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BritKat 21st March 2003 08:34 AM

200w Amplifier question
Hi guys,
I new here and Im building a 200w Amp for a subwoofer for my home theatre.
Here is the schematic for it 200w amp
Are there any specific mods I should do to it for low frequency use?

Also I ve bought the driver and its a 225wrms 4Ohm 87dB. I tried to run this off a 30w amp. It distorted badly. i know this is because the amp is being driven too hard. But when I run it from a 200w power amp I have it did the same?? Any ideas.
Maybe the power supply is not big enough? and can't supply the large currents?
Im pretty sure the driver is OK as its brand new

Thanks in advance for your help


audioPT 21st March 2003 07:14 PM

What are the transformer specs?
This could help a little :cool:

nw_avphile 21st March 2003 07:34 PM

What do you mean by "distorted badly"? Do you have access to another powerful amp to test the subwoofer with? Do you have it installed in a proper enclosure? Have you verified your 200 watt amp is working correctly? Have you tested it into a 4 ohm resistive load at full power? Do you have a way to measure distortion or at least look at the waveform with a decent scope?

You want about +/-45 volt supply rails (loaded) for 200 watts into 4 ohms. If your supply is much higher voltage than that, it would need extra VA capacity (and you'll need bigger heatsinks).

You need 7 amps RMS for 200 watts into 4 ohms. So if you have 50 volt (no load) supply rails, that would be about 35 volt secondaries on your transformer and it should be rated for at least 400 va or so. So that's 70 volts center tapped at 5.7 amps (or two 35 volt 5.7 amp windings).

The important part is the secondary current rating should be at least 5.7 amps on your transformer. If it's less, you'll likely saturate the transformer when you drive the amp hard.

Circlotron 22nd March 2003 07:47 AM


Originally posted by nw_avphile
You want about +/-45 volt supply rails (loaded) for 200 watts into 4 ohms.
The amp circuit is a *full* bridge so it has a single 44v rail. Neglecting voltage drops, it will still swing +/- 44v across the load.

nw_avphile 22nd March 2003 06:41 PM

Ooops! So it is a single rail design! I didn't look the first time. This amplifier appears to have some SERIOUS problems!

First of all, I doubt it can really make 200 watts into 4 ohms with a 44 volt max (no load) supply?

200 watts requires 80 volts peak to peak. A 44 volt supply only leaves 2 volts drop for each half of the bridge which is unlikely into a 4 ohm load given the higher current and resulting higher Vce(sat) as well as the losses in the TDA2030. The spec sheet for the TDA2030 shows about a 2 volt drop with external transistors at much lower power/current levels.

Further, with the typical series resistance (regulation) of the supply, it will likely sag to 39 - 41 volts at full power which would result in an output swing of around 66 volts if you assume a 3 volt drop for each half of the bridge. That's only 136 watts!

Worse, the TDA3020 is rated ABSOLUTE MAX for 44 volts, most designs don't run it at more than 40 volts. So a 44 volt supply is right on the edge of damaging the chip. A power surge could easily destroy the TDA3020's in this design. Further, you can't go any higher with the supply voltage to really get 200 watts or the 3020's will fail.

Finally, each half of the bridge is effectively seeing a 2 ohm load. Most bridged home amps are designed to run bridged only into 8 ohms which presents a 4 ohm load to each amplifier. I'm not familiar with the KD707 and KD708 output devices, but I would question if their safe-area parameters are up to this task and if the TDA2030 is up to driving them safely?

I also don't like using bridged amps for subwoofers in general because they have twice the output impedance and half the damping factor of a conventional amp (and they don't like 4 ohm loads which most subwoofers are).

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