Purifi Monoblocks

[Jaytor]

I'm starting a new monoblock amplifier build using the Purifi 1et400a module and thought it might be interesting for others to follow along and lend their input. This will be my third pair of monoblock amps built this year, after a pair of F5 Turbo v2 amps and a pair of Neurochrome Mod-286s.

My last exposure to class D was with a pair of Nord Hypex NC500 amps using the Hypex SMPS1200 supply and Nord's rev D input buffers. These amps were quiet, clean, detailed and powerful, but I also thought they sounded kind of flat - I never really had the emotional connection with the music that I do with other amps.

Virtually everything has changed in my system since then, and I've gotten much more into DIY, so I thought I'd give class D another try. But I don't want to just throw a couple of pre-built modules into a box and call it good. So my amps will be a bit different than the norm.

First, I am building these with a big linear power supply. This is partially because I can at least build this myself, but also to see if it has any affect on the way the amp sounds.

I'm also designing the enclosure so that I have room to experiment with different input buffers. I'm going to start with the Neurochrome board to get the amp operational quickly and to have a baseline, but then I plan to build several more designs using custom PCBs. I have enough room for buffer PCBs that are 130mm x 85mm which should allow for a lot of options.

The design approaches I am considering include:

• Simple discrete op-amp design using one Weiss OP2-BP for each balanced leg. This is similar in concept to many of the commercial Hypex and Purifi amps.
  
• Fully balanced design with quality op-amp (such as OPA627) feeding a single-ended class A mosfet buffer for each leg (similar to Whammy design).
  
• Fully discrete line stage with JFET input and MOSFET output stage for each leg (similar to Salas DCG3).
  
• Similar to above with bipolar outputs.
  
• Tube-based design (see this thread - Help with tube buffer).

I'm hoping to do a new input buffer every month or two (the first one will take a while since I'm learning KiCad). This will keep me busy for a while. I'm not sure I'll do all of these, depending on how it goes. If you've got thoughts on what you think might be the most interesting, please chime in.

I've sent the enclosure drawings to Modushop (Slimline 3U/350mm), so hopefully I'll have the enclosures in a few weeks.

I'll make follow-on posts with additional details.

[john61ct]

I would contribute toward costs for someone to evaluate / ideally create a DIY cookbook / design using this BTSB buffer, so that either true balanced differential or SE inputs could be used as desired.

Best Thing since Sliced Bread Buffer
BTSB Buffer - SE/Bal to SE/Bal Buffer GB

or balanced / differential on the power amp, with BTSB coupled with a clean SPL gain control pre-amp?

[john61ct]

link to your case design? Cost estimate?

Personally 2x mono in one case would be fine.

[Jaytor]

The Neurochrome buffer I am using as a starting point is pretty similar to the BTSB buffer, using a dual op amp to provide a differential input that can also handle single-ended inputs, and the same LME49724 as the final driver.

I've attached some pdf files of the drawings for the front panel and back panel. The bottom panel drawing was too big, so I attached a jpg showing the layout. This is a view from the top with the front toward the bottom. The two orange rectangles are the main positive and negative PS boards, the green rectangle is the op-amp supply (unregulated +/- about 18v), the light blue rectangle is the gate drive supply (regulated 15V referenced to the main negative supply).

The large gray rectangle in the middle is the mounting plate for a Toroidy 1500VA Supreme Audio custom transformer, with secondaries for all the required voltages.

The magenta rectangle is the Purifi module, and the dark blue rectangle is for the Neurochrome buffer. There are mounting holes for a larger 130mm x 85mm buffer board.

The enclosure includes internal panels separating the buffer and Purifi module from the rest of the board.

I am also planning to have a small 12V power supply mounted on the back panel that will drive an audio sense circuit for automatic turn on (also mounted on the back panel). There is also a soft start PCB mounted on the back panel that includes a low voltage power switch mounted on the front panel.

The front panel also includes a window to show a 32-segment LED power meter. Separate switches on the back panel are used to turn on and off the power meter and the auto turn on circuit.

I prefer monoblock amps because of my system layout. I don't want anything sitting between the speakers and it's a lot easier to route a balanced interconnect than big speaker cables.

I haven't added up the total cost, but this is obviously not a cost-conscious build. With all the machining, custom printing, etc. on the case, the cases alone were over $1000. I'd estimate that I'll be into these for a bit less than $3000 when I'm done (with the initial Neurochrome buffer).

[Koifarm]

Linear supply.

[cOz]

Cool project!

[Jaytor]

I built one of Mark Johnson's Quasimodo transformer snubber testers (thanks Mark) to determine the values to use for the snubbers. My Toroidy transformers have the following secondaries based on 115V input:

2 x 45V @ 15.6A
3 x 15V @ 2A

The optimal snubber values for the 45V primaries are:

Cx 10nF
Cs 150nF
Rs 8.2ohm

For the 15V secondaries:

Cx 10nF
Cs 150nF
Rs 9.1ohm

I have to order the correct value resistors before I can finish up the power supplies.

[mocenigo]

Originally Posted by Jaytor

I built one of Mark Johnson's Quasimodo transformer snubber testers (thanks Mark) to determine the values to use for the snubbers. My Toroidy transformers have the following secondaries based on 115V input:

2 x 45V @ 15.6A
3 x 15V @ 2A

The optimal snubber values for the 45V primaries are:

Cx 10nF
Cs 150nF
Rs 8.2ohm

For the 15V secondaries:

Cx 10nF
Cs 150nF
Rs 9.1ohm

I have to order the correct value resistors before I can finish up the power supplies.

Be careful with supply pumping. You can easily get an increase of voltage out of the transformer. 45*sqrt(2) will give you 63.6V and if you need to pull a lot of power depending on your speakers, you may easily get to 70V or more and trip the over voltage protection of the Purifi module.

Also any local utility AC variations may make this even more critical.

The solution for monoblocks is to use MOSFETS and not diodes for rectification. Then the power supply will not suffer from pumping. This applies to both switching and linear supplies.

[Jaytor]

This is a good point, but I'm not too worried with my speakers and PS implementation. My current speakers are 93db/w and a fairly easy 8ohm load. The new speakers I'm building are 98db/w efficiency (also a fairly easy load) and are line sources, so sound output doesn't drop as fast as point sources. In both these cases, the amp will only drive the speakers about about 80Hz (fed through a cap to roll off the lowest frequencies), so it will mostly be coasting along driving my speakers.

Also, my power supply is a CRCRC design, so there will be several volts dropped across the resistors under higher current. So I don't expect the voltage will get close to hitting the max on the Purifi modules from charge pumping. But I will try to do some testing to test this.

[Jaytor]

I'm using the DIYAudio Store Universal Power Supply boards for the main power supply. I've split the positive and negative supplies so they will mount across the front of the amp, shown in orange in post #4. I actually have two boards, one mounted about the other, so that I can use eight main filter caps per rail, and set up a CRCRC supply.

I just got the shipping notice from Modushop so I should have the chassis sometime next week.

The photo shows just the positive rail for one monoblock.