Low signal-level preamplifiers...

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Low signal-level preamplifiers...

Postby Steve Anderson » Thu Jan 26, 2023 7:24 pm

Low signal-level preamplifiers are now so easy to do, though within limits...

In the case of a few tens of microvolts or many millivolts these days comparatively easy...forget the BC109Cs and the like from the late 60s, there are many op-amps today that can easily out-perform them, and I don't mean in just Audiofool applications. One might think of the LM833 or the NE5532/4, but there's a comparatively new arrival, the NJM2068. It is quieter, to a degree, than the previously mentioned devices, and around half the cost, though generally that's not an issue.

I've built a few dozen of the following mic pre-amps for a large stage audio mixer, hence me being quiet over the past months, and it lives up to expectations. It's available in the UK (RS for example) and presumably others too. It's a dual device in DIP8 as well as SMD.

There is, or more correctly, were, historically quiet devices, e.g. the LM394, SM2210 and many others, but they're made of Unobtianium these days, and this is a realistic alternative to them. The circuit is for balanced microphone inputs, but at these signal levels, including NBTV/SSTV it's in the best interests to keep low-level sources balanced to reject hum and other common-mode interference. As a mic pre-amp it's hard to beat, even with high-level (loud) sources, e.g. drums and 'energetic' vocalists...but in the quietest of passages it's as good as absent, a straight piece of wire with gain...as far as audio or similar frequencies go.

I'm not saying it's perfect, nothing ever will be, but it's another step forward...

It's designed around dynamic microphones with a source resistance of approximately 100-200 Ohms and a load impedance of 1-2k Ohms.

Phantom powered mics need some other considerations...

Steve A.

I'll come back to the CMRR pot (VR101) for those interested later...
It goes without saying, all resistors must be new 1% Metal Film...otherwise you're wasting your time...ignore the reference to C121...
I've included the NE5532 as it's a back-up to the NJM2068 if that's not available where you are, the pin-out is the same so a drop-in replacement is possible in the future....

Mic Preamps Initial 2-Model.gif


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Re: Low signal-level preamplifiers...

Postby Steve Anderson » Sat Jan 28, 2023 12:44 pm

If you don't mind opening your wallet wide, the AD797 could be a replacement for IC101a and IC101b, but in single packages only, not duals/quads. But at around 15 GBP (quid/Sterling) each you may have to call your bank manager....though it is quiet, it depends if you really need that. I would find that outlay hard to justify.

Whenever I see a part number that starts with 'ADxx', (Analogue Devices), I know it's going to be expensive...

Steve A.

AD797 datasheet attached, a very thorough read of it is advised, not for the faint-hearted...

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Re: Low signal-level preamplifiers...

Postby Steve Anderson » Sun Feb 12, 2023 3:59 pm

As a follow-up to the above, another alternative for IC101 is the drop-in LM4562, much cheaper (as you'd expect) than the AD797 - the LM4562 is from Texas Instruments (TI)....with a vanishingly low distortion of 0.00003% in most applications, and is stocked by the usual suppliers. One-off price around US$3.00. In 8-pin DIP and SOIC packages. It's a dual device, one package, two op-amps.

It's been around for a decade, the attached datasheet is from 2013, but this is the first time I've been aware of it.

Steve A.

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Re: Low signal-level preamplifiers...

Postby Harry Dalek » Wed Feb 15, 2023 5:48 pm

Could you use the idea with a solar cell Steve i have used the NE5532 before with Light sensitive diodes transistors .
Just wondering if the circuits could be adjusted to use for Nipkow cameras or such .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Low signal-level preamplifiers...

Postby Steve Anderson » Wed Feb 15, 2023 6:21 pm

Yes, absolutely, exactly one of the applications I had in mind. Whether photo-diodes, or other sensors. It is/can be DC-coupled eliminating the poor LF performance of previous designs. I'm not sure of the HF performance at this point, but I would guess more than adequate for NBTV, above 50kHz at maximum gain.

Steve A.
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Re: Low signal-level preamplifiers...

Postby Harry Dalek » Wed Feb 15, 2023 11:07 pm

Steve Anderson wrote:Yes, absolutely, exactly one of the applications I had in mind. Whether photo-diodes, or other sensors. It is/can be DC-coupled eliminating the poor LF performance of previous designs. I'm not sure of the HF performance at this point, but I would guess more than adequate for NBTV, above 50kHz at maximum gain.

Steve A.


Oh great i will give this a go for sure it will be on my to do list ,last few years been doing pmt cameras only made one flying spot will a solar cell camera pickup , would like to try some solid state versions if any thing drops the size of these things.
50 khz nice bandwidth Steve .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Low signal-level preamplifiers...

Postby Steve Anderson » Thu Feb 16, 2023 10:42 am

I'll do a simulation of the pre-amp a bit later, that should give us an idea of the bandwidth at various gains...that's if I can find a Spice model of the NJM2068, if not, I'll do a simulation using the NE5534, the NJM2068 should be a little quieter, otherwise much the same...

Steve A.

Later...there is a slightly quieter version than the NJM2068, the NJM8068, same manufacturer, New Japan Radio. RS in the UK stock them at around a quid each, 244-8620. However, unlike the NJM2068 they're not available in DIL/DIP packages, only SMD versions. The difference in noise levels is really academic...I'm not going to bother with them until a new batch of PCBs is required.

Later still...Bandwidth comes out much as I estimated, -3db at 44kHz with a gain of 60db (x1000). But simulators, or the Spice models used, can sometimes be wrong. So I don't take the results as gospel until several real-world examples are built and tested. Bandwidth simulation attached, showing results at 60db gain and down in 5db increments, i.e. 60db, 55db, 50db and so on...

NJM2058 1.gif
NJM2058 1.gif (10.29 KiB) Viewed 3209 times

The attachment filename is wrong - it should be NJM2068 1.gif

I've made some minor alterations to the circuit posted at the start of this thread, and they are minor. It's not worth posting an update at this stage...
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Re: Low signal-level preamplifiers...

Postby Steve Anderson » Thu Feb 16, 2023 3:19 pm

Just out of interest, and just for the heck of it, I feel inclined to put this mic pre-amp head-to-head with a pseudo-1950s valve/tube design. The outcome is a foregone conclusion, but I've been using a valve/tube mic pre-amp for Skype, PC applications and the like for years. Would anyone 'on the other end' notice any difference? I very much doubt it. I need to find the circuit for the existing valve/tube pre-amp first....

Steve A.

Found the circuit...you'll notice that it has an unbalanced input and a fixed nominal gain of 44db (x160)...but it has done me well for some 25 years so far, with the original tubes...though it's only on when I need to use it...perhaps one hour a week as a guess...a total of maybe 1400 hours so far...

The heater for the EF86 (V1) is powered by DC from a LM317 regulator, set up to provide 6.3V at 200mA, which eliminates any residual hum. V2, the cathode follower heater is powered by AC.

Unusually I didn't date the drawing, but it's probably from the mid-late 90s...

Mic Preamps Initial 3-Model.gif
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