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Old 22nd November 2009, 02:46 PM   #191
Werner is offline Werner  Europe
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Joachim,

"what is going on in the op amp stage. do you run it on only one supply ? is it a differential stage? where is the second ground connection for?"


Your drawing is correct. The symbol in my schematic is not an opamp, but a differential in / diff. out programmable voltage source with a huge gain. I always use these in simulations as opposed to full opamp models (my simulator is node-limited).

Years ago I started work on a complex Vendetta-inspired phonostage. It would become a two-box affair with big LC-filtered supplies, multiple transformers, a single-ended FET transconductance stage driving an inverting opamp. Getting it DC-stable and with zero offset ended up being more complex that I anticipated, and I dropped the idea for a while.

Recently I revisited it. With the aim of building a much smaller version I accepted the idea of an electrolytic cap in the input of the opamp (partially bypassed with one of the RIAA caps), and a split load on the transconductance output. One half (1k5) to ground, also for DC biasing, and the other half (1k5) through the big capacitor into the opamp.
I found the resulting simplification very appealing.

Now waiting on my just-ordered Eagle license to start the real work.

With two toddlers and an extremely demanding dayjob I have little time for DIY, and soldering must be done in the very rare moments I am home alone. Hence PCBs in favour of breadboards...
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Old 22nd November 2009, 03:54 PM   #192
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hi werner ! have you build it and listened to it ?
i would do the elcap with two 220uF back to back ( minus together) from Panasonic FM or Rubicon ZLH with a 0.1uF Röderstein MPP bypass and the sound should be just fine. i even found a maker of expensive Mic amps (Millenia) that claims that elcaps in the signal chain improve sound. strange is´nt it.
hi stinius ! thanks, there is a lot of complexity going on there so i need some time for digesting
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Old 22nd November 2009, 04:25 PM   #193
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i promissed more overload measurements of the original MPP and here they come. anyway, this is history now that i know that matching and improved current source can make much better distortion readings. again measurement are taken in dB/U RMS one leg swing only. they start at 1V and are compensated for RIAA gain this time.
Attached Images
File Type: jpg Mpp overload RIAA comp 100 Hz.jpg (89.7 KB, 496 views)
File Type: jpg MPP overload RIAA comp 5kHz.jpg (88.7 KB, 473 views)
File Type: jpg MPP overload RIAA comp 20kHz.jpg (84.7 KB, 464 views)
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Old 22nd November 2009, 05:00 PM   #194
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so here come measurements on the new MPP unbalanced. It has good speed and amplification. when i set the gain to 30dB with the 340 Ohm resistor - 3dB bandwidth is 3.5MHz so gain bandwidth is 105 MHz. i will calculate the slew rate later. a 100kHz squarewave is reproduced without overshot and very straight lines. i will show a photo later in this tread too. HF demodulation is virtually non existend partly because the circuit has quite a low input impedance. is this at last an atvantage of the transimpedance topology ? i show distortion measurements at different output voltages. they start at less then 50dB. that amounts to an equivalent input voltage of 0.1mV. i did this measurement to look at resolution problems at low volume. maybe when i find the time i will measure at even lower volume. 0.02mV should be posible. the circuit seems to behave quite monotonic so i am thinking that low level resolution is good. on the plots you can see some intermodulation products. partly they are measurement artifacts (under 1kHz) and partly it is radiation from two nearby toroidal trannies. the circuit seems quite sensitive to hum injection and my lab powersupply is not really good enough. i will work on grounding and shielding and also started to work with Salas on a really good supply. installed at my home hum and noise are not audible in the listening seat at high volume. with the ear close to the speakers i hear a very faint "light" sounding noise and some hum with an unpleasant overtone spectrum. hum is slightly dominating in terms of subjective volume. hum got better when i put the trannies farther away but i could not get them any further because the cables are not long enough. anyway, in the finished version i will put the trannies in a seperate box with pre filtering.
Attached Images
File Type: jpg MPP SE Headamp dist @ -53ddB.jpg (121.2 KB, 450 views)
File Type: jpg MPP SE Headamp dist @ -43dB.jpg (121.5 KB, 407 views)
File Type: jpg MPP SE Headamp dist @ -33dB.jpg (121.6 KB, 82 views)
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Old 22nd November 2009, 05:02 PM   #195
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here some more, up to clipping
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Old 22nd November 2009, 05:12 PM   #196
