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mechanical resonance in MMs
mechanical resonance in MMs
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Old 5th September 2017, 10:08 AM   #1581
billshurv is offline billshurv  United Kingdom
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mechanical resonance in MMs
Kids came over last night so no energy left for soldering so looking back at this. I realised I used the term 'self-inductance' where Rod Elliot used 'Semi-inductance'. Happy to use another term if people prefer. The question then comes as to whether this can account for the midband dip some cartridges see, rather then the mechanical resonance that is 'perceived wisdom' and started this thread. Looking at George's plots from post 279 I think there is enough information to answer that as we can calculate the deviance from a pure L+R series combination and see what that would do to the frequency curve.

For examples of midband dip see here New Lamps for Old . Some distortion vs level graphs to mull as well, albeit only at 315Hz. I am sure we have discussed this before and started re-reading the thread from page one, but the intriguing part is that the shures all show a teeny (1dB) midband dip and small HF peak, whereas the Ortofon 2M shows no midband dip and a much larger resonant peak. And the Rega is well, bleh. Now all of these are same guy, same test record, same loading so hopefully something we can infer, esp as George has his plots of the M97 to compare.

Then I compared Zev's measurement Shure V15V - ZevAudio . Now I think he has got a bit carried away claiming a 0.7dB dip is from mech effects, but he is showing some severe HF effects. Guessing he has a boutique phono stage. He also hasn't stated which HFN test record he has, but I have the first and george has the second so between us we can work this out.

Also note the AT440MLa Audio Technica AT440MLa - ZevAudio Now LD's simulation of this cartridge does not show this dip, based on a simple LRC model.

Other thing I did was look at errors in the Aurak if the load resistor is wrong. This was probably obvious to those versed in the art, but above the turnover point its tracks RIAA perfectly and the error can be viewed as a fixed offset below the turnover point. Worst case in my initial checks using a fixed 150 Ohms was 0.8dB. from 348Hz down. I can live with that as my little room is modal by then so I have bigger issues to sort .
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Old 12th September 2017, 12:28 AM   #1582
billshurv is offline billshurv  United Kingdom
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mechanical resonance in MMs
OK soldering is nearly completed and I must write to Russ asking him to open out the IO holes a litte as they were effectively press fit for the power. It's not the prettiest* due to a couple of last night ordering sessions where I picked low tempco r*esistors without checking their girth, but at least everything is matched to a sane level**. Last resistor to fit is the gain resitor (R23 on Hans schematic). Now I have some odd gain requirements as I'll have cartridges from 4mV to 20mV and ADC inputs from 2V to 8V. Would be nice to have 6 switchable gain steps. Looking at the circuit doesn't suggest that an extra 3-6" of wire in that location would have any ill effect, but no direct experient of these devices and wondered if anyone here did?

Now to test!. LD have questions for you on final setup. PM or email best?

*Pics to follow.
** Critical resistors are as close to 0.01% as my test rig would allow. I figured worth seeing if good balance actually does anything.
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Old 16th September 2017, 11:38 PM   #1583
billshurv is offline billshurv  United Kingdom
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mechanical resonance in MMs
Pics just to show I have actually been doing some soldering. Need to find my hot glue gun then work out best way to test these with the limited setup currently to hand.

Other than the RIAA and gain modifications the rest goes together quite easily. I think as the number of people who would actually do something as bonkers as this is limited from an assembly point of view we can claim this as doable. I think a PCB for the single ended version would be good. I might challenge myself to do a layout for that sometime.
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Old 5th October 2017, 11:20 AM   #1584
billshurv is offline billshurv  United Kingdom
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mechanical resonance in MMs
OK a particularly dumb moment today. I have been fretting that I have too much gain in the preamp. The miniDSP has an 8V balanced input which gives me some headroom, but I hope to someday move to the TI EVM board that Wayne recommended on another thread with is half that. One of the combinations (at least until I can work out how to get the S-120 stylus to actually fit into a superOM body) is the S-120 body series wired for mono, which gives a whopping 20mV in that configuration. So I finally sat down and calculated the gain across the different parts until with a loud clunk the penny dropped that the cartridge series resistance directly affects gain and so, to a first order spherical cow calculation the series strapped input will give the same output as the stereo configuration. Additionally because output is generally related to turns on the coil (give or take magnet differences) the S-120 will give same output as the OM in stereo. I feel so silly...
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