New Bench Amplifier Project

[Jack Crow]

Hi all,

We all have problems. I tend to invent / build things to get away from life's irritations.

One of those irritations is this big Harman Kardon AV amplifier on my work bench.
Don't get me wrong, it's a great amp. Works very well.
The units annoyance for me is the total lack of soul.
The perfect appliance, it has a totally flat personality.
Takes up a lot of bench space that can be used for other things.

Wanted to make a smaller amp for use on the bench and have it powered by a high efficiency switching Lambda power supply I already have. The HK has a big power consuming linear PSU in it.
The last thing I need in my workshop is a Harmon Kardon room heater.

Wanted a physically small high performance amplifier that will work on 12 volts and can work with RCA level inputs.

Did some youtube research on these inexpensive (cheap!) amplifier boards.
Most anything with the TPA3116 chip seems to get good reviews.
Had good luck with Drok products in the past so I selected the below items.

Has a few main parts.
One, is the amplifier pcb.

https://www.amazon.com/gp/product/B01N1 ... UTF8&psc=1

According to specs, it's a 50 watt stereo amp, class D.
If I ran the thing at 24vdc yeah that can happen.
This is going to run on a 12 volt bench supply and I truly don't need all that power up close and personal.

Two, is the input selector.
It's a four input RCA stereo jack.

https://www.amazon.com/gp/product/B06XG ... UTF8&psc=1

This kit is made in two parts.
One is the selector switch / indicator LED's board, and the second is the board for the RCA inputs.
For some reason the OEM wanted this to run on an AC input power.
Had to remove the bridge rectifier and voltage regulator IC (12vdc).

Took a few try's before I got the mechanical spacing just right on the RCA jack field.
Learned a useful technique. Used 3D printing to make the box.
On complex projects, I print a cheap 'sketch' by setting Cura (slicer program) to make thin walls and very little fill, then printing just enough to see how it's all going to fit.

Once I got the dimensions right I went 'full spec', thick walls (.8mm) , good fill (30%) small (.15mm) layers.

Three, is a small high quality fan, made by Noctua.
This is almost useless because of the efficiency of the Class D amplifier.
It will not heat up much even at full throttle.
I was given the fan and wanted to make use of it.
It's a male ego thing.
If you have a cooling fan, your working with power.
2-24-2018 031.JPG


To save time I used one of my 'stock boxes' as a starting point.
Then reworked them to fit the parts.
The box started out as 145x75x50mm, I re worked it to 145x75x75mm to allow all the parts to fit.

Put the holes and mounting bosses of the switch unit and the amplifier onto the front cover.
2-24-2018 025.JPG
2-24-2018 028.JPG
The back box was a significant effort.
Getting those RCA jacks to fit flush and properly took some doing.
The OEM did not supply a mechanical drawing.
It installs with a couple of 4-40 screws driven through the mounting holes of the RCA jack matrix.
When it's done. you have a very firm mount.

2-24-2018 032.JPG

If you look at the first photo you get an over view of the items mounted to the cover.
The amplifier mounts via it's shaft nut of the on/off volume control.
There are some circuit board guides designed into the cover to hold the board from rotating.
We can also see the back of the four channel selector switch.

The second photo is the front.
The switch on the left chooses the input jacks and turns on a neat blue LED.
The other one is the volume control.

Yeah I know, not much to look at.

Was thinking of a dual VU meter similar to the my previously talked about stereo audio compressor project.
If I go for that option, it's a simple matter to rework the front panel to accept the new LED board.

The third photo is the back.
Here we can see the fan opening, the RCA matrix, and the nine pin Molex plug I used for the DC input and speaker outputs.

Used the Molex simply because I had them and the correct pins in stock.
The Midland Land Mobile Radios I also work in, use the same jacks and plugs.
In fact I wired the DC points the same way as Midland did to prevent an amp burnout should I confuse a radio plug for the amplifier.

Anyhow, wire work, nuts, bolts, and screws it all goes together.
Find out I have a out the gain issue.

The amplifier board has three inputs.
The 'walkman' style input, and two others.

Since there are no specs for these inputs I learned the hard way, that the most sensitive input is the one for the headset jack.

For the most part I am driving this thing with the output of a sound card.
I could not get enough gain out of the others but the headset jack allowed the unit to produce enough watts to be loud, even with 12 volts of input.

Once that was figured out, had to cure some some minor fan rubs by dressing wires out of the way.
Now on the path to success.

It works rather well. Hit my 97% design goals at this point.

So far it's been fairly porky in terms of sound.
At full volume on YouTube music programs (the greatest hits of 1977) I can get it to just about draw an amp from the bench supply. This is more than sufficient to drive a pair of Minimus 7 speakers to "shout" level sound.

Just did a check with the VHF ham radio at 50 watts.
The new amp played right through a transmission w/o noise or distortion.
No sign of EMI/RFI.
That is very good news.
Someone in the design section was thinking.

If I had to do it all over again what changes?

It would be in a bigger box.
Made a minor error of using a stock size.
It was a little tight getting all those parts stuffed into that package.

We talked about adding a VU meter.
Mostly it's eye candy, but that is why we have it, to make things look good.
To the human mind 'if it looks good, it sounds good'. Just the way we are.

Had a mechanical conflict during assembly, and needed to trim a little bit of the RCA jack matrix board to get around the space the fan takes.

In the unlikely event I need to make a second example, it would be easy enough to move the jack field out of the way, or move the fan opening.

Would have recessed the Molex jack so that it won't stick out the back, rather it be flush to the rear panel with enough clearance to get it's mate into place. With that plug sticking out the back it was hard to balance the amp to work on the insides, tended to lean over and fall. If it was flush it would stay put.

Anyhow, the plan for the moment.
Need to pick up some RCA cables today.
Going to make an adapter to go from the audio generator to the new bench amp.
This will provide 'exciter signal' for the amplifier.
Will connect to a restive load bank.
Let the thing run at full tilt on the over night.
If it's still working by Sunday AM, mount it on the workbench and get ready to remove the HK amp.

The things we learn by experience.
Keep it safe
Jack Crow in Virginia Beach VA