I think it is always a debate - whether I need a digital audio player (DAP)? Can't I use my phone as a source? Personally, as a DAP user, I would say I use DAP as a music source because I do not want my phone battery to be spent on music playback. I would rather bring a DAP with me solely for music listening. The other concern is memory. As an iPhone user, I want my 256GB onboard memory to be well-spent for photos etc.
If the above two concerns are irrelevant to you, I think using a smartphone as a music source is definitely not an issue. But how? I tried this yesterday and I think it is worth sharing. This is for iPhone but I think can be relevant to Android users. First of all, the music player application on your phone will be important. The application should be able to playback most of the file type with different bit and sampling rate. I spent SGD$15 to purchase the Onkyo Singapore HF Player from App Store and it fulfil all the basic requirements.

Now, moving your recordings to the phone. For Android users, a SD card would be useful so the recordings do not consume your onboard memory. For iPhone users, no choice, you have to mess around with what you have. I am glad that I have 256GB onboard and normally I store my photos on iCloud (with subscription). So, I have plenty of memory space for my recordings. Connect your iPhone to iTunes via PC, go to file sharing and drag your music library into Onkyo HF Player. Once the copying is completed, you are good to go.
Next, getting an external USB DAC and amplifier. I think as an audiophile, everyone should understand the capabilities of an external USB DAC and amplifier. It helps to decode and amplify the recordings better. I use Lotoo Paw S1. The retail price is SGD$269 for bundle with iPhone OTG cable. It connects to iPhone easily and works flawlessly.

Last but not least, a pair of decent headphones or IEMs. Since you save some money from investing a DAP, probably you can get a more decent headphones or IEMs here. I am pairing this setup with my Itsfit LAB - Custom IEMs Fusion CIEMs. With the 4.4mm balanced stock cable, I get to utilised the balanced output implemented in Paw S1. It gives a lower noise floor and greater output power.

To conclude, the total cost for this setup is SGD$1584 (Fusion is retailing at ~SGD$1300). The total price is dependent on the USB DAC and IEMs chosen. This could be a good path for you especially if you have a stricter budget. Audio is no longer a rich man's hobby. We have tons of alternatives. Let me know if you have any doubts, we can discuss them together! Cheers!
If the above two concerns are irrelevant to you, I think using a smartphone as a music source is definitely not an issue. But how? I tried this yesterday and I think it is worth sharing. This is for iPhone but I think can be relevant to Android users. First of all, the music player application on your phone will be important. The application should be able to playback most of the file type with different bit and sampling rate. I spent SGD$15 to purchase the Onkyo Singapore HF Player from App Store and it fulfil all the basic requirements.

Now, moving your recordings to the phone. For Android users, a SD card would be useful so the recordings do not consume your onboard memory. For iPhone users, no choice, you have to mess around with what you have. I am glad that I have 256GB onboard and normally I store my photos on iCloud (with subscription). So, I have plenty of memory space for my recordings. Connect your iPhone to iTunes via PC, go to file sharing and drag your music library into Onkyo HF Player. Once the copying is completed, you are good to go.
Next, getting an external USB DAC and amplifier. I think as an audiophile, everyone should understand the capabilities of an external USB DAC and amplifier. It helps to decode and amplify the recordings better. I use Lotoo Paw S1. The retail price is SGD$269 for bundle with iPhone OTG cable. It connects to iPhone easily and works flawlessly.

Last but not least, a pair of decent headphones or IEMs. Since you save some money from investing a DAP, probably you can get a more decent headphones or IEMs here. I am pairing this setup with my Itsfit LAB - Custom IEMs Fusion CIEMs. With the 4.4mm balanced stock cable, I get to utilised the balanced output implemented in Paw S1. It gives a lower noise floor and greater output power.

To conclude, the total cost for this setup is SGD$1584 (Fusion is retailing at ~SGD$1300). The total price is dependent on the USB DAC and IEMs chosen. This could be a good path for you especially if you have a stricter budget. Audio is no longer a rich man's hobby. We have tons of alternatives. Let me know if you have any doubts, we can discuss them together! Cheers!