In Reply to: Dynaco ST70 - adding a volume control posted by snooper on March 18, 2003 at 06:42:01:
Yes. I saw one installed as a project in Audio Amateur many years ago. I dont remember the particulars. As I recall, it was installed at the input of the ST70 under the assumption that you are using a preamp with a high level output, say above 775 mv or about about 1 volt Pk-Pk. The project was done using a PAS pre-amp with the preamp volume control out of circuit. This changed the LF response and the total circuit impedence, but there were small mods to account for those changes. Not knowing exactly why you want to do this, but if you're using a PAS or similar preamp - and you're not satisfied with the fidelity, a better approach is to mod the PAS that would include bypassing the tone controls. The EQ is flat but the total range expansion across the audible bandwidth is extrordinary and you'll still retain bass boost function for low gain settings. The mods simply Poodge the board with metal film Rs and polycarbonate caps, add a few simple circuit changes and the power supply is regulated. If you are using a solid state preamp, then there will be an impedence matching problem between pre-amp and amp interface. That will have to be accounted for, probably in the selection of the new gain control as well as elsewhere at the preamp output. There will also likely be an issue regarding input level from the solid state pre-amp. It may not be high enough with a 1 Meg audio or higher gain control in-circuit. You may also want to contact the Audio Amateur folks in Peterborough , N.H. They have a file of all articles published in AA and Glass Audio and Speaker Builder, indexed by title and key word(s). They can probably find the appropriate article(s) for your reference. Mr. Welborne may also have insight. Meanwhile, I'll look for that article I mentioned. I still may have it. Good Luck.
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