Hi folks, I'm currently cleaning up an old kenwood amp and this thing is brittle and difficult to work on and is only coming g apart this once! I'm after some expert advice on whether this phono stage can be improved? Maybe with the Riaa curve?
What do you hear that is wrong?
I see a thorough design with three generous supply voltages, three JFET stages, some very specific resistor values (415k is 1% series and now hard to replace, and not likely to need replacing), by a company which blew hot and cold over the years but this is sure NOT one of their 99c preamps for the discount store amplifier market.
If you must play with it: there's at least one typo in that drawing (Cd27 vs Cd28) so it can't be taken too literally.
Under no circumstance do not state the model of this amplifier 😉
If this is "now or never again" situation then:
If this is "now or never again" situation then:
- MM cartridge load seems unorthodox;
- I would replace all lytics for new ones;
- output capacitor could be a film capacitor;
- switches need probably good cleaning.
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I also wonder why Rd3 and Rd4 are 100 kohm rather than 47 kohm or 50 kohm. Is there another 100 kohm straight across the inputs, outside this schematic fragment?
"why Rd3 and Rd4 are 100 kohm rather than 47 kohm or 50 kohm." - in old amplifiers you can meet all sorts of strange values in these places.
Please do not attempt repairs unless there is a problem, these were highly regarded sets.
As above, changing capacitors, switch cleaning and so on can be done.
The components are now simply not available for the most part, so if you damage one during your modification, you are in trouble.
You can make a circuit that feeds the main section from the output of this stage, and do your trials.
Or build another amp, of course.
By 1970 or so (PRR would know), most sets had curves that matched the RIAA curves, so there is no need to modify the circuit to match them.
At least, I think so.
As above, changing capacitors, switch cleaning and so on can be done.
The components are now simply not available for the most part, so if you damage one during your modification, you are in trouble.
You can make a circuit that feeds the main section from the output of this stage, and do your trials.
Or build another amp, of course.
By 1970 or so (PRR would know), most sets had curves that matched the RIAA curves, so there is no need to modify the circuit to match them.
At least, I think so.