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Old 17th July 2004, 12:32 AM   #1
bob123 is offline bob123  United Kingdom
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Default is this design sound?

hi i am a bit of a newbie when it comes to designing amplifers and i would like to know if this design (atached) will work. i am sure there are many mistakes in the design and i would very much apreciate if they could be pointed out.

bare in mind this amp is for a sub and therefore does not need to be high quality or have low distortion. it just needs to be loud.

i have simulated this design and it works. but that is just with a cheap computer program.

oh and what is the maximum voltage that a line in will be?


thanx
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Old 17th July 2004, 02:45 AM   #2
anatech is offline anatech  Canada
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No, your DC offset will drift all over the map with temperature and supply voltage drift. There are more problems but ....
There are lots of simple, basic amps to copy from. Why not?
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Old 17th July 2004, 02:54 AM   #3
hughmon is offline hughmon  United States
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I haven't bothered grinding any numbers, but just looking at it the zero crossing distortion in the output is going to be huge. And with no negative feedback to help, it probably wouldn't even sound right driving a sub-woofer. Try looking around at some other output stage designs for some biasing ideas.
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Old 17th July 2004, 05:46 AM   #4
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Quote:
Originally posted by anatech
No, your DC offset will drift all over the map with temperature and supply voltage drift. There are more problems but ....
There are lots of simple, basic amps to copy from. Why not?

Yes this is quite a problem. If you direct couple the speaker to the outputs, then you need some type of feedback loop, or really precise transistors and very accurate biasing

If high freq. responce is not required, and stable DC offset IS required, then try using an OP- Amp as a differential, but be sure that it has a very high open loop gain, and no internal neg. feedback loop. Like the good ol' 741 for cheapness.

Take this circuit topology for an example, and feel free to modify it.
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Old 17th July 2004, 05:57 AM   #5
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Here is another example.

Just be sure to mount the bias servo transistor(the one with the pot from B to E) with the outputs so that they are same temp.
Very important!


I have not simulated this circuit, but the topology is fairly good.

Chris
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Old 17th July 2004, 07:59 AM   #6
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Arrow Commentable Thoughts

hi ,
This amp is only good for simulation purposes . Not in reality due to temperature instability leads to thermal runaway , no feedback.

Regards
AmPmAn
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Old 17th July 2004, 01:43 PM   #7
bob123 is offline bob123  United Kingdom
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i designed a circuit because i want to understand how it works. simply copying a circuit is pointless as this will give little understanding of how it works.

thanks for all the replies, i am going to modify one of the desigins that have been posted and see where i go from there.
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Old 17th July 2004, 02:20 PM   #8
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Quote:
Originally posted by hughmon
I haven't bothered grinding any numbers, but just looking at it the zero crossing distortion in the output is going to be huge.

How many of us have tried to listen to an amp with zero bias in the output stage with real music and how many of us can tell the difference between zero bias and optimal bias?

I had a hard time telling them apart, especially during casual listening sessions and without knowing the level of biasing.
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Old 17th July 2004, 03:47 PM   #9
anatech is offline anatech  Canada
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Default Crossover distortion

Hi millwood,
Boy, I can hear it. Maybe because I was a service tech for years and I naturally listen for it. There are lots of amps I can't listen to.
In this application you may still hear it if the woofer is forward firing and there is no acoustic filtering.
-Chris
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Old 17th July 2004, 05:03 PM   #10
hughmon is offline hughmon  United States
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Hi millwood,

I wasn't referring to a bias current per say, but just a bias voltage on the base-emitter junctions. As it stands, there's around a two and a half volt dead zone in the output structure, and with no feedback, that's likely to be very audible.
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