Ultrasimple mm/mc RIAA preamp 2

[wrenchone]

This looks like a neat little preamp, and easy to throw together. I will try cascoding both FETs, though, since it's only a few parts more.

Nothing is quite as good as the 2SK170 (for a single FET), in terms of transconductance or noise, but the 2N4393/PN4393 or the J110 might be worth trying just for grins. Both of these FETs are generally used as analog switches, but are also reasonable low noise amplifiers. The J110 is better than the 4393 in terms of noise and transconductance, but may have an IDSS much higher than you expect, since there is no spec maximum. The J110s I currently have are all pretty hot, with IDSS measurements over 100mA. A reasonable source resistor will tame the savage FET, though. The next batch may not be as hot...

[wrenchone]

I just did a PSpice simulation of the preamp using J110 FETs instead of 2SK170GR, and the results look pretty good. Indeed, you have to use about 150 ohms of source resistor to get the bias current down to reasonable levels (~5-6mA/device). You also need 1000uF of bypass around the source resistor to get the gain back, and higher V+ voltage to have both the gain and the dynamic range. I simulated the rest of the circuit using the same R and C values as Mad_K, but adding a current source loaded JFET output buffer (two J110s) biased at about 7 mA. I got about 0.05% distortion at 1KHz, 1mV input, 10k output load, with a very smooth distribution of harmonics dominated by the 2nd and 3rd. Not bad for an open loop design.... The thing that remains is to try both the 2SK170 design and the J110 design and see if I can hear the difference. I tried modeling a cascode version of the design, but the results were pretty icky - odd order harmonics dominated for some reason, and they weren't too small, either. I'll try revisiting that simulation and see if I can't get better results.

BTW, I'll probably use "dead bug" prototyping (transistors head down, legs in the air) on a PCB copper plane. I have double-stick taped a couple of skinny pieces of copper clad to the main ground plane to serve as the DC power bus for each channel.

Mad_K's schematic printed out on my ink jet printer looking just like a pencil drawing, smears and all - quite charming.

[wrenchone]

A little question - I've been rooting around on this site a bit , since I'm new around here, and found a thread regarding a feedback-type RIAA preamp using an OPA637, also built by Mad_K a month or two ago. I'm curious, Mr. K.... Have you done a comparison between the discrete and op amp design? If so, how was the comparison? Inquiring minds want to know...

[Mad_K]

wrenchone,

good job on the sims/other info!
-have you done a bode-plot? (If so, how did the eq perform?)

-Yes, I have done a very good comparison between them, as I merely swapped the boards (El Cheapo/Pacific) in my phono-pre between the two, and built them with much of the same parts-quality.

I can tell you right away; this one stays in the box!

I didn't notice any big difference in tonal balance between the two (both seems to give a very correct RIAA eq), but it is no doubt that I am now listening to a Single Ended, no-feedback, very simple and good phono amp. The Pacific is a little "nicer"/more romantic/musical sounding; I haven't listened to a record that didn't sound good yet. While the 'Cheapo is a very good and correct amp, it can be a little "analytical/matter of fact" as Thorsten puts it.

This little amp gives you THE MUSIC, nothing more, nothing less..

[Peter Daniel]

I will definitely try this one as well. I've always thought about building this simple preamp: http://www.metaxas.com/pages/masnewfiles/index.html (check diy section)

but the one you have here is even simpler

[wrenchone]

I did a frequency plot, but it was not set up to look at deviation from the RIAA standard. I wiggled pretty much in the right places, though. I just picked up Beige Bag Software's Spice package. Maybe I can get it set up to display RIAA accuracy. Once I find my FET sorting jig (I just moved), I can build up a couple of things.

[Mad_K]

Originally posted by Peter Daniel
I will definitely try this one as well.

Good for you! I think it'll be a good companion to your amazing chipamps

[vinylflo]

hello just want to solder together that nice and simple preamp. but i just have a bunch of 2n3819 jfets..is that ok or do i need something better???

thanks
flo

[ThorstenL]

Konnichiwa,

Originally posted by vinylflo
hello just want to solder together that nice and simple preamp. but i just have a bunch of 2n3819 jfets..is that ok or do i need something better???

The Circuit is speced for 2SK170 with an Idss (selected) of 4..6mA absolute with no more than 0.1mA difference between the two channels (per stage). That is what MUST be used if you want a working phonostage from the discussed circuit without problems.

You can of course change the design of the phonostage any way you like, change all sorts of parts and values, you are on your own though to get it working.

Sayonara

[Stanco]

I do not find the jfet 2sk170… but LSK170 is equal? better? worse? for this phono
thanks…. Marco