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Old 10th August 2011, 07:46 PM   #2481
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Thanks, so it is also made to safe some fuel because the currents are rather low in that design. I will try the 5V version with the offset control of the 1.5v version.
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Old 11th August 2011, 02:51 AM   #2482
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I have now prepared the input transistors. Why he chose those i can only reason about.
They seem to have a rather Low Rbb´because he gives the noise impedance even on the low current version as under 30 Ohm and that includes the feedback resistor of 18 Ohm.
So it must be 30 - 18 x 2 = 24 Ohm pro transistor under the current he chose. Rbb´must be even lower in fact because he runs them rather starved under modern view. Even in the "high" current version he runs them only on 1.1mA. I would use at least 5 to 10 x as much. The output stage on the other hand runs on a healthy 20mA plus ( on the high current version ) so there is potential for lower noise by raising the input stage current and using a lower impedance feedback loop. I have seen the same phenomenon on the original Hiraga Pre-Pre so both designer may have had the DENON DL103 on mind that limits the noise performance anyway because of the rather high self impedance. I have not measured it myself but i heard that the value of 40 Ohm given in the specs is not correct and it has around 20 Ohm instead what would be a good thing. For my 6 Ohm Titan this is sure not the optimum bias but as i said, i build it first as close to original spec that i can. These BD transistors are also rather slow. The Ft is only 3MHZ. That really surprised me but they are also very robust with 36W to burn.They where originally design for car audio by the way so this is funny. Hfe on the other hand is surprisingly high for a medium power device with around 130 on the PNP and 180 on the NPN. That minimizes offset of the direct coupled input. That can easily be beaten of cause by the special MC input stage devices by ROHM, Toshiba and Hitachi that have more the 500. They are not made any more and that is a pity. When i would do a modern version i would use Sanyo video transistors. They are also made by Fairchild and cost around 0.60 €. Concerning the schematic i think i found the latest version. That is the most logical to me and runs on plus-minus 5V.
Attached Images
File Type: jpg LJH NPN HFE.jpg (84.1 KB, 291 views)
File Type: jpg JLH Input Transistors.jpg (66.7 KB, 269 views)
File Type: gif jhlmmaud.gif (13.1 KB, 282 views)
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Old 11th August 2011, 01:01 PM   #2483
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Quote:
Originally Posted by Joachim Gerhard View Post
Homemodder, so you use a Szikley pair at the output. I use that sometime too. Did you had any stabilty problems? Anyway, i will build the circuit as is although with other transistors at the output because i do not have that old Motorola BCs. I have the correct input trannies though.
Lauret, when i look at the idle currents, 5V is much more logical.
Joachim, I apologize the right book is in fact sinclair and JLH also mentions the circuit in his own book. According to the text its correct that he states the use of a pair of dry cells but I think he intended them in series as it clearly also states that the design is optimised for 3 volts according to the text in sinclair s book which was written by JLH himself.

I indeed did have some stability problems with prototype the cure was simple, 5 pf caps accross Base Collector of each output cfp s. Also a single cap from output to ground worked just fine. Later with revised layout and proper PCB I didnt need any compensation at all. I used BC550 560 at inputs and slave of cfp s and 2sa1837 4793 and run the cfp s at 75ma but this is to power my sennheisers.

Looking forward to hearing your results, I havent found a circuit by JLH that didnt sound good, for him a circuit should measure at least reasonable and sound good too.
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Old 11th August 2011, 01:56 PM   #2484
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Thanks for further information. I have all of his books and will look it up. Today is building day.
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Old 11th August 2011, 09:35 PM   #2485
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I build the JLH stage and i must say that i am impressed by the technical performance and the sound. Bandwidth is wide, -3dB at 10Hz and 1MHz. The 10kHz square wave is one of the best i have seen. Putting 50mV into the input did not clip the output. I made some small changes and simplifications and i will talk about that soon. Noise is around 0.7nV/qHz and commendably free of flicker and shot noise. Offset is trimmable to under 2mV and stays stable. I use a different trimming method then JLH but without changing the topology. That will be described too in due course. I did not select the BJTs in any way. I needed around 3 hours to build it and it worked right away. Even somebody that is not so experienced then me can build it over a weekend for minimal outlay. Recommended and lots of fun. Made my day.
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Old 11th August 2011, 10:37 PM   #2486
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here are some pictures
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File Type: jpg JLH Pre-Pre 10kHz square.jpg (97.6 KB, 243 views)
File Type: jpg JLH my build.jpg (925.6 KB, 236 views)
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Old 11th August 2011, 10:50 PM   #2487
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The simplification i made was suggested by AS. The current limiter as he calls it is not necessary so i did not connect the 10kOhm resistors. I thought this is a kind of bias control but bias is rock stable without it. The DC offset i simply set by inserting a trimmer of 100 Ohm instead of the 80 Ohm resistors. After trimming i measured the value and it was 80.2 Ohm in one channel and 79.9 Ohm in the other. This makes the trimming kind of redundant. He also made the experience that the Elcaps can be unreliable so the Elcaps can be replaced with transdiodes with a 3dB noise penalty. The second stage could also be configured as a folded cascode for more speed, but ...... I just keep it as it is. I have a better idea. A balanced transimpedance input with nearly zero Ohm up to over 10Mhz. Even at 100MHz it simulates with only 21 Ohm. How that ?
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Old 11th August 2011, 10:56 PM   #2488
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By the way i run the stage slightly hot on plus-minus 6.8V triggered down from buffered 12V lead cells with a 220 Ohm 330uF RC combination. Hum is nil.
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Old 11th August 2011, 10:59 PM   #2489
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That set the input idle at 1.5mA. Input offset is under 0.1mV but i will measure that more precise when the batteries arrive for my precision meter.
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Old 11th August 2011, 11:01 PM   #2490
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I will post the balanced transimpedance circuit tomorrow, so good night boy and dolls.


There is chaos in the order, random things you can not change.
Joni Mitchel
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