Well, I published the single-loop circuit with the subsonic filter on this forum so whoever wants to try it can do so. I've never built it myself, but JRA has built a version of it (which works) and Chris is working on one. You can build whatever you want, including this circuit. When used with an OPA1656, I recommend including the 220 ohm-15 pF damper.
Okay.
I won't forget that with the OPA1656 I need to keep the 220R/15pF dumper.
Just a couple of questions, for now (let's make it three..)
1 - For this design, is the OPA1656 a better choice or would you consider alternative op amps, like the OPA2192 suggested by Nick?
2 - As for the input capacitors (220nF/2.2uF), and more specifically for the larger value, which type would you choose (or which would be better to use)?
3 - In Post #17 you wrote:
"..and keep the loop area from the input connector to the amplifier input and back small..."
Please, can you explain more about what you mean by loop area?
Are you talking about the distance between the input connectors (Left / Right) on the PCB, or are you referring to the distance between the pins (In / GND) of the RCA connector, and the input pins on the PCB?
I won't forget that with the OPA1656 I need to keep the 220R/15pF dumper.
Just a couple of questions, for now (let's make it three..)
1 - For this design, is the OPA1656 a better choice or would you consider alternative op amps, like the OPA2192 suggested by Nick?
2 - As for the input capacitors (220nF/2.2uF), and more specifically for the larger value, which type would you choose (or which would be better to use)?
3 - In Post #17 you wrote:
"..and keep the loop area from the input connector to the amplifier input and back small..."
Please, can you explain more about what you mean by loop area?
Are you talking about the distance between the input connectors (Left / Right) on the PCB, or are you referring to the distance between the pins (In / GND) of the RCA connector, and the input pins on the PCB?
Last edited:
Hi Nick,
What exactly do you mean with a transadmittance cascade at the output?
Hans
Hi Marcel,
Almost without exeption, most interlinks are having a characteristic impedance of 100R.
Your “diazepam” didn’t seem to be very critical for the used resistance value, but would a 100R resistance not be a more logical choice ?
Hans
See post #16. The RC time constant was chosen to give the maximal conductance-to-capacitance ratio at the guessed quarter-wave resonant frequency of the cable between JRA's muting switch and the amplifier. The required capacitance was determined experimentally and then doubled to have some safety margin. The characteristic impedance of the cable doesn't matter much, because the impedance of a quarter-wave cable that is shorted at the other end goes through the roof no matter what its characteristic impedance may be.
By the way, where did you get that 100 ohm characteristic impedance value from?
By the way, where did you get that 100 ohm characteristic impedance value from?
I got this from many measurements.
Not only Interlinks, also many speaker cables, but of course not all, have ca. this characteristic impedance (when long enough).
It is simply to be measured with a VNA, capacitance and admittance open ended and resistance and inductance shortened at the end.
Hans
Interesting, they must either be quite thick or have a quite thin centre conductor then. I expect cheap 280 pF/m PVC-dielectric cinch cables to be closer to 23.8 ohm (assuming an epsilonr of 4), but those are not very suitable for turntables anyway.
Characteristic impedance is pretty easy to calculate from geometry, most twisted pairs are in 100 to 160 ohm range, most coax in 30 to 100 ohm range, unless very unusual geometry or dielectric. However all of this is moot at audio frequencies as a lumped model will do.
Both can work fine. The OPA1656 has slightly better noise specs and can have slightly better RIAA accuracy due to its higher gain-bandwidth product, but it is also more prone to oscillations with awkward source impedances due to its high gain-bandwidth product and high differential input capacitance. The RC damper is meant to solve that.
220 nF is still available as NP0 a.k.a. C0G multilayer ceramic SMD capacitor - a big SMD, that is. For 2.2 uF, I would go for a through-hole foil capacitor. The very best of those are polypropylene capacitors, but MKT will also work fine.
The centre pin of the RCA connector will be connected to the PCB, and the corresponding PCB ground pin will via some path be connected to the shield of the RCA connector. I mean the area enclosed by this loop: RCA to PCB and back.
PP is huge 🙁
SMD acrylic from Rubicon/Panasonic/CDE are also works OK, and are small (1206, 1210).
Alex.
But aren't the most linear, and have poor dissipation factors, and poor tolerances. PPS is a good SMT capacitor dielectric, temperature stable, good tolerances available, very low dissipation. However 2.2uF is a big ask, a THT cap might be the best option and PP is fine for that.
I mean common emitter with open collector (DC loaded to active current source), closed via RIAA overall feedback to input . - Both output stage and looped feedback make fine accuracy RIAA with almost constant loop gain from 1 Hz to 50 kHz [ https://photos.app.goo.gl/vYSeG2Pue7oSopJb7 ], = no THD [ https://www.patreon.com/posts/o-klire-fonokora-120452578 ], simple (2 bipolars), good PSRR, ...
Yes, acrylic is not so good as polypropylene (FKP/MKP) or NPO ceramic, but it is much better than PET (MKT).
But of course PET is much cheaper.
Alex.
P.S. BTW, what Nick showed in the second link in the previous message - C4 (in the center of the board at the bottom) is an acrylic 1.0 µF 16V Rubycon MU: 16MU105MA23216
On another PCB version, THD is even better. Capacitors are the same.
Last edited:
I'd rate them similar, the best tolerance you can get is +/-10% with acryllic, +/-5% with PET. There are far more choices with PET too.
The only active film capacitors available to 1% tolerance seem to be PP these days, even PPS seems to be limited to 2%, a few 1% obsolete devices.... Digi-key list only one NP0 ceramic cap with 0.5% tolerance.... Plenty of 1% NP0/C0G caps upto 560nF though.
I'm more concerned about THD than tolerance.
Here you can find a comparison: https://passive-components.eu/minia...rs-selection-guide-are-they-as-good-as-mlccs/
For acrylic capacitors feature mentioned:
Here you can find a comparison: https://passive-components.eu/minia...rs-selection-guide-are-they-as-good-as-mlccs/
For acrylic capacitors feature mentioned:
- outstanding performance for noise reduction in measurement circuits
- ultra-low distortion characteristics ideal for coupling in audio circuits
Harmonic distortion of just about any film capacitor is almost immeasurable, frequency response deviations of a few tenths of a decibel at midband frequencies are audible in double-blind tests. I'd say that tolerance is far more important than the minuscule distortion of film capacitors.
Thd in a phono pre is insignificant next to the distortion in pressing and playback.
NJR2068 , got 20 to upgrade my EQ, works brilliantly in this circuit. THD less then 0.01%
NJR2068 , got 20 to upgrade my EQ, works brilliantly in this circuit. THD less then 0.01%
Exactly - it's the same discussion applied to speakers.
All these electromechanical devices have THD around 0.1% and 1% (or more!) in a good scenario.
I don't think is worth to push electronics, to milimetric THD levels.
Anything below 0.05% for electronics, which is easy to achieve, will be ok.