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Old 9th December 2011, 01:20 PM   #3321
hesener is offline hesener  Germany
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that'll make three... looks like this design is popular already ;-)
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Old 9th December 2011, 01:21 PM   #3322
brianco is offline brianco  Ireland
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Location: Scottish Borders - Kelso; on the famous Tweed River!
.......and you know what I need!!

highest gain with low impedance input!
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Old 9th December 2011, 01:52 PM   #3323
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Fine, i will post a circuit that shows the principle later today.
It has a balanced input with BJts. I use 4 plus 4 in parallel so a lot of standard parts can be used, also BC550, BC560. Then there is a current mirror and a transimpedance RIAA, driven from both sides. The RIIA has only 40dB of amplification. That lowers distortion and does produce less problems with offset. The output is an INA that makes the rest of the amplification ( 20dB ) and has a servo.

Last edited by Joachim Gerhard; 9th December 2011 at 01:55 PM.
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Old 9th December 2011, 02:27 PM   #3324
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So we have a team. MiiB, you are good at simulating so i would like to give that burden to you. Miib, Bksabath and Hesener have all offered to lay the circuit out. I would say that Bksabath and Hesener should discuss about who does what.
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Old 9th December 2011, 04:46 PM   #3325
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Here is the basic circuit. At the input i use a Faulkner circuit with 4 plus 4 transistors. That did not fit on the diagram so i put it in a separate file. The BJTs are degenerated. That linearizes them and helps in current matching. No selection should be ok. With BC550,560 we get a noise impedance of 40 Ohm and with the japanese transistors it is 15 Ohm so the BCs are a bit more noisy, on par with an AD797 still. I am not going for absolute minimum noise here but i try a good combination of low noise and low distortion. Each transistor runs on 5.5mA, that is a good value for low distortion. More current would not bring a big benefit because this are no ultra low Rbe`devices. Due to the balanced nature we loose about 3dB of S/N for the same amount of input devices but we gain in hum suppression and suppression of common mode distortion, so this may be more important. Some values are not set yet, simulation will bring out the correct dimensioning. i would like to setup the RIAA for 40dB gain, that would bring 60dB in total. The gain set resistor at the INA could be substituted with a 500 Ohm resistor in series with a 1,5kOhm trimmer for example and then we can vary gain over a wide range from 54dB to 66dB aproximately. I would not run the input transistors on more then 10V because of avalanche noise that can set in at higher voltage. I have an idea for a PSU too that i already tested out with good result. The input transistors are supplied with low noise current sources and are shunted to ground with big electrolytics.
I use 10.000uF, 6.3V Panasonic FC ore two 4700uF Panasonic FM. The Ina is an INA217, setup for 20dB gain. It is a current feedback structure with distortion cancelation so speed is not affected much by gain and distortion has a flat spectrum up to over 20kHz.
It cost something over 5,- so this is the most expensive part but it contains 3 high quality Opamps and precision trimmed resistors so i see no reason to use a discrete or hybrid circuit. It will be more complex, more expensive and likely not better in performance. The INA217 comes in a convenient DIL8 package, so no fumbling around with SMD.
Attached Files
File Type: pdf Paradise Lost.TSC - TINA.pdf (14.8 KB, 116 views)
File Type: pdf Paradise Lost Input Gain Cell.TSC - TINA.pdf (10.9 KB, 95 views)
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Old 9th December 2011, 05:36 PM   #3326
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Benedetto my PM box is full, i can not answer. Have to erase some.
Order 2 x INA217 in DIL8, 2 x OPA134 for servo and some TL431 for PSU. The rest is pocked change except the transformers and Elcaps. Also the new OPA1641 is very good sounding, not needed for this project though.
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Old 9th December 2011, 05:45 PM   #3327
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Sorry boys to bother you again but i think i have now an open loop transimpedance line stage that should work. After a lot of thought i think an emitter follower, though not standard, is the key if we do not want that the output stage is heavy into class a. There are of cause non switching arrangements possible but i am not an expert on that.
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Old 9th December 2011, 06:01 PM   #3328
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OPA134 in DIL version is "Obsolete" !
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Old 9th December 2011, 06:18 PM   #3329
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Is INA217 still available in DIL ?
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Old 9th December 2011, 06:24 PM   #3330
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Jepp
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