From post #253
I recognize that topology from Walt Jung's stuff. In my case with the active feedback RIAA approach, in order to get rid of the input buffer I struggled with putting the S-K filter behind the RIAA EQ like in your post #253. But I reasoned that why let that sub-sonic garbage go through and be amplified by 10 - 20dB just to then lop off 18dB downstream..? And I know that preamps with phono stages have "rumble filters" that do the same thing as in afterwards. But I decided to actively filter before the RIAA EQ which then needed the front-end buffer as well.
I though it interesting that you put a capacitor-input HPF that would be first to greet a MM cart with all of its RLC complexities instead of 47K. My original musing was is there a way to put a passive 2nd order HPF up front of the active RIAA EQ stage?
Also, referring back to Walt Jung's RIAA preamp stuff can a "R4" be placed in-series with the "N" feedback network to increase overall 1KHz gain without disturbing the existing time constants? Presently I have 40dB gain and I'd like that to be say 46dB due to my earlier intro-explanation.
And oh, BTW there is nothing magic about a 3Hz cone flap. Don't know where that came from.
Yes, darn it.😒 He reversed U1 and U2 in #244. Simple fix is to go with the OPA627 there instead.
Yes, that should also work. It reduces loop gain about five times, while cutting out the 56 pF does not.
I still don't get why I can't use OPA637, as gain on both stages on both preamp versions is more than 5x. In any case I also have OPA627s.
I also have some bipolars that I wish to try, on one of both stages, like OP27/37 and LT1115. Also several AD797s, but as I am using just MM carts I better don't go for that opamp path.
I also have some bipolars that I wish to try, on one of both stages, like OP27/37 and LT1115. Also several AD797s, but as I am using just MM carts I better don't go for that opamp path.
You can use OPA637 when you get rid of the 56 pF that reduces closed-loop gain at frequencies in the megahertz range.
LT1115 is very similar to LT1128, which is the unity-gain-stable variant of the LT1028. They all have too much equivalent input noise current for the first stage of an MM preamplifier, worse than the AD797 actually. See the section "Base Current Compensation Example" of https://audioxpress.com/article/boo...spice-models-to-simulate-vintage-op-amp-noise if you want to know the details.
LT1115 is very similar to LT1128, which is the unity-gain-stable variant of the LT1028. They all have too much equivalent input noise current for the first stage of an MM preamplifier, worse than the AD797 actually. See the section "Base Current Compensation Example" of https://audioxpress.com/article/boo...spice-models-to-simulate-vintage-op-amp-noise if you want to know the details.
Last edited:
So I get rid of the 56pF cap only when using OPA637? Not with others like OPA627 or OPA1642?
What spec should I look for on the spec sheets to know when not to use the cap?
Which bipolars can I use?
What spec should I look for on the spec sheets to know when not to use the cap?
Which bipolars can I use?
Unity gain stability. When the op-amp is not stable for unity gain but stable for a gain of five, leave out the 56 pF. When it is unity gain stable, you can either place the 56 pF or leave it out anyway.
NE5534A (without the 56 pF), OP210, OP27, OP37 (without the 56 pF). The DC servo op-amp can best be a FET op-amp, although an OP27 might also work.
NE5534A (without the 56 pF), OP210, OP27, OP37 (without the 56 pF). The DC servo op-amp can best be a FET op-amp, although an OP27 might also work.
I'm confused as well. But calmart why do you think you need blinding speed in both or either stage? Race cars can cause problems on new-design PCBs that didn't show up on the mighty stimulations. You sound like access to parts and PCBs is limited so why not step back and take a look at what you actually need vs. "want". You might avoid 2nd and 3rd PCB runs as well as for parts and components. I've been at this op amp rolling game for a long time and for me the winner is the 627.
I did not realize that one could drive a S-K filter with such a high impedance source. 10K really..?
I'm sorry if I cause confusion with my proposals and ideas. I want the preamp to sound as good as possible, and nothing else. Slew rate is not a great concern of mine for this project, but I would like to try, for sound, opamps like the OPA194x and OPA195x.
The pcbs I have for this first try are those made by Aliexpress, on which I will change the parts and improve the power supplies. What I needed was a good design for the preamps, because the original ones simulated quite badly on LTspice. Things got much better with other schematics and active parts.
The only part I needed to solve was the 3Hz frequency I had, which Marcel handled wonderfully.
Now comes the construction part, which I hope will help other DIYers that do not want or do not know how to design pcbs.
And don't worry that I listened to your advice and experience with the OPA627.
He did indeed but where again did you get the definitive opinion that your cone waffle was 3.0Hz? IOW, the waffle could be anything. It's linked to a resonant environment which includes the cartridge to arm, the arm to plinth, the plinth to the floor, the floor to the room and back again, round and round. You never measured the flap freq did you..? And you never said what the Thorens was mounted on (i.e it has a touchy spring isolation mech).
Yes. With NE5534A, I get only slightly worse total noise (measured with MM cartridge) compared to OPA627/637.
When an op-amp is said to be gain-of-five stable, it means that its feedback network needs to attenuate at least five times around the frequency where the magnitude of the loop gain (*) becomes unity. That's around 16 MHz for an OPA637 with five times attenuating feedback. The 56 pF reduces the attenuation of the feedback network from five times to only one time between about 1.6 MHz and 8 MHz, so that won't do.
(*): not to be confused with the open-loop gain.
When you look at the schematic and list of values of post #253, you will see that RA and RB are substantially larger than 10 kohm and that their ratio is a bit larger than the factor of two that you would need for a Butterworth response if the driving impedance had been zero. The highish values are chosen to keep the damping due to the high source impedance small and the larger ratio corrects for the remaining damping.
To find out what op-amp is most suitable noise-wise and assuming it is the RIAA- and A-weighted noise you are interested in, you can calculate the cartridge impedance at 3852 Hz, multiply the equivalent input noise current with it, and take the square root of the sum of the squares of this and the equivalent input noise voltage. For a 500 mH cartridge, the impedance at 3852 Hz is about 12 kohm, so:
NE5534A: √((12 kohm • 0.4 pA/√Hz)2 + (3.5 nV/√Hz)2)
OPA637AU: √((12 kohm • 0.0025 pA/√Hz)2 + (5.6 nV/√Hz)2)
You have to be careful with op-amps with base current compensation; sometimes manufacturers specify the equivalent input noise current for equal impedances driving the positive and negative inputs. That never happens during normal use, but it cancels the common-mode noise of the base current compensation, making the datasheet numbers look better. Example: LT1028, LT1128, LT1115: 1 pA/√Hz or 1.2 pA/√Hz specified, 3.4 pA/√Hz measured under realistic conditions.
Talking about bipolar input opamps, LT1028 is noisier with MM than AD797, both are not a good choice. Very good result re noise is with OP177, but in this case I would be afraid of very low SR.
Even the 1028 datasheet mentions the necessity of equal input impedances to achieve the declared noise specs.
Just as a curiosity, below are total noise (flat 22kHz) results with Shure M35X followed by flat 30dB gain amplifier with various opamps. The noise scale is plotted re amp input.
Even the 1028 datasheet mentions the necessity of equal input impedances to achieve the declared noise specs.
Just as a curiosity, below are total noise (flat 22kHz) results with Shure M35X followed by flat 30dB gain amplifier with various opamps. The noise scale is plotted re amp input.
Last edited: