Ideas and projects that don’t fit in the other categories
- Automatic power off circuit for active speaker
Thinking about ways to automatically turn off the PSU of an active speaker. I have two options on my mind:
- Locally switching PSU, with a floating input detection circuit
- Remotely switching PSU, with a power signal line from audio source
Option 1 has the advantage that if it’s not connected to anything, the speaker will turn itself off. Also, no remote power signal is needed, so it can be connected with just a mono jack. However, to switch it off remotely, the source has to disconnect its output. An audio signal detection circuit would prevent the need for that, but I don’t like to turn off the amplifier when there is no signal e.g. between songs. This option needs a little battery to keep the detection circuit powered.
Option 2 has the advantage of being a little simpler, especially on the amplifier side. There is also no need for a battery. It does however need a stereo jack. Disconnecting input turns on the PSU, but I guess a simple “on/remote/off” power switch next to the jack could solve that.
1. Locally switching PSU, with a floating input detection circuit
Idea is to inject a miniscule DC sense signal (e.g. 4V through a 1MΩ inject resistor) onto the amplifier input, and use a comparator to measure impedance. If impedance is very high (input is floating so voltage is high, especially if amplifier has a coupling capacitor), keep PSU off, if impedance is lower (input connected so voltage is low), turn on PSU.
To prevent audio signals turning off the PSU, filter out the AC at the comparator (e.g. low pass using 1MΩ sense resistor plus 1µF capacitor). I doubt 4V DC at ~4µA would do anything to the source output so I don’t think I need coupling capacitors there.
Comparator would power an SSR that turns on/off the PSU. To power the sense signal and comparator as well as SSR, use a small Li-Ion battery. Float the battery from PSU using an isolated buck converter when powered on. Preferably use a protected battery. If I stay under 100µA, a 2000mAh 18650 could last over 2 years without power. Unfortunately, the most widely available comparator (LM393) draws closer to 400µA which brings battery life down a bit. Could run it with a timer and latch circuit to bring down average current to maybe 100µA, but that seems like a lot of work. Either get a lower power comparator, or just use a protected cell and accept that once every couple of months the amplifier needs to be turned on.
Would have to figure out how to drive the SSR module directly from the comparator without much current draw in off state.
To remotely switch off from the source, the source would need to lift its outputs. Putting anything in the signal path introduces noise, but something like a TS5A23157 analogue switch module with relatively low R_on and very low distortion should be inaudible.
Parts list:
- 18650 protected cell
- isolated buck converter set to battery float voltage + output diode
- LM393 comparator module (plus SSR drive circuit)
- SSR module
- 1MΩ Resistors and 1µF capacitor
- TS5A23157 module for source
2. Remotely switching PSU, with a power signal line from audio source
Use a stereo jack cable; TRS with control signal on the ring. 5V turns the active speaker off, 0V turns it on. In active speaker, use 5V NC SSR board in two modes:
- Off / Control = 5 V:
Power the SSR board directly from the 5 V signal line. Current draw is negligible (few µA or less, just leakage). - On / Control = 0 V:
Signal line can’t supply 5 V any more. Instead, power the board from a small 42 V→5 V DC converter.
Buck converter must be isolated to prevent ground loop, and use Schottky diodes to prevent backfeeding into the control signal line. Use a capacitor by the SSR large enough to bridge the gap between powering on PSU and buck converter giving 5V.
There should never be 5V and signal at the same time through the wire, so no added noise expected as long as 0V is really 0V. I guess I can use an optocoupler, another isolated 5V buck converter and a pulldown resistor to audio gnd, to prevent injecting noise onto 0V. To prevent damage from DC with wrong connection, limit 5V output to just enough current to keep the SSR board powered.
Alternatively, directly inject 5V DC control signal onto audio signal. This allows using a mono jack cable instead, and again, audio and 5V should never be on the line at the same time. However, this would require filtering out AC at the SSR, and DC at both the amplifier and the source, and might be more difficult to prevent ground loops and noise. Also, higher chance of damage if connecting a different amplifier. Separate wire seems like a much better idea.
