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Old 21st November 2009, 07:43 PM   #181
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hi jan !
yes, the topology is quite similiar but with a transimpedance input (common base) and not a common emitter stage. that is simply not awaillable in IC opamps to my knowlage.
when you look at the MPP "Ultimate" you will see some similarity with the AD797.
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Old 21st November 2009, 08:01 PM   #182
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This is WHY I asked you the question.
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Old 22nd November 2009, 01:42 AM   #183
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i do not want to leave you in the shade so i designed an MPP unbalanced with RIAA for DIY use. i plan to sell stuffed and tested PCB board because the low noise transistors are not easy to get and matching is very critical for best performance. the source impedance to the second stage is a "medium" 340 Ohm so you have a lot of choice for the OP amp. For OP amp 2 (servo) i recommend the OPA134. Other FET types can be substituted. even precision bipolar devices like the LME49710 can be used. For OPA1 i tested AD797, LT1115, LT1028, LT1468, OPA211, OPA827, OPA134, LME49710, NE5534 and others. itīs your choice. I would us the LT1468 here for lowest distortion but i also like the sound of the OPA827 in that position. I found the LT1468 very honest and dynamic, the OPA827 had a bit more character in the way it presented tonality. i like both and differences that seem big after fast swapping disappear quick because both of them play music good enough that i simply forget about the hifi.
P.S. LT1115 (or LT1028 for that matter) is good in that position too because it has resonably low current noise.
P:P.S. : the first version of the unbalanced phonostage is not valid any more
(the one with the shunt RIAA) because i think DC offset is simply to high in the inputstage so i use a capacitor now. be not afraid to use it. the cap plus bypass i use is extremely transparent. i would say it gives you 98% of the resolution of a DC coupled design and avoids a servo in the input stage that could create itīs own problem. as i said, the design is good enough to get totally lost in the music.
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File Type: pdf MPP SE RIAA another return from the abbys.TSC - TINA.pdf (70.7 KB, 308 views)
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Old 22nd November 2009, 03:21 AM   #184
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JG, what I was trying to determine, is whether you had an EXTRA problem with record warps, which you would have if you had too low a tone-arm resonance and if the arm was overdamped. This does not be the case, so I don't see how you could overload your preamp with low frequency warp signal.
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Old 22nd November 2009, 04:45 AM   #185
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Quote:
Originally Posted by Joachim Gerhard View Post
and matching is very critical for best performance.
As I stated in the past providing two degrees of freedom via resistor trims can usually trim offset and gain/residual seconds both to zero with parts picked at random out of the bag. I posted a circuit and some scope plots a while back. A simple input/VAS stage could do 5E6 Aol and uV offsets with just about any 2SK170/SJ74's and 4401/4403's

As for the sonic signatures of passives and various op-amps, it is best to just let it be, nothing but trouble.
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Old 22nd November 2009, 11:11 AM   #186
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Quote:
Originally Posted by Joachim Gerhard View Post
tonight i had an idea for two varieties of the MPP that are even simpler.... The fet version is slightly noisier and has a high input impedance. noise in the fet version can be reduced by paralleling.
That is getting very close to what I've been working on on/off for a long time now.

Here's my latest incarnation.

Click the image to open in full size.
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Old 22nd November 2009, 12:26 PM   #187
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hi john ! no, subsonic is not a big problem in my system, anyway i will design a subsonic filter that could also help when digital transfer is wanted. my idea is a 6th order bessel at 16 or 18 Hz.
hi scott ! where can i find your posts ? i have added here two circuits that have adjustable gain and offset. one is FET and one is bipolar-transimpedance. i think the bias to the bipolars should change offset and gain in one trim. would you do it that way or do you have a better idea ?
hi werner: yes, that looks much as an even more simple single ended version. should work well too and can not be beaten in simplicity. i can not really understand 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? anyway i send you a drawing how i understand it. thanks guys for contributing, i have a lot of fun
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Old 22nd November 2009, 12:40 PM   #188
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yes scott, this listening tests to passive and active components are time consuming and not very pedictable nor very repeatable but ones you have made your choices it gets much more easy and you have a nice toolbox with building blocks. i work as beta tester for several companies so i do that all the time (actually not, i also listen to music 1 to 2 hours a day just for relaxation. i get payed for that work in the form of free samples. the problem that i see is that single components may not make a big difference (i swear nobody will hear a difference between a AD797 and LT1115 in a blind listening test) but a circuit is build from hundreds of components so these differences albeit small can culminate.
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Old 22nd November 2009, 12:43 PM   #189
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actually they may hear differences but it is very unlikely that they pick out the right one,
anyway i do think that a typical double blind ABX test is not very revealing of small differences but this is a whole new thread.
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Old 22nd November 2009, 01:05 PM   #190
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Joachim

You will find Scott’s circuit here.
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