K&K - Pure XLR Preamp

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
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Bononoob
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Post by Bononoob »

I have this Preamp because someone wants me to modify it. So I traced it
K&K makes preamps and mics for acoustic guitars, it's a nice and simple design if you're looking for something like this.
Here's the Eagle Schematic and some gutshots. Tell me if I made mistakes!

edit: Changed C14 polarity
Attachments
K&K Pure XLR Preamp - Schematic.png
k&kup.jpg
k&kdown.jpg
Last edited by Bononoob on 29 Jun 2020, 21:41, edited 1 time in total.

lukatosh
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Post by lukatosh »

Hello, thanks so much!! Just one question, the 22uF capacitores are non-polarized?

marshmellow
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Post by marshmellow »

Are you sure about R20/R21? I would rather expect 4,7k, 47k, ... There isn't much to point to having an additional pad at the output with a volume control that goes to zero anyway. It only makes it harder for the 4558 to drive such a low load. I don't know if this is meant to plug into a microphone input or a line input, but the reality is many budget mixers or recording interfaces don't always feature line inputs. So with a low impedance microphone input of 1-2k connected, the load gets even worse.

Bononoob
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Post by Bononoob »

lukatosh wrote:
28 Jun 2020, 04:59
Hello, thanks so much!! Just one question, the 22uF capacitores are non-polarized?
Hey, no probs! No, all 22µF are polarized and oriented like in the schematic.
marshmellow wrote:
28 Jun 2020, 11:51
Are you sure about R20/R21? I would rather expect 4,7k, 47k, ... There isn't much to point to having an additional pad at the output with a volume control that goes to zero anyway. It only makes it harder for the 4558 to drive such a low load. I don't know if this is meant to plug into a microphone input or a line input, but the reality is many budget mixers or recording interfaces don't always feature line inputs. So with a low impedance microphone input of 1-2k connected, the load gets even worse.
Yes I'm sure. I checked again they are SMD marked 471. I don't think R20/R21 are pads, the preamp can be powered by phantom power. Maybe you thought V+ was ground? Anyway thanks for checking it out!

marshmellow
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Post by marshmellow »

You're right, I completely overlooked the phantom powering :slap:.

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