AKSA's Lender Preamp with 40Vpp Output

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So I was able to get 28.8vpp out of this preamp to drive the MoFo to 11Wrms.

Here is scope shot.

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It’s interesting what that big 67mH choke can do to allow 27vpp swing from a 19v power supply.

So this seems to exceed the 20v limit that Adason was talking about?

Nice work X, as always. Do not forget that some of those amplifiers you mentioned have mosfets on the input which do not like to exceed 20 volts on the gate, hence the zener diodes protecting the gate.
 
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Wow! Nice boards! And phenomenal work AKSA, X and JPS64! I'm always amazed at the level of dedication to the art of DIY on this forum and especially all the very charitable contributions everyone makes on a daily basis. Can't wait for GB #2!


Now back to the preamp discussion! I have plans to build a MoFo eventually but right now I am building several MyRef-Fremen amps and plan to use them in an active setup with 1 or 2 MiniDSP 2x4HD's. I'm no EE or circuit expert, but I know I don't need 40v to drive any of these devices. Is it as simple as adjusting the bias down so I don't blow anything up or is the pre best suited for high drive applications like the MoFo?
 
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Hi Jwarch,
If you are aftraid of the high drive level damaging your amp inputs you can reduce the gain to say, 12dB, and you can reduce the Vcc to 24v. I can give you recommendations for which resistors to change. Basically you want to maintain the same bias current in the LTP legs and keep the same in the main VAS stage. It will work fine (even lower THD) at more modest drive levels. Probably the easiest thing is to just drop the gain but keep Vcc the same. Assuming your source puts out about 2.5vpp a gain of 3x is no more than 7.5v which shouldn’t damage a miniDSP.
 
That makes sense. Thank you X. I was hoping I could set it up to more or less switch between MiniDSP/My_Ref and Mofo on the fly. I wouldn't want to limit the potential of either system by compromising to get both to work with the same pre. Sounds like I'll need to order another set of boards. :D


It would be great to have a build guide with these options in it for various gain settings. Maybe that'll happen once people start populating boards and optimizing the BOM? ;)
 
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I wouldn't mind swappable daughter boards. That would be quick enough. Great idea!


I edited my previous post but you beat me to the punch. I was saying how a build guide with various gain settings would be awesome. Or maybe it would just be included on future BOM spreadsheets.
 
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There is truth to "Burn-In"

Well I let the Aksa Lender Pre stay on overnight for burn-in, mostly because I was lazy to turn it off. I come back in the morning and remeasure it to make sure everything is working and thought I made a mistake or something, but the distortion went down significantly.

Here is the FFT for 20Vpp into 7kohm load, the THD is now below 0.001% into the single digits ppm, whereas was at 0.0016% before. 38% reduction of an already low level...

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Hi X,

find attached schematics for checking before producing manufacturing data set.

JP

Hi JPS64,
Here are the suggested values that could be ordered in addition to everything else already shown. This will give the user some choices for level of gain (R10), emiter degeneration on R5/R6 LTP (more or less H2/H3 ratio), and relative level of H2 (R9). Otherwise looks great. The R10 is not a MELF so perhaps can simple 1/4W axial CF be used here with bent pins?

Thanks,
X

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Hi JPS64,
I noticed that the header pin spacing on the new 0805/1206 SMT version is different than the previous boards, can you make sure it is compatible? A few practical comments as I am attempting to build the MELF version using 0805. The diode part is unusually small and very close to capacitor and transistor pads. Solder paste from nearby pads can pool and surface tension from uneven heating can pull parts out of alignment easily. Would it be possible to make 1N4148 an 0805 part and move it farther away from others? Maybe place to the left where ground plane vias are - reduce ground plane there is OK. Also, would it be possible to cover all vias with solder mask? They are prone to sucking up loose solder and forming solder bridges.
Thanks,
X

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