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Removing/replacing series resistors/output imedance in receiver headphone output

post #1 of 9
Thread Starter 

The general consensus on using cheap/average receivers as headphone amps is that they suck due to the general practice of creating the headphone output by stepping the speaker out down with large (~200~800 ohm) resistors in series, creating an unfavorably high output impedance. There are products sold which take the speaker output and give you a headphone jack. This comes with two problems:

 

1. The high power can blow your phones or your ears if you're not careful with the volume. Solution: Lower the source or just be careful!

2. Pulling the headphone out of the jack will SHORT the speaker output, possibly damaging the amp. Bad bad bad, no way to avoid it apart from taping the plug in, but even then it's an issue if the cord gets pinched/cut/shorted.

 

So the only issue is #2. Now, a 2 ohm headphone output is still favorable right? Following a common "1/8ths" rule it would provide sufficient performance, much better than say a 680 ohm out. Replacing these series resistors with 20W 2ohm resistors would protect the amplifier if the jack is pulled/shorted, right? Would a single large wirewound resistor preserve audio quality here, or is using for example, five 10 ohm 5W metal in parallel okay? A load approaching 20W would probably never occur on the resistors at any reasonable listening volume, and would only be present for a second or two.

 

Does it make sense to do this? I can't find anything on this with a search; provide some input and I'll edit the first post to be more of a guide.

post #2 of 9
Quote:
Originally Posted by k00zk0 View Post
 

The general consensus on using cheap/average receivers as headphone amps is that they suck due to the general practice of creating the headphone output by stepping the speaker out down with large (~200~800 ohm) resistors in series, creating an unfavorably high output impedance. There are products sold which take the speaker output and give you a headphone jack. This comes with two problems:

 

1. The high power can blow your phones or your ears if you're not careful with the volume. Solution: Lower the source or just be careful!

2. Pulling the headphone out of the jack will SHORT the speaker output, possibly damaging the amp. Bad bad bad, no way to avoid it apart from taping the plug in, but even then it's an issue if the cord gets pinched/cut/shorted.

 

So the only issue is #2. Now, a 2 ohm headphone output is still favorable right? Following a common "1/8ths" rule it would provide sufficient performance, much better than say a 680 ohm out. Replacing these series resistors with 20W 2ohm resistors would protect the amplifier if the jack is pulled/shorted, right? Would a single large wirewound resistor preserve audio quality here, or is using for example, five 10 ohm 5W metal in parallel okay? A load approaching 20W would probably never occur on the resistors at any reasonable listening volume, and would only be present for a second or two.

 

Does it make sense to do this? I can't find anything on this with a search

 

Yes, it makes sense to do this. Sort of. 

Headphones are not affected QUITE as significantly as speakers by "incorrect" damping factors. The issue is complicated by the fact that some headphones actually sound better (subjectively, and in the absence of a clearly defined standard all we have is subjective) with *MUCH* higher output impedance than the rule of thumb would suggest.

 

2 ohms may still be too low. Grado used to sell an adapter to plug its headphones into a speaker amp. There is a wiring diagram in my pictures folders...

 

Too much gain from speaker amps is largely a myth bordering on FUD tactics, but the Grado adapter accounts for this by using a resistor divider. 

 

Quote:
Originally Posted by k00zk0 View Post
 

I can't find anything on this with a search; provide some input and I'll edit the first post to be more of a guide.

 

I consider editing posts after otehrs have contributed to a thread incredibly rude. 

 

At the very least run the thread and compile it into a blog post. This has the advantage of letting you look like you accept your OP as it is, AND you look cool in the blog post. 


Edited by nikongod - 9/23/13 at 6:56am
post #3 of 9
post #4 of 9

The only 2 things I would add is to make sure you know which terminal is ground, and if there is really a ground.  Some amps invert and ground may actually be the (+) terminal.  Some amps may be balanced, and you will need to ground to chassis if your headphones are not balanced.  It's unlikely that a 20W amp will be balanced, and is probably just single ended.

 

The 2nd is, that depending on your headphones, there may not be enough voltage swing without clipping.  Though, I would think 20W is safe.  I haven't done the math, so I don't know.  A 5W amp would probably not be enough voltage swing for 600 ohm headphones, for example, even though it would probably have a gain of around 5.  The resistor divider would cut that down more, but 5W you can probably connect it directly as the voltage gain is probably around 5 into speakers.  There's generally a reason why headphone amps have higher voltage rails (+-30V) for high Z headphones, and low Z headphones don't need much voltage gain (if at all).

post #5 of 9
Thread Starter 

Yes Nikon, I accept the OP as it is. The ultimate point is that someone googling this would preferably want the answer in the OP. Many forget that long discussions about some methods often won't reveal a clean answer. A blog makes sense though.

 

Actually its a 125 WPC Amp. The model is STR-DE197.

 

Start by googling the model number and the words "service manual". This provides the following document for my receiver:

 

http://sportsbil.com/sony/STR/STR-D/STR-DE197_v1.5.pdf

 

Scrolling to page 17 we find a circuit layout, where we can see that the speaker output negative terminals are connected to ground. To be super sure, you can trace the common ground plane all the way to the headphone ground, indicating the amp is not balanced, and that it is okay to short the two ground/negative of the speakers together (since they are common on headphones).

