Feeler: BBC AM9/4 or AM9/5 Germanium mic pre project

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Lee_M

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Hi everyone,

I've designed a PCB for some clones of the BBC AM9/4 (or AM9/5) mic preamps, would anyone be interested in some boards (and possibly other parts) if I place a larger order?

The AM9/4, AM9/5, and AM9/6 circuits are basically identical, the only major difference is that the AM9/4 has an output transformer fitted (AM9/5 and AM9/6 are unbalanced out) and an optional low cut filter. The AM9/6 is just two channels in a single module. These are based on the AM9/4 version, with onboard output transformer. I haven't included the low cut, but it could be implemented very easily (it's a toggle switch and a capacitor) if there's demand for it.

The preamp has switchable input impedance, the low-z setting is designed for 30Ω output ribbon mics, and gives an additional 9.7dB of gain.

All of the caps are axial types, I'm using Vishay 021 series for electrolytics. There's a few small value film caps, I've designed the board to accommodate Mullard 160v "mustard" caps (I've got some sitting around that I want to use up), but modern axial film caps (Vishay MKT1813 series, for example) are much smaller and will fit with ease. Resistors can be whatever you want, as long as they're 1/4W or more. The board can accommodate the larger 0.5W resistors if you want to go big.

It uses 3x germanium transistors per channel, there's a few different types that are potentially suitable but I need to do some testing to figure out what the best specs are for each position, I might do a bulk buy and sell pre-sorted sets with the PCBs (if there's enough interest), the board is designed for mounting the transistors upside down, nested in rubber grommets, just like in the original BBC preamps. I'll be able to supply the rubber grommets with the boards.

I'm using original BBC LG/59MSC and LL/76MSC transformers for mine, but I don't have any spares to sell. The board will fit canned transformers up to 30mm diameter (the BBC ones are 25mm, but I've left some extra margin just in case), they're mounted via threaded grommet through the board. The LL/76MSC are basic 600:600 outputs, so there's lots of possible alternatives if you don't have any of the BBC transformers. The LG/59MSC (mic input) types are a bit more difficult, they have a fairly low (1:2.78) impedance ratio, with a low impedance tap on the primary (for 30Ω mics). Sowter should be able to make some replicas quite easily, if you can't find original BBC transformers.

The boards are 107mm by 107mm square, power is -24V, I'm racking mine up in some empty BBC modules, but the PCB could easily fit onto one of the 500 series "L bracket" modules (along with an edge connector) and work with a 51x rack. Regular 500 series won't work, as these need the -24v rail. They could also be mounted horizontally in a 19" chassis, the PCB mounts on 4x M3 standoffs, connections designed for hardwiring, but you could fit 5.08mm pitch connectors (screw terminals, Molex, etc.) if you want. Everything is clearly labelled, so you'll have a hard time messing up the offboard wiring.

Component layout is fairly close to the original AM9/5 preamps, although not exact. All of the tracks are on the underside of the PCB and both sides have copper fill, with the entire top layer as a ground plane for improved shielding. The boards will be ENIG finish with black solder mask.

I've attached a pic of the 3D render of the board, some of the component visual models are a bit of a bodge (there's no canned transformer models so I used toroids instead, there's no models that are remotely close to the transistors so they're completely absent, and the models for the yellow film caps are a lot chunkier than the actual capacitors I'm using) but it should give you a rough idea for how the boards will look.

Price will depend on how many PCBs I order, but I guess it should be about £10 per PCB once postage and customs fees are factored in.

Happy soldering!
 

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Lee_M

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Nov 23, 2013
Messages
308
Location
Dorset, UK.
Thinking about four :) have a ton of Soviet germaniums, hope they can be used.

What PSU are you gonna use?
Very nice, what type are the Soviet germaniums?

I've got an old BBC PS2/49 PSU that I'm recapping and rehousing in an external box, which will connect to a module rack via a 4-pin XLR cable (it doesn't need 4-pins, but using a different connector means I can't accidentally plug the PSU into somewhere it's not supposed to go), but any -24v PSU should be fine as long as ripple is fairly low.

This is the schematic for the BBC PSU, it's all built on tag strips so shouldn't be too much trouble to replicate.


If you want to clone the BBC PSU, I'll take some pics of the tag strip layout and upload them on here.
 

Urskov

Active member
Joined
Jan 6, 2020
Messages
41
Location
Denmark
MP20B's and MP21A's. Mostly bought 'em for fuzz pedals.

Nah, the schematic looks to use some outdated components 🙂 I think a modern 24v PSU will be easier to make. I assume it's like with PNP Germanium guitar pedals where you can just swap the ground and positive wiring for negative voltage?
 
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