Hi all,
I make small funky speakers using eBay chip amps, and have really been enjoying it! There's a 50w/50w amp board with a single volume knob and bluetooth that Ive used a lot - mainly because it will go from 5v to 24v so can be powered from a variety of sources.
However, I find myself wanting to add a single mono sub to my designs but its not easy to see just HOW i can do this without things getting much more complicated. I know there are 2.1 amp boards out there (and for not much more money that the one I use) but they don't work down to 5v, which I really want.
So, how can I make a 2.1 crossover? I always thought such a thing MUST exist as I had a passive sub back in the day that had stereo inputs and stereo outputs, but I can't find such a thing "ready made" for the life of me.
Any ideas for a circuit?
Regards,
Jim
I make small funky speakers using eBay chip amps, and have really been enjoying it! There's a 50w/50w amp board with a single volume knob and bluetooth that Ive used a lot - mainly because it will go from 5v to 24v so can be powered from a variety of sources.
However, I find myself wanting to add a single mono sub to my designs but its not easy to see just HOW i can do this without things getting much more complicated. I know there are 2.1 amp boards out there (and for not much more money that the one I use) but they don't work down to 5v, which I really want.
So, how can I make a 2.1 crossover? I always thought such a thing MUST exist as I had a passive sub back in the day that had stereo inputs and stereo outputs, but I can't find such a thing "ready made" for the life of me.
Any ideas for a circuit?
Regards,
Jim
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hi,
solution 1:
take a sub-plate amp (or similar) and connect it to the output of your amp. some plate amps filter the mid-high speakers passively, some don't.
you can also build the "plate amp" yourself by summing the signals with resistors, including an active crossover and connecting an amp.
solution 2:
you take an active sub-crossover and insert it between the volume pot and your existing amp (you need to split the pcb up or desolder some connections). then use an additional amp for the sub-output. by doing so you can still use the existing volume pot.
solution 3:
you connect your active sub-crossover before your existing amp and use an added amp for your sub. the volume pot on the existing amp will now only regulate the sat-speaker volume, so you may need another volume pot before the active crossover (or included there).
solution 1:
take a sub-plate amp (or similar) and connect it to the output of your amp. some plate amps filter the mid-high speakers passively, some don't.
you can also build the "plate amp" yourself by summing the signals with resistors, including an active crossover and connecting an amp.
solution 2:
you take an active sub-crossover and insert it between the volume pot and your existing amp (you need to split the pcb up or desolder some connections). then use an additional amp for the sub-output. by doing so you can still use the existing volume pot.
solution 3:
you connect your active sub-crossover before your existing amp and use an added amp for your sub. the volume pot on the existing amp will now only regulate the sat-speaker volume, so you may need another volume pot before the active crossover (or included there).
As an aside, you do know that the available maximum power falls dramatically with lower supply voltages, yes? With a 5 volt supply and 4 ohm speakers, that "50w" amp can only deliver about 1 watt RMS to each speaker.
(And that's assuming that each channel is actually two amps in bridge mode, which is typical for these sorts of class-D chips nowadays. If not in bridge mode, each channel can only deliver about one quarter of a watt with a 5 volt supply and 4 ohm speakers.)
The easiest way is to simply sum the left and right channel input signals to make a low-level mono signal, and feed that to a powered subwoofer.
This will work if the powered subwoofer has its own adjustable low-pass filter (most do), and you carefully adjust this filter so that the sub blends well with your woofers.
The woofers themselves will continue to operate as before, down to the lowest frequencies they can handle; they are not high-passed in any way by this simplest of circuits.
You can crudely sum the left and right channels using nothing more than two resistors, but this is not a good idea for several reasons. A much better way is to use an op-amp mixer stage.