 

Following one of the headphone signal traces, we see it connects directly to the speaker output (through some relays for the speaker A/B setting), with a 680 ohm resistor in between. This is the resistor which gives the amp high output impedance.

 

The question is, would a 2 ohm resistor (what value?) prevent damage to amp if the headphone is shorted while at regular listening volume? Is changing this to a 2 ohm resistor going to help SQ significantly? Sure there are the phones that work well with huge output impedance but the majority are designed for zero Zo right?


Edited by k00zk0 - 9/24/13 at 4:21pm
post #6 of 9

Is the amp stable into a 2ohm load? If so get a resistor rated for the amount of power the voltage you put into your headphones is into 2ohms. 

 

If for example you use 2.5v(rms)* to drive your headphones:

V^2/R=P

(2.5vrms^2)/2ohms=3.25wrms

Since it is good to have some safety factor in your part selection it would be prudent to get a 5watt resistor.

All of this is of course predicated on the assumption that your amp is 2ohm stable, and that you have ever put 2.5v into a set of headphones. 

 

What is the output impedance of the Grado adapter I posted? I'l give you a hint - its about 2ohms. The attenuation it provides could help you with possible noise issues. 

 

Part of the reason receivers suck so much for headphones is that they just suck. Attributing the issues with recievers to the output resistors is sort of like saying "Yugos suck as formula-1 race cars because they have such small wheels." No, yugos just suck. Back to headphones - the very high output impedance certainly hurts some (I would not even say most) headphones, but how much noise do you think that 680ohm resistor protects you from? If you think I am trying to convince you not to do this, I am absolutely not. I would absolutely try this (and have). Try lots of things (I have). You are asking too many questions, and not trying enough stuff already. Just dont really expect much to work. The 2 or 3 things that do work make all the failures worth it. 

 

 

*2.5vrms is not a realistic value, Just pulled it off the top of my head to make the math easy. 2vrms would have been easier, and no less realistic really, but I didn't think before I typed. 

post #7 of 9
Thread Starter 

I may be wrong but the adapter appears to provide a 20 ohm series resistance with the headphone? What is the role of the 2 ohm? This isn't my area of certainty which is why I'm posing the question; many amps are decent and provide great power for people with hundred dollar big hungry cans and a laptop as a source. The grado adapter hasn't been explained by anyone and doesn't appear to be ideal. It does recommend wire wound resistors however some claim elsewhere that is a bad type for audio chain due to inductance. There is a simple straight theoretical answer here requiring no testing, suitable for today's headphones designed for zero output impedance, which I reckon is something along the lines of "replace the huge series resistor with 5x 5W 10ohm metal film in parallel and keep your volume set carefully". Is it common for amps to do something wonky when pushing low power into 2ohms for the second or two a channel may be shorted during insertion/removal of the headphone?

post #8 of 9

Output impedance is the 2 resistors in parallel. 20ohm||2ohm=~1.8ohm. The 2ohm resistor reduces the output impedance and combines with the 20ohm resistor to create a basic voltage divider. This voltage division is nice because some power amps have too much gain. Others have much more gain than you really need. Dunno how to explain the Grado adapter better than that. One needs to note that in the mid 90's, when many head-fiers were in diapers and Grado was selling those adapters for use with the HP1000, it was very hard to source anything but wirewound resistors for high power applications. Well, you could always get carbon composition, but these are rife with compromises too. Use newer resistors if they satisfy your requirements. One could always measure the inductance of a wirewound resistor for themself and see where the inductance starts to have an effect on their system. Being that its 2013 at the time of writing, we could even get non-inductive wirewoound resistors, designed with very low inductance (like it implies in the name) for RF applications. Unfortunately there are very few straightforward answers in DIY audio, and those are often arrived at by extensive testing. In commercial audio there are even fewer answers at all just more skilled salespeople who lead you to believe there are just a few ideal paths to nirvana. Who ever said that today's headphones are designed for zero ohm output impedance? There are quite a few headphones in production & unleashed onto the market AFTER you joined here that sound reallllllly sweet, I would argue much nicer, when driven from an IEC 120ohm output. As I subtly hinted by saying that many power amps suck for headphones simply because they suck, you will likely run into issues if you try to *simply* replace the 680ohm resistor with a significantly smaller one - even if you do keep your volume set carefully. It is common for amps to have issues driving loads below a certain impedance. Its your amp, if your going to muck about with it you should figure out what it is by reading up before you figure out what it is by exceeding the ratings. I feel dirty writing that all in one paragraph but I was bored enough to decipher your post so I thought I would try it for myself. Don't like. 


Edited by nikongod - 9/24/13 at 9:26pm
post #9 of 9
Thread Starter 

Thanks for the highly respectful input.

 

For anyone ending up her with this question, the results may be terrible, depending on your amp.

 

The TRS connector avoids short situations so there isn't much fear with shorting the amp unless your headphone cable or driver fails.

 

The problem is that irrespective of input presence or volume level, the output stage in some amps generates a significant amount of noise, which is almost silent on speakers. Therefore, running sensitive headphones with low or no series resistance rather than the original 680 ohm, means you hear constant, very noticeable noise. Totally unusable. The grado adapter wouldn't do any better. Some speaker amps are crap.


Edited by k00zk0 - 11/3/13 at 2:00am
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