Reflection, issues
Option 1 seems elegant but very sensitive, e.g. to floating ground and false triggering. Switching module at source will likely add pops/noise when turning off amp.
Option 2 seems more robust, but need to think about wiring. For instance, can the source handle connecting a mono cable, shorting the 5V to gnd? I guess with good current limiting, that shouldn’t be a problem.
Ultimately, both options seem like a lot of work and can potentially introduce noise, without solving any real problems. Is it really so bad to have to get up and control a physical on/off switch, instead of a software one?
- “Subwoofer” conversion of bass practice amp
Thinking of designing a plug for my 2×6″ horn reflex bass cabinet, so I can use it as a closed cabinet subwoofer in a 2.1 system with software EQ.
Checking Hornresp, if I plug the end of the horns, I get a closed cabinet with a Qtc of 0.70. Max SPL (quarter space):
Original Closed 16L 30 Hz 74 dB 90 dB 40 Hz 85 dB 95 dB 50 Hz 94 dB 99 dB 60 Hz 102 dB 102 dB 100 Hz 126 dB 111 dB That’s actually a useful level, I think? Max SPL is mostly independent of volume so I don’t have to think too much about the shape of the plug(s), driver volume etc. It would however need a lot of EQ to get some bass out the 12db/oct declining response. Actually a 2nd order low pass at 30 Hz might already give a decent response as well as could function as a 2nd order crossover around 150 Hz? Phase is then -90 degrees around 150 Hz, might have to reverse polarity.
Let’s look at power. Assuming the above active EQ, and aiming for max excursion at 30Hz, power requirement would be 60W total into 4W. A TPA3116D2 or TPA3221 module according to their datasheets should be able to reach that at 24V, but without headroom – a TPA3255 module >32V might be a better fit. Or TPA3223/TPA3250/TPA3251 at 32-36V.
- Kallax bass speaker with passive radiators
Idea: dedicated woofer for 2.1 system. Opposing drivers to cancel vibrations.
Kallax compartment is about 335x335x390mm. A 335x335mm baffle could just about fit an 8″ passive radiator and a 6.5″ driver, but it’s very close – 8″+5″ would be a better fit. Assuming 12mm plywood, internal volume using maximum dimensions is 3.11×3.11×3.66=35.4L. Subtracting driver volume, that’s about 34L cabinet.
Let’s run some options in Hornresp (including max SPL, quarter space radiation)
Driver (2x) Drone (2x) F3 F6 F12 SPL@35Hz SPL@60Hz SB16PFCR25-4 SB20PFCR-00 54 Hz 42 Hz 33 Hz 100 dB 110 dB Visaton W130X SB20PFCR-00 48 Hz 33 Hz 29 Hz 97 dB 103 dB Dayton DA135-8 SB20PFCR-00 47 Hz 37 Hz 29 Hz 95 dB 101 dB W5-1138SMF SB20PFCR-00 46 Hz 35 Hz 29 Hz 107 dB 113 dB W5-1138SMF none 66 Hz 46 Hz 29 Hz 101 dB 110 dB SB20PFC30-4 none 69 Hz 54 Hz 36 Hz 104 dB 113 dB Ok so assuming I can do some EQ in the low end to bring up the response, for the same money I can better just put in a pair of more powerful drivers and no passive radiators.
Looking at smaller drivers, I’m intrigued by a 2×5″ SB13PFC25-4 sub that could potentially do 91+ dB (quarter space) down to 30 Hz at xmax and would need less than 30W for it – at only 6.1L internal volume. That could fit in a 22cm cube, including space for a TPA3221 amplifier and a laptop charger to power it. But if I have the space in a Kallax compartment, 2×8″ SB20PFC30-4 looks great, much louder so probably cleaner at lower levels, and at Qtc of 0.75 should be relatively easy to EQ and crossover.