John Curl's Blowtorch preamplifier part II

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#51,821
gerhard said:
Amplifier and measuring object are each in a massive hammond box.

I wished I had a few dozen of these ferrite tiles that a friend of mine
used to construct a small hall (with doors for people & rolling in devices)
for fieldstrength certifications.

Gerhard
Click to expand...

Too man variations to chose among:  EMI Absorbers: Electronic Solutions : 3M United States There are more exotic available and they are heavier than they look.

There are some other options including a roll material that may be much easier to use.


Paralleling opamps-
One of these http://www.ti.com.cn/cn/lit/ds/symlink/lme49724.pdf gives you 4 2.1 nV/rtHz opamps to parallel but that only gets you close to .7 nV? (too much for my head at this hour), but since they are all on the same substrate they may not need any balancing stuff. They are cheap as well ($3.30 USD at Digikey) That and correlation of several would get pretty deep into the noise for measurement.

Scott:
What do you mean by transformer feedback to get low noise? It sounds intriguing.
 
#51,822
1audio said:
Too man variations to chose among: *EMI Absorbers: Electronic Solutions : 3M United States There are more exotic available and they are heavier than they look.

There are some other options including a roll material that may be much easier to use.


Paralleling opamps-
One of these http://www.ti.com.cn/cn/lit/ds/symlink/lme49724.pdf gives you 4 2.1 nV/rtHz opamps to parallel but that only gets you close to .7 nV? (too much for my head at this hour), but since they are all on the same substrate they may not need any balancing stuff. They are cheap as well ($3.30 USD at Digikey) That and correlation of several would get pretty deep into the noise for measurement.

Scott:
What do you mean by transformer feedback to get low noise? It sounds intriguing.
Click to expand...

4 in parallel would only give you 1.05nV/rtHz.
 
#51,825
#51,826
EUVL said:
Why use ferrite, instead of a highish gate resistor like 1k or even 10k ?


Patrick
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PMA answered this for you already Patrick. Noise from a 1k resistor resistor is very significant in the context of a low noise active device.

The SMD ferrites are great because you have very low DC resistance at LF (so up to a few hundred kHz) but above that Z rises quickly - there's plenty of options that will give you hundreds of Ohms in MHz region (they usually quote Z at 100 MHz IIRC).

If you are using LM4562/49710 opamps and similar on input stages, a ferrite followed by a suitable cap to ground gives a very effective RFI filter. A quick test is to place you mobile phone on top of your pre or power amp and if you hear the characteristic zzz . . zzz . . .zzz you have a problem. BTW, not many devices pass this test - its brutal.
 
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#51,827
I understand the benefits of the ferrite, but aren't you worried about linearity or saturation ?

I know that the gate has no current, and it is very small signal, and probably noise is much more than lineairty in this case.
But still, I have a prejudice against ferrite I find difficult to overcome.

Maybe I should just try it one day to convince myself, especially SMD .....



Patrick
 
#51,830
Bonsai said:
PMA answered this for you already Patrick. Noise from a 1k resistor resistor is very significant in the context of a low noise active device.

The SMD ferrites are great because you have very low DC resistance at LF (so up to a few hundred kHz) but above that Z rises quickly - there's plenty of options that will give you hundreds of Ohms in MHz region (they usually quote Z at 100 MHz IIRC).
Click to expand...

I discussed it with Ovidos when he was doing his preamp. One SMD inductor (.06 Ohms DC) for each 4 - BF862's and there were no problems. No sure of the exact ones he used. It's not just RFI, massively parallel devices can be underdamped and make (usually) something like an RF Colpits oscillator with the interconnects acting as L's. The lossy L on the gate kills the forward gain at MHZ. If you obsess 10 turns of #32 wire on a 1K might work too.

A customer had a 1024 channel EGC (JFET op-amps) board spontaneously oscillate which was immediately fixed by gate dampers.
 
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#51,832
scott wurcer said:
I discussed it with Ovidos when he was doing his preamp. One SMD inductor (.06 Ohms DC) for each 4 - BF862's and there were no problems. No sure of the exact ones he used. It's not just RFI, massively parallel devices can be underdamped and make (usually) something like an RF Colpits oscillator with the interconnects acting as L's. The lossy L on the gate kills the forward gain at MHZ. If you obsess 10 turns of #32 wire on a 1K might work too.

A customer had a 1024 channel EGC (JFET op-amps) board spontaneously oscillate which was immediately fixed by gate dampers.
Click to expand...

Thanks - Ovidiu's approach is the one I'm looking at - with a bit of Denis Colin and DS as well although I wont use an opamp to remove the offset between the front end ant the opamp stage - I'll use a good quality film cap for that.
 
#51,833
EUVL said:
I understand the benefits of the ferrite, but aren't you worried about linearity or saturation ?

No.

I know that the gate has no current, and it is very small signal, and probably noise is much more than lineairty in this case.
But still, I have a prejudice against ferrite I find difficult to overcome.

I've tried them. They have no (zero, zip,nada, SFA) effect at audio.

Maybe I should just try it one day to convince myself, especially SMD .....

Yes!
Patrick
Click to expand...

😎🙂
 
#51,835
Yes. There are a lot of things Charlie says. The other ones are that feedback is bad and that IC's are 'dirty little grains of sand' (but he still uses them anyway in critical parts of his circuits!).

Best to takes your advice from more grounded individuals IMV.
 
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