Adapting a replacement speaker

[noise]

I have an old 63 ohm computer speaker that I want to replace with an 8 ohm model. It's a small speaker, about 57 x 57 mm at the frame. What is the best way to adapt the 8 ohm to give it a similar impedance?

 

[somebodyelse]

You could do that with a transformer of the same ratio. If you gave more info about the application there may be a better option though.

 

[noise]

The application is a vintage computer from the late 80's. It has a single internal speaker that says on the back "Regal 63Ω 0.25W TAIWAN R.O.C." and measures 57mm square at the edges of the frame.

Here's a copy of the schematic of the sound circuit. It looks like it is stereo output, but I thought that the specs are mono. J11 looks to be the internal speaker connector at the logic board.

 

[somebodyelse]

Stereo to the TRS socket which will disconnect the signal to the speaker if a jack is plugged in. It's mixed down to mono on its way to the amp input for the speaker.

You should be able to find a speaker driver that's a closer match electrically, and use something like a 3d printed bracket to match the mounting. R10 seems an odd thing to have unless they needed the extra resistance to stop Q1 and Q2 overheating. A quick search at one of the usual online suppliers turns up a few 50R speakers - you could stick an extra 13R inline with it to make up the difference, making it a little quieter, but it's probably close enough. If you try some of the other suppliers you may find something even closer. I'm assuming you'd prefer to keep R10 original if possible.
https://uk.rs-online.com/web/c/offi...-drivers/?sra=p&applied-dimensions=4294828169

 

[Tom C]

Isn’t R10 to match impedance? It’s on the output to the external speaker jack. 63 ohm would be unusual for an external speaker, 8 ohm more common. If you’re replacing the internal speaker anyway, I would think you could just wire the replacement to the terminals for the external speakers.

 

[somebodyelse]

I may have misunderstood, but I don't think there is an external speaker jack, and J11 is jsut the connector for the internal speaker. Or is that where SND-M2 goes to on the other sheet that's not in the post above? The output transistors are only little TO-92 200mA ones, so I think it needs the unusually high load, and wouldn't cope well with 8R.

 

[Tom C]

I see. noise says J11 looks to be the internal speaker connector on the logic board. I was taking SND-M1 and SND-M2 as hardwire connections to the internal speaker.

 

[noise]

The internal speaker is plugged into the board on J11. There is an external jack (J1) but I want to leave that port alone. I have a new transparent reproduction case for the computer, so I want to install a speaker with a clear cone to go with it like this one and can't really hide 3d printed brackets, etc. I'm concerned with stressing the audio circuit with a much lower impedance speaker and want to maintain a similar volume profile to the original. Would a transformer be the best option?

https://www.mouser.com/datasheet/2/670/ga0576m-1778375.pdf

 

[somebodyelse]

If you can't find a more suitable speaker then give the transformer a try - it converts impedance as well as voltage.You're aiming to step the voltage from the amp down 8x to drive the speaker. I don't know where you'll hide the transformer though.

 

[noise]

I am thinking I could glue it to the back of the speaker if it is small enough, but I haven't found a transformer with the right ratio that's low cost.

 

[gfx_1]

Maybe add a resistor in series? 47 Ohm (or 63-8 = 55) If it isn't loud enough lower the value a bit.

 

[noise]

I could try a resistor as an experiment, but I don't think I would do it as a permanent install.

 

[Doodski]

I could try a resistor as an experiment, but I don't think I would do it as a permanent install.

Why not? If the load resistance is in specification and the speaker outputs enough sound then it should be fine?

 

[JDW]

I assume this thread was inspired my the MacEffects Clear Case for the SE & SE/30. I released a YouTube video review about that in July 2020. I discussed the clear replacement 8Ω speaker. After all these months of use, that clear speaker still works, but it isn't as loud as the stock 63Ω speaker. I discuss voltages across the MacEffects 8Ω speaker in this 68kMLA thread (scope measurements are on page 2).

Using a resistor is a bad idea because of how lossy it is. It will drop the volume such that the end result is no different, or perhaps worse, than just using the 8Ω speaker without the resistor.

Using an audio transformer is something I haven't tried, but in theory it should work. It's been a number of months since you gentlemen were discussing this topic, so have any of you given a transformer a try? It would need to be roughly an 8:1 turns ratio transformer for proper matching.