Class-T amp with subwoofer - impedance? mono or stereo? - diyAudio
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Old 10th July 2008, 10:58 PM   #1
pdvrog is offline pdvrog  Switzerland
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Default Class-T amp with subwoofer - impedance? mono or stereo?

I'm still a beginner in diy-electronics and have some uncertainties. I could not find the answers to my qustions in other threads, or did not understand them. If it's already said, please be kind and link me to there.

This ist my setup: Class-T amplifier (TA2020 or TA2024) and a Stereolith Duetto.
(http://www.alldatasheet.com/view.jsp...A2020&q=TA2020 - http://www.alldatasheet.com/view.jsp...A2024&q=TA2024)
The Duetto is a monolithic speaker with two woofer/midrange and one tweeter per channel.

I'm quite happy with these parts, but the Duetto is a bit weak for low bass. So I want to build a subwoofer. Just a simple box for the low frequencies with a low-pass-filter. I don't want to build a crossover, because the Duetto has no problems with the low range, it's just too weak.

I'm not sure about the following points:

1. It is said, that the outputs from the Tripath chips "may not be shorted". This means one may not pull them direct to ground. But if I take only one subwoofer speaker, I have to combine the signals from both channels. Is that also forbidden?

2. What happens to the impedance, if I take two 8 Ohm subwoofers - one for each channel - and wire them in parallel to the Duetto, which has 4 Ohm? I'd think, this would give an impedance of 6 Ohm to the amplifier - is that right? What happens, if I only take one mono subwoofer speaker? What effect has the low-pass-filter for this configuration?

3. And to get a bit a deeper understanding: Let's look at a normal 3-way speaker. It has three speakers with 8 Ohms each. The overall impedance is also 8 Ohms, although the speakers are wired in parallel. Is it right to think, that this is because the crossover seperates the signal into three bands for low, mid and high, which gives virtually three circuits, one for each band?

4. I have read contradictory meanings about adjustable low-pass-filters. Some say they give not as good results as fixed ones. What is your thinking about that?

Thanks for any advice and feedback.
robert
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Old 13th July 2008, 04:59 AM   #2
HiSPL is offline HiSPL  United States
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If I am understanding you correctly, you want to power the sub(s) off of your T-amp along with the mains, is that right?

You certainly can do that, but I don't think you will be thrilled with the performance. A sub will "steal" alot of the power that was used for your mains. It takes either (a) a bunch of power or (b) a bunch of subs or (c) horns, to make bass. You can't really have your cake and eat it too.

If you still want to do this, you will have to use a low pass crossover for the subs. You can get a dual voice coil (DVC) sub to do exactly what you are talking about. The left channel goes to one voice coil, and the right goes to the other coil. Never the two shall meet, saving not only your amp from meltdown, but also your stereo image from the horrible world of mono!

The easiest and probably the best way to do this in your situation is to buy a dedicated plate amp to power your sub with. You can run it from a line level source like your preamp, or you could run your speaker wires to it and then out of it to your mains. This lets you use the plate amp for the heavy lifting (sub) and then use your T-amp for your mains.

One more thing to point out is that by the time you buy the hefty crossover components to make a passive subwoofer, you'll find that you are not far away price-wise from a plate amp...

Hope this helps!
-=Tim=-
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Old 13th July 2008, 03:46 PM   #3
pdvrog is offline pdvrog  Switzerland
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HiSPL, thanks for your answer, it has helped me. I already suspected that I may not wire the outputs together, now I'm sure.

Till now I don't have any experience with subwoofers, but I understand that it will take a lot of power.
The reason why I wanted to avoid a seperate amp is that I will try several different power amps and I don't have any equipment to do proper accustic measurements for the tuning of the subwoofer's amplification.
In fact, I'm still in an experimental phase and I don't know how the final setup will be.

I have found a Canton 12" woofer with dual voice coils. It's an auction item, but I hope I can get it because this should be a good fundament for my experiments. I have also found a suitable lowpass filter, but it's incredibly cheap (about 16$ per channel). Probably it's scrap. I absolutly agree with you about the price for a good filter versus price of a subwoofer amp.

So it's very likely that I will come to a seperate amp as final solution, but I'm still concerned about the tuning between subwoofer and speaker. I can just rely on my listening.

Thanks for any further advice,
robert
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Old 13th July 2008, 08:15 PM   #4
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Default Re: Class-T amp with subwoofer - impedance? mono or stereo?

