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Hi InmatesI've searched the archives, but I can't find much on the subject of stray and wiring capacitances when determining cap values in RIAA networks.
Is there any rule of thumb when assigning a value for strays? Morgan Jones' "Valve Amplifiers" quotes "a few pF" for strays, which isn't much help. Should it be based on the total length of the wiring used in the network? Or should a rough figure, eg. 5, 10 or 20 pF be assumed? It seems a bit futile to aim for accurate cap values in the network when the strays are so difficult to determine.
Any help would be appreciated.
Cheers
Doug
Follow Ups:
Why not do it backwards ... remove all timing capacitors from the actual circuit (except the interstage ones of course), and plot frequency response. The actual treble rolloff pattern can then be reverse calculated into model capacitances for each grid / grid+plate.
I assume you are not talking about cartridge loading but rather values for the parts in an RIAA EQ network.Doug, from what I've learned, the best you can do (with the calculator) is the best you can do. Match your parts tightly, figure out a best guess, and you will be VERY VERY close and get good sound. Then to get perfect measured accuracy requires adjusting the final parts values (obviously, resistors are much easier to do this with) while feeding in the signal from a reverse RIAA EQ circuit of known accuracy. There is info on this in Allen Wright Tube Preamp Cookbook.
In practice, I followed Jones' approach (though not his circuit) and I like the tonal balance so well I have not gone that extra mile. I intend to build another RIAA using the Jones or OZLoesch circuit but right now I have a version of Bob Danielak's Octal Phono preamp running which is a sweet sounding phono pre. Probably NOT the state of the art, though.
I found it ESSENTIAL to get channel-to-channel matching as close as possible - buy lots of caps and measure them.
-j
Hi,
I suppose "a few" pF wouldn't matter that much as the total loading should be about 200-350pF for a MM cart (or did I goof here?). So wether the total figure is 250 or 262 pF is quite uninportant compared to the variation in the cart (from brand to brand and from item to item).my 2-öre
mvh /Pär
Hi,
Parden me for answering the wrong question :-) But still... (network values - in a passive - are quite large compared to strays, and the riia scheeme isn't that exact but differs (from brand to brand and mastering to mastering))
mvh /Pär
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