31-01-2019, 10:49 PM
Hi Mike,
That's a lovely bit of woodwork!
I'm guessing you prefer to use the Sinclair module because it will be quick and easy to implement. But if you're building tone controls as well, it's not much extra work to bung a power amp IC on there while you're at it. Looking at the schematics of that Sinclair, it's no wonder they were unreliable, given the complete lack of thermal feedback in the biasing circuit. And I know that you didn't mean it, but there's no way in this universe that they'd get down to 0.001% THD+N
The ICs I suggested earlier would be OTT for that drive unit. As you say, a couple of watts will do. There are loads of ICs that will do that. Years ago I used a lot of the once-ubiquitous TDA2002 - they will happily run from the single supply you have, and are easy to use. As well as the supply rail, the impedance of the speaker will determine the exact power output, naturally. Failing that (or equivalent), old PC speakers can often yield a suitable amplifier - ready built, of course. I recently took the time to desolder a TDA1517 from a 1990s soundcard before it went in the WEEE skip. With today's instant access to datasheets, old ICs can easily be given a second life.
In case you haven't already seen it, I built an "active" speaker a while back. If nothing else, you might find it an interesting read. The final instalment was added when I got a measurement setup - you can see how I tailored the frequency response to give a reasonable response from the small drive unit: https://www.markhennessy.co.uk/activespeaker/
Incidentally, I still haven't got around to properly measuring those 4" drive units that you kindly sent me a while back. A quick test looked very promising indeed. I plan to build another bench amp with several inputs and a built in mixer, based around one of those. I need to measure the T-S parameters to work out what kind of box they'd work best in first. No idea when I'll get the chance though...
Meanwhile, I started replying to some of the earlier posts before I starting putting the kids to bed. Now that we know about your specific project, a lot of it is redundant, so it has been edited down considerably.
---
The definition of an active speaker is simple: the crossover is implemented at line level, and there is a power amplifier for each of the frequency bands following on from this. A single power amp is a powered speaker. Yes, I (accidentally-on-purpose) made that mistake when I titled the article linked to above. I'll probably correct it in due course.
Meanwhile, there are many additional things you might choose to do while you're at it, including motional feedback, active servo technology (Yamaha), or a Linkwiz transform, but all of these are separate to the basic question of active vs powered vs passive. You could choose to do any of the above to either a powered or active design.
"Hybrids" exist - a good example is the Dynaudio Air 20, which is a three-way that uses a pair of power amps - one for the woofer, and the other for the mid and tweeter. The crossover between the latter is done using a simple series inductor for the midrange and a series capacitor for the tweeter. The low level crossover is actually implemented using DSP. They need occasional reboots.
Likewise, a "hybrid" powered speaker could use a passive crossover to split between LF and HF, but the BSC could be performed by a line-level bass boost circuit. That reduces the size of the inductor in series with the woofer by typically a factor of three, which equates to a significant cost saving.
The main benefit of MFB is to reduce harmonic distortion at low frequencies. If you use it to get bass extension, then you trade output level at LF. If you're smart, you do that dynamically (I've played with that sort of thing on an experimental sub years ago).
We're not terribly bothered by LF distortion, which is why MFB and similar schemes have failed to achieve significant market presence. Not helped by the fact that good drive units today have lower THD than the units available back in the '70s - motor design has come a long way... Far better to concentrate on mid-range distortion, where the ear is most sensitive.
That's a lovely bit of woodwork!
I'm guessing you prefer to use the Sinclair module because it will be quick and easy to implement. But if you're building tone controls as well, it's not much extra work to bung a power amp IC on there while you're at it. Looking at the schematics of that Sinclair, it's no wonder they were unreliable, given the complete lack of thermal feedback in the biasing circuit. And I know that you didn't mean it, but there's no way in this universe that they'd get down to 0.001% THD+N
The ICs I suggested earlier would be OTT for that drive unit. As you say, a couple of watts will do. There are loads of ICs that will do that. Years ago I used a lot of the once-ubiquitous TDA2002 - they will happily run from the single supply you have, and are easy to use. As well as the supply rail, the impedance of the speaker will determine the exact power output, naturally. Failing that (or equivalent), old PC speakers can often yield a suitable amplifier - ready built, of course. I recently took the time to desolder a TDA1517 from a 1990s soundcard before it went in the WEEE skip. With today's instant access to datasheets, old ICs can easily be given a second life.
In case you haven't already seen it, I built an "active" speaker a while back. If nothing else, you might find it an interesting read. The final instalment was added when I got a measurement setup - you can see how I tailored the frequency response to give a reasonable response from the small drive unit: https://www.markhennessy.co.uk/activespeaker/
Incidentally, I still haven't got around to properly measuring those 4" drive units that you kindly sent me a while back. A quick test looked very promising indeed. I plan to build another bench amp with several inputs and a built in mixer, based around one of those. I need to measure the T-S parameters to work out what kind of box they'd work best in first. No idea when I'll get the chance though...
Meanwhile, I started replying to some of the earlier posts before I starting putting the kids to bed. Now that we know about your specific project, a lot of it is redundant, so it has been edited down considerably.
---
The definition of an active speaker is simple: the crossover is implemented at line level, and there is a power amplifier for each of the frequency bands following on from this. A single power amp is a powered speaker. Yes, I (accidentally-on-purpose) made that mistake when I titled the article linked to above. I'll probably correct it in due course.
Meanwhile, there are many additional things you might choose to do while you're at it, including motional feedback, active servo technology (Yamaha), or a Linkwiz transform, but all of these are separate to the basic question of active vs powered vs passive. You could choose to do any of the above to either a powered or active design.
"Hybrids" exist - a good example is the Dynaudio Air 20, which is a three-way that uses a pair of power amps - one for the woofer, and the other for the mid and tweeter. The crossover between the latter is done using a simple series inductor for the midrange and a series capacitor for the tweeter. The low level crossover is actually implemented using DSP. They need occasional reboots.
Likewise, a "hybrid" powered speaker could use a passive crossover to split between LF and HF, but the BSC could be performed by a line-level bass boost circuit. That reduces the size of the inductor in series with the woofer by typically a factor of three, which equates to a significant cost saving.
The main benefit of MFB is to reduce harmonic distortion at low frequencies. If you use it to get bass extension, then you trade output level at LF. If you're smart, you do that dynamically (I've played with that sort of thing on an experimental sub years ago).
We're not terribly bothered by LF distortion, which is why MFB and similar schemes have failed to achieve significant market presence. Not helped by the fact that good drive units today have lower THD than the units available back in the '70s - motor design has come a long way... Far better to concentrate on mid-range distortion, where the ear is most sensitive.
