Germanium microphone preamplifier

So here's the story: A long long time ago [silent:arts], Jensenmann and I met at Jens' place for a drink, a chat and also to [listen to Pres for comparison]. The RCA BA71 and Neumann V476 pres were among the best ones we heard that night. While nicely chatting Jensenmann suddenly grabbed two BA71 input transformers out of his workshop and handed them out to [silent:arts] and me. Later, [silent:arts] forwarded his transformer to me, because he himself had no use for it and so my intention to buid an RCA BA71 clone tightened more and more. Jensemann was also kind enough to find out appropriate replacement transistors for me and since the output iron for my clone originated from a groupbuy that Jensenmann organized, I was determined to call my clone...

... Jensamp

I try to avoid to call it an RCA BA71 clone, because the circuit is heavily modificated:

And here's the usual pr0n:

That should give you an exact idea of what a star ground is about - in case you were ever in doubt:

The signal board:

The PSU:

Tons of dust in the corners... But it's all good for a more "vintage" sound ;-D

The transformers that everything started with:

Jensen-Caps to stick with the name ;-D

That's art, the signal board from the bottom: Point to point, the true spirit of DIY at its best (Well, maybe I'm just to lazy to design and etch a PCB ;-D)

The switchable feedback resistors and caps to control the gain:

Everything you need for everday's use:

The sound??? Big bottom end (read: balls!).
Feels almost like the proximity effect known from good condenser microphones. It's that in your face sound that you might know from the narrator's voice in all the education movies you've been shown decades ago at high school, of course not that heavily overcompressed, but you get the idea. And it's still very clean and detailed.

Update 03.01.2009

Viele fragten danach, hier ist der Plan:

R1: 22k (Termination des Eingangsübertragers, siehe Schaltplan für Erläuterungen)
R2: 33k
R3: 91k
R4, R5, R10, R16, R23: 10k
R6: 470 (kann auch als 1k-Trimmer für die Feinjustierung ausgeführt werden)
R7: 4,7k
R8: 68k
R9: 56k
R11: 200
R12, R21, R15, R22: 22
R13: 2k
R14: Siehe Schaltplan
R17, R20: 680
R18: 330k
R19, R24: 6,81k (werden direkt am Eingangsübertrager befestig)
R25: 20

C1, C2, C3, C7, C10, C11: 470u (Nichicon 470u/50V) + 100n (MKS02 100n)
C4, C5: 100u (RAD100/35) + 100n (MKS02 100n)
C6, C15: 1000u (RAD1000/25) + 100n (MKS02 100n) (C15 wird direkt am Eingangsübertrager befestig)
C16: 4700u (RAD4700/35) + 100n (MKS02 100n)
C8: Siehe Schaltplan
C9, C12: 6.8u (MKS-4-250 6,8u) + 10nF (Keramik!)
C13: 330p (siehe Schaltplan!)
C14: 3,3u (MKS-4-250 3,3u)

Q1: BC109
Q2: ASY26 (BC560 geht auch, falls jemand Germanium nicht mag)
Q3, Q4, Q5, Q6: BC140

PCB Layout

Looks like working, not tested though:

[Back to main]

Darius Kubarth 2008