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Richard Lee's Ultra low Noise MC Head Amp
Richard Lee's Ultra low Noise MC Head Amp
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Old Yesterday, 08:48 PM   #701
kgrlee is offline kgrlee
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Quote:
Originally Posted by Hans Polak View Post
So far I examined Richard's Duraglit mainly on noise, collector current and Rs.
Now I looked at the FR and noticed that depending on Rs and collector current, there can be a nasty bump below 10Hz when using 470uF caps.
The lower Rs, the larger the bump.
With Rs = 5 Ohm, it can already be as much as 7 dB, getting even larger for lower values.
The remedy is to alter these caps into at least 5mF as can be seen in the image below where 0.5mF and 5mF are both used for Rs=5R and Rs=17.5R
Thanks for this Guru Hans. I was about to exit beach bum mode, get LTspice running on a computer and see if I can still pre10 to use it

Your Fig4 in #655 shows Duraglit actually has the topology of a fancy HP filter (whose name I've forgotten) which I've used in an unrelated commercial product. I would want to build proper subsonic filtering into a matched device so might do more work later. Da beach bum stuff is a boat so this might never happen

What current is this sim at?

Last edited by kgrlee; Yesterday at 08:52 PM.
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Old Yesterday, 08:58 PM   #702
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Hans Polak View Post
Now I looked at the FR and noticed that depending on Rs and collector current, there can be a nasty bump below 10Hz when using 470uF caps.
Exactly the reason why in Richard Lee's Ultra low Noise MC Head Amp I used 7,800uF (2x3,900uF polymer) on each half.
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Old Yesterday, 09:12 PM   #703
Hans Polak is offline Hans Polak  Netherlands
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Quote:
Originally Posted by syn08 View Post
Exactly the reason why in Richard Lee's Ultra low Noise MC Head Amp I used 7,800uF (2x3,900uF polymer) on each half.
O.k. In that case, everything under control 👍

Hans
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Old Yesterday, 09:59 PM   #704
Hans Polak is offline Hans Polak  Netherlands
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Originally Posted by kgrlee View Post
What current is this sim at?
Richard,
Sim was at 3mA.

Hans
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Old Yesterday, 10:32 PM   #705
gerhard is offline gerhard  Germany
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Be careful with these Poly Alu electrolytics.

I gave up on feedback for my wideband lo noise amplifier because I could not get
1 or 2 MHz BW without negative input impedance which I find unacceptable for
a lab tool. Yes, damping everything to death works, but then the BW is gone.

I wanted really high input impedance, so I grounded the gate via 10 to 100 Meg with
an analog switch to decrease that, if the operating point is out of bounds.
Gain with a FET goes with rt(Id) and over a range of 30C the change was not acceptable.

I enforced the source current to 27 mA with a CCS made from a FZT851 mirror.
That's not too much current for 2 IF3601. To keep the source at AC GND, I used
Poly alu 4700uF / 2V5 * others for RF. DigiKey 493-4582-1-ND
2V5 should be OK for +500mV for Vgs. Did I think.

That resulted in the purple function in the picture. 30 dB/decade from 300 Hz downward.
Low cost normal ALUs 3300u/10V healed that. Wet slug tantalum was not better.

There is still 0.8dB/10K gain change in simulation. Attributable only to the FET.
I'll heat it to constant 35C.
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Last edited by gerhard; Yesterday at 10:49 PM.
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Old Yesterday, 11:14 PM   #706
syn08 is offline syn08  Canada
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Quote:
Originally Posted by gerhard View Post
Be careful with these Poly Alu electrolytics.

I gave up on feedback for my wideband lo noise amplifier because I could not get
1 or 2 MHz BW without negative input impedance which I find unacceptable for
a lab tool. Yes, damping everything to death works, but then the BW is gone.

I wanted really high input impedance, so I grounded the gate via 10 to 100 Meg with
an analog switch to decrease that, if the operating point is out of bounds.
Gain with a FET goes with rt(Id) and over a range of 30C the change was not acceptable.

