I have been studying vintage phono preamps. I am looking at the Sansui AU-717 phono stage and I see parts that are drawn as two series diodes in a single package. According to the parts list, these are SS-3 and SS-3R. Are these essentially two series diodes in a single package, or something different? I couldn't find a datasheet.
Thanks,
Don
Thanks,
Don
Little mixup there, SS-3 and SS-3R are 3-pin dual rectifier diodes (bridge rectifier halves) in the power supply. The ones in the phono stage are VD1212s. These were called "varistor diodes" but I guess were more or less exactly what you suspected (or perhaps a pair of transdiodes, transistors with B-C tied together), and people commonly replace them by a pair of 1N4148s as VD series parts have a history of less than stellar reliability (which is not what you want when they are responsible for power amp biasing).
I used a pair of series 1N4148 diodes in place of the VD1212 diodes to simulate the circuit. The simulation shows the DC bias for the input diff pair matches measurements given on the schematic. So these parts are indeed just two series diodes.
The schematic implies the output stage is biased at ~3.6mA. I had to tweak the current source to get the output biased close to that. I guess that is to be expected due to how dependent the biasing is on transistor VBE.
I would have attached a screen shot of the simulation, but I don't see how to add an attachment (Insert Image button wants a URL??)
The schematic implies the output stage is biased at ~3.6mA. I had to tweak the current source to get the output biased close to that. I guess that is to be expected due to how dependent the biasing is on transistor VBE.
I would have attached a screen shot of the simulation, but I don't see how to add an attachment (Insert Image button wants a URL??)
Use the "normal" reply function, not the quick reply field. Then you'll get the "Manage attachments" button and misc. other things.
Once the attachment shows up in the main reply form, you can also copy its URL and feed "insert Image" this way.
If you only attach an image, it will show up as a thumbnail that can be enlarged by clicking on it. If you use "Insert Image", you can place the image anywhere in the text; it'll then disappear from the thumbnails.
Or-- "Quick Reply" to get started, except then click "Go Advanced" to gain access to more formatting frills and the file-upload function.
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