Internal Speaker

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Centurion
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Internal Speaker

Postby Centurion » Wed Sep 16, 2020 10:00 pm

Hey Next'ers,

I wanted to start a new topic on the use of an Internal Speaker. I have searched the forums and have dug out bits and pieces of helpful information, but it's challenging because a lot of it is spread out over the "DIY Upgrade Components" topic and probably bits buried in the Next Facebook group as well.

I have been trying to figure out which route is the best choice for using an internal speaker, but am not having great luck. I purchased the Piezo speaker from Rich at his ActiveConsult website, but the audio is so quiet that I can barely hear anything even holding the Next up to my ear.

Since I typically use an HDMI computer monitor and keep my external speakers connected to my PC, I would like to use the Next's Internal Speaker for casual use.

So I'm curious. What Internal Speaker do you folks suggest? Is there a volume level I can adjust somewhere in settings?

Someone here suggested they had good luck using a 28mm 8 Ω 1W speaker without any added resistors. Would that be a wise choice?

Thanks!
ZX Spectrum Next Backer №1,104 / ZX Spectrum Harlequin Rev. G owner / USA ZX Spectrum enthusiast

Alcoholics Anonymous
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Re: Internal Speaker

Postby Alcoholics Anonymous » Thu Sep 17, 2020 6:31 pm

I don't think the piezos are the best choice as they have very narrow bandwidth.

The Next operates the speaker in one of two modes. One is where all beeper audio is diverted to the internal speaker. This means it's removed from hdmi and the audio jack so that external speakers play AY, DAC, PI audio and the internal speaker would do the beeper. In this mode, the beep sounds should be louder. Maybe the piezo is ok here. The other mode plays all audio through the internal speaker: beep, AY, DAC, PI. This mode is going to be more quiet as the audio sources have to be mixed in a way that doesn't cause clipping and you want a speaker with wider bandwidth.

The internal speaker output circuit of the Next is a BC846 NPN transistor which can carry 100mA. Total resistance in the output shouldn't be below 33 ohms. There is a 10 ohm series resistor there so that's 23 ohms. Speaker impedance is rated around 10% above its minimum value so seeking an ~30 ohm speaker is probably right.

I bought a couple to try out but haven't had time to put them in yet. Both are about 30 ohms, about the right diameter to be placed on the pcb, and have relatively wide bandwidth (5kHz - 8kHz iirc).

If these are working well enough, I'll probably look at trying to get the volume louder in a later core.

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Centurion
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Re: Internal Speaker

Postby Centurion » Fri Sep 18, 2020 12:13 am

Thanks for the information AA, that's helpful. Please reply with your findings after you've had a chance to test the speakers you've purchased. I'm curious to know what you think of them.

Here are some videos I've found from Mikes Retro Tech on YouTube showing his experiences trying to find a suitable internal speaker for the Next. In case anyone would like to see them:

https://youtu.be/iBCfLxnWVsg (ZX Spectrum Next Upgrades PT2 RTC & Speaker &wifi)
https://youtu.be/J_SMDQdm0yI (Spectrum Next upgrades PT3 - ram & speaker)
https://youtu.be/-vrRJEAMZuI (8ohm 0.5watt Speaker in the ZX Spectrum Next)

Also a helpful diagram from Andy P here showing the polarity of J3 which came in handy when I was installing the piezo:
Andy P wrote:
Sun Dec 17, 2017 4:22 pm
The speaker pin header (J3) can be connected as follows:
spkr-pin.jpeg
spkr-pin.jpeg (143.63 KiB) Viewed 3550 times
Pin 1 or 2 = negative, pin 3 or 4 = positive for Piezo type. For normal coned type speaker the polarity does not matter.
One more bit of information I have found is that it looks as though a 20mm speaker should fit pretty nicely, if we can find one that sounds good.

I appreciate the help, thanks! :)
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Re: Internal Speaker

Postby Alcoholics Anonymous » Fri Sep 18, 2020 2:35 pm

A new post on fb where someone's attached an amplifier for the internal speaker.

https://www.facebook.com/groups/specnex ... %2CO%2CP-R

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Centurion
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Re: Internal Speaker

Postby Centurion » Sun Sep 20, 2020 6:01 am

Alcoholics Anonymous wrote:
Fri Sep 18, 2020 2:35 pm
A new post on fb where someone's attached an amplifier for the internal speaker.

https://www.facebook.com/groups/specnex ... %2CO%2CP-R
interesting! Thanks for the link :)
ZX Spectrum Next Backer №1,104 / ZX Spectrum Harlequin Rev. G owner / USA ZX Spectrum enthusiast

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Centurion
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Re: Internal Speaker

Postby Centurion » Sun Sep 27, 2020 7:17 am

Further findings...

This evening I installed a 20mm 8 Ω 1W speaker. The volume level was even weaker than my Piezo speaker. So much so that I thought it wasn't even working. I was pretty excited to try it, so it's quite a disappointment.
20200927_020101.jpg
20200927_020101.jpg (247.58 KiB) Viewed 3307 times
As you can see, the size is great and even had a sticky ring to adhere to the board.

I reconnected the Piezo and experimented with settings and some different games and found that I could actually hear a bit of R-Type about a half meter away using the BEEPer setting to Int.

So one thing I'm finding is that 48k games that use the beeper are definitely better than 128k AY and Next games. But still too quiet for myself with the Piezo.

