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Old 24th May 2010, 07:09 PM   #1381
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This is the circuit i am working on. First i will build it with integrated Opamps and later i will use the Josi that is made of discrete components. Circuit values of the RIAA are not shown yet because i first will build and measure it. The output can be a transformer, a differential stage or passive. Coupling from input stage to RIAA could be made DC coupled but i decided on the most simple solution.
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File Type: pdf Transformer Balanced Inductive.TSC - TINA.pdf (98.7 KB, 95 views)
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Old 24th May 2010, 07:13 PM   #1382
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Hi Joachim -

what is your experience with signal transformers, have not you find any sound degradation. Thanks.

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Old 24th May 2010, 07:50 PM   #1383
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I use a transformer with only 6dB gain. Used that way it is very extended in the treble. In the bass you nead high inductance and for low noise you nead low impedance so you can imagine that i use a big core. What you may like to know is how high the didtortion is. That depends on the core material. A core that delivers high inductance may have problems at low level because of Barkhausen effect so the lower the level goes, the more noise and distortion you get. My goal is to get lower noise then 0.3nV. The transformer we use has 1.8 Ohm DC impedance, so it may be posible. Distortion at high level should be not a problem because an MC cartridge does not deliver high voltage, but at very low level resolution could be finite. Anyway, an active MC input stage with NFB stands under suspision too concerning ultra low level resolution but i should not say that here right in the open because it it hard to prove that with measurements.
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Old 25th May 2010, 11:54 PM   #1384
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I have a long term project to substitute all integrated Opamps in my system for discrete circuits. I am perfectly happy with the performance but a discrete circuit is more fun and a bigger challenge. One project is the JOSI Opamp but it is made with a lot of expensive
and partly obsolete parts. I was able to buy them NOS but sometimes i may need something more afordable and simple. Of all the circuits i have seen lately i like PMAs Fet Dispre very much. It uses a diamond buffer and i have alaborated on that circuit in my headphone amp. It works very well but is not really simple any more. I had made good experiences with my modified JLH buffer that is much simpler and performs very well. I hear it each day in my default JGSelf preamp. I had made a version in the meantime with medium power Video transistors that can stand up to 70mA of idle curent with +-24V supply so i thought this could be a candidate for the Fet Dispre. The circuit i show here has no values and i omitted all compensation, filters, servos etc. just to show the basic topology. You have to scroll to the second page to see it.
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File Type: pdf Fet Dispre JLHJG.TSC - TINA.pdf (40.0 KB, 145 views)
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Old 26th May 2010, 06:39 AM   #1385
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Hi Joachim -

the buffer looks good. You made some changes in the VAS - are you sure they are right.

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Old 26th May 2010, 01:30 PM   #1386
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I do not exactly know what you mean. Maybe a resistor to ground is missing on the input of the buffer. Here i post again one of the linestages i am using. Here i use darlingtons at the output. - 3dB is at 16MHZ and distortion is low. For higher current i use the 2SA1407 and 2SC3601 video transistors.
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Old 26th May 2010, 02:40 PM   #1387
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1) If the buffer has +1 gain (or any positive gain), then the feedback in your circuit is positive.

2) what is the reason of resistors in emitters of VAS transistors.
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Old 26th May 2010, 04:14 PM   #1388
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Sorry Pavel, i have drawn the circuit wrong. Here comes the correct version. In my buffer the resistor from base to emitter sets the idle current in the Fet. ca 300 Ohm is ok.
The resistors from source to output set the idle current in the bipolars. Depending on transistor they can be as low as 10 Ohm. The bipolar does a DC bootstrap on the Fet and provides 100% local feedback so linearity of this simple circuit is very good.
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File Type: pdf Fet Dispre JLHJG.TSC - TINA.pdf (43.5 KB, 125 views)
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Old 26th May 2010, 04:19 PM   #1389
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Hi Joachim -

this is much better. Thanks for the correction

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Old 26th May 2010, 09:38 PM   #1390
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I am preparing for the Frickelfest. I will bring my MPL light with X-Dipole woofer and will use my LA Audio Monoblocks plus the JGSelf that is now working in a nice cabinet. I have now an active double mono PSU and that sounds better then the two lead acid batteries i was using. Maybe it is because the double mono use. I will also bring the cheap musicians phono and the MPP Nova. I also have a little Tripath amp to compare it to the tubes. Testing the MPP Nova with the linestage of the JGSelf i was not totally happy with the hum performance. I solved that issue with rearanging the elcaps that filter the Leds of the input cascode. I connected them now to ground instead of the PSU and the Humm disappeared. Bias after trimming ( ideally with the cartridge connected because that has a small impact ) i can now DC couple. I had to raise the servo authority somewhat, i am using now 2kOhm at the output of the servo. I finetuned the RIAA with aditional 10nF caps. Over 200Hz accuracy is better then +-0.1dB now, my measurement limit. There is a tiny hump ( +0.3dB ) at 100Hz but i left that in.
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File Type: pdf MPP Nova High Z 2.0.TSC - TINA.pdf (93.1 KB, 121 views)
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