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That’s great Brach, yes I’m deliberately setting the gain sensitivity high as I am thinking of using cortados within a sculptural installation, it’s a screen of fine elastics that produce very little volume in themselves. Our existing pickup system could do with improvement, hence my interest in your product.
Thanks, Nick
…..ahh but now I wonder whether setting the bias this high will damage the FETs?? I note from the original instructions that 0.15 should be top limit.
I was wrong about changing the bias current not making a difference….I was winding the pot the wrong way!! I’m able to get rid of the oscillation when used in conjunction with my Fireface by setting the bias current high: 0.3v
Happy to have found a solution and thanks again. NickSo I have discovered a new issue with the cortado when used with my RME fireface soundcard. The cortado works fine when plugged into my Mackie 12/02 mixer, but when plugged into my RME fireface soundcard it immediately goes into very loud oscillation. I’ve checked the interface with other audio sources and I also compared the phantom power voltages and I note that my mixer gives 45 volts whilst the audio interface gives 49 volts. I assume this higher voltage is causing the problem. If so, is there a way to reduce that voltage within the cortado?…..(adjusting the cortado potentiometer makes no difference to the oscillation).
Thanks Nick.…..and now I’m kicking myself…..my problem with the mixing desk was simply that I had forgotten about a compressor I had inserted into the channel I’ve been using to test the mic. and as the compressor was turned off I was not getting the signal through the desk as I should have done. Now all is well and the cortado does its job splendidly!! Many apologies for having made such a silly mistake.
Righto….now I’m getting somewhere. I finally got around to making up the Expresso phantom kit and the contact mic now works perfectly. I get 41V. from the expresso, sound quality is now very good… so it seems like my mixing desk gives too many volts for the cortado. If I want to use it with my desk how do I get around that? (This has been an interesting process…frustrating at times but I’ve learned a lot)
ps… I then went back to 150 ohms and biased up to .085v using the little blue pot. Audio quality the same.
So I’ve broken the circuit at R8 and wired out to a breadboard so I can try different resistors I have in stock.
adding a 1k resistor in series with the existing 150 ohms gives a voltage at tp of .039v
substituting a 2.1k resistor for the 1k gives me .25v
substituting a 4.6k resistor gives me .71v
as the volts go up the audio level seems to drop!! Audio quality however remains very much the same: totally distorted.
Interesting…..
disc is screened and there are no shorts. I have fitted a second disc that I made up just to be sure the disc was not faulty, but sound is still totally distorted. For testing I have the disc taped to a silver cup that resonates like a bell but even the most gentle striking of the object produces sounds akin to, well… distorted farts and high pitched squeals. To give you some background to my project I’ve been working with piezos for quite a while using an unbalanced buffer circuit from a book by Nick Collins. The sound produced is very good, but unfortunately the white noise level is unacceptably high, so I’ve been researching a high gain low noise alternative which led me to your product. The eventual application is to attach to a large sculpture object made from a wood frame and fine elastics that pass over a sort of bridge such as you would have in a guitar, but the sound levels produced acoustically are very low compared to a musical instrument, hence the need for a lot of amplification. I very much appreciate any further help you can give. I notice in an earlier topic the resistor R8 needed a different value?? Could my problem be anything to do with that?
Yes that’s right, 46v between red cable (xlr pin 2) and earth (xlr pin 1) and the same between white cable (xlr pin 3) and earth. I shortened the cable from piezo to board which I thought might be too long. (one of my own) Still very distorted.
Many thanks for your reply and sorry for the delay as I’ve been away for a few days. I get 46v from the phantom power on my mixer…I’ve checked for shorts and reflowed the joints. Audio quality remains the same. As if the circuit was gated and then scratchy bursts of distortion when I touch the piezo or attach to a resonating object.
