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Old 27th April 2008, 01:20 AM   #1
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I'm wondering if is possible to do Class G with LM1875?
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Old 27th April 2008, 06:27 AM   #2
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Quote:
Originally posted by danielwritesbac
I'm wondering if is possible to do Class G with LM1875?
No.

Let's concentrate on the output stage to see why: Two sets of output transistors are connected to two different rail voltages in class G. Since all of the output devices of the LM1875 are tied to one voltage, and since we can't really muck with the guts of the chip, there is no way to generate class G since we are "stuck" with only one rail voltage.
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Old 27th April 2008, 11:57 AM   #3
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Quote:
Originally posted by dfdye
No.

Let's concentrate on the output stage to see why: Two sets of output transistors are connected to two different rail voltages in class G. Since all of the output devices of the LM1875 are tied to one voltage, and since we can't really muck with the guts of the chip, there is no way to generate class G since we are "stuck" with only one rail voltage.
Oh right! Oops. Sorry about that.
Thanks for the really good answer!

So, howabout an "extra power" discrete output stage? Maybe that still doesn't make way for class G (no idea), but a little power boost could be a fun add-on.
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Old 28th April 2008, 05:17 AM   #4
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Quote:
Originally posted by danielwritesbac
So, howabout an "extra power" discrete output stage? Maybe that still doesn't make way for class G (no idea), but a little power boost could be a fun add-on.
See LM4702 and related.
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Old 28th April 2008, 05:47 PM   #5
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You can do Class G and H with the LM1875 (or any chipamp) but you have to build the rail voltage controls external. I have done both with LM3886. The drawback is a tick up in THD when going into different modes but I never did extensive listening test to see if the difference was audible. There are circuits in some ST chipamps showing how to do it, although may either not be the best or too complicated or something else. I have never used them or looked at them in great depth.

The best option is to use something like a LM4702 or one of it's cousins (LME49810/811/830) so that you can keep the rail voltage changes only on the output devices and the rest of the amplifier is fixed. This should give better performance but I haven't gotten to it yet. Someday. . .

-SL
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Old 28th April 2008, 06:17 PM   #6
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Look at LM12CL datasheet page 10 for higher rails operation.
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Old 28th April 2008, 07:27 PM   #7
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Quote:
Originally posted by SpittinLLama
You can do Class G and H with the LM1875 (or any chipamp) but you have to build the rail voltage controls external.
My (admittedly limited) understanding of class G amps is that a separate output device is used for each rail voltage. What you are describing is more of what I thought class H was, IE variable rail voltages modulated by output voltage requirements. As such, it would follow that a class H chipamp would indeed be possible, but a class G amp wouldn't. If, however, you define class G as just stepwise adjustment of rail voltages (as opposed to continuous in the case of class H) then your description does make sense.

I guess I am getting tied up in the technicalities, and like I said, I am definitely not the ultimate expert on these definitions.

PS Either way, I agree with only varying the output stage voltage of an LM4702 or related amp would be the best chip based solution. Not really sure how to do that, but I am guessing a variable regulator would be involved somehow. . . .
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Old 28th April 2008, 10:13 PM   #8
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I'm relatively new to this, but I imagine it would be possible to design a PSU whose supply voltage varied according to the output voltage of the amp.

Doing this using a voltage regulator (I think probably easy to implement) seems fairly pointless though as you'd just be burning off all that extra heat in the regulator instead of in the chip
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Old 28th April 2008, 11:47 PM   #9
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dfdye, Class G & H is not about how to implement but what happens at the supply rails. Class G is stepped voltages, 2 or 3 tier type of thing. The idea, obviously, being that for most signals under 'normal' listening conditions are small enough to use lower supply rails and waste less power. When the signal demands more the rails jump to a higher level. If you go to infinite number of rails then you hit Class H. There is plenty of confusion about which is G and H so no point in arguring that just that one is stepped rails and limited in number and the other is output tracking or envelop or infinite steps. Stepped rails can be done with a FET acting as a switch and some diodes. Output tracking is not done with a redulator as all the power saved in the output stage is wasted in the regulator. Instead some sort of SMPS is needed to gain efficiency in the system not just the output stage. With chipamps, you have to do a hybrid because there is some minimum operating voltage. So you set solid DC rails at say +/-20V and then envelop tracking for signals that need more. The point of doing either Class G or H is all about efficiency but it comes at a cost in THD and sonic performance, cost, etc.

-SL
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Old 29th April 2008, 01:47 AM   #10
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Sounds good to me.
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