Ha folks,
I am currently rebuilding a Sansui AU-505. A former owner has converted the phono stage to a regular input and took out the RIAA correction components.
These consist of two resistors and two mylar caps. Now these mylar caps are unobtainium meanwhile and in a rather uncommon capacitance, I was wondering if anyone could help me out finding the right components to bring the phono stage back alive. No need for anything special, as close as possible to the original state is perfectly fine.
There seem to be two versions of the RIAA correction part, one with R621 being 180k and one with R621 being 330k.
Anyway, I guess the main issue is to determine replacement values for the caps, I hope someone can shed some light on it!
Images of the two schematics;
I am currently rebuilding a Sansui AU-505. A former owner has converted the phono stage to a regular input and took out the RIAA correction components.
These consist of two resistors and two mylar caps. Now these mylar caps are unobtainium meanwhile and in a rather uncommon capacitance, I was wondering if anyone could help me out finding the right components to bring the phono stage back alive. No need for anything special, as close as possible to the original state is perfectly fine.
There seem to be two versions of the RIAA correction part, one with R621 being 180k and one with R621 being 330k.
Anyway, I guess the main issue is to determine replacement values for the caps, I hope someone can shed some light on it!
Images of the two schematics;
I’m guessing that one of the diagrams has a misprint for R621.
In regards to the caps, as they are so cheap to buy I tend to buy loads and measure them. 2 x 2.2n caps in parallel will be close an if you measure a batch of 10 - 20 there will be a good chance that at least 4 will actually be 2n.
For the 12n cap chances are from a batch of 15n caps you’ll actually have some that measure 12n
In regards to the caps, as they are so cheap to buy I tend to buy loads and measure them. 2 x 2.2n caps in parallel will be close an if you measure a batch of 10 - 20 there will be a good chance that at least 4 will actually be 2n.
For the 12n cap chances are from a batch of 15n caps you’ll actually have some that measure 12n
The values in either of those circuits are pretty out for an RIAA network.
Better values you could use are (270k, 12nF) and (22k, 3.3nF), keeping the 330 ohm
bottom leg.
Better values you could use are (270k, 12nF) and (22k, 3.3nF), keeping the 330 ohm
bottom leg.
Nope, that's not going to happen with good film caps. 12nF is a standard value, should be obtainable, if not then 10nF || 2n2 would do at a pinch, or 10nF || 1nF || 1nF of course.
This sets the 50Hz pole.
A 2-Q phono preamp really does not have enough gain to cover 50Hz precisely.
The math (assuming infinite amplifier gain) says about 180k (perhaps 162k) here.
The real 2-Q amplifier does not have enough excess gain for that to work out. Maybe when they realized that they were nearly 6dB shy at 50Hz they doubled the resistor to 330k. My guess is that 250k-500k is as good as it gets, the exact value depending muchly on transistor parameters.
There are better phono preamps. Audio Amateur had an upgrade back in the 1970s which could be built on the PCB of most commercial preamps. eBay is full of 5532 preamps but most are dual-rail.
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