BTSB Buffer - SE/Bal to SE/Bal Buffer GB

[xrk971]

Edit Feb 19, 2024: The BTSB was tested and verified to produce 49Vpp with balanced 4.9Vpp input and balanced output. This means it can easily drive 0dB amps like F4 to clipping levels. The LME49724 datasheet says that it should be able to drive 600ohm loads to 52Vpp with low distortion levels.

Edit Nov 23, 2020: Tips on videos on how to do your own SMT/SMD soldering:
BTSB Buffer - SE/Bal to SE/Bal Buffer GB

Edit Oct 22, 2020: new boards v1.3 for TH and v1.2 for SMT have corrected them layout error and verified to be working properly in all aspects.

Edit Sept. 4, 2020: Stop Press! Please note that there is an error on J36 (the phase is flipped). I am ordering new boards and will send out replacements to folks who have already purchased these. This only afftects one of the SE outputs. If you have no need for the SE output, it doesn't matter. There is a workaround to it here. So sorry about this but it was the one thing I did not check on the verification build.

Edit Aug 18, 2020: Mouser shopping carts for the BOM here. Thanks to Vunce for making these for us!

This is the Best Thing since Sliced Bread (BTSB) Buffer that I will be offering as a GB. It was designed by Jhofland to address a need that I had with my TPA3255 Class D amp which required balanced input with gain. Jhofland designed a prototype that had just balanced output with gain (selectable via jumpers) with either balanced or SE inputs. I tested it out and it works great.

Here was the v1.0 verification prototype that I built:

Member Redjr has already used the prototype version on his very nice TPA3255 amp build here.

I thought that a more universal buffer would provide a choice of either SE/Balanced input and be able to driver SE/Balanced outputs simultaneously. This might be useful for driving a subwoofer amp, for example. The topology uses a state of the art OPA1656 for the input buffers and state of the art LME49724 balanced line drivers and another OPA1656 for the SE outputs. Another OPA1656 provides a bootstrap for the input amp to allow lower distortion drive at higher drive levels close to rail voltages. The power supply is provided with a very low noise Murata isolated DC/DC converter to take 12vdc from a wall-wart to +/-15v using the DC/DC followed with a CLC filter. The BTSB is absolutely silent and adds no noise or coloration. It is very transparent and can be used as a simple signal booster in cases where you need just a little bit of extra bump in the output from your source or existing preamp.

With the ability to accept SE or balanced and to drive SE or balanced, and with your choice of 0dB, 6dB, 14dB, or 20dB gain selectable with a convenient 8-position DIP switch - you will find it an indispensible part of your electronics toolbox kit. I think that you will agree that is may be the best thing since sliced bread...

To see the schematic, please click below. It is too wide of an image to display inline.

Here is the layout:

Here is a photo of the PCB with a Neutrik XLR jack (plain Molex KK terminals are also provided if XLR is not needed):

Here is a measurement of a 1kHz low distortion signal from Victor’s oscillator and 20dB gain from SE to Bal mode and fed to XLR input of my Focusrite - the so-called loop back test with BTSB in the loop. It has noise and distortion below the measurable limit of the Focusrite.

Here is a measurement of the distortion of my TPA3255 w PFFB as driven by the BTSB in SE to Balance out with 20dB gain applied - the THD is lower than the TI factory data at the same condition. The noise baseline is very low at -130dB:

I'll post a photo of the populated board as soon as I get back from vacation.

We can get the GB interest list started. Boards will be 1.6mm thick, 1oz copper, ENIG finish, and green color solder mask. Construction is straight forward as it is mostly through hole except for the opamps. Price is $23 ea for the bare PCB. The BOM cost is about $65. Please see BOM below - I will post a Mouser shopping cart to make ordering easy (soon).

I have received questions about whether or not a version will be provided with the SMT parts pre-installed. Yes, I can do that and the cost for a BTSB with 4x OPA1656 and 2x LM47924 pre-installed will be $69 ea. The 6 opamps cost $21, and I am charging $25 for the SMT soldering service.

Also available in all SMT - 70mm x 70mm. No XLR jack. Same price as TH version (Schematic and BOM here).

Please add your name, desired boards, SMT pre-pop'd Y/N, and country below.


BTSB Buffer GB Interest List:
-----------------------------
Name No. of boards SMT pre-populated (Y/N) Country

Please place a pre-order in my shop if you add your name to the list above.
BTSB Buffer SE/Bal to SE/Bal Buffer | Etsy

There is a new panel mount version of the BTSB that has separate auto-switched RCA inputs. Anytime something is plugged into XLR/TRS, a relay automatically disconnects RCA's. This version also uses OPA1637 for balanced output driver (uses less quiescent current). Mounts directly to rear panel and no mounting standoffs needed. Very compact and conserves chassis real estate on bottom panel. Also reduces flying leads that can pick up noise.

