[nikongod]

The 10 ohm resistor is there to protect the AMP when you plug in headphones with the volume way up high. 
A 10ohm resistor will not protect the headphones from much of a fault in the amp. 
 
You could PROBABLY use a smaller resistor, but why? Lots of headphones sound great with a bit of source impedance and compared to what a high voltage tube operated at low voltage and current does to the signal a 10ohm resistor is nothing. 
 
Keep a bit of safety factor, leave it in.

 

[Evshrug]

The 10 ohm resistor is there to protect the AMP when you plug in headphones with the volume way up high. 
A 10ohm resistor will not protect the headphones from much of a fault in the amp. 

You could PROBABLY use a smaller resistor, but why? Lots of headphones sound great with a bit of source impedance and compared to what a high voltage tube operated at low voltage and current does to the signal a 10ohm resistor is nothing. 

Keep a bit of safety factor, leave it in.

Safety is definitely important, don't want to ruin your equipment or, worse, hearing!

I have read that impedance matching headphones to 1/8 the output ohms is one more thing to help drive headphones to their full dynamic potential... I think that many people who see the 10 Ohm output and then may immediately write the amp off without even hearing it. A lot of the most affordable headphones are rated at 32 Ohms. But then, this is the Internet, full of conflicting reports and misinformation. Some people actively seek out things to raise impedance between the amp and headphone, like the Etymotic adapter to "convert" the ER-4p into the ER-4s. They say that it helps to dampen the background noise, which obviously would be great for tube amps (and vinyl LP record albums). Now you, Nikongod (reference to the camera brand? Love Nikons, almost got one instead of my Olympus), are saying that tubes for headphone amps (low voltage & current) add so much of their own distortion, that the output ohms doesn't even make a noticeable difference in comparison.

So, is there a grain of truth in all three perspectives, or what? How are we defining the effect of this distortion, anyway? When I would use the word "distortion" to describe a sound, I would say something like "The overdriven bass in that bad Kanye West song called 'Gold Digger' usually produces really bad distortion in low-fidelity headphones." Am I actually thinking of "clipping" instead of "distortion?"

Thanks very much for the information so far - I think I would have to take classes to learn all this for myself.

Also, my curiosity about possibly lower output ohms is purely selfish... I have a pair of 62Ohm AKG Q701 headphones, if the output was within that target of 1/8 the ohms of the headphone, I would have both the benefits of lower distortion AND amp protection. As they say, "have your cake, and eat it too."

 

[nikongod]

The 10ohm resistor wont do anything to protect your hearing, and probably wont do much to protect the headphones either. The 10ohm resistor is just there for the amp. 
 
The objectively correct way to drive headphones implies that there is an objectively corect headphone. Ask the objectivists to identify the objectively correct headphone some time, its amusing if you like making fun of people. 
 
The fact of the matter is that the largest difference between any 2 amps with any aspiration of "quality" is smaller than the difference between almost any 2 headphones... So until there is an objectively correct headphone (there will never be - measures flat VS sounds flat VS diffuse field) building an objectively correct amplifier is an interesting academic exercise but ultimately an exercise in futility. 
 
Where does the damping factor rule come from? Certainly not from the IEC standard for driving headphones that specifies a 120ohm output impedance - which gives only a damping factor of ~3 with HD650 or 2 with any 250ohm beyer headphone... The damping factor rule comes from multi-way speakers where it only makes any sense because there NEEDS to be a standard so that things work across brands. When multi-way speakers started to become the norm in the 60/70s the companies that explained it best happened to be the ones that were very good at building ~0ohm output impedance amps. The companies that built VERY nice systems with everything designed as a system got left in the dust. 
 
In headphone land where there are no crossovers (except in multi-way IEM's, which realllllllly need 0ohms, which just proves the need for 0ohm amps for things with crossovers...) the importance of achieving an arbitrary damping factor could even be counter productive. Quite a few headphones sound subjectively better with a decent bit of output impedance... 
 
In the end the only way you will find the right answer is to experiment. Well, at least until the objectivists can identify an objectively correct headphone*. Heh, considering that there are half a dozen ways to measure a headphone (In onter words they cant even agree how to measure) you still have plenty of time on your own. 
 
*How ironic would it be if the objectively correct headphone only measured correctly if it was driven by a 120ohm source impedance? 

 

[Zigis]

Yes, 10 ohm resistor at output is to protect FETs, if output is shorted by plugging and unplugging phones on working amp.
On the other hand, I just see Millett SS amp schematic, similar output stage (OK, different CCS, twice voltage, same IRF510) no output resistor. At least my SS amp work flawless several years now, and I often change phones on working amp.
 
B.t.w., I got PCBs today, 3 are spares, if someone is interested.

 

[Evshrug]

NikonGod,
Just wanted to thank you for your highly instructive post. I have many posts where I write a lengthy analysis and response to what I've just read or been asked... But this won't be one of them. Do you mind if I link to your post in a thread I'm thinking about starting? I want to pull together interesting and useful information I read into a kind of blog that I can use as a reference, and share with others who ask the same questions.

Zigis,
Updates? You got the PCBs on the 16th...