I enforced the source current to 27 mA with a CCS made from a FZT851 mirror.
That's not too much current for 2 IF3601. To keep the source at AC GND, I used
Poly alu 4700uF / 2V5 * others for RF. DigiKey 493-4582-1-ND
2V5 should be OK for +500mV for Vgs. Did I think.

That resulted in the purple function in the picture. 30 dB/decade from 300 Hz downward.
Low cost normal ALUs 3300u/10V healed that. Wet slug tantalum was not better.

There is still 0.8dB/10K gain change in simulation. Attributable only to the FET.
I'll heat it to constant 35C.
Not a problem with the bipolars in Richard Lee's Ultra low Noise MC Head Amp, although I did not care of the HF response. Gain is 27dB, noise RTI about 0.25nV/rtHz (best try). Average was around 0.3nV/rtHz. Gain is 27dB, input shorted.
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Old Yesterday, 11:54 PM   #707
syn08 is offline syn08  Canada
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Quote:
Originally Posted by syn08 View Post
Not a problem with the bipolars in Richard Lee's Ultra low Noise MC Head Amp, although I did not care of the HF response. Gain is 27dB, noise RTI about 0.25nV/rtHz (best try). Average was around 0.3nV/rtHz. Gain is 27dB, input shorted.
That is 4ohm equivalent: 4/2=2ohm Rbb', 1/2=0.5ohm emitter, 1.2/2=0.6ohm collector current noise contribution (1/2gm @ about 10mA), about 1ohm is everything else. Not much left for the alu polymer caps PLG0E392MDO1 Nichicon | Capacitors | DigiKey from a bag of 20, I must have been lucky.

Transistors were extinct Rohm 2SD786/2SB737, the Zetex ZTX851/ZTX951 devices are as good... if you are lucky. I found 2 competitive (with the Rohm devices) pairs of Zetex devices out of 10, the variation is quite large, and it's not Rbb' at fault. Rather a high (for bipolars) noise corner frequency and apparently high G-R noise. Exactly what I was expecting from devices not intended for low noise.

Last edited by syn08; Today at 12:07 AM.
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Old Today, 12:50 AM   #708
gerhard is offline gerhard  Germany
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There is probably no problem with the DigiKey 493-4582-1-ND Poly Alus in a
low impedance environment, such as filtering a LiPo battery, but when used to keep
the output of a dc current source at AC 0V, every charge packet that defects through
the cap produces telegraph noise. (is telegraph 1/f**3 ?)

I'm not interested in circuits here, it's just that el Cheapo 1189-1056-ND 3300u/10V
ordinay alu electrolytic does not produce that effect.

I got some mechanically larger caps in the mean time that cost more but are promised
to survive longer at 105/125C (still 12.5mm dia). The results are still out.
I have used them in the second channel of the cross correlation machine; if they
perform there, the first channel will be updated.

One could also try 10V organic, but I'm seeing signs of operator exhaustion.
There is always so much one could test additionally.


That could be used for stereo, at least the input stage, but it's for cross correlation.
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Old Today, 03:27 AM   #709
syn08 is offline syn08  Canada
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Quote:
Originally Posted by gerhard View Post
the cap produces telegraph noise. (is telegraph 1/f**3 ?)
This would be strange, since telegraph noise (aka popcorn noise) is normally associated with micro defects in semiconductors and/or heavy metal contamination, being therefore strongly process dependent. Not much to do with the electrolytic capacitors physics. Typically, popcorn noise doesn't have a Gaussian distribution, but a trimodal distribution inherited from 1/f and Johnson noise, shifted by the popcorn noise. The power spectral density of the popcorn noise is ~1/f^a where a=1...2, usually closer to 1/f^2, definitely not 1/f^3. It is fair to say that modern semiconductor manufacturing processes made the popcorn noise largely a non issue.
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