If you're unfamiliar with the textbook op-amp mixer circuit, take a look at the "Op-Amp Audio Mixer" section of this page: Op Amp Summing Amplifier: Virtual Earth Mixer >> Electronics Notes
If you want to run all the way down to 5 volts power, you will need an op-amp capable of operation on a single-rail 5 volt supply. These days there are op-amps designed to run on a single lithium cell (4.2 volts nominal), and one of those will work for you.
Beyond the above simplest of simple subwoofer crossovers, things start to get rapidly more complex. But if you're building little 1-watt speakers, you probably don't need a more complicated subwoofer crossover.
-Gnobuddy
So basically I was thinking along the lines of a pair of two-way woofer/tweeter crossovers, except the woofer outputs are summed to a single mono output.
Is that a complex beast?
Sound quality isn't imperative here. I find that by using good speakers (Faital Pro) as opposed to off brand rubbish, I can make things that sound five times better than the "competition" anyway.
Is that a complex beast?
Sound quality isn't imperative here. I find that by using good speakers (Faital Pro) as opposed to off brand rubbish, I can make things that sound five times better than the "competition" anyway.
it is, if passive, as far as i know. and it might be lethal for your amp.
but you can simply use a double coil woofer.
each channel goes through passive crossover an into one of the coils. visaton W130 X for example.
you will notice however that coils for passive subwoofers are immensely huge and extremely expensive. that's why most subwoofers are active.
edit: one way might be to build a bandpass-woofer that will acoustically act as "crossover". you are driving the woofer and mid/high speakers in parallel, in this case, with resulting reduced total impedance.
edit2: or this one: Skar Audio EVL-65 - 6.5" Subwoofer
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I mean, I'm using the term "subwoofer" but perhaps that's slightly misleading.
My speakers generally use the 3" or 4" Faital Pro full range drivers. They are bloody BRILLIANT, but of course, they don't go that low.
So, adding a 5" or 6" woofer would change things up significantly, but it's not like we're talking mega power here.
My speakers generally use the 3" or 4" Faital Pro full range drivers. They are bloody BRILLIANT, but of course, they don't go that low.
So, adding a 5" or 6" woofer would change things up significantly, but it's not like we're talking mega power here.
Moved to Multi-Way.
The dual voice coil woofer certainly looks the way to go.
Commercial dual voice coil woofer/satellites crossovers look like this: Order the Subwoofer Crossover 130 Hz - SoundImports
Alternatively, bass in recordings is not separated between the right and left channels below around 300 Hz, so you could experiment connecting a woofer via a low pass filter on one channel only without risk of damage to your amplifier.
Commercial dual voice coil woofer/satellites crossovers look like this: Order the Subwoofer Crossover 130 Hz - SoundImports
Alternatively, bass in recordings is not separated between the right and left channels below around 300 Hz, so you could experiment connecting a woofer via a low pass filter on one channel only without risk of damage to your amplifier.
Problem is, that crossover costs nearly as much as the dual coil visaton driver, and isnt available in the UK as far as I can tell.
Im wondering if coaxial drivers may be a better bet - say 5" speakers with centrally mounted tweeters and built in crossovers?
Basically a driver that does bass and mid, and then a tweeter to add sparkle, rather than a full range driver and then a separate sub, just to add that real low end.
Different way of achieving the same end result, but it's a case of which is cheaper and also which retains the most speaker sensitivity.
Im wondering if coaxial drivers may be a better bet - say 5" speakers with centrally mounted tweeters and built in crossovers?
Basically a driver that does bass and mid, and then a tweeter to add sparkle, rather than a full range driver and then a separate sub, just to add that real low end.
Different way of achieving the same end result, but it's a case of which is cheaper and also which retains the most speaker sensitivity.
Very nice speaker projects you´re building.
I´d guess you don´t want to spend too much for the whole package so looking at coaxials and larger fullrange drivers might be an option.
Maybe one of the Mark Audio´s fullrange options like CHR/CHN/CHP-70 or Tangband W4-655 or similar.
