John Curl's Blowtorch preamplifier part II - Page 3317 - diyAudio
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Old 21st January 2013, 02:52 PM   #33161
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Good Idea Bonzai. It will only triple the power supply draw, and 3 times the heat. This original design that I put up was for insertion into the Parasound JC-2 preamp to provide phono input. More heat and more 3 times more current was not recommended.
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Old 21st January 2013, 02:53 PM   #33162
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Good Idea Ed!
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Old 21st January 2013, 02:54 PM   #33163
SY is offline SY  United States
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Originally Posted by simon7000 View Post

I think you are going off on a tangent again.
Nope, just correcting your false assumption about my measurements and Jones's. And you may also be incorrect about datasheet methods (confusing ESL with ESR). I haven't looked up any IEC standards.

See Figure 19.

The point remains, whether you care to deal with it or not, a 10,000uF cap has impedance in the milliohms at 3k, 6k, or wherever you decide to move the goalposts. With practical, properly engineered basic regulated supplies (which is not a high hurdle), the noise from the 3k, 6k, or whatever ripple component at the output of a good IC based preamp is lower than -200dBV, deeply buried in the thermal noise of the universe. It doesn't make as much fun for articles or internet postings, but it's nonetheless true.
“Just as soon as we get our opposable thumbs on an interesting concept, the rest of our chimp brains hijack it for a drunken joyride down the interstate.”
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Old 21st January 2013, 02:54 PM   #33164
bcarso is offline bcarso  United States
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Originally Posted by john curl View Post
Bcarso, your concern is noted, but the emphasis in this design was performance with a large number of phono cartridges, BUT NOT EVERY POSSIBLE CARTRIDGE. It is virtually impossible without significant compromise in op amp loading or extra cost.
There is no 10dB penalty, because the moving magnet cartridges have their own 500 ohms or so DC resistance, so even IF I reduced my 500 ohm feedback resistor to 50 ohms, I would gain only 3dB in the MM gain position at best. Of course, current noise will make it even noisier with the very old phono cartridges, as their inductance is so high.
Unfortunately, there is no popular 'in between' IC op amp, so we are stuck with either relatively high noise jfet input op amps or very low noise bipolar op amps that can't drive their own optimized feedback resistors without compromise.
It is naive to think of this design as an all-out assault on phono noise. It is a mid-range product, designed to fill a niche in the marketplace.
I'm sure it's fine for what it is, and indeed most MM cartridges have high internal thermal noise. One Ortofon product described as a high-output moving coil managed 80 ohms and 450mH, if memory serves, and wanted to see 47k damping, so it's an example of something that could benefit from low current noise, including the contribution of that termination.

To Bonsai's comment: well, the motivation is, "because it can be done", and secondarily, because it hasn't been done all that well yet. And beyond that, because other inductive sensors may be able to benefit from the various techniques brought to bear. And as John points out, there is no one-size-fits-all opamp for the purpose, that can manage the drive requirements and provide a good noise match.

Also, I think noise in different bands and with different statistics can be perceived differently. A persistent component of high frequency noise, for those of us not too affected by presbycusis, may be noticeable even in the presence of other masking at other frequencies.

Last edited by bcarso; 21st January 2013 at 03:24 PM. Reason: dept of redundancy dept
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Old 21st January 2013, 02:59 PM   #33165
jcx is online now jcx  United States
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from 30 mW to 100 mW??? - in something plugged into the wall?

really doubt you have to jump to the next xfmr VA or add fins to the case

and as for the quiz - is that layer wound, bifilar, toroid, EI, 2 section split bobbin or "hum bucking" dual split bobbins on UI or R core?
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Old 21st January 2013, 03:06 PM   #33166
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Hello Ed, it's not like you to far as i can see those two schematics are identical ????

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Old 21st January 2013, 03:09 PM   #33167
PMA is offline PMA  Europe
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Originally Posted by bcarso View Post
I wish we could persuade JA/Stereophile to measure phono noise with the input open, as well as what they do now (input shorted), and then back out the current noise contribution based on various cartridge loads. MM applications in particular, given their high inductances, will be affected significantly.

JC's (and some others') JFET-based preamps have low current noise, as well as very low voltage noise. Most op amp-based designs have high current noise, unless they are JFET input, in which case they usually have rather higher voltage noise.
I agree that the measurement with input shorted is telling not much for MM preamps. I measure the noise with input shorted and also with input connected to the real cartridge. Above 2 or 3kHz we may see a contribution of the 47k loading resistor. I would also appreciate if JA used log frequency scale. Lin scale masks eventual poor supply line induced noise rejection.
Gain of the measured preamp is 40dB/1kHz.
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Old 21st January 2013, 03:14 PM   #33168
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Originally Posted by Bonsai View Post
Just place a 3 transistor discrete buffer on the output of the op-amp inside the feedback loop.
I will go for 4
I don't understand the john's problem. Or you need low noise at low levels, and you can set the resistances value low, or you need output buffer, and you don't care about the resistance's noise and/or you can add a buffer as in this schematic.
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Last edited by Esperado; 21st January 2013 at 03:20 PM.
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Old 21st January 2013, 03:16 PM   #33169
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Originally Posted by Max Headroom View Post
Hello Ed, it's not like you to far as i can see those two schematics are identical ????.
Senses of the trasfo coils.
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Old 21st January 2013, 03:20 PM   #33170
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Figure 19 is impedance vs frequency a different measurement. You will never see -200 dB (re 1 V) ripple ever in a real circuit. It just ain't possible.


They are different, it is a where's Waldo difference! But very important. (Hint, there is a reason why a center tapped transformer is not used.)

BTY Thank You for being willing to venture out into the the dungeon of DIY Audio.
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