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Re: Gonna Try Building a 1kW Class D Amp for Only $396
Ken.C #335363 01/22/11 11:38 PM
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I just thought of something. To bi-amp I would need to split the input signal, so the pr-amped signal voltage would be cut in half as well. I would then need to raise the receiver volume to get the same db output. The only was I guess this really works is to have a pre-amp voltage doubler as well.


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
SRoode #335374 01/23/11 03:29 AM
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Steve, I haven't been following this closely because I'm not very interested in DIY in general or class D amplifiers in particular, but I've now looked at the last couple pages. First point, as we've discussed at some length in the past, is that although two 8ohm loads driven in parallel result in a 4ohm net load, that isn't what's happening in a "4ohm" speaker. Say, at 100Hz if the impedance of a bass driver is actually 4ohms, because of the effect of the crossover circuitry the impedance of a "4ohm" tweeter section at that frequency might be more like 100ohms. The resulting impedance at 100Hz would then be 1/4+1/100=1/x; 25/100+1/100=1/x; 26/100=1/x; 26x=100; x=3.85ohms. So no, a "4ohm" speaker(which the M80s aren't at most frequencies, of course)isn't the result of two 8ohm sections being driven in parallel.

You ask "Am I missing something?" on the bi-amp question. Yes; although you refer to "each amp", there's only one in a two channel amplifier with one power supply section. The output transistors in each channel have no power of their own, and can't supply added "headroom".

Also, note on the voltage splitting question that two voltages in parallel don't form a voltage divider(series connection does that), and the output voltage to each load hasn't been cut in half.


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
SRoode #335378 01/23/11 04:41 AM
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Originally Posted By: SRoode
If I leave the receiver setting at -12b, and change to bi-amp, both low and high still dissipate 10W, but now each amp is only "delivering" 10W.

Yes-ish, with the caveat that the split would probably be more like 4/16 than 10/10. Systems designed for bi- or tri-amplification generally have much larger amps for the woofers.

Originally Posted By: SRoode
In the end, if I had my sound meter measuring before and after, the sound level should be the same, right? And, I gained headroom on each amp.

Yes to the sound level, probably no to the headroom. There are a bunch of different ways an amplifier can "reach its limits" - output voltage, output current, temperature are the main ones. Most clipping happens when the amp reaches its voltage limit**, although shutdowns tend to happen as a result of current or temperature limits.

Since you don't have an electronic crossover before the amplifiers you're sending the same signal into both amps, and so the voltages will be the same and voltage limits will be hit at the same point as before.

If, however, the amp is "shutting down with 4 ohm load" as a result of current or overtemp limits, then the splitting of *current* could help a bit although this is where the 4/16 split bites you and you don't gain that much.

** it's actually not quite that simple because higher output currents tend to drag down the power supply voltage so you hit voltage clipping a bit sooner, but you get the idea...

Last edited by bridgman; 01/23/11 04:44 AM.

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Re: Gonna Try Building a 1kW Class D Amp for Only $396
bridgman #335384 01/23/11 06:04 AM
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I'm going to be over here with Ken, chanting in the corner.


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
tomtuttle #335386 01/23/11 06:40 AM
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Bet you have the kit page for Class D Audio open in a tab in your browser, too, though...


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
Ken.C #335390 01/23/11 12:14 PM
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Thanks for all of the great information guys, I'm really learning a lot. One thing I have not done is put my meter on the power supply to see if the voltage is drooping at high output. I would bet the toroid is just fine, but the DC power supply may be reaching it's limit. I'll try measuring on both sides of the dc power supply to see if the ac or dc (or both) dips on high output. I'm kind of hoping the dc output is drooping because that may be able to be fixed with the addition of another dc power supply (use one for each amp board).


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
SRoode #335391 01/23/11 12:16 PM
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I have affixed the additional heatsinks that I provided a link for several posts back on one of the amp boards. With no forced air cooling, the channel with the additional heatsinks is about 5 degrees C cooler than the one without. So, it's some progress.


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
SRoode #335395 01/23/11 03:11 PM
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I think I'd use some ear protection on these 'tests'.

Re: Gonna Try Building a 1kW Class D Amp for Only $396
CatBrat #335398 01/23/11 03:33 PM
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I just ran a test with the left speaker "bi-amped", and the right speaker single amped. The "biamped" speaker can play with the receiver set 1.5 db higher before cutout than the single amped speaker. As bridgman predicted, when I raised the "bi-amped" speaker volume higher, it was the woofer amp that cut, while the mids and tweeters kept playing.


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Re: Gonna Try Building a 1kW Class D Amp for Only $396
SRoode #335426 01/23/11 07:43 PM
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Neat. Sorry, I hadn't realized you were able to drive the amps hard enough for them to shut down, or I would have been a bit more enthusiastic about your bi-amping experiment wink

I guess someone not fighting a flu bug could probably use that 1.5 dB number and work out the actual power split between woofers and mid/tweeters, but that isn't going to be me, at least not for a few more days.

EDIT - I assume we're getting into REALLY FREAKIN' LOUD territory here. You might want to build a small insulated structure on the outside of your listening room similar to what Axiom uses for high volume testing.

Last edited by bridgman; 01/23/11 07:45 PM.

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