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What's going on at the input stage? Electrolytic capacitors minus to minus

neteraser

Feb 5, 2017
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Hello dear experts!
I'm having a hard time trying to figure out what's going on at the mic input here:
http://schems.com/bmampscom/art/art Tube Pre M3 sch.pdf
Could anyone please explain me what's going on with polar electrolytics connected minus to minus? Does this config let the signal pass through?
Next. What are the diodes? I know it should be some kind of the input protection (limiting), but how does it really work? Why do you see two diodes connected to +33V and one to the ground?
And more importantly, is there a way to optimize this circuit (i.e. remove unnecessary complexity)? I'm of the opinion that this circuit doesn't sound really good, so I'm looking for a way to mod it.
Could I just remove the 47uF caps and first diodes and replace the 100uF ones with 33uF ?
I'm not very good at electronics, but usually it's not hard to understand the basics with some help or googling. This time I have troubles. Hope someone can help.
Thanks in advance.
 

duke37

Jan 9, 2011
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C12 and C14 are connected back to back to protect each capacitor from reverse voltages. The nett result will be about 33μF as you assume. At 15 years with no bias, they may need reforming. I would think that the amplifier input stage will have quite a high input impedance and a modern unpolarised foil capacitor could be used as a substitute.

The diodes CR 11 and CR13 are to keep the input voltage between 33V and 0V to protect the input stage. They will do nothing if the input is resonable.
 

neteraser

Feb 5, 2017
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Welome to the forum
Thanks! Always glad to learn more!

C12 and C14 are connected back to back to protect each capacitor from reverse voltages.
Seems reasonable. But why anything like this isn't discussed in this paper for example ?
http://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf
They use just one polarized cap there...

At 15 years with no bias, they may need reforming.
I heard about coupling electrolytic capacitors bias before, could you please explain me what is it?

The diodes CR 11 and CR13 are to keep the input voltage between 33V and 0V to protect the input stage. They will do nothing if the input is resonable.
Now I do understand that because I found a couple articles about the diode clipping, but what's the reason behind also placing CR9 ?

P. S. I'll look into unpolarized caps! Thanks again!
 

neteraser

Feb 5, 2017
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Btw, in my opinion this circuit can become much more stable soundwise with C40 removed (that is grid to cathode 330pF cap at the tube). I'm considering also removing C35, or the NFB loop altogether. These mods probably increase the noise floor, but who cares, it's better to let the noise reduction plugin do it's job than to have nothing from the original amp when recorded.
Also I noticed that with this mod (C40 removed) the presets (which are essentially EQs) work more like EQs, rather than "presets" which are supposed to change the concept/application that ART developers intended (with a questionable result).
Please do tell me what you think of this mod and whether or not you think it's technically correct. I do care. In my opinion 12AX7 tube shouldn't be used that way (with C40).
 
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