RJM Audio B-Board Project

[rjm]

I've been into buffers lately. Diamond buffer, from the LH0002 datasheet (and the related app note), and, a more recent find, a variant from the TI document "Current Feedback Op Amp Applications Circuit Guide" which has a few more damping resistors**. They are really versatile circuit blocks. Line driver, unity gain preamp, preamp output stage, headphone amp output stage...

So I'm going to try and develop a circuit board or two, the founding idea being "small, cheap, simple, easily available, reliable." This is the sketchpad thread, I'll post the changes as they come... don't expect anything in final form.

Right now I've got three distinct variants in mind, depending on whether you want to drive 10k line impedances (low current version, TO-92, no thermal feedback), headphones (high current version, TO-126, heatsinks) and with or without voltage regulation.

First up is the the most basic: low current version, no regulation. The "B Board Mini". 4x7cm board. I'm thinking people can retrofit this into their VSPS and Phonoclone projects - or anything at all, really. You are looking at the pre-alpha version below, not even worth posting the eagle files yet.

**These extra resistors are nice because they naturally give you a few more jumpers to work with, greatly simplifying the board layout.

[rjm]

Finished the unregulated version of the buffer board, though I will later "metricize" the final board dimensions and hole spacing.

- using TO-92/SOT-54 EBC transistor packages, BC547/557 shown but any similar can be used

- extra set of redundant power supply connections, so boards can be daisy chained or rotated 180 degrees and the supply wiring stays neat

- Q1/3 Q2/4 back-to-back so the transistors can be glued together for thermal feedback

These boards are designed to run from a regulated, split +/- 9-12 V supply.

Dual sided boards 4x7 cm.

[rjm]

"And for my next trick", said the Magician, waving his magic wand...

With the last board, we had the line-driver buffer circuit powered by a regulated supply. What we have here is the same buffer but for use with an unregulated power supplt. It has an on-board voltage regulator made up of a Zener reference and series pass transistors (a circuit block I have just dubbed the "Z-reg".)

This is the smallest version, with TO-92 packages. It's 4x10 cm, or half the size lengthwise of the Phonoclone 3 - or 1/4 Eurocard I believe. It's designed as a bolt-on upgrade for the Phonoclone, VSPS, any line level source components, phono stages, DACs, etc. It could also be used as the output stage of a pre-amplifier.

The regulated voltage is available from the pads on the right side, to power an external op amp voltage amplifier stage. As long as the circuit doesn't need much more than 10 mA... i.e. it's an op amp or two. As such there are all sorts of uses.

Next up, to see if we can replace the TO-92 with TO-126 type packages and some small heatsinks without increasing the board size all that much. If so we have a headphone buffer, the ultimate drop in upgrade for a chu-moy.

[rjm]

Well, that was easier than expected...

Now we have 1000uF filter caps, 2W "411” resistors R3, R4, and TO-126 ECB transistors　(BD135,BD136 or equivalent). Heatsinks can be used, the clip on or screw on types have clearance underneath for the resistors.

This can either be a headphone buffer or a line buffer, depending on the value of resistors R3, R4 which set the bias current in the diamond buffer.

[rjm]

For those following along, the boards ("GZ" [Grand, Z-reg] variant, rev "10e") are now "RTM" status. I should get the first batch in a couple of weeks, then I'll have a look though and do some evaluation to fine tune the BOM values.

Also putting together a project page, here.

[hags]

A nice, clean, purpose built pcb would be nice as opposed to the perfboard I use.

These would come in handy for expeimenting with diamond, or other buffers/output circuits.

Let us know when you get the boards.

[rjm]

Sure thing. I expect to start shipping by the end of Feb.

I'm glad you see things the same way I do re. having a PCB. Though I would say the B-board is multi-purpose built rather than purpose built. It's a go-anywhere, do-anything kind of board.

[rjm]

ltspice files for both the line driver and headphone driver variants of the B-board circuit attached.

[rjm]

A photo of the finished boards.

The Q1/4 and Q2/3 are glued together for good thermal contact.

Notice In+ and In- are tied together with a resistor across the input. This required for testing, as the B-board expects to be attached to an input load normally.

Regulator output V+/- are 11 V and -11 V. Output offset voltage about 50 mV in both channels. 2 mA bias current in both output stages.

Everything working according to plan.

[rjm]

Is it is hard to write instructions that say "do this" or "do that" when so many different configurations are possible.

Shown in the attachments are the basic use cases as I see them. Either the B-board is going to be the "front end" in the chassis, accepting the line input signal directly, or it is going to be a traditional buffer, bolted on to the back of another circuit in the chassis.

Both cases are covered, I hope.

Any questions, please ask.