Little bit of Hi-Fi action

[stecykmi]

Just completed a perf version of this JFET phono-preamp. using 4 of the 2SK170's I got from the group buy a while back. That particular circuit is based on this one if anyone is interested.. Phono preamps are made to connect the extremely low output of a record player (only about 30mV) to other amplifiers (which is why most receivers have an input designated "phono"). Since I typically connect my turntable to a mixer, I decided to put in a simple balanced line driver output in addition to the regular RCA output.

Beavis has some interesting info on preamps on his Hi-Fi site so I decided to implement a switchable input loading scheme. The resistive loading is a point-to-point wired rotary switch with a few different values. The capacitive loading options are in parallel (which is simple addition when adding the values of capacitors) and it allows you a few values between 0 and 200pF.

the enclosure is an old parallel port switch box that i sealed with bondo, which is why the inside is all red. the paint is some textured automotive spray paint which seems pretty durable. the power supply is a simple LM317 regulator running at 24V. it's filtered with some big electros and a 220uH inductor in series. the whole thing is star grounded at the binding post. except for the extra differential output, which is a textbook non-inverting buffer and an inverting unity gain stage, there are no mods other than minor part substitutions (47ohm in place of 50ohm, 4.7k // 10k = 3.19k ~ 3.16k). i chose a 10uF + 0.1uF combo for C6 and C7 on the schem. the Rload is switchable, but i used 2.2Meg for R2 to insure a reliable ground reference on the input. Parts are mostly non-mojo!

i haven't played much with the loading options yet but i'm very happy with the sound. it beats the pants off my old cheapo audio-technica preamp both for clarity and noise performance. I would recommend this circuit to anyone interested in building a nice little pre. except for the 2SK170's, none of the parts are too hard to find.

[pickdropper]

Very cool.  I've thought about doing one, but I've been slightly put off by the cost.  My phono cart is a very low output MC (250uV) so I'd probably need step-up transformers to get the gain I need.

[lincolnic]

This is rad. Have you noticed any appreciable results from switching the input loading?

[Vallhagen]

Yes, very cool. I immidiately want to build one, so i guess i will go there:)

What shall i focus on when lookin for transistors? I notice that my main source Banzaimusic has two different 2SK170-s, one with "BL" extension.

Cheers

[stecykmi]

Very cool.  I've thought about doing one, but I've been slightly put off by the cost.  My phono cart is a very low output MC (250uV) so I'd probably need step-up transformers to get the gain I need.

i believe you can also use a 3rd gain stage instead of an xformer which would be easier and almost certainly cheaper. i think i saw one on this site, which is, in general, excellent source for info and projects: ESP projects.

This is rad. Have you noticed any appreciable results from switching the input loading?

i played around with it a bit today, all the controls are subtle but the resistive element seems to act like a brightness control with high values being brighter than low values. the capacitive loading is even more subtle but it also cuts high end. the cut seems to act at very high frequencies. i will have to try some more settings with a few more records. i think it might be useful for scratchy or noisy records. i currently have it set to 60k and 47pF. if i were to build it again, i'm not sure i would include the capacitive loading, but i need to try it on a few more "test subjects".

Yes, very cool. I immidiately want to build one, so i guess i will go there:)

What shall i focus on when lookin for transistors? I notice that my main source Banzaimusic has two different 2SK170-s, one with "BL" extension.

Cheers

i'm not sure, both those seem identical, the specs are basically the same, just written out slightly differently. Google it, the datasheet for the 2sk170 is available. it's a fun, reasonably simple, very nice sounding project, i recommend it.