 

[Zigis]

Yesterday I made test about 10 ohm output protection resistor necessity.
I short amp's output to ground before output resistor on working amp, while music playing. First I try ground output on short moment, imitating TRS connection, then I shorted playing amp on 10 seconds. I try it with both channels in a row. Output FETs survival, nothing are damaged in the amp.
So, my conclusion is - output resistors are not required for safety reason, at least with IRF510 at output (original China amp use different output FETs)
 
If someone use my PCB, just solder piece of wire in R9a and R9b position.

 

[Evshrug]

Yesterday I made test about 10 ohm output protection resistor necessity.
I short amp's output to ground before output resistor on working amp, while music playing. First I try ground output on short moment, imitating TRS connection, then I shorted playing amp on 10 seconds. I try it with both channels in a row. Output FETs survival, nothing are damaged in the amp.
So, my conclusion is - output resistors are not required for safety reason, at least with IRF510 at output (original China amp use different output FETs)

If someone use my PCB, just solder piece of wire in R9a and R9b position.

Interesting. Is there a way to use a fuse (either a self-sacrificing burn out fuse or a mechanical circuit breaker) in place of the resistor without sacrificing sound quality, thus gaining protection and low output resistance? Personally, I already am in the habit of turning my volume down before turning off my audio equipment and putting things away, because I hated the pop from the powered desktop speakers I used to have when I turned them off.

Btw I do have some experience building things. I learned several different construction techniques in my 3D design class, I took a class on metal smithing, and I put together my own computer. I just don't know much about circuit design and soldering that doesn't go with a blow torch

 

[KimLaroux]

Honestly, if you're gonna claim the resistor is there to protect against short circuit, the MOSFET will be the last thing I'd worry about. The IRF510 can easily push 20 Amps on a pulse. So everything before the MOSFET is more likely to fail before the MOSFET does. And even then, with the amount of capacitance on the power rails, the capacitors will not even have the time to discharge before you're done plugging your headphones in.
 
I am more willing to believe that adding a resistor at the output will lower the overall gain of the amplifier, which may give you better S/N ratio.

 

[KimLaroux]

I just randomly thought about something. Since the resistor is after the output decoupling capacitor, it adds to the impedance of the RC network. This translates into less bass roll-off if you use the amp with low impedance headphones.
 
Though a resistor as low as 10R will not change much if the output capacitors are over 500µf.

 

[Evshrug]

Quote:

I just randomly thought about something. Since the resistor is after the output decoupling capacitor, it adds to the impedance of the RC network. This translates into less bass roll-off if you use the amp with low impedance headphones.
 
Though a resistor as low as 10R will not change much if the output capacitors are over 500µf.

Did you think of this after reading TKdockweller's (sp) ODAC review? I'd like to actually see measurements and hear it for myself, but I'll probably never work up the ambition to clear the myth for myself.

 

[KimLaroux]

Well, no, I haven't read his review. I'm steering away from reviews nowadays, since I don't need to buy more stuff. Though now that you seem to make a link between what I said and his review, I think I'll have to go read it just to see how what I said matches what he said.
 
I actually thought about it during my shower. I always get the most random Eureka moments in my showers...

 

[Evshrug]

Well, no, I haven't read his review. I'm steering away from reviews nowadays, since I don't need to buy more stuff. Though now that you seem to make a link between what I said and his review, I think I'll have to go read it just to see how what I said matches what he said.

I actually thought about it during my shower. I always get the most random Eureka moments in my showers...

Lol, indeed reading reviews are wallet-dangerous. Just so you know, I tend to make obscure links between things... I actually see the link in a comment posted on the last paragraph of the review. Here is the comment, so you don't have to read a review/temptation:
-------
doktorsteve 9/28/12 at 3:22am
Science guy here - re the last paragraph. The output of the ODAC has a fairly large capacitor to ground right on the output. Most of the difference between cables is due to capacitance (IMHO). This interacts with the output impedance of the DAC (or whatever) and can upset the output buffer to some extent. In the ODAC the capacitor swamps the cable capacitance in most cases so the cables all sound the same.
-------

So... Basically I don't think what I read has much to do with what you though of. For someone like me, with former engineering aspirations before a major change in focus, it is interesting to juggle these concepts around in my head.

P.S. I also have eureka moments in the bathroom, though usually while on the porcelain throne/ thinking stool.

 

[nikongod]

Quote:

P.S. I also have eureka moments in the bathroom, though usually while on the porcelain throne/ thinking stool.

 
My best ideas come to me while dropping a deuce. 

 

[Evshrug]

My best ideas come to me while dropping a deuce. 

And my best singing is done in the shower! However, I keep my audio gear (and any microphones!!!) away from such a humid environment.

I should be able to enjoy this amp sometime next week!

 

[Evshrug]

Well, amp received. The craftsmanship is beautiful and luxurious. The damping on the volume knob reminds me of the damping on the zoom rings of high-grade camera lenses. I love the fit and finish of the unique and charming enclosure. But unfortunately, the tube didn't survive the mail. Anyone seen good 6DJ8 tubes for sale recently?