Add Volume to BBC Master control

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andrewvideo
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Add Volume to BBC Master control

Post by andrewvideo » Sat May 04, 2019 1:03 pm

I want to add a basic Volume to BBC Master control (Speaker). I bought a Volume nob.
The question is can I just wire the speaker to volume Potentiometer with the two wires from the board.

How kind I need. I got a 1K Potentiometer or do i need buy other one.

I seen this video and I don't fancy messing around with board and don't need a Audio jack out.
https://www.youtube.com/watch?v=lb05bcuafK4
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1024MAK
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Re: Add Volume to BBC Master control

Post by 1024MAK » Sat May 04, 2019 1:43 pm

Just to be clear, are you saying that you want a plug-in arrangement rather than carrying out any modifications to the board?

Mark

andrewvideo
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Re: Add Volume to BBC Master control

Post by andrewvideo » Sat May 04, 2019 2:28 pm

Yes I want to just the plug-in. Basicy I am just soiding the wirles and not touching anything on board part from plug into the speaker 2 pin socket Would that set up work? I don't want to blow any chips.

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1024MAK
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Re: Add Volume to BBC Master control

Post by 1024MAK » Sat May 04, 2019 3:12 pm

I would suggest then, assuming you wish to use the existing internal speaker, that you get the following: You pick up the audio sound from the Master, which is then wired to the 10k logarithmic potentiometer. Then wire from this to the Amplifier Module input. The output from the Amplifier Module then goes to the existing internal speaker.

Mark

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0xC0DE
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Re: Add Volume to BBC Master control

Post by 0xC0DE » Sat May 04, 2019 4:02 pm

Any reason why this wouldn't work on an Electron?

Could you please explain to me why you need the amplifier and not just the pot?
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andrewvideo
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Re: Add Volume to BBC Master control

Post by andrewvideo » Sat May 04, 2019 4:33 pm

Thank you very much your help guys. Sorry My wirting skills are not the best. I have dyslexia.

yes I am using the internal speaker. Do I really need amp as It loud already? I only want to turn it down when its too loud.


Would I need to remove the potentiomete off the amplifier and soilder the 10k logarithmic potentiometer? Also where is the best place to get power from? I am going to fit in the Datacenter in at somepoint (Will I am going to get mark - RetroClinic to do it). I am very new to this stuff. What you call those little black pin headers so I can plug in the cables to the Amp? I know they are the same on PC cases for LEDs, Power swtich etc. I guess I could soilder them on with some heat shink around each pin.

Sorry for the dump questions.

1024MAK wrote:
Sat May 04, 2019 3:12 pm
I would suggest then, assuming you wish to use the existing internal speaker, that you get the following: You pick up the audio sound from the Master, which is then wired to the 10k logarithmic potentiometer. Then wire from this to the Amplifier Module input. The output from the Amplifier Module then goes to the existing internal speaker.

Mark

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1024MAK
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Re: Add Volume to BBC Master control

Post by 1024MAK » Sat May 04, 2019 5:39 pm

Yes, there is a built in amplifier on the main board, but there is no provision for a volume control :(
The output of this built in amplifier is designed to directly drive a loud speaker, which is exactly how Acorn supplied it.

The impedance of the loud speaker is less than 16 ohms. The human ear perceives different sound levels in a non-linear way. Hence in audio amplifier applications, logarithmic potentiometers are used.

To wire a potentiometer in-between the existing amplifier output and the existing loud speaker requires a value below 30 ohms and which can handle the power output of the amplifier. It’s difficult to find suitable linear potentiometers with a suitable value, let alone suitable logarithmic types.

It is possible to use a fixed value resistor, but that only offers a one volume level.

Hence my alternative solution...

The amplifier module will work on either +5V or +12V DC, so you can pick up power from any convenient power connection point.

The jumpers/links and pin-headers on the Acorn boards are 0.1” (2.54mm) pitch. Lots of manufacturers produce connectors with this pitch.
On eBay there are lots of different length “dupoint cables” that you can use. Or various other multi-way connectors that are suitable. E.g. search for “2.54mm connector”.

You don’t have to desolder the preset from the amplifier module.

Mark

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hoglet
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Re: Add Volume to BBC Master control

Post by hoglet » Sat May 04, 2019 6:41 pm

1024MAK wrote:
Sat May 04, 2019 5:39 pm
To wire a potentiometer in-between the existing amplifier output and the existing loud speaker requires a value below 30 ohms and which can handle the power output of the amplifier. It’s difficult to find suitable linear potentiometers with a suitable value, let alone suitable logarithmic types.
Do you think something like this might be suitable?
https://uk.rs-online.com/web/p/potentiometers/1790668/

It's:
- linear
- 25 ohms (there is 50 ohms available as well)
- handle 5W
- costs £3.10 (including free delivery)

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Re: Add Volume to BBC Master control

Post by 1024MAK » Sat May 04, 2019 7:14 pm

hoglet wrote:
Sat May 04, 2019 6:41 pm
Do you think something like this might be suitable?
https://uk.rs-online.com/web/p/potentiometers/1790668/

It's:
- linear
- 25 ohms (there is 50 ohms available as well)
- handle 5W
- costs £3.10 (including free delivery)
Yes, that will result in a user volume control. Because it is a linear track, the useful range may be limited to a smaller degree of rotation than is ideal. That is, the change in volume will not appear to be in proportion compared to the amount of rotation of the pot over some of the range of the pot.

Mark

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