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Old 19th November 2009, 03:19 PM   #151
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ok. i will publish more measurements
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Old 19th November 2009, 05:22 PM   #152
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me sorting transistors for the MPP unbalanced. i sort them in 6 groups with +-10% Hfe matching.
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File Type: jpg IMG_0516klein.jpg (179.4 KB, 585 views)
File Type: jpg IMG_0517klein.jpg (181.6 KB, 605 views)
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Old 19th November 2009, 11:46 PM   #153
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Here come some more measurements.
I measured the MPP at 20Hz, 1kHz and 20kHz with varios input levels.
The RIIA in in action with these measurements. I do not have a passive inverse RIAA at hand momentarily but i plan to build one and then do the measurements again. Gain is very high in this measurements at 20 Hz (+84dB, e,g. 64dB plus 20dB RIAA) and i could not reduce the output voltage with the equipment that i had on hand spontaniously, so take this measurement with a grain of salt.
What do this measurements show?
Basically that the MPP can not swing much more then 5V cleanly at any frequency. so overload margin is around 20dB with a cartridge that puts out 0.5mv at cruising level. distortion is only 2nd harmonic at low level and rises with level. higher harmonics are showing up monotonically at higher levels. i found this typical with open loop designs. designs with high feedback seem to behave a little different in my experience because they stay clean until a certain level and go into overload more suddenly. if that is a disadvantage or not i do not know. i think overload has simply to be avoided. Does that lead to the conclusion that open loop designs have better low level resolution and feedback desigs have a resolution boundary ? i think that is much to early to say and neads more reseseach.
If good enough is not good enough the remedy whould be simple if you want more overload margin: the gain structure in the MPP could be changed for example giving the intrumentation amplifier more gain and build an outputstage with more voltage swing unburdining the inputstage. distortion in the MPP (high level distortion, that is) comes mainly from the inputstage. this is contrary to the observation that in other designs distortion comes more from the following stages that have to swing more voltage.
by the way i ran the MPP including the op amps on a +- 12V supply.
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File Type: jpg MPP Overload @ 20Hz.jpg (88.8 KB, 540 views)
File Type: jpg MPP Overload @ 1kHz.jpg (90.6 KB, 483 views)
File Type: jpg MPP Overload @ 20kHz.jpg (100.7 KB, 95 views)
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Old 20th November 2009, 12:04 AM   #154
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at last i show a sweep from 20 Hz to 20kHz of a level of 1V RMS @ 1kHz.
The MPP runs out of steam under 60Hz because the output voltage is over 5 V. this will not happen in reality because low frequencies are suppresed on the record.
distortion is mainly 2nd again. some 3rd harmonic is intruding over 6kHz due to the ballanced-parallel symmetric nature of the design. i hope it is high enough in frequency and low enough in level to not be audible. lumba will of cause complain that the bad nature of this circuit will bite it`s horrible teath into his delicate ear tissue.
you may wonder why the 3rd harmonic does not show in the 20kHz overload plot. it is because my 96kHz-24Bit A-D converter simply does not capture it. this tone is at 6okHz so it is suppred by the steep 48kHz anti aliasing filter, not to talk about the Shannon theorem.
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Old 20th November 2009, 12:32 AM   #155
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This small changes will improve the overload margin by 6dB to +26dB. the MPP should now be able to swing 10V RMS :
Change R8 to 400 Ohm (just visible on the left of the second page).
Change R11 and R12 to 10 kOhm.
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File Type: pdf MPP2 1.0 Structure with +6dB overload.TSC - TINA.pdf (92.2 KB, 160 views)
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Old 20th November 2009, 12:35 AM   #156
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I wouldn't be happy with this kind of performance, but for open loop it's about what one can expect.

I would be more concerned about those 20dB headroom. To my experience, that's totally insufficient. Try and build one with 26dB (that is, 10V swing) and give it a shot. Play a used record and you'll hear a huge difference out of the bat.
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Old 20th November 2009, 12:53 AM   #157
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I take your comment serious and will try the improved version. Thanks for contributing.
i do this for fun triggered by Malkolms notion that there may be a resolution boundary in feedback circuits. anyway i will give my best to improve the circuit further and as i already mentioned feedback could be added too. My commercial products have very low distortion, need no selection and are competitively priced if not cheep because we build them by hand (even soldering) in Germany. Anyway you are an inspiration for me and i find your approach thorough. Thank you for sharing this hobby (yes, i think this is still my hobby although i make money with it) with such talented people.
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Old 20th November 2009, 03:38 AM   #158
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one little thing that bugs me. syn08 i have studied your website again and you talk about
32 V Peak to Peak output capability of your best phono stage. this is quite a spectacular reading compared to my meager 5V or 10V in the improved version. first i use RMS readings and second i measure only only one side of the voltage swing. so that translates into 5 x 2 x qrt2 = 14.1 V P-P and 10 x 2x qrt2 = 28.2 V P-P. i run the circuit on +-12V supplies ( also the OPAs but i may change to +-15V) out of reasons i did not discuss here so more is simply not possible without redesigning the gain structure and the output amp again. Anyway, for the time being i won´t make any big change until i have the full measure sound wise. i know, i should be forbidden to listen to it. it is simply not good enough.
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Old 20th November 2009, 03:43 AM   #159
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errata: 28,2 V P-P is of cause only possible with a +- 15V supply
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Old 20th November 2009, 10:36 AM   #160
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Quote:
Originally Posted by Joachim Gerhard View Post
one little thing that bugs me. syn08 i have studied your website again and you talk about
32 V Peak to Peak output capability of your best phono stage. this is quite a spectacular reading compared to my meager 5V or 10V in the improved version. first i use RMS readings and second i measure only only one side of the voltage swing. so that translates into 5 x 2 x qrt2 = 14.1 V P-P and 10 x 2x qrt2 = 28.2 V P-P. i run the circuit on +-12V supplies ( also the OPAs but i may change to +-15V) out of reasons i did not discuss here so more is simply not possible without redesigning the gain structure and the output amp again. Anyway, for the time being i won´t make any big change until i have the full measure sound wise. i know, i should be forbidden to listen to it. it is simply not good enough.
HPS 3.1 and HPS 4.0 are in fact allowing 40Vpp at the output. Supplies are +/-22V and I'm using 44V opamps (OPA552, LT1115, OPA2604, LME49870). This leads to a 32dB headroom, at all frequencies in the audio band (due to the way the 60dB gain is split between the head amp and the distributed RIAA correction).
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