boards in the past, where the resistors have overheated and taken everything else with them. The best place to get these in the UK is Blue Aran, where a 4.7 ohm 20W resistor costs 79p.

Yes, a bigger resistor is very cheap. Back when I did some serious design on PA speakers I used the 20w axial ceramics (paralleling when I needed more safety margin) and they survived years of thrashing without a single failure.

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When using heavier parts (like 20 watt resistors), provided the smaller part is properly specified, I see more mechanical damage from the larger parts being not well secured to the PCB. With the larger resistors, they must be mechanically secured in addition to being soldered. All it takes is the cabinet falling over to cause such mechanical damage.

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On 11/04/2022 at 05:27, Stub Mandrel said:

Make sure the resistor is mounted so air can move freely around it.

It still needs to be mechanically secured or damage to the leads and/or PCB will result when the cabinet takes even a minor tumble. Most 20 watt parts have bumps in the bottom for this purpose, most 10 watt parts do not, and don’t need it with FR-4 PCBs.

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Another option is to use an array of smaller, lighter resistors, calculated to have the same overall resistance and power handling, but less subject to shocks. As the power is shared, they have less need to be 'in the air', and thus less prone to damage. Four 4.7 Ohm 5W resistors in series/parallel, for example. Just a thought. B|

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1 hour ago, Dad3353 said:

Another option is to use an array of smaller, lighter resistors, calculated to have the same overall resistance and power handling, but less subject to shocks. As the power is shared, they have less need to be 'in the air', and thus less prone to damage. Four 4.7 Ohm 5W resistors in series/parallel, for example. Just a thought. B|

Yes, this is sometimes done for mechanical reasons (especially with surface mount components, which obviously doesn't apply here). Smaller components have lower individual mass combined with a lower center of gravity.

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8 hours ago, agedhorse said:

It still needs to be mechanically secured or damage to the leads and/or PCB will result when the cabinet takes even a minor tumble. Most 20 watt parts have bumps in the bottom for this purpose, most 10 watt parts do not, and don’t need it with FR-4 PCBs.

 

My concern was that it doesn't end up surrounded by acoustic wadding.

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This is all a bit of a strawman. The instructions for assembling the crossover are on page 3. We're not using a PCB; we're using a chocolate block. The resistor is fitted against the chocolate block and taped (or tie-wrapped) to it, and the chocolate block is then screwed to the cabinet. If you wire it up this way, there is no chance of the resistor wires breaking.

 

The heaviest component isn't actually the resistor - it's the coil. And the advice there is to screw the coil directly to the cabinet.

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7 hours ago, stevie said:

This is all a bit of a strawman. The instructions for assembling the crossover are on page 3. We're not using a PCB; we're using a chocolate block. The resistor is fitted against the chocolate block and taped (or tie-wrapped) to it, and the chocolate block is then screwed to the cabinet. If you wire it up this way, there is no chance of the resistor wires breaking.

 

The heaviest component isn't actually the resistor - it's the coil. And the advice there is to screw the coil directly to the cabinet.

 

My comments were general in nature, all heavy components need to be securely mounted regardless of how they are used.  I would not recommend using tape to mount a component that's likely to get hot. 

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  • 3 months later...
  • 7 months later...

Resurrecting this as I'm looking to put two of these cabs together. Have the drivers so need the wood and crossover components. Never did I think it'd be so hard to find decent plywood in Northern Ireland. Thought I was sorted with a company that would cut panels of 12mm Baltic Birch to size and post for a reasonable price (as in sub £100 for the 2 cabs), but of course they won't post to Northern Ireland. Anyone got a lead on somewhere that will? While I COULD buy some rubbish plywood locally and do the cutting myself, buying in "kit" form would increase the likelihood of me actually doing it 10-fold! 

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looking at the Woodsheets site it looks good. They seem to be using high quality plywoods and from their offer are CNC machining from consistent quality materials, and charging accordingly. One of the indicators is that they are using sheets with multiple cores. The cheaper boards generally use fewer thicker plies. Let us know how you get on with them and if they provide a good service others will want to go to them. Looking forward to your build :)

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On 25/03/2023 at 08:22, Phil Starr said:

looking at the Woodsheets site it looks good. They seem to be using high quality plywoods and from their offer are CNC machining from consistent quality materials, and charging accordingly. One of the indicators is that they are using sheets with multiple cores. The cheaper boards generally use fewer thicker plies. Let us know how you get on with them and if they provide a good service others will want to go to them. Looking forward to your build :)

 

It MAY look good... But looks like I'll never know. "Birch plywood is out of stock" email just received...

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