Fit check with 3D print:

Here is panel hole template. Note that the vertical CTC distance below is for the Elecaudio RCA jacks. If you purchase the RCA jacks from my shop or the RTR BTSB Panel Mount then the distance is 7.25mm:

Board dimensions:

BTSB TH v1.3 board dimensions are 100mm wide x 88m high.

BTSB SMT v1.2 board dimensions are 70mm wide x 70mm high.

BTSB Panel Mount SMT v1.2p is 132mm wide x 45mm high.

 

[astromo]

Nice one X ..

I'll swing a pre-order in along the lines of ..

BTSB Buffer GB Interest List:
-----------------------------
[Name No. of boards SMT pre-populated (Y/N) Country]
astromo - 3 bare PCB (SMT = N) - Australia

 

[xrk971]

Just wanted to thank the folks who are placing Pre-orders in my shop - but noticed they have not added their names to the list here.

 

[wineds]

Nice one X ..

I'll swing a pre-order in along the lines of ..

BTSB Buffer GB Interest List:
-----------------------------
[Name No. of boards SMT pre-populated (Y/N) Country]
astromo - 3 bare PCB (SMT = N) - Australia

I am interested in one bare PCB. Can we share shipping?

 

[outtek]

I would be interested in two boards only if you have or can provide a drawing that I could use to cut with a CNC machine the holes for the connectors used. Thanks in advance and ai will be waiting for great news from your side 🙂.
Florin

 

[xrk971]

Hi Florin,
You can find a DXF file for the Neutrik connector here:
NCJ6FA-H-0 | Neutrik
That should give you enough info for cutting the holes on the panel.
Cheers,
X

 

[outtek]

Hi, that is true but I would require some reference points in the pcb like distance between the pin of each connector this way I know that pin 1 of connector 1 is at x mm from the same pin from the connector 2 and also if possible in relation to the edge of the board.
Thanks.

 

[jhofland]

@outtek,
Perhaps I can help. The leftmost connector, J4, centerline is 21.5mm from the board edge. The center to center spacing for J4 and J3 is 45mm. And J3 is 21.5mm centerline to right board edge.

 

[xrk971]

Hi Folks,
Jut wanted to let you know that I am finally back from vacation and tested the BTSB v1.2 as a verification build (by jhofland). It sounds fantastic! Very nice sounding, super neutral, super transparent. Adds exactly the amount of gain you need with a flick of a DIP switch! 0dB (1x), 2x (6dB), 5x (14dB), or 10x (20dB). Listening to them with my TPA3255 amp right now.

Please note that PCBs you will receive are 1.6mm thick, green, 1oz copper, and ENIG finish. Like this:

 

[poseidonsvoice]

XRK,

What is overall current draw of the whole circuit? Will the MURATA DC to DC converter accomodate that as well as higher swings?

Thanks,
Anand.

 

[xrk971]

I’ll let Jhofland answer this but note that this is being used as a preamp application meant to drive high impedance amplifier inputs - not headphone amps. The LME’s draw 10mA max and I suspect the OPA1652’s are similar 10mA. So maybe 60mA total for full current. Those Murata’s were about 67mA rated for +/-15v I think.

I think OPA1652 can be used as a headphone amp chip as they can drive 100mA - but that’s not what we are using them for.

 

[poseidonsvoice]

Thanks XRK, I'll wait on Jhofland's response. I think it's important to consider the different scenarios, not just idle but at clipping as well. And in addition, consider the small current increases for a 10k ohm volume pot (~2ma). Thankfully you are not driving a 600 ohm load (~50mA).

That being said, one can easily feed into C4, C5 if the currents aren't enough, and/or we have our own separate +/- 15V DC supplies.

Best,
Anand.

 

[jhofland]

Minor clarification. The op amps are OPA1656's, spec'ed at 3.9mA per section. So there's 39mA. Each of the LME49724's are another 10mA typical/ 15 mA worst case. So we're pretty much at the specified power for the Murata converter.

 

[xrk971]

Sorry about the 1656 to 1652 mixup! Totally different chip.

 

[TNT]

Are there any measurements done?

//

 

[xrk971]

Not yet. You don’t believe that two state of the art opamps will give THD below that measurable by conventional DIY gear? 🙂 I’ll give it a measure when I get some time. Basically a loop back test with the unit in series acting as a “cable”.

 

[xrk971]

I just ordered a bunch of the opamps that go with the BTSB in preparation for those of you who want the pre-populated SMT version.

 

[xrk971]

Did I mention that the BTSB is available in all SMT (except bulk caps and inductors)? 70mm x 70mm compact size. No XLR jacks.

Great for use inside your preamp case with panel mounted jacks.

 

[poseidonsvoice]

The SMD option looks cool and compact. I will be curious to see your measurements.

Thanks again,
Anand.

 

[astromo]

So, with the shop ordering is the bare option through hole by default?

There doesn't appear to be bare THT and bare SMT options.

How to pick one or the other in bare mode?