November 19, 2020

ZX Spectrum Next - Internal Speaker Mini Amplifier Mod

I have recently been adding upgrades to my new ZX Spectrum Next computer. Getting it all maxed out with Ram, Wifi, Raspberry Pi Accelerator, Real-Time Clock, Internal Micro SD slot and what I will be focusing on today, which is the Internal Speaker.

If you're one of the lucky people who added an internal speaker to your ZX Spectrum Next, you most likely found out the hard way, that the output volume is way too quiet. I did.

Before realizing this, I did a lot of research on trying to figure out which speaker I should use. I tried a Piezo first and could barely hear any sound(which was a surprise, having assumed it'd be similar volume wise to an original Speccy), second a 20mm 8 Ω 1W and finally settling on a 23mm 32 Ω 1W speaker(Model: ABS-231-RC). This was the same speaker I used in my Harlequin, but was still barely audible.

After some discussion with a few others on the Spectrum Next Forums, I was directed to a Facebook post in the ZX Spectrum Next group where Neill Mitchell used a small TDA1308 Headphone Amp to boost his internal speaker audio output. This was intriguing to me, but was also something I was hoping to avoid, at first thinking I just needed to find a suitable speaker.

Well, I gave in and accepted after quite a bit of discussion along with trial and error that this was the only way the Next will be able to have good enough volume output for the Next's internal speaker. But I also wanted to try something different. When I was shopping around for different mini amps, I found the LM386 amplifiers with small potentiometers which seemed like a nice addition.

With postal deliveries across the globe being effected by Covid-19, I was impatient to wait 2 months for the very small coin sized LM386 to arrive from China, so I found a domestic LM386 board, with a much larger through-hole build, to use as a test board and received it in just a few days.

My first attempt at connecting it to the Next was not good and I could have easily fried my Next! I had connected the Negative line on the amp input to Pin 1 on J3 and the Positive line to Pin 4 on J3. This is WRONG! I Repeat WRONG! Neill Mitchell had looked over my photo and noticed I had it connected this way and informed me that although Pins 1 & 2 on the Next's J3 are Negative, they are actually connected to the 3.3V line on the Next and that will short the 3.3V rail. Yikes! Thanks Neill

Be careful, these two Negative lines are connected to the 3.3V rail on the Next!

Having sorted this out, I disconnected the Negative input completely and left that wire out. All of a sudden, the Next was pumping out the Manic Miner tune from the internal speaker. Yes! Progress!

Doing all of these tests with the bare board, I was curious to know how it'd sound all cased up, but the board I had was way too large. I modified it to fit inside the case, although very tight. The volume and clarity definitely suffers somewhat enclosed in the case(FYI, removing the expansion door on the back helps a bit), but it was still good enough for what I have been going for on this project. I don't expect a small 23mm speaker to have great audio quality and I still have the option of using the output jack on the back. The internal speaker for me is nice to have for a quick casual gaming session when I don't feel like switching to my powered speakers that are connected to my PC. It's also nice and neat this way with minimal wires and connections when resting on the desk.

Size comparison of the two different LM386 Amplifiers I tried

So... Finally, I received the very small coin sized LM386 amplifier from China. It took about 1.5 months or so to arrive. I studied over the board layout and came up with a nifty way to connect it all to the Next with minimal effort and without having to do any major changes to the Next's PCB. My hope is that anyone that can do some minimal soldering will be able to use this design to amplify their Next's internal speaker.

This mod allows me to hear the internal speaker from up to several feet away, it's removable and not permanent, very inexpensive and easy to do.

So, having said all of this boring stuff, grab a beer, fire up your soldering iron and let's get to work...

First you will want this version of the LM386 amplifier board. Sure, other versions will most likely work(like the large one I tested first), but this mod is designed for this particular version of the amp. I believe on eBay it had the description "DC 5V-12V LM386 electret microphone power amplifier board gain 200 times mic amp".

This is the model I am using in this project. Different LM386 boards will probably work the same, but will need to be adapted to fit the motherboard differently.

We will power this board using Pins 20 (GND) and 19 (5V) on the Next's J15 GPIO socket.

Since the Next's 2 Pin Keyboard Membrane connection occupies Pins 4 & 17 of the GPIO, it was a bit tricky figuring out how to keep this board connected directly to the Next motherboard(which I had preferred for cleanliness and stability), while keeping it from blocking the 2 Pin Keyboard Header. My solution was bending a 2 Pin Male Header, so that it could mate with the 2 Pin Female Header I soldered onto the amplifier. This keeps the amplifier from touching the membrane, while also keeping it low profile enough to fit inside the case.

This bent 2 Pin Male Header is soldered to Pins 20 & 19 on the Next's J15 GPIO connections

For the the amplifier connections:

1. solder a 2 Pin Female Header onto the amplifier's VCC Positive + and Negative - inputs. This will be what powers and secures the amplifier to the Next's motherboard. 

2. Solder the speaker wires directly to the OUT connections on the amplifier.

3. Lastly, solder a single wire directly to the Positive + MIC input on the amplifier with a 1 Pin Female Header on the end. This connects to either Pin 3 or 4 of the Next's J3 internal speaker connection. I used a Right-Angle 4 Pin Male Header for J3(which allows you to close the case when all connected).

Notice here how the angled 2 Pin Header allows clearance for the keyboard membrane. Also how it's all wired together.

This mod is about as easy as it gets for getting suitable(for me at least) volume output from the Next's internal speaker. Anyone with some soldering skills should feel pretty comfortable handling this mod. It's low profile, inexpensive and removable. It fits inside the case perfectly and securely. Doesn't get in the way of the keyboard membrane and really improves the volume output.

Low angle photo so you can see the clearance for fitting inside the case

Post installation photo. If you look closely, you can notice the potentiometer is accessible with this install.

Some final thoughts and helpful information I learned along the way:

1. It was discovered that the ESP Wifi module does add a slight bit of noise. For me personally, it wasn't very noticable with this setup. You can disable the ESP by typing REG 2,128 into Basic.

2. After everything is connected, you will most likely need to adjust the potentiometer to get proper volume output. Use a small screwdriver and very gently turn the pot clockwise while you have some audio going(I prefer the Manic Miner title screen). You can see in the above photo that after connected, the pot is visible with the bottom of the case removed. Just be gentle and careful not to short any connections while it's powered on!

3. The ZX Spectrum Next has different audio settings in the Next's NMI Menu under Settings/Sound. "IntSpeaker" allows you to turn the Internal Speaker ON/OFF and "BEEPer" allows you to use the Internal Speaker for "All" sounds (AY, Beeper, Etc) or "Int" for original Speccy beeper sounds only. The "Int" setting is slightly higher in volume because it doesn't need to split the signal between AY, Beeper, Etc. I have found that with this setup I can keep it set to the "All" setting and still get suitable volume output.

 Special Thanks to everyone at the ZX Spectrum Next forum who helped me with their helpful discussion and in particular Neill Mitchell, who had the original idea of using a small amplifier and took the time to post about it on Facebook.

LINKS:

ZX Spectrum Next Forum - Internal Speaker Topic

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