Quote:
Originally posted by pdvrog


2. What happens to the impedance, if I take two 8 Ohm subwoofers - one for each channel - and wire them in parallel to the Duetto, which has 4 Ohm? I'd think, this would give an impedance of 6 Ohm to the amplifier - is that right? What happens, if I only take one mono subwoofer speaker? What effect has the low-pass-filter for this configuration?


8 Ohm and 4 Ohm parallel are about 2.5-3 Ohm.

p.s. use a parallel resistor calculator
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Old 13th July 2008, 08:59 PM   #5
HiSPL is offline HiSPL  United States
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It is possible to get a passive crossover for about $16 dollars a channel, in fact here is one.

http://www.partsexpress.com/pe/showd...number=260-221

But pay attention to the details, like the fact that it is a 12dB per octave crossover for the low pass centered at 130 hz for a 4 ohm voice coil.

130 hz is a pretty high crossover point for a home hifi sub. You will not have a sub that "blends" into your system very well, because you will be able to locate the subs placement in the room with your ears. Also the sub will be playing into the midbass region above 200 hz. You will have male vocals coming out of your sub instead of the mains where they are supposed to be.

Also, if your sub has 8 ohm voice coils, this crossover will be at 260 hz. You need bigger coils to crossover an 8 ohm sub.

Alternatively, you could get something like this.

http://www.partsexpress.com/pe/showd...number=300-782

For just a fews dollars more than the passive XO, you could have this fully active amp, with crossover built in. This amp will supply 25 watts into a 4 ohm sub, which is about 50% of your mains power. That should be enough to keep up nicely if all you want to do is supplement your mains' low frequency output. It's like giving your main amp 50% more power down low where you want it.

I would also suggest a small sealed box woofer like this.

http://www.partsexpress.com/pe/showd...number=295-480

This is an excellent little woofer in a small box. It provides just the right amount of low end fill in an easy to hide size.

I'm sure you can come up with something comparable to all this on your side of the world. Most of these things are very generic and made mostly in China. They are sold all over the world under different brand names, so with a little looking around you'll find something useful I'm sure....

Good Luck with your project!
-=Tim=-
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Old 14th July 2008, 03:16 PM   #6
pdvrog is offline pdvrog  Switzerland
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@pikkujöpö: Thank you for correcting me. It's a pity, I should have known better. Let's say I was tired and confused because with two 4 Ohms the result is 2 Ohms. The formula is 1/R = 1/R1 + 1/R2...

@HiSPL: Tim, Thank you also for your tips and links.

My diy-ing began with microprocessors and now I'm doing my own preamp. The other components like dac, phono preamp and power amp are kits with calculated parts and instructions or even finished pcbs. Unfortunatly I had absolutly no electronics in school and so I do the learning now step by step with these projects. The big goal is a selfmade system but I have great respect for building loudspeakers. My plan was to build a fullrange speaker till I ran over that Duetto.

These are the links to my subwoofer components (all in german, sorry):
The woofer: http://www.ricardo.ch/Article5422409...8527/P/0/1/0/1
It was fitted in such a box: http://cgi.ebay.ch/ws/eBayISAPI.dll?...MEWA:IT&ih=012
More info: http://www.hifi-wiki.de/index.php/Canton_Plus_C
And these are the lowpass filters I found in ebay: http://cgi.ebay.ch/ws/eBayISAPI.dll?...MEWA:IT&ih=023
and: http://cgi.ebay.ch/ws/eBayISAPI.dll?...MEWA:IT&ih=022

My simple plan is to copy the original box in size and with its filter (12 db at 120 Hz). But I will build the filter as an external part, so I can test other configurations or even an active mode. In that way I will also be able to measure the implications of the crossover on impedance and power.
And perhaps I will than do my own box designs.

So thank you for wishing me good luck, I hope I will get a better understanding of all that stuff.
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Old 14th July 2008, 05:04 PM   #7
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Not sure if better,but there are active subwoofer crossovers for cars.Sometimes the prices can be surprisingly cheap,ap.15Euros or so.
One with "speaker level input" would work nicely.Of course a transformer is needed as well...

I also recommend a proper plate amp,later if you ditch your current speakers you are still left with nice pair of active subwoofers.I wish I had one myself ,instead of an old stereo amp !
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