I am discouraged. My Harlequin pumps and I guess I assumed the Next's internal speaker would do the same. I Really hope this is something that can be addressed with a future software update at some point as I'd really rather avoid installing a mini amplifier. :?
Last edited by Centurion on Thu Oct 01, 2020 2:19 am, edited 1 time in total.
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Z80Man
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Re: Internal Speaker

Postby Z80Man » Mon Sep 28, 2020 7:04 am

Just a note, the paper printed documentation quotes a 28 to 32 ohms speaker while the new PDF version quotes a 8 to 16 ohms speaker, so I guess the output impedance has been reduced on the issue 2B version motherboards, which makes it easier to select more commonly found models.

Now the output power is obviously quite low, but since there's apparently no setting for this (unless there's a Next registry handy somewhere, I didn't read through all of it yet), I guess it's still better to have a discrete output level than nerve wracking beeps yelling through the house, unless of course you're living alone with no neighbours around... :p

The best solution IMHO, especially now the output impedance has been standardized, would be to connect a small, 5V to 9 V powered mini amplifier board and connect a 4 to 8 ohms miniature wide range speaker to it. The kind meant for laptop computers.

You'll get both a better output power and a volume setting potentiometer, which is absolutely mandatory IMHO.

I tried ordering a few models from eBay, but since most, if not all of the providers are from China and my country is currently blacklisted (because it's the only one with no coronavirus at all), shipping is absolutely impossible as the parcels get rejected at the sender's post office, so I couldn't do anything as planned. I'll probably either build a raw audio amplifier using a couple of transistors or stick with my headphones in the audio output jack. Which has the added advantage to not piss my wife off. :p



I have a question, though : are ALL the sound devices outputs from the Next present on the speaker connector ? I mean, beeper (of course), all 3 AY, Covox, Raspberry Pi ?

Thanks...

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Centurion
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Re: Internal Speaker

Postby Centurion » Mon Sep 28, 2020 2:38 pm

Z80Man wrote:
Mon Sep 28, 2020 7:04 am
Just a note, the paper printed documentation quotes a 28 to 32 ohms speaker while the new PDF version quotes a 8 to 16 ohms speaker, so I guess the output impedance has been reduced on the issue 2B version motherboards, which makes it easier to select more commonly found models.
I had noticed this as well and one reason why I chose to try an 8 ohms speaker(also a lot more common for domestic shipping). As per AA's suggestion in this topic, I was able to track down some 23mm 32 ohms speakers here in the states. The same type that I believe I used for the Harlequin build. We'll see if I have any better results with those.
The best solution IMHO, especially now the output impedance has been standardized, would be to connect a small, 5V to 9 V powered mini amplifier board and connect a 4 to 8 ohms miniature wide range speaker to it. The kind meant for laptop computers.
This is definitely an option and one I might go with if I have to. It looks like we might be able to use the 5v on the J15 GPIO for power:
Spectrum_Next_J15.jpg
Spectrum_Next_J15.jpg (151.05 KiB) Viewed 3233 times
You'll get both a better output power and a volume setting potentiometer, which is absolutely mandatory IMHO.
The volume pot is a good point and makes that solution even more appealing!
I have a question, though : are ALL the sound devices outputs from the Next present on the speaker connector ? I mean, beeper (of course), all 3 AY, Covox, Raspberry Pi ?

Thanks...
Not sure about this either, but thanks for chiming in on this. Maybe someone can clear that question up.

I will report what I found out about the 32 ohm speakers after I have a chance to test them out.

Cheers!
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Re: Internal Speaker

Postby Alcoholics Anonymous » Mon Sep 28, 2020 5:50 pm

Z80Man wrote:
Mon Sep 28, 2020 7:04 am
Just a note, the paper printed documentation quotes a 28 to 32 ohms speaker while the new PDF version quotes a 8 to 16 ohms speaker, so I guess the output impedance has been reduced on the issue 2B version motherboards, which makes it easier to select more commonly found models.
It must be about 30 ohms. The output transistor is only rated at ~100mA.
I have a question, though : are ALL the sound devices outputs from the Next present on the speaker connector ? I mean, beeper (of course), all 3 AY, Covox, Raspberry Pi ?
There are two modes:

1. Beep only with beep sound removed from hdmi / 3.5mm audio jack.
2. All audio including the AY, dacs and pi.

Number two is a lot more quiet than number 1 as the mixing is done in a way to minimize clipping.

The internal speaker is driven by an open collector transistor only intended to be on or off. We changed how it's driven in order to attempt to play all audio via the internal speaker. An open collector transistor is very difficult to drive as there's no feedback and what the hardware is trying to do is generate voltages between about 0.6 and 0.9 into the transistor base.

Operated as on/off long ago was considered too loud by people trying it. The new method (#1) is bearable. Some people say it's loud enough for their liking, a lot of people seem to be saying it's too quiet. They are all using different speakers and some have the expansion bus blank at the back removed and some don't.

#2 is considered quiet by most. But I do have feedback from a few people that it is loud enough on their setups.

I don't have the internal speaker set up in my own machine yet to test directly myself. All dev up to now has been done using feedback from users.

Better audio volume can be added using an amplifier as you are talking about and at least one other has done. It might even be possible by adding a feedback resistor to the existing open collector circuit.

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Centurion
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Re: Internal Speaker

Postby Centurion » Mon Sep 28, 2020 7:04 pm

Thanks for keeping up on this AA. Your knowledge and support is helpful. Here is the datasheet for the ABS-231-RC speaker I have on order. Hopefully this falls into the specifications you've suggested.
20200928_135833.jpg
20200928_135833.jpg (198.92 KiB) Viewed 3203 times
I'm going to give it a try and test it out with the different modes and various games to see how it does without any amplification.
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