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Old 13th August 2011, 02:02 PM   #2521
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Here as JPEG, right in the open.
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File Type: jpg MPP Summer 2011 BJT official concept.jpg (115.6 KB, 315 views)
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Old 13th August 2011, 02:15 PM   #2522
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I found one mistake in post 2519. The servo should be inverting.
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Old 13th August 2011, 02:40 PM   #2523
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Here is the two stage approach. The BJT version can be modified in the same way so we have our 4 versions :
1 : Fet High Z AIOG
2 : Fet High Z Two Stage
3 : BJT Low Z AIOG
4 : BJT Low Z Two Stage
So lets call them MPP2011 1.0
MPP2011 2.0
MPP2011 3.0
MPP2011 4.0
for simplicity. If we make any changes or improvements we just call them 1.1, 1.2 etc.
When you ask me what i like better, AIOG or Two Stage i would say they have equal merit. The AIOG is simpler so potentially more transparent and the Two Stage has higher dynamic range and lower distortion, not that the AIOG has high distortion in the first place.
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Old 13th August 2011, 02:55 PM   #2524
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Quote:
Originally Posted by Joachim Gerhard View Post
Here is the two stage approach. The BJT version can be modified in the same way so we have our 4 versions :
1 : Fet High Z AIOG
2 : Fet High Z Two Stage
3 : BJT Low Z AIOG
4 : BJT Low Z Two Stage
So lets call them MPP2011 1.0
MPP2011 2.0
MPP2011 3.0
MPP2011 4.0
for simplicity. If we make any changes or improvements we just call them 1.1, 1.2 etc.
When you ask me what i like better, AIOG or Two Stage i would say they have equal merit. The AIOG is simpler so potentially more transparent and the Two Stage has higher dynamic range and lower distortion, not that the AIOG has high distortion in the first place.
Thank you for your efforts.It is the first time I have noticed that you are not giving all the components details. Circuits are probably the most complex phono stages I have ever seen. Are they very low noise? They use single complementary transistors, without usual paralleling.
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Old 13th August 2011, 03:43 PM   #2525
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In fact you can use up to 4 pairs on each side in the Fet version. I have drawn with only one pair for clarity. With one pair you get 0.7nVq/Hz with two pais 0.5nVqHz and with 4 pair you get 0.3nV/qHz, then effects set in that prevent an even better result . That is 3dB, 6dB and around 10dB better then an AD797 so they all qualify as low noise. The BJT version is 0.5nV/qHz with one pair and 0.3nV/qHz with two pairs. It has higher transconductance ( BJTs have around 10x the Gm of Fet stages with the same idle current ) and lower 1/F noise so the open loop gain is higher and the bass is less noisy. Subjectively noise is about similar because the ear looses sensitivity in the lower reaches where 1/F noise sets in in Fets.
This has all being beaten to death here many time but i understand that you do not follow each and every post. Anyway, i will start with version 1 and we will see a full schematic soon.
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Old 13th August 2011, 05:48 PM   #2526
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The JLH Homage JG is progressing slow. First i made a mistake. The transdiodes conduct as much current as the input transistors and also there are 4 mA flowing in the feedback resistors so the output transistors where starved because the voltage over the collectors
of the input transistors dropped accordingly. I have fixed that. I also had a strange idea to bring down the DC offset at the output. I simply revered the transdiodes so that the P channel is working on the N channel and vise versa. That dropped the Dc offset to 50mV.
I also will insert a potmeter and try to trim the offset further that way. I start to understand better this simple circuit. There is a lot of entanglement going on here so it is not that simple in detail.
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File Type: pdf JLH Homage Pre-Pre 13.8.2011.TSC - TINA.pdf (10.8 KB, 85 views)
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Old 13th August 2011, 07:15 PM   #2527
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After mucho experimenting i come to the conclusion that the original JLH circuit is the best compromise. With my version there is a DC offset problem i can not solve easy at the moment and a 3dB noise penalty because of the transdiodes in series with the signal. That is a too high cost for the advantage of DC coupling i think. I can lower the noise of the original circuit though by using higher currents, better transistors and lower value feedback resistors. That requires a higher value elcaps. I do not see the reliability problem if good quality caps are used like Rubicon ZL.. Pannasonic FM, FC, Nichcon KZ Muse or the superior Elna Silmic 2. I will draw the circuit how i whould build it and then move on to a DC coupled circuit that does not have some of the problems of this one. It is the Piglets Dad Crophead, made for Tac McLaren. This is now double differenntial so one step up on the ladder for parallel symmetric amps.
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Old 13th August 2011, 07:35 PM   #2528
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This is how i would build it. Noise is 0.5nV/qHz so 6dB better then an AD797.
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File Type: pdf JLH Homage Pre-Pre final.TSC - TINA.pdf (10.1 KB, 113 views)
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Old 14th August 2011, 12:44 AM   #2529
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So how is DC feedback made easier ? Simply by using a differential amplifier. See Piglets Dad MC design. PSSR is not great in that design so i would use constant current sources.
Idle current in the input stage is 3mA so this is closer to what i would use. The disadvantage again is a 3dB noise penalty because the input transistors are in series.
That is the reason i arrived at parallel symmetric designs like MPP2011 1.0 to MPP2011 4.0.
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File Type: pdf Piglets Dad MC stgae.pdf (24.3 KB, 88 views)
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Old 14th August 2011, 01:38 AM   #2530
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Where do we move on from here ? As i said, i arrived at parallel symmetric. That is where the Fets come in. They are "self biasing" and do not have the DC offset problem into the cartridge. The most simple version would be A. This has better PSSR then Piglets Dads because the Fets work as a constant current source. A better way is B, not much different then a Vendetta. Here the second stage is a folded cascode , giving much better speed.
I build many nice phonos that way. A good example is the MPP FF2010 you can see earlier in this thread. Since then i have moved on to current mirrors and other more advanced stuff but when you need a quick fix, version B is a very good start. Version A and B can be build with bipolars at the input when you solve the biasing problem and the input offset if you are afraid of that. The second stage can be Mosfets. Vertical or better lateral. I have shown all this before. Now you understand that the JLH is only interesting on an historical basis and i have no motivation to max it out any more. Its good, very good, and costs nothing but i prefer my latest designs. They are more quiet, higher speed, less distortion, higher dynamic range, better PSSR and so it goes.
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File Type: pdf Parallel Symmetric Fet Stages.TSC - TINA.pdf (9.0 KB, 110 views)
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