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In the Forum: Melquiades Amplifier
In the Thread: Planning my DSET
Post Subject: Good luck... :-)Posted by Romy the Cat on: 8/21/2019
 anthony wrote:
Yes, the second DSET is still not biasing properly and I cannot find enough consecutive time to look for the fault.  In the meantime I've got a couple of minor side projects off the electronics bench, and started thinking seriously about preamplifiers.  Part of the thought process is to do with analogue volume controls and to make some decent decisions I need to know the input impedance of the DSET's.  So I measured it...see below.

Romy, the back-of-the-envelope estimation of about 5k Zin is roughly correct.  You can see in the graph above that at 20K Zin gets as low as 3.5k.  Phase is flat until the inductors kick in circa 600Hz, 3.2kHz and 10kHz which causes it to roll off or less linearly with frequency.
   
Anthony, I know about the shortage of “enough consecutive time”. I am in the same boat here… I think that fact that you took some time off is good for debugging, what you come back to it you will find what you connected wrong. It is most like one of the emails of the filter you connected to the wrong point. You juts need to fine where it is. When you have time to go back to it then write up the voltages and we can go over it together again.
The impute impedance of 5K is was very predictable, 5 line of 30K in parallel. The drop of the impedance at HF is irrelevant in my view as damping at HF is not so effective. If you do not have low enough out impedance at your sored then the whole character of lower octaves get screed and it is very auditable. Also, you filter will not work properly. The thinking seriously about preamplifiers is hard and frankly you are at your own. I still cannot fathom why Placcete does not work in you case, but it is what it is. All that you need to have a good buffer + gain that can drive current and have as low impedance as possible, DC coupled would be the best, or the simplest to archive the lowest impedance. If I were you then I would not build but try a few of the shelf cortical preams. It is not that you need to buy them but going over the trails with commercial pros you will develop an acquired taste of what you need. Also, some of them might be good for you. Do what I did: connect a few manufactures, give them the specification you need and see what they will offer to you. Most of the manufactures if they see an unusual and original need will bypass the worthless dealers and will work with you directly. Some small makers will even do some custom work for you… 
 
Anyhow, it is hard to find a good pream generally and the one that can drive a couple kOm in particularly. There are a lot of options out there, do not re-invent, adapt and improve, unless you are in a driving sit and know what and how to do the thing and how to evaluate/interpret the results properly. The last one is tricky and you need to know you own playback VERY well to decipher and discriminate the preams contribution. It took for me years to learn it and I am still not immune from mistakes.

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