My desk amp - “Amp Camp” - Aleph current source OR??? - diyAudio
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Old 8th April 2013, 04:39 PM   #1
knutn is offline knutn  Norway
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Default My desk amp - “Amp Camp” - Aleph current source OR???

In the process to build myself a desktop amp, I decided to "steal" the "Amp Camp" output stage and you my J175 JFET input stage from my NOStalgia amplifier. The schematic is shown with the simulation result.
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File Type: jpg deskamp1.jpg (240.8 KB, 366 views)
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File Type: txt deskamp1.txt (879 Bytes, 24 views)
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Old 8th April 2013, 04:46 PM   #2
knutn is offline knutn  Norway
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However, I also tried the output stage with an Aleph source (or similar), but ended up with the schematic shown, which is something between the Aleph source and the "Amp Camp". But the strange thing is: The distortion is much lower (says the simulator) for this modified amplifier shown.

So my question is: Have someone tried a similar configuration and has an idea of how it sounds?
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Old 8th April 2013, 04:59 PM   #3
Zen Mod is offline Zen Mod  Serbia
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simulated THD spectra is .... ?
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Old 8th April 2013, 05:53 PM   #4
knutn is offline knutn  Norway
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The idle current for the input JFET is about 4.5 mA and about 1.1 A for the MOSFETs. As can be seen from the attached files deskamp1.txt and deskamp2.txt, the distortion drops from 0.3% to 0.02%.
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Old 9th April 2013, 05:40 PM   #5
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Try this on the deskamp1 sim. Change R10 to 0.82 ohm. Change R11 to 0.15 ohm. Add a 1K (or thereabout to maintain Iq) resisitor from base to emitter on Q13. That should drop distortion significantly. Feeling lucky? Change M9 and M16 to smaller devices like IRF520.

It's not hard to drop the measured distortion of the ACA when you change the gain/feedback ratio. Nelson likes the gain reduction to be around 10dB because that keeps the distortion low order. I find that when I listen to a CD through the ACA (even with feedback disconnected), I want to listen to another one instead of do something else.
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Old 9th April 2013, 06:32 PM   #6
knutn is offline knutn  Norway
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Thank you for your advice! I have tried it on the simulator, and you are perfectly right. I used an extra resistor of 1.5k and got the results attached in deskamp3.txt.
To be sure, I checked my first sim run (deskamp1.txt) and found that the capacitor C14 was too low. The right sim result is attached in deskami_rev.txt.
So, what is the conclusion? I think I will stick to deskamp1, but maybe I should add your extra resistor and try with different values of R10 and R11 when coming to real life. Simulations can be fun, but you can't listen to them....
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Old 10th April 2013, 07:16 PM   #7
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Default deskamp 1 build

I like your design and will give a try. 2SK170 will be the jfet and c14 will be 2200uf low esr. also will add the resistor between gate and emitter probably 1.21k. hold up will be R10&R11 increase as values in 2-3 watt resistors seem to be hard to find these days. I have at present .33 ohm .22ohm and .47ohm. I could use parallel and series grouping to gain proposed values but that takes up so much space. Also hav 24vct 10 amp tranny from MPJA.
Any further development clues would be appreciated.
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Old 10th April 2013, 07:54 PM   #8
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Default further on build

also (trying to use what's here) will use 5k multi turn trimmer set iniiately at
2k. Question why 1meg at input??
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Old 10th April 2013, 10:42 PM   #9
juma is offline juma  Germany
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Quote:
Originally Posted by sandyhooker View Post
... 2SK170 will be the jfet ...
No it won't (unless you wanto to turn the whole circuit upside down) beacuse the k170 is a N-channel JFET (J175 is a P-channel device)...

Take care !
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Old 11th April 2013, 04:04 AM   #10
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Default oops!!

Thanks Juma... i actually knew that as i had checked mouser for the device..just forgot.
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