John Curl's Blowtorch preamplifier part II

[syn08]

Ok, so I took the best power supply buffer, the PN4392, measured here and added a gate-source resistor of 360ohm. Under the circumstances, the drain curent went down from 47mA (Idss) to about 8mA.

PN4392
-------
Datasheet:
Idss=25...75mA
Vt=2...5V
Vn@100Hz=Unspecified

Measured:
Rs=360ohm
R=2.2kOhm
Vo=17.8V
Noise=139.7nV/rtHz

Noise increased about 40%. This is an important increase, but somehow less than expected. Does it make theoretical sense? Yes it does, though. It is enough to imagine the JFET with a source resistor as a new device, having an equivalent Vt' (that is, the algebraic sum of the original Vt and the voltage drop on the source resistor) and an equivalent Idss, that is the drain current at the new Vgs'=0 (that is, about 8mA). Now, for such a device, a strictly parabolic relationship between Id' and Vgs' doesn't really hold, unless Rs is very small (not the case here). However, we can get an image on what's happening; by adding a 360ohm resistor we added a significant noise source (has 2.4nV/rtHz). However, the noise gain is now 2*Vo/Vt' rather than 2*Vo/Vt and certainly Vt' is larger than Vt (again, because of the voltage drop on Rs). Therefore, adding a source resistor increases the equivalent device noise, but decreased the noise gain. From this tradeoff, the overall noise increases only 40% rather than in the ratio of device and equivalent device noises. The first picture below shows the noise spectra. As usual, the high imput impedance leads to some 60Hz pickup.

Now, (drums are rolling) the much expected case of a LED fed by a JFET. I used the same setup as above, but replaced the 2.2k resistor with the same red LED made in ACME China. The noise (in the second picture below) is marginally larger (less that 10%) and, if you consider that the current was increased to 8mA, could actually be thought as unchanged. This is again to be expected; after all, the JFET current source injects some noise by itself, as above, but the dynamic impedance of the LED is rather low. What is though remarkable is the effect of a combination of LED's rather low impedance with the good isolation with respect to the input voltage (due to the current source, rather than a simple resistor). The ratio between the LED dynamic impedance and the JFET output impedance is now very small, therefore the 60Hz pickup almost vanished.

And a last comment on LEDs. It is not unexpected that LEDs *may* have lower noise than any zener. After all, LEDS are forward biased junctions, and there's not much intrinsic sources (excluding second order effects) that would make a forward biased junction very noisy (as compared to impact ionization, etc... in breakdown junctions as zeners). Perhaps a forward biased diode is one of the best (from a noise perspective) reference device, but has only 0.6...0.7V. LEDs have more than that only because they are not build on silicon, but on a material with a much larger bandgap, otherwise, physically, they are, again, forward biased junctions. Now, there are good reason why I would not blindly trust LEDs as a low noise device; they are not designed for such, and the noise variations from manufacturer to manufacturer and batch to batch can be significant.

P.S. If I'll be in the mood over the weekend, I'll build such a buffer on a small board with SMD parts (I have some MMBFJ177 (J177 in SMD) from OnSemi) and then measure the Sij parameters up to 6GHz. That will end the myth of such a buffer having some amazing RF rejection properties. I did this before so I already know the answer The best rejections are for the LED and... for a simple resistive divider connected to the serial device base or gate (of course, decoupled to the ground).

[Bonsai]

This a very interesting discussion. I would tend to agree with Syn08's comments about the LM3xx not 'filtering'. These are active series regulators with a finite GBW, so they run out ouf steam quite quickly (approx 1 -2 MHz). However, in my view they are great devices that do a solid job.

For my part, I recently built an LM4562 op-amp based line pre-amp. I used simple 22 Ohm in series with each op-amp supply pin and then a high quality 100uF to ground (capacitor return paths are critical for noise in this approach). Since the LM4562 has outsranding PSSR, this is a valid approach. If you ar e designing discrete gain stages and/or using topologies with less PSSR (e.g. current feedback based), then you need something better in my view - series shunt arrangement.

[Bonsai]

"I have had good results placing a simple RC filter before the 317/337. It helps roll off the RFI before it reaches the first transistor in the regs. Perhaps we could insert another RC after the 317/337 to filter before the 2nd reg."

Another approach is to do this on the primary side of the transformer.

[GK]

Originally Posted by Bonsai

"I have had good results placing a simple RC filter before the 317/337. It helps roll off the RFI before it reaches the first transistor in the regs.

I now that inductors are evil around these parts from the audiophile perspective, but is anyone here aware of just how effective a little LC RF filtering on the input side of the regulator can be??????????
I'm not talking 10H chokes here.

[john curl]

You changed the load resistor (Rl) and that is why you got more noise. Noise (total) is related to Gm Rl, low Rl, low noise.

[syn08]

Originally Posted by john curl

You changed the load resistor (Rl) and that is why you got more noise. Noise (total) is related to Gm Rl, low Rl, low noise.

Nope. Changing only the resistor Rl would lead to a contribution of SQRT(2.4k/0.36k)=2.6 times more noise, or 260%. This is not the case.

Obviously, you forgot what you read a couple of days ago. Read again about the noise gain in this configuration: http://www.diyaudio.com/forums/solid...ml#post1963702

[john curl]

PMA, thanks, once again, for your input. I might say that Jan asked a proper question, just because it is true the higher frequency gives higher self noise on a scope. We all agree to that. However, you are referring to a lot of RF from the power line getting through, I suspect, and you are right. Sometimes we have to let others come to that conclusion, before it will 'stick' and later be obvious in future to 'everyone'. Keep up the good work, PMA

[john curl]

Gm Rl does not have any square root in it, so far as I can see.

[syn08]

Originally Posted by john curl

PMA, thanks, once again, for your input. I might say that Jan asked a proper question, just because it is true the higher frequency gives higher self noise on a scope. We all agree to that. However, you are referring to a lot of RF from the power line getting through, I suspect, and you are right. Sometimes we have to let others come to that conclusion, before it will 'stick' and later be obvious in future to 'everyone'. Keep up the good work, PMA

[syn08]

Originally Posted by john curl

Gm Rl does not have any square root in it, so far as I can see.

Then the noise should be x6 larger, which is not. Really John, I know it's hard for you, but you need to go back and read.