Contest: Linear Power Amp in a mint tin (class Aa, class AB, or class B) - Page 4 - diyAudio
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Old 23rd November 2012, 04:27 PM   #31
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What about a TDA1521
http://www.datasheetcatalog.org/data...s/TDA1521Q.pdf
Very few components. Room for a bottom fan with ventilation holes trough the bottom (Allowed?) each side.
External symetrical supply.

Last edited by bobodioulasso; 23rd November 2012 at 04:36 PM.
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Old 23rd November 2012, 04:42 PM   #32
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That makes sense. With 9mA bias you get 410mR-600mR Ro for the CFP's, much closer to 1R.

Emitter resistors MUST be relatively high in this application to achieve low bias current without excess distortion. 1R is a good compromise for 9W. The dynamic emitter resistances are such that they can never perfectly cancel. However the best crossover occurs in an EF when the emitter resistors have a voltage drop of 26mV or so. We bias the current of the output stage so that at crossover, the emitter resistances will equal the degeneration resistances and this way, when both outputs are conducting and the degens are seen in parallel, the added emitter resistances make up for the halved degen resistance.

This design is interesting from a design angle perspective, but what are you comparing it to? What is it supposed to be better than? If it is not supposed to be a competing design then maybe you should make your own thread, after all this is a contest.

Your CFP does evidently have the advantage that it can use smaller emitter resistors than an equivalent EF. An equivalent EF would need degeneration of 2.66-8R, which is too much if you want to push 2A. However it has worse current gain and I suspect it will have more distortion overall than a simple EF biased at 29mA or so with .82R degeneration.
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Old 23rd November 2012, 04:47 PM   #33
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We have the potential to get WAY lower distortion than chipamps if we go discrete. My current design uses 8 transistors and should get around .001% THD at 9W. 5 transistors for the output stage, 3 transistors for the VAS and input.
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Old 23rd November 2012, 04:49 PM   #34
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Quote:
Originally Posted by keantoken View Post
FETS are a good idea, and you can bootstrap the drivers to get full rail voltage swing. However at low bias currents the large transconductance will cause massive capacitive current spikes and I don't think that will sound good. BJTs can be biased low and still have reasonably low crossover distortion. I don't know, maybe there is a suitable FET pair.
I wonder what is the discrete parts version of DMOSfet outputs?
Quote:
Originally Posted by qusp
linear... poetic license? or you are simply referring to a non-switching amp?
Yes, a non-switching amp will do nicely.
Quote:
Originally Posted by bobodioulasso
Two of these in a mint tin?
That size stereo amp will fit on a credit card size PCB or can be made point to point even smaller.

There's also the available option of NuVistor and subminature-8 tubes/valves boosted with a solid state output.
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Old 23rd November 2012, 04:58 PM   #35
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Is the contest about distortion numbers?
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Old 23rd November 2012, 04:58 PM   #36
bigwill is offline bigwill  United Kingdom
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Interestingly, I removed the emitter resistors altogether, and the crossover artefacts completely vanished from the error signal at optimal bias. Bias stability has gone out the window though!
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Old 23rd November 2012, 05:09 PM   #37
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The contest is not about distortion numbers. Well, partly.

Quote:
Originally Posted by danielwritesbac View Post
The goal of the contest is a useful efficient linear amplifier that works well inside a mint tin.
I take "useful efficient linear" to mean, it can get loud enough for computer monitors or a bedroom, it has low enough distortion to be considered enjoyably transparent, and doesn't smoke, burn, or explode.

I think for a discrete design the transistor count is one of the most important qualities. For AB outputs we will need at least 4 transistors for two channels. 8 if we use drivers. 10 with a VAS. At least 12 with all this and a singleton input.
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Old 23rd November 2012, 05:13 PM   #38
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Bigwill, from your current probe position I don't think you will be able to see the distortion. If you probe C1 in LTSpice I think you will be able to see it, in your prototype you may try probing across R2 or R3, if this does not cause your amplifier to become unstable.
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Old 23rd November 2012, 05:43 PM   #39
bigwill is offline bigwill  United Kingdom
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keantoken,

These are oscillograms taken from the amp with no emitter resistors.

Here is the error signal with zero bias:

Click the image to open in full size.

Here is the error signal with what appears to be optimal bias.

Click the image to open in full size.

It's amazing how the artefacts aren't visible in the error signal. Note that the funny notches on the peaks of the sine wave are from the signal generator.

The picture was far less pretty with 1R emitter resistors, sadly, though it was possible to bias to an optimum. Sadly some resistance is needed in a real amp.

Last edited by bigwill; 23rd November 2012 at 05:48 PM.
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Old 23rd November 2012, 06:08 PM   #40
bigwill is offline bigwill  United Kingdom
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Am I going crazy or didn't this thread start in the Solid State section and not Chip Amps? If anyone is wondering why I've been posting discrete circuits, I've been under that impression the whole time
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