Mottlefeeder

And replied. David

And I'm back in the room. My basses are all 5-string, and vary from two-pup designs to a bridge Piezo acoustic bass, so I'm not planning to do any further analysis on output vs frequency. I'm in the right ball park so I'll stop there. So, firming up the design, with an 18dB/octave HPF set to 60 Hz, and limiting the input power to 20 W per speaker, and using a vent with a 2:1 cross section, I get an 80 mm x 40 mm vent 160 mm long giving me a maximum air flow of 15 m/s at 65 Hz. So far, so good. For an overall box depth of 305 mm, with an amp on the back, and a grill on the front, I have an internal depth of 220 mm for a port of 160 mm. Is the port end too close to the back wall? David

Thanks for that - I'll be away for the next few days, so I may not respond to further comments. David

Just to make sure I understand - the output from the pickup is 6dB down at 30-40 Hz because there isn't as much signal to pick up, so to model that into a port calculation, if I have say 10 watts at 60 Hz, putting me on the limit of port air-speeds, I should check that at 30 Hz, the port can cope with 4 watts without reaching that air-speed limit? Talking of compromises, and refining my design slightly, I was aiming for a similar sound and volume to my 8 ohm Eminence basslite S2010 in a small cab, taking about 10 watts from one channel of a stereo amp. This two speaker box will do it, with two 4 ohm drivers taking 20 watts each - i.e. half the size of box , but taking 4 times the power to reach the same volume. The next step up in volume would be to add a second S2010 and run one per channel, taking 20 watts in total. But the increase in output is only 3dB, and I am now carting a mini stack around. The alternative next step is to use my bigger battery-powered amp - 80 watts into 8 ohms / 150 watts into 4 ohms. At this point, the single S2010 will deliver 114 dB but the twin speaker box, with the drivers in series, will only deliver 108dB, so it is 6dB quieter. Beyond that, a pair of S2010s will deliver 116 dB, but I would also be carrying a bigger battery (27 kg!), so it would no longer be a portable rig. On that basis, there is no point in designing the twin speaker box to take more than 20 watts per speaker - if I want it louder, there are better ways to do it. David

If the port calculations are based on area times length, that implies that the volume of air is the important bit? Yes, a sharp 90 degree corner will cause turbulence, but the prefabricated pipes have radiused corners to reduce that. David

And I'm back in the room... My existing 10 inch speaker box has a thin slot port, but very low air speeds, which may be why I have never heard any problems due to turbulence. Moving away from a thin slot in this design, to something like 1:4 or 1:3, and big enough to bring the airflow down below 17 m/s, gives me a port that is longer than any of the box dimensions, so I'm now looking at a port turning a corner inside the cab. One option is a drainpipe with a 92.5 degree angle, and the other is a rectangular extractor fan duct with a 90 degree bend. Should I be aiming for a 'middle of port' length as per WinISD, or a port volume as per WinISD? And are there any rules of thumb for compensating for whatever problems are caused by bends in ports? Thanks David

Thanks for your observations and effort on my behalf. I'm on holiday at the moment and have no access to WinISD but I will be checking it out when I get back. David

Thanks for that - a commercial 3:14 ratio of port height and width is a useful benchmark. Stevie 's suggestions of 69mm diameter or 50:90 don't sit so well in the baffle size I had in mind, although that may change as the design matures. David Correction - since there are two ports, the 'combined port' ratio is 6:14, or slightly over 1:2.

My original plan was to use a slot port between the two speakers, so it would double as bracing for the cab sides, so I may have to rethink that. My existing rig uses Eminence Basslite S2010 speakers each in a 0.9 cu ft cab with ports tuned to 60Hz (based on an Eminence design). They used twin 3 inch ports, and I went for a 12 inch wide 1 inch high 7.5 inch long port instead. I haven't had any problems with my sound at the volume I play, so I was aiming to get a similar response curve with this design. I play a 5-string with a low B, and often use an HPF at about 50-60 Hz, but I tend to go for punch rather than rumble, so that may be why I haven't heard any problems. David

Thanks for that - I'll check it out with the new figures. Win ISD suggests 330cc's per speaker, so with an internal width of 150 mm, and the amp and battery strapped on the back it's looking like this - Briefcase speaker calculations.pdf

I had in mind to build a 'Joe Blogs Briefcase' using FaitalPro 5 inch drivers, but when I feed their parameters into WinISD I'm getting port lengths of over 300mm which does not look right. Can anyone be bothered to input the TS parameters for the FaitalPro 5FE120, for a cab with two 4 ohm drivers and a volume of 9900cc tuned to 70Hz with a 12dB/octave HPF with a 30Hz turnover. My clculations give - For 30 watts I need a slot port 150 mm*20 mm by 132 mm long giving me 17 m/s air flow. For 80 Watts I need 150 mm*35 mm by 260 mm long giving me over 16m/s air flow. For 150 Watts (my maximum amplifier power) I need a slot port 150 mm*45 mm by 345 mm long giving me just over 17m/s air flow. Do you agree with these figures? Alternatively, can anyone tell me how they split the power between the fundamental and the harmonics so that they don't have to design for full power at every frequency? Thanks David

Rod Elliott recommends a connection to ground via a 10 ohm resistor bypassed with a 0.1 mfd capacitor. Covers most situations and doesn't not need a switch David

