Monday, January 6, 2025

R2R ELECTRIC: Germanium Pre-amp

It's no secret as to what the R2R Electric germanium preamp is based upon, which is an old and very rare Hofner preamp. The Hofner is a full-range germanium booster, which was once used as a way to plug guitars into old console radios way back in the day.

As this is a simple Rangemaster style circuit, any layout used for a Rangemaster could be used instead of the one below.

R2R ELECTRIC GERMANIUM PREAMP LAYOUT

R2R spends a bit of time testing every component used, to get the sound that they're chasing. Since not all components are created equal, it’s worth dedicating time to testing and selecting components to ensure you get the most out of this simple circuit.

One notable choice is the AC107, which R2R recommends as a low-noise, higher-quality alternative to the OC44 or OC71 commonly used in Rangemasters. This decision is practical: while OC44s are highly sought after due to their use in Rangemasters, they’ve become increasingly expensive and are often prone to noise issues.

Side note: An excellent podcast featuring Chris from R2R can be found here on the Fretboard Journal. Chris talks about his journey and process.

And thanks to the person who very kindly helped out by sharing some photos of their pedal, confirming that the boost pot is wired the same way as a Rangemaster.

I built it on tag strip, using the same layout - works just fine. If you do the same, careful with the size of the caps either end as it’s a very snug fit in a 1590B

Boost / Volume pot: R2R implemented the volume/boost control in the same way as the Rangemaster. The stock Hofner circuit uses a 10k resistor to the collector and has no volume control - R2R replaced the 10k resistor with a 15k pot. If a 15k pot is hard to source (as it's an uncommon value), try a 25k pot with a 39k resistor across pin 1 & 3 - this will drop it down to 15k. Any resistor between 33k - 50k will get you close enough to 15k. Pots are rarely bang on spec, so not something to stress about. You could also experiment with a 10k pot to see how it goes.

Input resistor: There's no need to worry about using the exact value for the input resistor. The stock Höfner schematic specifies 37k, which is an unusual value. R2R opted for 36k. As long as your choice is close, it should work fine.

It’s worth noting that the input resistor forms a voltage divider with the 10k resistor connected from the transistor base to ground. Together, these resistors reduce the input signal by about 80% before it’s amplified by the transistor.

Emitter resistor: The stock Hofner circuit uses a 1k5 emitter resistor, but there's room for variation, which can be tuned to match your chosen transistor. The R2R layout uses a 3k9 resistor, like the Rangemaster. Smaller values are more likely to produce some dirt, while larger values will result in a cleaner sound. This is a great spot to use a trimmer pot if you want adjustable control.

Capacitors and Bias Resistors: The capacitor values and the 100k/10k bias resistors remain consistent across designs. However, there's flexibility with the capacitor values:

The input capacitor can be reduced significantly without noticeable changes in response. For instance, 1u is nearly indistinguishable from 4u7.
The 100u emitter bypass capacitor can be reduced to 47u with minimal impact.
Similarly, reducing the output capacitor slightly will have negligible effects on the sound.

Of course, collectively changing a lot of component values at once will increase differences, but ultimately, it is just a fairly generic single transistor booster in therms of the circuit.

Drop down resistors: R2R included 1M input and output drop-down resistors wired to the stomp switch, which you can see in the photos below. These are not present in the Höfner circuit but are a common addition to prevent switch pops. Including them is recommended.

Board: I’ve not listed the size of the board or any other info on the point-to-point layout - I’m sure you can work that out yourself.