MAX9709 class D amplifier (2x25W) + MAX5440 encoder controlled volume

[thunderer]

I decided to give a try to the MAX9709 as a class D amplifier. And as the idea was small volume, I opted also for a 32 step logarithmic volume by encoder with MAX5440.

Sounds better than expected, so here it is.

Things to know and to do:
1. The MAX9709 has a power pad that should be soldered to the PCB (ground plane). In amateur life it is difficult to impossible to do it through vias home made. So, I did it this way: drilled a 3mm hole under the pad (chip). Soldered the chip and then filled the hole with molted solder until flown under the pad realizing the thermal bond. Then filled even more until reached the bottom side PCB. This explains the solder island in the middle of the PCB (bottom side). Soldered then a 20x30mm brass plate (0.3mm thickness). I used my skills to make some tentacular heatsink. For image ONLY I threw a Raspberry Pi heatsink on the plastic package.
2. The thermal alarm (programmed at 80C) is not a shutdown, so it should be linked to the mute or shutdown input (pin 30 or 31) as you'd prefer. They're available on the PCB with headers, so easy to hook to.
3. The schematic I used is identical to the evaluation kit (see it here: http://datasheets.maximintegrated.com/en/ds/MAX9709EVKIT.pdf). I used the filters as the output looked funny without filters.
4. The PCB is ready for bridge connections (outputs in parallel and jumper to place). Refer to schematics and evaluation kit documentation.

On the linearity with frequency, I could not entirely test with a good system as my generator gets -3dB variation from 20 to 20000Hz. It presented fair rectangle shape response at 1kHz, you can see the picture in attachment.

Top view

Bottom view:

Input 1kHz:

Output 1kHz:

[Vineyards]

You say the sound is better than what you expected but that doesn't tell much. I don't know what you are driving with this but perhaps you can tell us a few words about good and bad aspects of the sound output using the hi fi jargon.

[thunderer]

Listening to it now  . I am no hifi guy (lacking the ins and outs). Note that this is not a hifi amplifier, nobody labelled like this.

I tried, as hifi guys asked for it, to see the answer of a rectangular signal at 10kHz. I could do it only to 7kHz, see the attachments for input signal and output signal. The hifi guys asked me to decrease the 33uH inductance to about 15 or even 10uH. I did not do it, as it sounds OK for my hearing (found out lately that fairly low volume - shall I say like whispering - signals above 15-16kHz are not anymore heard by me). I intend to use it as a 5-10W ambient music. Actually I drive some satellites at 6ohm. I am looking for some shelf speakers to match this "raging moster".

I even input sine from 50 to 20kHz, mine it shown like attached. About -3dB attenuation at 20kHz. See the out_1.JPG below. Like I said in my original message, I should test the frequency response with a better generator, but I do not have one available for the moment. Some other guys on the Internet had it linear in 20 to 20kHz. See here: http://www.360customs.de/en/2014/08/max9708max9709-25w50w-class-d-messungen/

I built for it a Baxandall preamp (the 10dB version of ESP) powered by single supply. Works great too. Even built a clipping indicator, but not tested yet so I will not publish it yet.

I bought Hammond box (model 1455T2201) to house it as final product sometimes soon. I will connect it to a 20V laptop supply.

NOTE: The diode in the ground pin of the 78L05 was intended to provide additional 0.6V to the output of 7805. It is wrongly shown on schematic. I finally put a 78L06 and jumped the diode. Beware!

For the records, my audio experience is limited to:
1. A TDA2030 amplifier built in the beginning of the 90s.
2. The Universal Tiger amplifier built in mid 90s.
3. The TDA7294 amplifier built in late 90s.
4. The MAX9709 and TDA2050 amplifiers built last year.
Well, this tells you about my experience with amplifiers.

[thunderer]

Months later, the amp got a box.


and


Next to the MAX9709 we have:

1. A buffer and a Baxandall (bass+treble) based on a AD8656 (for 5V operation). Preset, no outside the box settings.

2. A MAX5486 volume circuit. Adapted with a PIC to accept a rotary encoder. (I do not like the LEDS controlled by MAX5440).

3. The headphones amp with MAX4410 (http://www.sonsivri.to/forum/index.php?topic=61122.0).

4. A clipping detector on each channel.

5. An ON/OFF + speaker connect control module (PIC based).

Powered by a 20V laptop PSU.

[mike_au]

Do you happen to know how quickly the amp powers on from either complete power off or shutdown mode? I am looking to set up an amp for music, etc. and also to use it for announcements (doorbell, phone calls, etc.) but I don't want to keep it running at full power all the time.