I'm hoping for something modular so that I can fit the amp into the speaker casing, and power it from 12 Vdc when there is no supply available, or 230 Vac when there is. In the briefcase, PJ drops the mains down to 12 Vdc and feeds that, or the battery into a +/-X volt SMPS to power his class D amp. On 12 Vdc you have one voltage conversion, but on mains you have two in a row. It's inefficient having two conversions, but with mains power it is not that important. Looking at what is readily available, you can use a pure sine inverter with a 230 Vac rig, but that is one voltage conversion on mains, and two on battery, so it is inefficient and it is using up your battery power. As an example, my Ashdown MyBass Mk1 is Class D and with an inverter, it draws about an amp at 12 Vdc while idling, which is about a third of the capacity of an easy carry 7AHr battery. You can also use an automotive booster amp with an SMPS feeding a class AB amp, which has a lower quiescent current, but takes more on peaks. In summary, an automotive class D amp fed from a 12 Vdc SMPS would be the optimal solution. Unfortunately they don't do them in smaller power output units, and with a bigger unit you are back into the wasted power problem again. David

Apologies for the slight thread diversion, but are any of you Class D module users aware of a 50-100W Class D module with a separate power supply, and a 12V dc SMPS to suit, to allow something like a PJ briefcase design to be built. The nearest I have got is a car stereo booster amp with a SMPS and a pair of bridged Class AB amps. Thanks David

Lead acid batteries refer to the basic chemistry, in the same way that 'alkaline' or 'Lithium something or other' does. Lead acid technology is heavy, but also considerably cheaper than the other options. Within the family of lead acid batteries there are minor differences in design depending on what the battery is intended to be used for. Car starter batteries are intended to be kept fully charged unless they are cranking the engine but golf cart batteries are intended to be run down to 'empty' and then recharged. Batteries designed for discharge and recharge are known as deep cycle batteries. Using the wrong battery will not damage the rest of your gear, but it will result in the battery having a relatively short life. David

I have an Ashdown MyBass mk1 500w amp, and I can run it from a 'modified sine wave' invertor rated at 300 W. The invertor has a continuous rating, and the amp is never used at full power, so it works. That setup draws about 2 Amps from a lead acid battery, so for a half-brick-sized 7AHr battery you can expect 2-3 hours of use before the voltage droops and the invertor shuts down. That type of invertor uses a very crude approximation to the mains waveform, and it is rich in harmonics, so it may cause a buzz through your gear, and it may invalidate your amp warranty if you have a problem. A better solution is a pure sine wave invertor, which does not generate a buzz, does not invalidate your amp warranty, but costs a bit more, £80 vs £30 for a modified sine wave invertor of comparable power. An alternative would be to buy a car stereo booster amp for about £50 and run that directly from the battery. Current designs from the reputable manufacturers will deliver 150 W into 4 ohms, or about 75 W into 8 ohms. An active bass will provide enough output to drive it directly (but not drive it to full power - for that you will need a booster, or an FX pedal with some gain). Something like https://www.caraudiocentre.co.uk/product_m-pioneer-gm-a3702_p-40132.htm would probably do (mine was a Kenwood, no longer in production), but you would need to check the maximum sensitivity was about 200mV. Anything complying with CES 2006 will specify RMS values, regardless of what the marketing department painted on the lid or put as a headline. David

You also cut away some wood and added a plastic tube... David

I was wondering why you had a two channel balanced feed circuit, and then I read this bit and it all made sense. Having said that, Rod Elliott has modified the DI output on his bass pre-amp design so that it could tolerate anyone accidentally applying phantom power to it - series resistors and zener clamps if I recall, to take care of the inrush. David

The ASX pcb has 400mA fuses on the aux supplies, so you may be able to replace them (SMD fuses?), or short them out and make sure you have smaller value fuses downstream. David

I'm showing my age - the last drive-in movie I went to was in the US in 1975. David

My understanding is that the 2 m rule is for short duration exposures, and if you spend a long time with people, e.g. a rehearsal, 2 m spacing will not help you. As others have said, a drive in would be a safer option, but if you had to hang a speaker on your window, you could pick up infections from that, or if you leave your windows open, then you lose the 2 m when someone (drunken idiot) walks past. David

Another couple of options would be to use a 230V ac fan, or a 12 v fan on a capacitive divider and bridge rectifier as here (shudder) David

Have you checked that your preamp will not be affected by the fan EMF if you have a common supply. I put a fan on a 12 v battery powered system and found the preamp became unusable. I now have to use a pedal preamp fed from its own battery. David

OK, A bit more information. It's two almost separate circuits - Firstly, I use a passive bass and long cables/not hi-z inputs, so it is a simple bass preamp, giving a hi-z input, an HPF, and an output to FOH or backline amp. That's two jack sockets on one side, (one in and one out) and a gain selector: 0dB or 10dB. Secondly, I usually create a headphone monitor mix with a small mixer on top of the back-line amp. I take a feed from the main PA, and a feed from my bass amp, and an ambience mic, and feed them back to me in stereo (acually two channel mono since I put me on the left and everything else on the right). With this box, I can select left only, right only or both and I mix them down to mono and feed them to the headphones. That's the other two jack sockets, (stereo in and headphones out) and the selector switch. Since I have the bass signal in the same box as the headphone mix, I added the switch in the battery compartment which gives me the option of substituting my live bass sound for the left monitor channel, so I can take a mono feed from the FOH mixer, or a stereo feed from a Zoom or Tascam recorder, and still control 'me or them' at my end of the cable. I use a 'figure of eight' twin microphone cable to connect my bass/IEM to my bass amp/monitor mixer. It's reasonably discrete and keeps the two sets of signals apart. The compromise is that it isn't an instrument cable, so I need the hi-z buffer - that was the starting point. David