Other fullrange options:
http://www.spectrumaudio.de/breit/tabelle.htm
Have a look at these projects for inspiration:
Search speaker kits
Click "shelf-speakers" and give a max. price or size and search.
That might give you some ideas for drivers.
Or look for a 2.1 amplifier module. There are lots of them although I haven´t investigated if and how they realize the crossover.
I´d guess you don´t want to spend too much for the whole package so looking at coaxials and larger fullrange drivers might be an option.
Maybe one of the Mark Audio´s fullrange options like CHR/CHN/CHP-70 or Tangband W4-655 or similar.
Other fullrange options:
http://www.spectrumaudio.de/breit/tabelle.htm
Have a look at these projects for inspiration:
Search speaker kits
Click "shelf-speakers" and give a max. price or size and search.
That might give you some ideas for drivers.
Or look for a 2.1 amplifier module. There are lots of them although I haven´t investigated if and how they realize the crossover.
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you might also consider a bigger full range driver that combines added bass response while keeping most of the upper end (maybe with whizzer cone?).
monacor SPH60X, just as a 5" example.
When you say "woofer output", do you mean the line-level, high-impedance signal at the *input* of your amplifier, or the low-impedance, high-current signal at the actual woofer voice coil?
If the former, the solution I suggested is pretty painless and simple. It uses one op-amp, but avoids having to design and build any active filters at all.
If you're planning the latter - a passive low-pass subwoofer filter summing watts of audio power taken from the woofer terminals - that is a can of worms I will stay well away from.
(For one thing, the class D amps you're using are almost certainly operating in bridge mode. If so, neither end of your loudspeaker voice coils is grounded - they are driven in antiphase, and both ends are floating with respect to ground. Attempting to sum these signals in a passive filter is likely to fry your chip amp.)
The simple solution I suggested earlier assumes you'll be using an external, commercially available, powered subwoofer with its own internal power supply, amplifier, and active low-pass filter.
If you are talking about starting with two bare tweeters, two bare woofers, and a bare subwoofer, and creating a complete powered 2.1 system entirely by yourself, then things get considerably more complex.
You'll need to first sum the two inputs. Then low-pass the summed signal, and feed the result to a subwoofer power amplifier (presumably another chip). The low-pass filter should probably be at least 4th-order, needing two op-amps.
Meantime, the unsummed input signals are fed to your woofers. Do you plan to high-pass this signal? If so, you'll need to design and build an active high-pass filter for each channel. Second order minimum, 4th order is better - which requires two more op-amps per channel, or four op-amps for both L & R high-pass filters.
Now we're up to five op-amps along with a number of precision capacitors and resistors. And the crossover between woofer and tweeter is still up in the air. I presume you're currently using passive crossovers of some sort there, in which case, those can stay.
At about this point, paying for the factory-made 2.1 channel amplifier board starts to look pretty attractive!
Incidentally, I worked in a speaker R&D group at one time, shoulder to shoulder with two guys who designed pro-audio speaker systems. What I found out is that designing a good passive crossover between woofer and tweeter is the hardest part of the entire design. It's a complex and long-drawn-out iterative process - you measure, calculate, build, measure again, and keep doing this until you finally get the crossover right.
An off-the-shelf "textbook" passive crossover built out of a few inductors and caps, sized using simple 1/(2 pi RC) sorts of formulae, never works properly. This is because tweeters and woofers don't behave like pure resistors, with constant impedance and infinite bandwidth.
Instead, the tweeter will have its own acoustic high-pass filter response, an impedance peak at its fundamental resonance, and a slow impedance rise at higher frequencies. All this will ruin the calculated high-pass response of a "textbook" passive filter.
There are similar problems with the woofer. It has its own acoustic low-pass behaviour, and its own rising impedance curve, which will mess up the calculated frequency response of the passive crossover. Typically, the acoustic output of the woofer will fall too early and too fast to match up to the tweeter response.
There are ways to correct all these issues, but all of them involve having the ability to measure speaker acoustic frequency responses. That is a can of worms in it's own right.
