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Please explain the difference in impedance measurements

Hacdrag

Member
I've attached a picture of the project in question. Without pairing the Bluetooth receiver with a Bluetooth transmitter, If I measure the impedance of the inputs (bottom of the picture) on the amp board, it measures 4.0 ohms, which makes sense since the speaker is 4 ohms. However, still without any Bluetooth pairing, if I measure the impedance of the terminal block mount connector, (put the positive multimeter lead on either of the combined channel outputs and the negative lead on the ground output) this reads between 3.65 and 3.8 ohms. Two questions about this discrepancy. Why is there a discrepancy between this and the input ohms reading, and why don't I always get the same reading.

As soon as I pair the Bluetooth receiver, the ohms reading on the amp board goes down to either 0.9 or 1.0, and the ohms reading on the terminal block mount connector goes to 0. Please also explain why the Bluetooth pairing lowers the ohms reading for both measurements, and why it affects them differently.

In this thread (https://www.electro-tech-online.com/threads/capacitor-to-eliminate-speaker-hum.163451/) which deals with a similar circuit where I used two amp boards to make a stereo system, I mentioned how the Bluetooth pairing was beneficial, since it eliminated most of the speaker hum. With that in mind, I have an additional question about this setup. If I were to remove the Bluetooth receiver and directly connect an audio device to the circuit with a 3.5 mm connection, would the sound be any different, or would the battery life be any different? I'm guessing the answer is no, but I asked because I'm curious whether these different impedance readings the Bluetooth pairing causes are doing anything to change the character of the amplification, and/or the sound.
postit.jpg
 
Last edited:

rjenkinsgb

Well-Known Member
Most Helpful Member
The input of the amp alone should read probably 10K or more? You are normally just measuring across the volume control pot.

If the bluetooth module is connected, then the output of that may show lower resistance, especially if it is powered up and not muted - the output will be actively trying to hold the correct signal level to drive whatever it is connected to.

It's not actually an "ohms" reading if there are semiconductors or especially active electronics involved, as the external circuit can clamp the voltage at a point, or force the voltage on the probes to be similar, making the meter "think" it is connected across a low resistance.

[To read Ohms, the meter puts a small current through the probes and measures the voltage that produces across whatever is connected, in effect doing an Ohms Law calculation to determine the resistance that would cause the voltage drop].


Technically, a direct connection rather than bluetooth should give rather higher quality, at that point at least. Bluetooth stereo audio is quite heavily compressed using a lossy system similar to MP3.

The loss of quality is even less than with 320K MP3, so it is a rather minimal effect, not generally objectionable when using such as bluetooth headphones.

Whether it's possible to hear any difference using a direct connection instead depends on both the quality of the audio source [never compressed using a lossy system such as MP3 etc] and the amplifier / speaker noise levels and frequency response; very unlikely, with a simple amp and open speaker as you show above.
 

Hacdrag

Member
The input of the amp alone should read probably 10K or more? You are normally just measuring across the volume control pot.

If the bluetooth module is connected, then the output of that may show lower resistance, especially if it is powered up and not muted - the output will be actively trying to hold the correct signal level to drive whatever it is connected to.

It's not actually an "ohms" reading if there are semiconductors or especially active electronics involved, as the external circuit can clamp the voltage at a point, or force the voltage on the probes to be similar, making the meter "think" it is connected across a low resistance.

[To read Ohms, the meter puts a small current through the probes and measures the voltage that produces across whatever is connected, in effect doing an Ohms Law calculation to determine the resistance that would cause the voltage drop].


Technically, a direct connection rather than bluetooth should give rather higher quality, at that point at least. Bluetooth stereo audio is quite heavily compressed using a lossy system similar to MP3.

The loss of quality is even less than with 320K MP3, so it is a rather minimal effect, not generally objectionable when using such as bluetooth headphones.

Whether it's possible to hear any difference using a direct connection instead depends on both the quality of the audio source [never compressed using a lossy system such as MP3 etc] and the amplifier / speaker noise levels and frequency response; very unlikely, with a simple amp and open speaker as you show above.
Thanks for the detailed reply. This was an older, proof of concept project. My newer setup which I'm using as my bathroom sound system has properly enclosed speakers which, needless to say, sound much better than the one I mounted in air. :)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks for the detailed reply. This was an older, proof of concept project. My newer setup which I'm using as my bathroom sound system has properly enclosed speakers which, needless to say, sound much better than the one I mounted in air. :)
In free air you get no bass, as it cancels out, which is why you need a baffle or a box (a box is a folded baffle).

You also need to understand what impedance is - you can't measure impedance with a multimeter, only resistance - and they are usually very different things. Essentially resistance is DC, and impedance is AC - all your meter is reading is DC resistance.
 

Hacdrag

Member
You also need to understand what impedance is - you can't measure impedance with a multimeter, only resistance - and they are usually very different things. Essentially resistance is DC, and impedance is AC - all your meter is reading is DC resistance.
Ty - I get it, but the two seem to be closely related. I've measured the same way, but with an 8 ohm speaker, and the reading is either 8 ohm or very close.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Ty - I get it, but the two seem to be closely related. I've measured the same way, but with an 8 ohm speaker, and the reading is either 8 ohm or very close.

The DC resistance of a loudspeaker is usually about 3/4 of it's rated impedance, so an 8 ohm speaker usually reads about 6 ohms, and a 4 ohm one about 3 ohms.

Meters are also notoriously poor on low ohms readings, due to the leads and contact resistance etc.

Try measuring the impedance of a capacitor with your meter on ohms! :D or an inductor (and a speaker is just a type of inductor).

If you test a plain (inductance free) resistor, then it's resistance and impedance should be the same.
 

audioguru

Well-Known Member
Most Helpful Member
A speaker with a coil and magnet is at its rated impedance only at about 400Hz. Its impedance at resonance is higher.
Its DC resistance is the resistance of its coil and and its impedance rises as the frequency rises due to its inductance.
Here are the details on a graph:
 

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