No AUX input for some devices when bluetooth module connected - Page 1
Forum section: electronics / projects

author: willPowell
author_rank: Contributor
timestamp: December 15, 2021, 11:03:19 pm
content: Hi everyone,I have recently added Bluetooth functionality to my micro HiFi amplifier (Sony CMT EH15) by adding this audio Bluetooth modulehttps://www.amazon.co.uk/Bluetooth-Wireless-Receiver-Amplifier-Modification/dp/B0881V53QY/ref=sr_1_8?keywords=bluetooth%2Baudio%2Bmodule&qid=1639608514&sr=8-8&th=1in parallel with the aux/headphone jack input. Sorry, no datasheet! I am powering the BT module with a 5V dc power supply.The Bluetooth works great, no problems at all.But weirdly only some devices can play music using the aux lead, whilst others can't play at all, no sound. E.g. my phones aux can play music but my iPad, or pre-amped record player cannot.Any ideas / ways to solve this issue?P.S. I have also tried to attach the aux in parallel with the input of the cassette player 'Head', but unfortunately that's too quiet.Also to the CD left and right output, after it has been inverted, but that requires CD to be read before amplification.P.P.S is it possible to connect it to the F.M. antenna? Unsure if I would get stereo though.Thanks in advance,Will

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 16, 2021, 12:03:40 am
content: QuoteProbably the reason for that is the bluetooth module, which I assume you've just soldered in parallel with the aux jack, has a low output impedance.This means that any device that also does not have a very low output impedance will have it's signal severely attenuated. Even the ones that work, may sound a bit distorted as well.A solution that could work would be to put a 1K resistor inseriesbetween both the L and R leads from the bluetooth module to the Aux jack. (Two resistors total). When using bluetooth, you'll have to disconnect the other device from the aux jack.EDIT: 1K is just my best guess. You may have to experiment a bit with the value.

author: willPowell
author_rank: Contributor
timestamp: December 16, 2021, 09:25:34 am
content: This is a great response and I look forward to trying this this evening.Excuse my naiviety but what is the best way to measure the low impedance from the BT module?On the user manual for the HiFi, it says that the audio input has a "Sensitivity of 800mV and impedance of 47kOhms".From this discussionhttps://electronics.stackexchange.com/questions/6846/how-important-is-impedance-matching-in-audio-applicationsit seems that impedance matching is not important, only that the source should be less than the input.Any rough ratio or guidelines to follow?

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 16, 2021, 05:44:02 pm
content: QuoteI don't think it would be necessary for you to do so. But the way you'd do it is play a sound file with a test tone via BT, measure it's open circuit output level, and then put a rheostat/pot on an output and slowly reduce the resistance until the output level drops to half of the open circuit value. Then you'd disconnect the pot and measure it with an ohmmeter. This reading would be the BT module's output impedance. I would guess that it would be quite low (10-100 ohms).QuoteThen the 1K resistors will work just fine, and not attenuate the signal from the BT module much at all.QuoteFor line level audio, the load/source ratio is usually more than 100:1 but it's not critical. Generally, what's more important is that the source impedance is low enough that any cable capacitance doesn't roll off the frequency response too soon. (Cables have to be quite long for this to be an issue) This won't be an issue for your BT module connection inside the HiFi.

author: willPowell
author_rank: Contributor
timestamp: December 17, 2021, 05:35:07 pm
content: This worked and sounds great on both BT and Aux input! I ended up adding two 100 Ohm resistors in series as 1k, reduced the BT audio too much.Thanks for all your help, I've learnt something today.