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Old 23rd August 2013, 07:34 PM   #10241
dvv is offline dvv  Serbia
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Nige, while I would never dispute the need for as quiet and noise free as possible, everywhere and anytime, I do think a lot of this debate is, for one reason or another, relevant to line electronics, and especially power amps.

After all, while noise is an issue, it is only of the issues which we need to weigh against other issues. Noise we can deal with by careful selection of our input transistors. I find, for example, that using 2SC2240, or 2N5551, or BC546B (better yet C when I can find it, higher gain) usually takes care of noise issues well enough without compromising other issues.
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Old 23rd August 2013, 07:45 PM   #10242
bcarso is offline bcarso  United States
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To a certain extent it's also about how much noise is in the given source component. When I was doing class D designs for a startup I got the amplifier noise down to extraordinarily low levels, of order 30uV rms in the audio band at the output for a ~100W into 4 ohm product. It turned out the best digital source we could find at the time was considerably noisier than the amplifier.
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Old 24th August 2013, 08:19 AM   #10243
dvv is offline dvv  Serbia
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Which should still make you VERY happy, Brad, because you achieved the goal of the amp not being the bottleneck.

I was thinking of oddities which can happen. To use an extreme example, say you find that your input differential pair behaves ideally with a bias current of say 0.5 mA per tranny, but the VAS stage is Miller compensated with say 47 pF. This would reduce your slew rate to just over 10V/uS. Depending on your point of view, this may be enough, but personally I'd still like to see say 40 V/uS as the working value, not because I can mathematically prove we need that much, but because in my experience faster amps tend to sound more coherent than slower ones.

So, this may cause you to investigate why do you need 47 pF and not say 22 pF, and that leads to examining the whole circuit, the layout and the very PCB. Some changes may be required, but neverthess, it would still require you to get some more bias in, say 1 mA. This is double the "ideal" value, so now you have re-examine the noise figure.

That sort of thing.
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Last edited by dvv; 24th August 2013 at 08:22 AM.
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Old 24th August 2013, 09:19 AM   #10244
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Quote:
Originally Posted by dvv View Post
Nige, while I would never dispute the need for as quiet and noise free as possible, everywhere and anytime, I do think a lot of this debate is, for one reason or another, relevant to line electronics, and especially power amps.

After all, while noise is an issue, it is only of the issues which we need to weigh against other issues. Noise we can deal with by careful selection of our input transistors. I find, for example, that using 2SC2240, or 2N5551, or BC546B (better yet C when I can find it, higher gain) usually takes care of noise issues well enough without compromising other issues.


In valve design there would be little point as the op amp style valve design is not universally liked ( read disliked ) . In transistors it is and makes sense to do one fits all . The good old 2SA1085 seems almost a good universal input device . 2N4403 on steroids . It's high gain means it might be possible to run 3 deceives coupled to a MC pick up and not exceed a sensible current flowing through the PU. As I think Stan Curtice said most caps leak more that that ( Naim type circuits ) .

On that . Seeing as no voltage swing is required at TR1 . What is the ideal discrete op amp voltage for MC gain 1000 ( pre 75 uS and further 3180 / 318 gain 16 @ 1kHz ) . +/- 6V seems to offer 50 db overload margin , stage two if op amp is less . That is I need 15 mV from stage 1 max . I would be using a VAS CCS .
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Old 30th August 2013, 08:09 AM   #10245
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For years I have used 2SA970 PNP ( 872A 1085 and 716 ) . 120 V , very low noise and high gain . Soon I will be forced to use something different . I notice in the book ( D Self ) that the input transistors are in the nothing special category . 2N5401 comes to mind as a possible substitute if nothing special is OK . I would like to have a >100 V capability if possible as sometimes I build high voltage amps that resemble hi fi use . LM4702 will do that for that , prefer it is my own design . I have seen MPSA 92 used ( HH ) . Any thoughts ?

Noise figure . 8 dB ? OK ?

2N5401G - ON SEMICONDUCTOR - TRANSISTOR, PNP, 150V, 0.6MA, | Farnell United Kingdom

Posted this question in another thread ( D Self book ) .
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Old 5th September 2013, 02:51 PM   #10246
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Years ago I was sent a LM3900N Norton amplifier quad package that keep appearing in it's little coffin in the loft . Noticing that TL074 is now cheaper than LM 324 I saw this one also and asked myself what if . Looking at applications some audio uses are stated . A mixer and sine wave oscillator of the SVF or similar type . In the thread about Douglas Self's new book current feedback was being lorded on the thread to the extent it got a bit heated . I was of the impression LM3900N was for making audio compressors of about 1% THD so never gave it a second look thinking the distortion was inherent . Is the Norton ( op ) amp worth a listen ?
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Old 6th September 2013, 12:54 AM   #10247
gootee is offline gootee  United States
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http://www.mouser.com/ds/2/405/slos059-122258.pdf

The LM3900N quad "current-differencing amplifier" is only $0.34 each in qty 10.
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Old 6th September 2013, 02:02 AM   #10248
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.. and noisy as all heck, as I found out many years ago when I tried to use it in a general purpose audio amplifier role.
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Old 6th September 2013, 02:24 AM   #10249
bcarso is offline bcarso  United States
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Originally Posted by Don Hills View Post
.. and noisy as all heck, as I found out many years ago when I tried to use it in a general purpose audio amplifier role.
confirm
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Old 6th September 2013, 04:13 AM   #10250
gootee is offline gootee  United States
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Yeah, the datasheet even admits as much.
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