RJM Audio B-Board Project

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

rjm

Member
Joined 2004
Paid Member
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.
 

Attachments

  • LH0002.png
    LH0002.png
    19.3 KB · Views: 1,575
  • snoa365b.png
    snoa365b.png
    26.4 KB · Views: 1,564
  • LH0002.pdf
    118 KB · Views: 254
  • BBoardmini10a.png
    BBoardmini10a.png
    15.1 KB · Views: 1,558
Last edited:

rjm

Member
Joined 2004
Paid Member
B-Board mini rev 10b

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.
 

Attachments

  • b-board10b-brd.png
    b-board10b-brd.png
    15.2 KB · Views: 1,528
  • b-board10b-sch.png
    b-board10b-sch.png
    21 KB · Views: 1,522

rjm

Member
Joined 2004
Paid Member
Self-regulated, with Z-reg

"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.
 

Attachments

  • b-board20d-sch.png
    b-board20d-sch.png
    24.1 KB · Views: 527
  • b-board20d-brd.png
    b-board20d-brd.png
    14.5 KB · Views: 616
Last edited:

rjm

Member
Joined 2004
Paid Member
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.
 

Attachments

  • bboard30a-brd.png
    bboard30a-brd.png
    14.4 KB · Views: 473
  • bboard30a-sch.png
    bboard30a-sch.png
    24.1 KB · Views: 358
Last edited:

rjm

Member
Joined 2004
Paid Member
B-board GZ 10e

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.
 

Attachments

  • pcb-bb10e-info.zip
    128.5 KB · Views: 152
  • b-board-GZ-10e-sch.png
    b-board-GZ-10e-sch.png
    43.1 KB · Views: 422
  • b-board-GZ-10e-brd2.png
    b-board-GZ-10e-brd2.png
    15.6 KB · Views: 329
  • b-board-GZ-10e-brd.png
    b-board-GZ-10e-brd.png
    22.8 KB · Views: 429
Last edited:

rjm

Member
Joined 2004
Paid Member
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.
 

Attachments

  • IMGP1949j 1600.jpg
    IMGP1949j 1600.jpg
    404.3 KB · Views: 611

rjm

Member
Joined 2004
Paid Member
B-board application note

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.
 

Attachments

  • B-board Application Note.pdf
    144.3 KB · Views: 215
  • Slide4.PNG
    Slide4.PNG
    30.6 KB · Views: 415
  • Slide3.PNG
    Slide3.PNG
    32.7 KB · Views: 404
  • Slide2.PNG
    Slide2.PNG
    25.3 KB · Views: 374
  • Slide1.PNG
    Slide1.PNG
    10.2 KB · Views: 541

rjm

Member
Joined 2004
Paid Member
@quan

Unless you have a DC-coupled amplifier, this offset, while slightly on the high side, is OK.

The VSPS output has an coupling capacitor, C3, so the DC impedance seen from the B-board input is high, equal to R7 or 75k.

If you connected a smaller resistor across the B-board inputs from IN+ to IN-, perhaps 10k, you would see the offset voltages very much reduced (<50 mV). Actually I'd recommend you do that if you can. Exact value isn't critical, roughly 10k to 22k.

The other option is a little more complicated: move C3 and R7 to the other side of the B-board. This is done by inserting the B-board in place of R6 (VSPS) or R8 (Phonoclone). The resistor is removed from the board, the inputs go to the pad connecting to the op amp output, the outputs to the pad connected to C3. See attached.

Richard
 

Attachments

  • pcb-vsps300m.png
    pcb-vsps300m.png
    28.6 KB · Views: 567
  • pcb-pc35f.png
    pcb-pc35f.png
    30.5 KB · Views: 559
Last edited:
@quan

Unless you have a DC-coupled amplifier, this offset, while slightly on the high side, is OK.

The VSPS output has an coupling capacitor, C3, so the DC impedance seen from the B-board input is high, equal to R7 or 75k.

If you connected a smaller resistor across the B-board inputs from IN+ to IN-, perhaps 10k, you would see the offset voltages very much reduced (<50 mV). Actually I'd recommend you do that if you can. Exact value isn't critical, roughly 10k to 22k.

The other option is a little more complicated: move C3 and R7 to the other side of the B-board. This is done by inserting the B-board in place of R6 (VSPS) or R8 (Phonoclone). The resistor is removed from the board, the inputs go to the pad connecting to the op amp output, the outputs to the pad connected to C3. See attached.

Richard

Will do Richard
 

rjm

Member
Joined 2004
Paid Member
I'm very glad to hear it's working now.

The short answer is "yes": 10k resistors across IN+ and IN- can be used for both phono stages to reduce the offset voltage.

It's the way the B-board circuit operates: the output offset is a function of the DC impedance (resistance) seen between IN+ and IN-.

If there is a blocking capacitor upstream, the B-board cannot "see" the low impedance output of the driving stage, and instead has to source/sink current from the resistor placed between IN+ and IN-.

(Full disclosure - I forgot about the influence on output offset voltage of the blocking capacitor, basing the performance instead on the Sapphire amp, where the buffer is driven directly by an op amp and the total output offset voltage is extremely low, less than 10 mV.)

Both the Phonoclone and VSPS normally have this blocking capacitor, so the output offset depends on whatever resistors to ground are placed after the blocking capacitor (C3). This is normally 75k for my phono stages. That's - a bit marginal. 10k is better, if only for peace of mind.
 
Last edited:
Buffer installed (for the time being after the coupling cap of the Phonoclone). Works perfectly, about 160mV offset on one channel and 180 on the other without adding additional resistors on the input.

Verdict - very positive. With the Phonoclone I had been swapping out the 2nd opamp between the OPA27 and AD797 baing unsure which I prefered. The OPA27 was more mellow and listenable for a long session, but the AD797 seemed to carry a little more weight as well as detail and resolution, but at the expense of a certain edginess, that despite adding power pin decoupling never went away.

Now with the buffer added, the OPA27 setup gains the detail and weight of the AD797, without the edginess. I am sure that the easier driving of the buffer for the OPA27 is the reason, plus the buffers immunity to what it is driving.

I can thoroughly recommend the buffer!

I'll post pictures of the completed set up in a few days, the wiring is messy at the moment due to the addition of the buffers.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.