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Old 14th January 2007, 04:49 PM   #1
pjpoes is offline pjpoes  United States
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Default Using a CRC to bias opamp

I want to try biasing the opamps in my phonostage project to class A a bit using a CRC, IE current regulating diode. I'm not very familiar with these parts, but understand what they are and essentially how they work. I understand that a cascoded jfet might be a better design, and possibly cheaper, but a little more complicated and I'm concerned it wont fit, since the board has no place for it, so I have to wire this in point to point under the opamp.

I figured the CRC route was the simplest. I have a few questions about this though. First, given the way inwhich most typical RIAA cuircits work in opamp based phonostages, does the slight capacitance of the CRC screw up that stage? If so, how do I know how much capacitance, I imagine I can measure it, and can I solve it by simply recalculating my values taking that capacitance into account.

In the case of my Hagtech Bugle phono stage, which uses three dual opamps, do I need two CRC's per opamp, or just one. I haven't looked and should, but do they have two negative outputs or one common for both channels, inwhich case I imagine I only need one. Also, does anyone know if, since he uses such large values in the phonostage, in the uf range, if the slight capacitance will throw off the RIAA much if any?

I also wanted to try this in my musical fidelity X-LP mod project, but without a schematic, I don't even know for sure where the RIAA stage is, if its active or passive, etc. I think, unless some of you know this makes no sense, that the opamps are used in the final gain stage, and that a set of Dip8 transistors are used for the initial gain stage. This is based on the fact that the opamps are located near the output wire, and the transistors are located closer to the input. Also, I believe I correctly traced out the outputs to the opamps. I also found a few similar topolagies on this phonostage to the example cuircit given by the LT1115 and AD797 phonopreamps, so I believe it might be a somewhat similar but optimized design.

Thanks for any help on this. If it looks like its going to be more work than its worth, I may just give up on it. I mean, its recomended in the tech papers from Linear, its recomended by various diyers and so on, but I never see it used in production or even kits really. I did use the jfet version of this trick on a headphone amp and really liked the difference, and I have also used it on CD player mods and preamp mods with similar enjoyable results.
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