John Curl's Blowtorch preamplifier part III

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Funky!

My next project works with AKM. No NDA's, just a company making good products and giving proper info on them hoping that other companies will use their product.

NS blue book was a good example.

AKM is barely any better. Their datasheets are not amazing compared to AD, TI, or Cirrus (at least for audio parts). I believe they hold back the full datasheet for serious customers. I will give them points for adding Digi-Key as a distributor and at least releasing a basic datasheet.

From personal experience, Maxim and the like have parts that aren't even commercially advertised and you have to sign NDAs for, but the quality of datasheet is much higher. I suspect it's easier to get access to a good FAE too.
 

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Nice. ;-)
Not sure that we don't all lose with the way we return back to electricity.
We could have standardized the batteries (plug and play) and transformed the service stations into charging halls. The way fuels are standardised.
All it takes is an automated mechanical replacement system so that in one minute, we have filled up with energy. Stopping the whole car to refill is stupid, don't-you think ?
 
the batteries would be owned by companies providing this service.
Is not this the case with some electric cars manufacturers, like Renault ?

It is the only logical way, as I see the things. Batteries need to have a second life, at a big scale, as energy tanks for renewable energies. And a third one as
lithium mines and other rare materials etc.

When everyone will have a Tesla, it'll be cool, the tail at the electric pump on the days of big departures on vacation. ;-)
Our world is more and more crazy.
 
What is the source you are using with 100K Ohms impedance? Maybe a ceramic cartridge but even then it has a reactive Z lower than 100K. Thermal Noise Calculator comes up with -104 dBV with 100K 20 KHz bandwidth. Subtract 20 dB for the 100 mV signal and you get 84 dB SNR. Except for some really nasty ceramics the distortion products would mostly be lower if there were a voltage across that cap.

One aspect not mentioned is the cumulative effect of cascaded high pass filters. An audio recording will have at least the following high pass filters:a microphone (1), a mike preamp (2) a DAW (typically 3) a digital link and then a DAC (4) a preamp (5) an amp (6) and a speaker (7) so a seven pole high pass filter. Looking at part of it would be misleading. and on a seven pole filter even a small change in a part can have a big effect on the overall behavior. Just maybe some of the cap sound comes from small changes in value?

Demian,

Current big project is microphones. As you pointed out capacitor types of signal sources are high impedance. Also capacitor coupled sources have the impedance rise as the frequency decreases. (Nothing new.)

As to noise level the thermal noise of a 100,000 ohm resistor at room temperature I get 41 nanovolts per hertz (doing in my head) sqrt (100,000/60), times 141 for the 20,000 hertz, so last night I must have made an arithmetic mistake.

Yes the distortion products are lower than the noise level, so it would make sense to lower the resistance to get the noise level down, probably to equal the capacitors distortion level. To raise the input impedance to not load the source can be done with positive feedback. (If I get it this does raise the noise level but by a square root of the noise function.)

When the TI guys stopped by they looked at where I put the DC blocking capacitor in the gain stage opamp feedback loop and were surprised I did not put it to ground but rather in series with the inverting input.

ES
 
Nice. ;-)
Not sure that we don't all lose with the way we return back to electricity.
We could have standardized the batteries (plug and play) and transformed the service stations into charging halls. The way fuels are standardised.
All it takes is an automated mechanical replacement system so that in one minute, we have filled up with energy. Stopping the whole car to refill is stupid, don't-you think ?
In a communist state maybe, far too egalitarian, or would you have different classes of battery for different classes of customer? I can't see Mr Marsh pulling up in his electric Bentley and having a battery that's been in any Tom Dick or Harry's jalopy, can you? He has an ego to maintain.
 
DPH, did you measure the DA or dielectric absorption of these caps? It is probably several %.

John, it's not my study (but one I remembered), not having an AP of my own. Personally, I do not worry about DA for filters such as DC blocking caps in the feedback network as nominally the DC value across it is very stable and the AC signal is very low. AC coupling cap size obviously depends on loading.

What is actually being proposed is to use coupling capacitors at least an order of magnitude greater in value than what would be required for any given low frequency cutoff.

So the actual trade off is noise vs low frequency distortion. My math says on a preamp with a nominal input level of 100 millivolts would have thermal noise at -75 dB with a 100,000 ohm input impedance. So the distortion of the capacitor would not be the limit! (A valid test would be to match the capacitor distortion at a load that would have noise matching distortion level.)

As we all should know, the best test is to apply the part in service, so it all depends on what you need. Not sure why you need a 100k input impedance.
 
In a communist state maybe, far too egalitarian, or would you have different classes of battery for different classes of customer? I can't see Mr Marsh pulling up in his electric Bentley and having a battery that's been in any Tom Dick or Harry's jalopy, can you? He has an ego to maintain.
So, you are jealous because you don't have a Bentley ?
(What kind of pleasure do you take from attacking a member that cannot answer-you putting the discredit on him ?)
 
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Derfy,

What value capacitor do you need to have say 120 dB down voltage across the capacitor at 3,000 hertz with a 10,000 ohm input impedance?

Typical for a CD signal figure .002 volts at both 3,000 and 20 hertz.

I get 80 uF for 20 hertz. Pretty big as a film capacitor.

As an electrolytic with no DC across it....

One of the projects in the back room is rebuilding a Syncon B mixing console. It uses NE5534 opamps with every stage capacitor coupled. As there is no DC across them they turn out to be pretty much the factor that reduces performance.
 
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