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Old 26th November 2012, 08:56 PM   #101
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Quote:
Originally Posted by keantoken View Post
. . . they don't store so much energy in their junctions.
Like FJPF5200/FJPF1943 pairs?
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Old 26th November 2012, 09:04 PM   #102
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Wow... Just priced up parts for the tda4935 and they roll in at a huge 5 including postage.

Now if the bank of wife allows it, I will see if it will all fit in a tin somehow... Can i use normal RCA input and output, kinda like a super micro, micro amp. I have a toriod that gives 18v after rectifier so all i need is a tin and a fiver..... Sorry it may not be as technical as you guys are used to, but i dont have a clue how it all works, i just have a go and see If i never post again... it went BAD!... real bad lol.
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Old 26th November 2012, 09:13 PM   #103
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Those are better, but AFAIK the MJLx302/x281 are the best in this category. The important parameter is not Cob but Cib. In the SPICE models it's Cje. Also Cbe. You want Cbe/Cje/Cib to be as low as possible for an output transistor. The actual size of Cbe is in the nF's. It is interesting to compare Cje in SPICE models you know are accurate.

Also, it is not the Ft at excursions that matters for crossover behavior but the Ft at Iq. I try to go for outputs with an Ft of over 20MHz at 100mA.

Cbe can also matter for small-signal transistors. Too much Cbe in the input stage will create a "glass ceiling" in amplifier BW that is not obvious. The BC5xx, MPSA18, 2N5210/5089 transistors have low Cbe. High-voltage transistors (japanese or not) tend to have larger Cbe and can actually lower BW if put in place of the others. So I shall proverbialize: Don't use high-voltage transistors for low-voltage applications.

Quasi-saturation and Vcesat are other parameters that get worse with higher Vcemax.
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Old 26th November 2012, 09:18 PM   #104
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I'll try to use suitable output transistors but this is going to be an amp in a tin after all.
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Old 26th November 2012, 09:35 PM   #105
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It is not necessary for a working circuit but I think it is necessary for a well-behaved one. I suggest the MJE150xx pair - Cordell has models for them - which are faster than the pair Dan mentioned and more suitable for a tin-size amp.
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Old 26th November 2012, 09:39 PM   #106
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The MJL/NJL might be overlarge. I think that the MJW/NJW are slightly smaller. Onsemi didn't seem to have much in the way of extra nice to220 outputs, however. . .

Can sturdy drivers be output devices for a little amp?
MJE15028/MJE15029
MJE15030/MJE15031
MJF15030/MJF15031

Edit:
I see that Kean mentioned them 4 minutes ago.

Edit2:
However, I wonder if the much lower voltage Tip31a Tip32a would be lower loss, more placid (easier for stable), and cooler running?
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Old 26th November 2012, 10:04 PM   #107
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Originally Posted by littlerick View Post
Wow... Just priced up parts for the tda4935 and they roll in at a huge 5 including postage. Now if the bank of wife allows it, I will see if it will all fit in a tin somehow... Can i use normal RCA input and output, kinda like a super micro, micro amp. I have a toriod that gives 18v after rectifier so all i need is a tin and a fiver..... Sorry it may not be as technical as you guys are used to. . .
I'm also doing a chip project, just as basic as possible and barely meeting the 9w+9w parameter. Since mine is for use with the computer, I'm thinking of installing 1/8" stereo jack so it is really easy to fit in the tin.
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Old 26th November 2012, 10:53 PM   #108
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Edit2:
However, I wonder if the much lower voltage Tip31a Tip32a would be lower loss, more placid (easier for stable), and cooler running?
I don't know why that would be the case. The TIP transistors are very old, they must be a different process. I would save them for lightbulb flashers when better types are easily available.
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Old 27th November 2012, 09:23 AM   #109
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To be fair - with the TIPs in place, in real life, I was able to get a decent looking 100Khz sine wave at full amplitude without much trouble. Even with no bias it still looked fairly decent.
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Old 27th November 2012, 04:28 PM   #110
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Probably, the MJE15028/MJE15029 (or little FJPF5200/FJPF1943) could give some very good (lower noise) performance in the no bias setup. I think they're worth a try. One of the projects here in the chip amp forum uses 5200, 1943 pushed by a TDA729x chip and I think those outputs are used without bias. It seems that the negative feedback was enough for noise reduction from such fine outputs? I don't quite know the particulars.
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