Designing *active* crossover filters that actually work the way they're supposed to, is much easier. At least impedance variations in the drivers won't mess up the filter frequency responses. (But the acoustic frequency responses of the individual drivers can still mess things up.)
-Gnobuddy
Thanks for the comprehensive answer! 
Ive ordered the 50w/50w board (that claims to be able to use 5v to 24v)
I'll check and see if the bluetooth works at 5v, what the range is like, and what the overall volume output is like, and compare to the cheap PAM 5v amp i have used in the past.
If it doesnt work at 5v then my point is moot, and the 12v 2.1 boards will be the way to go.
Ive ordered the 50w/50w board (that claims to be able to use 5v to 24v)
I'll check and see if the bluetooth works at 5v, what the range is like, and what the overall volume output is like, and compare to the cheap PAM 5v amp i have used in the past.
If it doesnt work at 5v then my point is moot, and the 12v 2.1 boards will be the way to go.
You're very welcome, Jim.jimcroisdale said:
I think these cheap class D boards are amazing in almost every way. Decades ago I used to build my own power amplifiers, with aluminium heatsinks the size of paperback novels, and big unwieldy 2N3055 transistors in enormous metal TO-3 packages. The power supply transformer was huge and heavy, and the big filter caps were huge too.
Then you needed a pretty big enclosure to house the whole thing in. In my starving-student days I could never afford an enclosure, so the guts of the latest amp often sat, unhoused, on a corner of my desk.
Now we can buy these amazing little class-D boards that are small, lightweight, inexpensive, come with tiny little heatsinks, have distortion levels better than the ear can hear, and produce plenty of output power - given sufficiently high power supply voltage.
That last bit of information is the one thing that virtually never seems to be provided by the seller, when looking at ads for these boards on Amazon or Ebay.
The fact of the matter is that a board that puts out a clean 50 watts RMS into a 4 ohm load when powered by a 24 volt DC power supply, is only capable of delivering just over 1 watt RMS of clean power, into the same 4 ohm load, when powered by 5 volts DC.
I suspect sellers sometimes deliberately omit this information, because nobody would buy one-watt audio boards that cost as much as 50-watt audio boards. Unfortunately, not all buyers of these boards have enough technical knowledge to realize that the output power available, varies tremendously with available power supply voltage.
-Gnobuddy
I know the power output is a LOT less at 5v.
The PAM8403 board is what I started this little journey with - purportedly 3w per channel from 5v (dont know if thats true or not, buy hey)
THIS IS ONE I USE A LOT
When I say "test at 5v", really what I mean is to test how the 5v-24v board compares to the little PAM8403 board when run at the same voltage.
Ive made a few boxes with the small PAM board, and when combined with Faital Pro 3" or 4" full range drivers (which are good quality and have high sensitivity) ive been amazed at the volume and quality thats achievable.
Thats sort of my benchmark
Jim
The PAM8403 board is what I started this little journey with - purportedly 3w per channel from 5v (dont know if thats true or not, buy hey)
THIS IS ONE I USE A LOT
When I say "test at 5v", really what I mean is to test how the 5v-24v board compares to the little PAM8403 board when run at the same voltage.
Ive made a few boxes with the small PAM board, and when combined with Faital Pro 3" or 4" full range drivers (which are good quality and have high sensitivity) ive been amazed at the volume and quality thats achievable.
Thats sort of my benchmark
Jim
Yes, just a simple passive design if possible!
I have article about 2.1 passive, but it includes custom made inductor. I can dig out the article if you are interested.
However sub with dual coil woofer is easier.
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I'm not surprised!
I reckon those Faital drivers are capable of producing around 90dB at 1 m for only 2 W of power.
This is more than adequate when listening to light music from small portable devices such as the ones you make.
I can see why you want 5 V DC as a the operating requirement - your devices run off a USB cable and 5.5mm/2.1mm 5V power input.
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