Ah, John never mentioned it. I'll ping him about it...
He's currently reworking a plinth for his TT using a composite structure that I suggested so we are in relatively frequent contact.
We got the best results with constant velocity sweeps and no equalization. Tone arm/bearing + counterweight resonances are bound to be non-minimum phase with respect to the motor.
John and I have a common friend who is a mod here, personally the TT fussing does little for me and I prefer to offer technical support for others. We did a couple of nice Python scripts, one to display instantaneous frequency on a polar plot using a 1kHz or 3.15kHz pilot tone (shows belt/motor issues quite nicely). Another to plot FR and the first two harmonics vs frequency using any sweep including letting a fixed tone spin down by itself (again to remove as many cutter/equalization issues as possible).
Also did a lot of work on recording flat and doing EQ in DSP, I got some pointers from Bob Orban (the Opti-Mod guy), lots of wrong folklore on doing this also floats around.
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Continuing with the measurement results- using the Cordell 3 tone in band TIM measurement method (900Hz, 1050 and 20k input tones) the TIM is -110dB relative to an output signal from the preamp which is 0.5dB below clip (+18.5dBv) and 1dB below FS on the measurement converter. That's pretty damn good. Normal IM using the SMPTE 60Hz, 7kHz method is -119dB under the same level conditions which is also pretty damn good.
Whomever designed the two opamps used deserves some kudos.
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It's pretty hard to find fault with the operation from an objective point of view.
Whomever designed the two opamps used deserves some kudos.
.It's pretty hard to find fault with the operation from an objective point of view.
Yes, I read the posts on flat/EQ. I have, as well as the RME, a KORG DAC10 that enables a DSP inverse RIAA operation, but the RME cannot do the operation directly.
Someone on AK asked for a suitable INAMP for a MC, but I can't say that I'm terribly motivated. Perhaps he should check in DIYAudio.
Continuing with the measurement results- using the Cordell 3 tone in band TIM measurement method (900Hz, 1050 and 20k input tones) the TIM is -110dB relative to an output signal from the preamp which is 0.5dB below clip (+18.5dBv) and 1dB below FS on the measurement converter. That's pretty damn good. Normal IM using the SMPTE 60Hz, 7kHz method is -119dB under the same level conditions which is also pretty damn good.
Whomever designed the two opamps used deserves some kudos.
It's pretty hard to find fault with the operation from an objective point of view.
By the way, the measurement output levels are 27dB above the rated output level of the preamp.
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Wayne Kirkwood has posted extensively on a classic Demrow current feedback in-amp with Zetex input devices for MC. BTW there are several well under 1nV step-up amps using the Zetex devices posted here (if noise is what you want). Low distortion is nice but seriously vinyl is straining at -60dB, I simply don't see the point.
Yes, I understand that vinyl is "straining at -60dB", in fact I think that you're being generous. I've conducted extensive measurements on several MC cartridges over the years, and I'm well aware of the manufacturers specs. and the literature.
However, if you can achieve sub 1nv/rtHz from an opamp based design, essentially unmeasurable distortion- harmonic, IM and TIM- excellent power supply rejection, superb compliance to the RIAA standard, easily adjustable cartridge loading, excellent load tolerance, outstanding overload characteristics, the ability to support balanced outputs etc. then what is the point in any of the rest of the designs?
Is it for the added distortion that many designs offer? Or do you believe in some ineffable quality that exists that cannot be captured in measurements? And if you can also put it into an easily built, essentially fool proof packet- then what's not to like from a less than technically astute public's point of view.
In any case, it was not my choice to put this on DIY Audio.
I've simply stated how it measures as a response to someone else's actions.
In my defense, I've made the whole exercise as user friendly as I can, because that was what was necessary for the intended audience.
I've gone to fairly extreme lengths in this- with complete measurements, design criteria, etc. etc, because people who are interested in building the thing have asked sensible questions, questions that I enjoyed finding and supplying the answers to.
Heck, you know I could design it at the device level, using a particular technology of choice, but the fun that I was getting was helping the people that were kind enough to ask questions and request solutions.
In any case, at this point I will depart from DIYAudio again.
Thanks for the discourse. It's always a pleasure.
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I would be interested in how you extracted those from the measured response, is that posted in the AK thread? We've been trying to do the same on MM for a while over here with limited success so far.
Got two sets of those boards in build at the moment. Completion data as ever depends on what time the sprogs allow
.Loaded question. In theory you only need a few designs to meet the requirements for a DIY 'hifi'. But that would take all the fun out of it. People chasing sub 0.5nV/rtHz phono stages is generally just for the challenge, but the world would be poorer if they weren't.
Please stay. Good to have another sensible voice on here!
Didn't you start out talking about comparing to valve based pre-amps?
Sorry we're used to having heated discussions and questioning everything here. There are advocates of fully differential class A (even open-loop) solutions.
Take Bob Cordell's Vinyl-Trak for instance, we're not talking adding distortion but so far below the LP, and vanishing monotonically with level, that I see no argument for that being a factor. The bare JFET inputs and the possibility of differential wiring could have a large impact on EMI/RFI/mains incursion. I did say it was my opinion that the full scale THD was probably not a perceptual factor.
As Bill said the cartridge mechanical models are an unfinished story, there are mechanical transmission lines modes and more.
Yes, please stay but don't say there is no point in anyone else's inputs.
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