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ok here they are
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File Type: jpg MPP SE Headamp dist @ -23dB.jpg (106.0 KB, 86 views)
File Type: jpg MPP SE Headamp dist @ -13dB.jpg (105.8 KB, 81 views)
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Old 22nd November 2009, 05:22 PM   #197
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i found clipping at -3dB/U so it can put out (head amp section only) 2.5V (P-P).
shown here is a sweep at 50mV (head amp only) and total distortion and noise including following RIAA stage at 1V dB/U RMS also done with my lab PSU. i think distortion is not a problem any more with the new design and i am very pleased that the MPP had such good potential for improvement.
Attached Images
File Type: jpg MPP SE Headamp distortion sweep @ 50mV.jpg (117.0 KB, 75 views)
File Type: jpg MPP SE + RIAA @ 1kHz 0dBV.jpg (118.1 KB, 72 views)
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Old 22nd November 2009, 05:50 PM   #198
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i set the system now up in my home with 4 LC Audio low noise supplies, double mono for Head Amp and RIAA amp in seperate metall boxes. the result : dead quit, i first thought there was a mistake but it plays very well.
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Old 23rd November 2009, 03:22 AM   #199
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Having now solved the noise problem i did now 2 longer listening sessions. one was with a modified bladelius MM stage that i know very well for many years and use as a benchmark (when it sounds good with that stage i know nothing is wrong) and one with my own MM stage. I listened to classical, jazz, pop, funk, hip hopp and south american folklore. the classical pieces where Lorin Mazel on Decca with Respigi and a Proffesor Johnson recording of Berlioz. the Berlioz is easy one of the best recordings availlable so most competent equipment makes it sound quite spectacular. i noticed slightly better depth of image and better focus and wider spread of the soundstage than i am used to. that came as a pleadant surprise because depth of image is a little restricted in my new listening room. this is the most serious flaw of my new room compared to the old one that on some records could throw a 20m deep soundstage. the new room is better in other regards. for example the sound can be very high and broad. i think the stage that i can populate is 8m wide, 6m high and 5m deep. on some recordings like the ones that are made in Q sound, the sound can come subjectively from my yard, in my head, around my head and even in the back of my head. that is posible because i sit very close to the widely spaced speakers. and speaker build i can ! that at least my customers tell me and i like them too. the Respigi is much more problematic because it has a very delicate string tone that can easily be destroyed. the MPP did good on the string tone but i have heard it more spectaculary fleshed out with tube equipment. it sounded a tiny little bit synthetic and that may be the way it was recorded. it is a quite late DECCA and i think not recorded with tubes. anyway during the last two days i heard the MPP mature quite nicely and the tone got rounder over time. where is did spectaculary well was on Brass and Percussion : this is a very dynamic phonostage. the music just jumps on you. Brass and Woodwind is pefect food for the MPP. So is vokal. Vokal rendition male ( Donald Fagen ) and female (Nona Hendrix, Mercedes Sossa) is very low in coluration and distortion, tightly focussed right there in front of you. Donald Fagen sounded 10 years younger then he used to on "Gaucho" , some unpleasant nasal tone cleaned up, well his voice is a little nasal but not unpleasant. Nona Hendrix sounded fresh and powerfull and Mercedes Sossa gave me a privat concert in the Argentinian Pampa. Well i could go on and on. Yes i am pleased and whould call the nature of the sound fresh, open, transparent, dynamic, wideband, natural (also spacecial because small things stay small and big thing are really big in terms of perceaved dimensions). I could imagine that flat eathers whould love the sound too because it has tremendous timing precission. i play live and hear a lot of live music so i was pleased to hear good interplay between bass and drum on Galiano "the plot thickens" for example. I could not hear subjective distortion, to the contrary, the sound is subjectively squiky clean. I think the "double-double" mono arangement of the powersuplies have really helped the soundstage, so does the very good low level resolution. This may not be the best choice for listeners that prefer a fat, round, golden tone that makes it´s precence known all the time but it can sound warm and intimate SOME time. it perfectly compliments my Lyra Titan that is also quite alive and kicking. it´s like riding in a Porsche or an AUDI A8 : It´s fast AND safe.
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Old 23rd November 2009, 05:19 AM   #200
Salas is offline Salas  Greece
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Its nice you like it. Have you been doing any professional reviews at a point BTW?
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