All JFET open-loop RIAA pre-amp

[Mr Evil]

My turntable has been sitting on a shelf for many years. Finally I have done something about this and designed an RIAA pre-amp. It uses JFETs since I recently bought a load and like them. It has no global feedback partly because it's an integral part of the functioning of the circuit and partly because I never go open-loop and want to do something different for once.

As per the attached schematic it consists of two differential stages each of which implements part of the filter, the first being the pole and zero at 50 and 500Hz, the second being the pole at 2122Hz (with an extra zero at 50kHz as I have seen suggested in a few places). A source-follower buffers the output.

Differential stages use 2SK389/2SJ109, output stage is 2SK170/2SJ74.

It requires some careful pot twiddling to balance the differentials (essential for correct frequency response) and zero offset. The various source degeneration resistors need to be chosen to achieve the desired gain with particular JFETs.

Since I have not listened to vinyl (at home at least) for a few years and have no other pre-amp to compare, I can't really judge how well the circuit performs, but it's certainly not bad.

[analog_sa]

You can actually get stable offset from this circuit?! Looks not bad at all. Have you considered adding current sources in the tails?

[Zen Mod]

reminds me to Hafler preamp........
I relly dunno what number,but it's on their site anyway

[Mr Evil]

Originally posted by analog_sa
You can actually get stable offset from this circuit?! Looks not bad at all...

Apparently yes. I wouldn't trust it with a power amp without an input cap, but the offset does seem to remain constant and low even after an hour or so of running. I suppose that using monolithic dual FETs takes care of thermal matching so offset of each pair should remain low, and since the same current flows in both differentials, any change in their offset should be roughly equal and opposite, leaving nothing at the output.

Originally posted by analog_sa
...Have you considered adding current sources in the tails?

Indeed I have. It's just a prototype at the moment, so still room for change. I will probably change R3 for a CCS (JFET, of course ) and possibly also R5, which will give a nice boost to PSRR.

[ljozsef]

Hallo,

Did anybody think of using it with a balanced pre/amp?
I wonder if it could drive them without the actual complementary stages. Using FET or MOSFET input amps (e.g. Pass or its diy clones) it could succeed.
And with dual FETs only in the circuit the offset will be minimized.

Any thoughts?
Laci

[jam]

You might be on to something here...........You might want to consider doubling up the fets in the first differential.

Jam

[jam]

......for noise reasons.

Jam

[Mr Evil]

Originally posted by choky
reminds me to Hafler preamp........
I relly dunno what number,but it's on their site anyway

I searched around but couldn't seem to find that one. However it is not an unusual design: I have seen plenty of double-differential JFET amps. The only thing about it that I haven't seen before is the way filtering is accomplished.

Originally posted by jam
You might be on to something here...........You might want to consider doubling up the fets in the first differential.

Jam

Do you think that would be worth it? Those JFETs are already low noise and not cheap. Maybe if I were going no-expense-spared, but this is something that will see only occasional use.

That makes me think of something: Is it better to have n-channel or p-channel for the input? The datasheets say noise is the same for both.



I've attached what the sims say the distortion spectrum at 1kHz should look like. Higher order harmonics are very low, which is nice.

[Onvinyl]

That makes me think of something: Is it better to have n-channel or p-channel for the input? The datasheets say noise is the same for both.

I recall borbely opts for npn in his papers (fets-the new frontier in audio), and I think he did for noise reasons (not sure).

The only thing about it that I haven't seen before is the way filtering is accomplished.

You may want to visit this thread:
Balanced MM phono stage : When Aleph Ono and Xbosoz meet...

Rüdiger

[EUVL]

Nice and Simple Circuit, but enough gain for MC ?

And how about making it fully symmetrical with differential output ? Probably not much more needed than adding another source follower on the opposite arm ??


Patrick