Audio - connecting headphone out to line in - Page 1
Forum section: electronics / beginners

author: npelov
author_rank: Frequent Contributor
timestamp: July 12, 2021, 10:51:11 pm
content: Hi everyone,Background: During the pandemic I learned to play the piano on a small syntesizer. I decided to look for better digital piano. The best I can test/hear live are Yamaha's Clavinova pianos. So I like the base model, but it sounds like crap when I tested it in the shop and it sounds really good through my speakers when I hear demos recorded through line out. However they decided the base model ($2000) would cost too much more if then put line out. The next model costs $500 more and I don't really want to pay that much for line out. I understand why they put crappy speakers - they have to sell their top of the line pianos. But I have to think how to cheat the system. So I decided if it sounds well on my JBL SE 30 speakers and I came up with the idea of using semi-decent amp + ok quality speakers to unleash the true potential of the piano.I tried a quick search on connecting headphone put to line in, but I've got audiophile answers "it'llprobablysound like crap". Which doesn't seam like a good explanation.Question(s): Can someone explain to me the possible implications of connecting headphone jack to line in? They will probably have different impedances. Headphones output usually has lower impedance than line out and should be able to drive line in input. But is that so bad? Like, what could go wrong? Can I do something to improve it?

author: Whales
author_rank: Super Contributor
timestamp: July 12, 2021, 11:17:55 pm
content: Plugging headphone out into line-in should be perfectly fine, unless they have intentionally nerfed how it sounds even with headphones.Headphone out: normally an op-amp controlled output with negative feedback. No DC offset. Maybe a small amount of series DC resistance. If there is an AC coupling cap: it will be quite large.Line in: normally a resistor (10's of K) + a capacitor (AC coupling) and then high-impedence opamps or voltage dividers.Doing this the other way around (line out -> headphones/speakers) will often lead to distorted audio, because the line out tends to have a reasonably high output impedance (esp if it has a capacitor to AC couple the audio -> your bass will disappear).EDIT: FWIW a "line out" is a cheaper version of a "headphone out".

author: Whales
author_rank: Super Contributor
timestamp: July 12, 2021, 11:27:14 pm
content: Obligatory extra: connecting ground between double-insulated devices cansometimeslead to buzzy or other annoying noises. A TRS audio lead has a ground wire.If you have problems like this: there are various ways of solving them. Sometimes the simplest is to turn around the power plugs in the units until the problem goes away (if they use the figure-8 style reversible AC plug).

author: John B
author_rank: Frequent Contributor
timestamp: July 12, 2021, 11:30:47 pm
content: A pair line outs would be balanced connections, headphones would be a combined set of unbalanced on the one jack. Perfectly fine to use for a line out for a small monitor setup.

author: npelov
author_rank: Frequent Contributor
timestamp: July 13, 2021, 12:09:46 am
content: Quote from: Whales on July 12, 2021, 11:27:14 pmI forgot about that. But the piano uses a AC/DC SMPS. It should be (in most cases) isolated from neutral/earth. Well sometimes there is a cap between primary/secondary of the transformer. I don't know how that affects the connection. I'm not sure how amps work. Are they connected to earth or the electronics is completely isolated. Maybe just the chassy is earth connected, but secondary is isolated?

author: npelov
author_rank: Frequent Contributor
timestamp: July 13, 2021, 12:21:05 am
content: Quote from: Whales on July 12, 2021, 11:17:55 pmYes, why did they leave this out? It's their better line of instruments. Well, I'm not sure how cheap it is to make a ballanced output, even though you don't have to drive a bigger load. Are all line-outs ballanced? I don't think so.Well technically if the headphone amp is a crappy one the line out would sound much better. For example the Casio CT-X3000 has so much noise in the headphones I can't hear my own thoughts. And that's with the volume all the way down.

author: Whales
author_rank: Super Contributor
timestamp: July 13, 2021, 12:58:46 am
content: If it's a TRS connector: it's unbalanced (except for weird cases)If it's an XLR connector: it might be balanced.I wouldn't worry about balanced signals unless you are either sending them over long distances (more than a few meters) or it's the only method you have left of fixing any buzzing problems that come up. An off-the-shelf audio isolation transformer could be used externally (ie without having to mod anything).

author: themadhippy
author_rank: Super Contributor
timestamp: July 13, 2021, 01:00:54 am
content: Quotecost,2 extra sockets and somewhere to mount them may only cost penny's,but on a large production run that soon adds up,also it takes away the opportunity to charge more for a model with them.Quoteno, and that includes some of the yammy models with line outsQuotetheres a fair bit of stuff out there thats weird,many of the cheaper mixing desks use trs for group/aux balanced outputs for example.

author: Whales
author_rank: Super Contributor
timestamp: July 13, 2021, 01:01:36 am
content: Quote from: npelov on July 13, 2021, 12:21:05 amSidenote: this is not always the fault of the headphone amp. Sometimes this is the noise floor of their synth and putting headphones on allows you to more easily hear it. I have not used that model.

author: John B
author_rank: Frequent Contributor
timestamp: July 13, 2021, 01:22:57 am
content: Low end keyboards have the most bargain-basement electronics design of just about any consumer good. The DSP is probably on a FR4 daughterboard, but the rest, ie power, audio power amps, I/O, and connecters, are probably just on a phenolic board. Every cent is pared away.

author: tooki
author_rank: Super Contributor
timestamp: July 13, 2021, 09:43:54 am
content: Quote from: npelov on July 12, 2021, 10:51:11 pmUsing a headphone output as a line out is a common thing. The biggest issue is often signal level, as many headphone outputs do not reach the voltages expected on line level signals, especially on portable doodads.You're not going to damage anything by trying it.

author: radiolistener
author_rank: Super Contributor
timestamp: July 13, 2021, 01:13:13 pm
content: Quote from: tooki on July 13, 2021, 09:43:54 amIt can be damaged if a lot of force will be applied to the connector at insertion

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 13, 2021, 03:51:04 pm
content: I am surprised that a $2000 digital piano is the base model. I bought a Yamaha CLP 860 20 years ago and it was $2300 and it's the second from top. It has 2 headphones out, line in and line out. The speakers are also decent I don't need extra speakers. Actually I plugged my keyboard into the piano and use the piano speakers playing it via the piano keyboard with midi.Any way headphones out is fine for line in. The output impedance for the headphones out is about 8 ohms and the input impedance for line in is 5K or so.