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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output
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Old 3rd December 2017, 11:48 PM   #341
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
So I was able to get 28.8vpp out of this preamp to drive the MoFo to 11Wrms.

Here is scope shot.

Click the image to open in full size.

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?

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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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Last edited by xrk971; 3rd December 2017 at 11:51 PM.
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Old 4th December 2017, 01:01 AM   #342
keantoken is offline keantoken  United States
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If the buffer-amp has 20V output then there is no problem with driving the input with more than 20V. After all there is no other way to get 20V out of a buffer.

Any design where such an error is possible is not a good investment anyway.
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Old 7th December 2017, 11:07 AM   #343
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
The GB boards have arrived.

AKSA's Lender Preamp with 40Vpp Ouput GB-aksa-lender-pcb-delivered-01-jpg

AKSA's Lender Preamp with 40Vpp Ouput GB-aksa-lender-pcb-delivered-02-jpg

AKSA's Lender Preamp with 40Vpp Ouput GB-aksa-lender-pcb-delivered-03-jpg

AKSA's Lender Preamp with 40Vpp Ouput GB-aksa-lender-pcb-delivered-04-jpg

Thank you for the superb layout JPS64!
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Old 7th December 2017, 03:15 PM   #344
pcgab is online now pcgab  United States
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AKSA's Lender Preamp with 40Vpp Output
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Originally Posted by xrk971 View Post
The GB boards have arrived.

Thank you for the superb layout JPS64!

Awesome! Those look great!
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Old 7th December 2017, 03:16 PM   #345
lehmanhill is offline lehmanhill  United States
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You were having fun with the beauty shots. Cool.
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Old 7th December 2017, 05:26 PM   #346
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
I always post glamour shots of new PCB arrivals.

It’s quite amazing what we can get as diy hobbyists nowadays. Our PCB’s look so much better than pro PCB’s of 15yrs ago and are so affordable.
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Old 7th December 2017, 07:19 PM   #347
lehmanhill is offline lehmanhill  United States
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And we are all a little bit geeky because we think pictures of PCBs glamourous. But I'm raising my hand and saying those boards look so fine. Congratulations on your photography.
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Old 7th December 2017, 07:30 PM   #348
jwjarch is offline jwjarch  United States
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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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Old 7th December 2017, 09:26 PM   #349
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
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.
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Old 7th December 2017, 09:35 PM   #350
jwjarch is offline jwjarch  United States
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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.


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?

Last edited by jwjarch; 7th December 2017 at 09:46 PM.
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