XRK TPA3255 Amplifier

This project started when a friend bought the commercial Warp One amplifier by LSA Audio.  As I researched the product, I discovered there were a number of positive reviews online:

Further reading showed that the Warp One was an evolution of a DIYAudio project by contributor XRK971 that started as group buy in 2019 for the populated amp board (less power supply, chassis, input buffer).  Link to that thread here.  There was a followup group buy thread here, and the assembed amp board is now available on Etsy, here.

The amp itself is based on the Texas Instruments TPA3255 chip, specs and datasheet here, which is very popular in the DIY community.  The chip contains 4 channels of class D amplification, and can be used in bridge tied load (BTL) configuration as a two channel/stereo amplifier.  BTL bridges the outputs of two of the internal amplifiers (no output connection to ground) allowing double the voltage swing and four times the power to the speaker load.  In the BTL configuration, the TPA3255 delivers 150 watts per channel into 8 ohms and 260 watts into 4 ohms, with extremely low noise and distortion.

This project was an attempt to build an amplifier like the Warp One, but in a smaller chassis and with a remote trigger capability.  I started with XRK's amp board, which included a thru-hole version of the input buffer PCB (unpopulated).  Although the amp can be powered by a linear power supply, or even a battery, I opted for XRK's recommended switching power supply (as used in the Warp One) from Micro-Audio, here, and placed an order with Mouser.com for the buffer parts.  I used a spare aluminum chassis I had from Par-Metal.com.  Assembly details follow. 

The XRK amplifier board, supplied as shown, fully populated and tested.

The DIP switch on the right is typically set once (e.g. for BTL mode) and not changed.

One thing to note: This board as supplied from XRK is configured with the TI Post Filter FeedBack (PFFB) circuit enabled, unlike the commercial Warp One amp, which has that circuit disabled.  Enabling PFFB reduces overall gain from 150WPC to 75WPC (a 3dB drop in level), but reduces noise and distortion by several dB.

For more detail, read TI's application note on the PFFB circuit here.

This is the BTSB (Better Than Sliced Bread) Buffer, also available fully assembled and tested from XRKAudio on Etsy, as used in the commercial Warp One.

Its basically all surface mount components.  Notice that it includes both XLR and RCA inputs, with automatic switching.

The TPA3255 amplifier board supplied by XRKAudio also includes an option to include an unpopulated PCB for the BTSB buffer, in this case a thru-hole version, shown here.  Its different only in that it doesn't include RCA inputs with automatic switching.  The schematic is otherwise the same as the surface mount version, just a lot easier to assemble.

The circuit uses a 12V to +/-15V dc to dc converter for power, and OPA1656 and OPA1637 ICs.

This shows the buffer after I assembled it, ready for testing/installation.  Only the ICs are surface mount, so not difficult to assemble.

The two switches on the board are used to set gain for the input stage, and include options for 0dB, 6dB, 14dB and 20dB.  That buffer gain is added to the 14.5dB gain of the amp PCB, so to get a typical amplifier gain, the buffer would typically be set to 14dB gain.

I didn't populate the components in the lower center of the board as that supplies unbalanced outputs from the buffer which will never be needed in this setup.

This shows the inside of my rear panel, below the buffer PCB.  You can see the rear of the two RCA jacks at the lower right, and just above them the DPDT switch used to manually short the inverted input to ground for use with single ended inputs.

One pair of speaker outputs is on the left, as yet unconnected.

This is the MicroAudio power supply for the amp, model SMPS630-SO.  Its a switching supply providing 51VDC for the primary power to the amp, as well as 12V for the buffer and and 15V for the amp.

This supply is the one recommended by XRKAudio, and is the same used in the commercial Warp One.

The rear panel being assembled.  The openings at the top left are for the buffer PCB-mounted XLR inputs, below those are Vampire gold/teflon RCAs and the switch for balanced or unbalanced.  The binding posts are Superior BP-30, and on the right is the power switch, IEC and fuse.  There is no line filter needed as there is one built into the power supply shown above.

The hole to the left of the power switch is for a 12V trigger input, more below.

Inside of the rear panel before any wiring was done.

Basic layout of the amp before wiring it up, with the power supply at the front (right in this photo) and the amp board at the rear.

The TPA3255 chip is mounted on the bottom of the XRK PCB, and needs to be heatsinked to dissipate heat.  XRK includes a nicely machined aluminum heatsink transfer block, shown here, that mates with the TPA3255 chip using heatsink compound.  

This shows the aluminum block mounted on the PCB, and 0.5mm silicone pad that transfers heat from the aluminum block to the chassis bottom plate, allowing the entire chassis to act as a heat sink. 

The aluminum heatsink transfer block mounted below the PCB, just making contact with the silicone pad when mounted on M3 10mm standoffs.  

The amp wired up, now including the buffer board to the right rear.  

Note that due to a parts availability issue, the amp board needs a +15V supply from the power supply, and needs a 2,200uf cap added (lower right) to surpress turn on/off noises.  

All wiring in the unit is teflon coated, and power supply and speaker wiring is all 16 gauge.

Note for anyone using the thru-hole buffer: The pinouts for its outputs are different than the pinouts on the amp PCB inputs, so refer to the schematics to ensure proper connections.  This was changed on the surface mount version of the buffer sold in Etsy.

This side view shows the 12volt trigger circuit I added (circled in yellow) to allow the amp to be turned on & off remotely by the preamp in my system.

Side panels added.

Top view of the completed amp, front to the left: power supply on the left, input buffer on the lower right and TPA3255 amp board center rear.

A little internal documentation: DIP switch setting info taped to the bottom of the top cover.

This is an old photo, and the sharp-eyed will notice the amp gain is shown incorrectly here, sonce corrected to be 14.5dB, not 22.

As usual, I ordered a custom faceplate from Frontpanelexpress.com.  Blue power LED not actually so bright in real life.

As noted, the amp is usually turned on and off with the remote trigger from the preamp.  The front panel switch is to put the unit in Standby mode (amplifier off and power supply in low power state).  


Rear view, all hooked up.

Some quick measurements indicate virtually identical channel balance (<0.01 db), and DC offset <3mv, and gain of 14.5 dB with the buffer in 0dB gain setting.