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Old 30th April 2005, 03:56 PM   #1
djdamix is offline djdamix  France
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Default Class B for low frequency ?

I plan to set my bias current to zero on my class AB amp so that it could work in class B. The main reason is I will not have to deal with thermal issues on bias current. The other reason is, I don't see the need of it because I will use this amp for low frequency (sub Beyma 18G550 )so <500Hz and at these frequencies the sinwave looks clean without bias current. (Amp loaded by 8 ohm)

Any comment is welcome.

David
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Old 30th April 2005, 04:01 PM   #2
Giaime is offline Giaime  Italy
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Well, if it works for you...

If I had to make a choice, I would go on class D for a sub

Pay attention that maybe under stress and heat the thermal compensation might want to reduce further the bias, you'll end up with a class C amp.
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Old 30th April 2005, 04:06 PM   #3
djdamix is offline djdamix  France
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nice advice thanks, I didn't think about it ... I'll check
I'll go class D but I'm looking for a simple schematic to start

David
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Old 30th April 2005, 06:03 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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It would be better to choose a very low but non-zero bias. Perhaps 0.4-0.5V Vbe. You will still get the thermal stability, but distortion will be a lot lower.
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Old 30th April 2005, 07:44 PM   #5
anatech is offline anatech  Canada
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You have to be careful here. Crossover distortion may be heard as a "buzz" from the speaker. Depends a lot on the speaker and amount of distortion.
Mr. Evil has the best suggestion, with a bias circuit.
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Old 30th April 2005, 09:32 PM   #6
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Why not just use a thermal compensation servo circuit like most class AB circuits do, and keep the bias in class AB but make the compensation factor larger so that when the outputs circuit get hotter the bias drops faster and into class B operation. Maybe use a higher gain device for the servo that has a larger temperature coefficient. This would reduce the "buzz" of crossover distortion, even to a low frequency. Of course if you are using a band pass speaker box, where the diaphram is not on the outside, who gives a hoot about a little crossover.

I think your bigger concern might be SOA and whether or not the circuit can handle the inductance, i. e. phase shift, of a heavy sub-woofer without letting the smoke out of the outputs.
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Old 30th April 2005, 09:32 PM   #7
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djdamix

Why? What's wrong with your AB amp that you need to set the bias to zero? Is it thermally compromised and you wan't to run it hard?
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Old 30th April 2005, 09:49 PM   #8
djdamix is offline djdamix  France
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cunningham =>can you explain further about phase sift and the SOA. Will I have to dissipate more power than with resistive load ??? How can I test it ? What inductance value ?

Thanks.
David
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Old 30th April 2005, 09:55 PM   #9
djdamix is offline djdamix  France
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amplifierguru=> In fact the thermal compensation could be ok, (thanks to diyaudio members) but I think the amp will be less hot and it could be better and maybe more relyable : On my previous amp, when the amp turn on the bias curent rise to 600mA and then stabilize to 400mA. Maybe in some bad condition (low temperature or else, it could rise up to 600mA when turn on) maybe it is not a good thing. Furthermore I have to put a transistor for the VBE multiplier on one of the output mos and I need to make a mechanical piece to do that.

Thanks
David
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Old 30th April 2005, 10:15 PM   #10
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Lets see if I can get this right... In an inductive load, current lags the voltage, and a capacitive load, current leads the voltage.

At any rate, the current and voltage is out of phase. So when the voltage rises up to peak and then to zero on a resistive load, the current rises up to peak at the same time. So when the current is peaked, the voltage across the output transistor is at minimum. When the voltage is around zero, the full rail voltage is across the transistor but the current is zero.

On a reactive load, the voltage on the transistor may be half way to zero when the peak current flows, thus possibly exceeding the SOA of the device. Or the output voltage could be around zero when like half the peak current flows and all the rail voltage is across the transistor and may exceed the SOA. Anyway, you get the point??

This is where some people use paralleled outputs to drive heavy (inductive) sub-woofers with lots of power.
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