Step-up after LCR

[dave slagle]

Hey Daniel,

The first thing that comes to mind is if the iRiaa filter you are using has the 50kHz time constant. Here is a LCR filter with the 50kHz Neuman time constant into a LCR filter with and without it.

Screenshot 2023-09-06 at 7.27.40 AM.png (19 KiB) Viewed 10577 times

I suspect if you add the resistor R6 you might be able to get some of that bandwidth back.

Screenshot 2023-09-06 at 7.29.46 AM.png (7.08 KiB) Viewed 10577 times

The other thing I notice is the difference between the notch/dip in the red and blue filters is an octave apart which suggests a change in the LC relationships... The 1:4 will reflect back 16X the miller capacitance of the following stage which can have a profound impact on the filter behavior.... particularly on extension and where it resonates with the surrounding inductances.

dave

[Daniel]

Hey,
Inverse RIAA is a software filter. It doesn't have the Neumann time constant. Currently I am still using the Allnic/Acoustic Dimension LCR in a can, which is potted, so I can't change the innards and add R6.
But I did try my luck with an RC bridging the filter and that (360pF + 39k) did away with the notch and extended the -3dB point to 80kHz.

The other 360pF for the second channel are missing in my drawers, so I haven't had a chance yet to give it a listen.

Capacitance getting transformed as well is something I have never thought about.

Thanks for looking into it.

Phase isn't pretty, so there still is motivation to switch to a 1:1 interstage into a higher Z LCR and getting rid of the 1:4 step up.

[walge]

@Dave: Sorry for my last (confused) Emails. I should have shut up and measured first.
To everyone of the smart people discussing LCR filters here: I built a phono pre:
Triode -> 4:1 interstage -> 600R LCR -> 1:4 step up -> triode -> 1:1 output.

Dave wound the 4:1 interstage and in circuit (measured at the input of the LCR) it is flat to 90kHz.

Here is a measurement of the output of the LCR. Red trace: output attached to probe/measurement gear, otherwise open. Blue trace: output attached to the 1:4 stepup (Lundahl 7903). Sorry for the noise around 40k, you still get what's going on...)

LCR Vergleich.jpg


I am not happy with the frequency response; the blue trace is pretty much what I get at the output of the preamp.

When I got the discussion right, so far it was about lf roll off, but not what happens to the hf. Docali writes in his paper "Who wants to further improve these results could bypass the whole LCR module with an RC combination. A 180k resistor in series with 150pF is a good starting point." (p. 13).
Would this be a proper step to get the output closer to the blue trace?

Or is the way to go to change the 4:1 to a 1:1, get a 1k8 LCR and get rid of the 1:4 step up?

@walge : your measured response looks pretty good. Did you have an issue like me along the way?

HI

I don't know the test set you are using but if you know the Zout of generator ( hope very low) you can trim it to reach 600 ohm then drive the 1:4 trafo + output circuit to understand it is linear enough.
I think this is the one of the firsts step to do
This beacue the first stage+ lcr seems to be fine.


Walter