[ExpatinJapan]

Could any head-fier share this info, or If you have the means to test it on my behalf it would be much appreciated.
 
 
 
SOLVED- see below.
 
_________________________________
Also has anyone every opened their Fostex unit to see what op amp etc it are using in the amplifier section?
 
Thank you in advance.
 
 
 
(Not sure whether this belongs in the Source section, or portable headphone amps - sorry).
 
_______________________________
 
SOLVED
 
 
Apparently its 10.20ohm
 
Originally Posted by tomscy2000
http://monoadc.blog64.fc2.com/blog-entry-99.html
 
Yay! solved.

 

[Norbert]

Headphone jack "applicable load impedance: 16 ohms or more." That is from the manual. Personally, I'd be terrified to crack my baby open to take a peek!

 

[ExpatinJapan]

Yes, that is correct from the manual, their website and the specs listed on for sale websites.
 
But it still doesnt tell us the output impedance.
 
I want my curiosity sated.

 

[Norbert]

Yea, does seem a bit vague doesn't it? But how would you measure it?

 

[ExpatinJapan]

Yea, does seem a bit vague doesn't it? But how would you measure it?

Yes, could anyone tell us how we could test this.

What equipment would we need.

Thanks.

 

[ClieOS]

Yes, could anyone tell us how we could test this.
What equipment would we need.
Thanks.

You need a fixed load, meaning two resistors, one channel each into the ground channel. They should be reasonably matched with reasonable resistance, try not to get too high or low .You need to be able to plug them into the HO of the amp as well. I soldered two leftover 47ohm 1% resistors from another project to a Neutrick 3.5mm plug to make mine. Also, get an interconnecting cable (IC)

Then you need a multimeter or a voltmeter capable of measuring AC to the mV range.

Last you need pure test tone (1kHz), which you can generate using Audacity and use your PC sound card as source.

Play the test tone, measure voltage of the amp's output with no load first (either left or right channel vs ground will do) using the IC. Then measure the fixed load. Mark down V-noload and V-load.

Now, calculate: output impedance = [fixed load resistance x (Vnoload - Vload) ] / Vload

 

[ExpatinJapan]

Oh noes! Sounds beyond my present technical skills.

Thank you though, i may persevere, Shigzeo please help

 

[Yukster]

Quote:

 
Also has anyone every opened their Fostex unit to see what op amp etc it are using in the amplifier section?
 
Thank you in advance.

Expat,
 
I think opamp used in the HP-P1 is the BB OPA 2134UA. 
 
You can find the inside pics here:  http://light-widely.blogspot.jp/2011/03/fostex-hp-p1.html

 

[ExpatinJapan]

Thanks. You are awesome.
Moving closer to finding out its output impedance.

I want to upgrade my headphones/iems and am going to spend quite a bit of
Money and dont want a terrible mismatch, us i am simply. Urious and want to know more
About my kit/rig.

Edit.
Yes it seems to have the burr brown BB opa 2134UA.

And of course the AK4480 chip.

Also noticed a cirrus cs8406-zz in there.

 

[qusp]

as far as I can tell, given the black PCB and black soldermask on a multilayer board (doesnt make it easy to read the circuit), the output impedance appears to be more than 100ohms, there is 100ohms series resistance on the output of the tpa6120, the opa2134 is after the dac as low pass filter and VAS (voltage amplification stage/gain), which then passes the signal to the volume control, then to the TPA. the opa2134 is probably also serving as the input for the line in.
 
its pretty much the design in the tpa6120 datasheet, but with the AKM dac instead of the PCM1792. the TPA6120A2 calls for minimum 10ohms series resistance on the output and up to 100 ohms for capacitive loads, given they are dealing with unknown loads, they probably played it safe and added 100ohms, not thinking about IEMs. the tpa will oscillate with capacitive loads, its a fast current feedback chip, high performance, but I have to say they sure could have done it better given the application.
 
ripe for mods though =) otherwise it seems a nice design, but the engineer obviously hasnt kept with the times as far as headphone amps, or still follows beyers guidelines of 120ohms. it doesnt really surprise me coming from a pro audio company as they deal mostly with higher impedance headphones, but still its a bit lame. now I would NOT suggest that people remove or lower this resistor without knowing what they are doing and having means to check for oscillation. you could fry your headphones with DC

 

[ExpatinJapan]

Thank you very much for your detailed reply.

Yes it is a wonder if it is around 100ohm, near unbelievable as you say
In this age of multidriver iems, esp for something sold as a portable device.

So i am looking for portable headphones of around 800ohm going by the possible stats and the 1/8 rule. LOL.

Groan. It Does sound good though, but i am sure it can sound better.

Need the headphone out tested properly Before i buy any new headphones.

 

[Gilles De Rais]

Hi Expat,
 
I note you've made this request in a number of areas and I think you're getting a little too hung up on it.
 
Most of the threads on the Hp-p1 have people indicating what phones do and don't work too well with it. Your own CK10s have an impedance of 55ohms and clearly work well, your EWS9s are around 42ohms, the Dt1350s everyone likes in combo with the Fostex are around 80ohms.
 
The only things I believe most people don't like with the Fostex are multi-BA IEMs which tend to have a difficult impedance curve. Even then at least one person likes the Fostex with the K3003 which have an impedance of 8 ohms (see the K3003 thread)! The problem with these types of IEMs is that impedance varies and is typically higher in the treble region than the nominal impedance which can lead to a situation where high treble is a few db higher than the bass - leading to a "peaky" sound". Jude's review of the Fostex and the CLAS obliquely referred to this when he said he preferred the CLAS when driving some IEMs. There is a thread on someone with JH13s (IIRC) not sounding good out of the Fostex which validates this point.
 
Unless you're looking at customs, I think it is just a matter of taking the Fostex along to auditions and seeing what tickles your fancy...
 
regards,
 
Giles

 

[ExpatinJapan]

Thanks Gilles. 
I will chill on it.
I just hate loose ends and unanswered questions  because of my curious nature.

 

[Gilles De Rais]

Hi Expat,
 
know the feeling...I've just been going through the same process choosing an ultra portable amp to go with the K3003s and having the same issue of output impedance...
 
Funny thing is, I'm (right now) sitting at my desk with the Fostex, a TTVJ Slim, a Nuforce Mobile Icon,a HRTiStreamer, Ortofon Eq-7, Sony Xba-3, Stax Srm001 and the K3003 as well as the Dt1350, At EW9 and M50 and AKG K271 (not to mention various interconnects, a Nano, a touch 2g and 2 touch 4gs) and I think my preferred sound is the K3003 into the Fostex. Now science tells me this combo should sound 'hot', but it doesn't - the bass is full, the midrange clear (not recessed) and the treble sparkly. I suspect what this means to me is that despite the impedance issue, the damping factor of the Fostex is keeping the K3003 drivers under control. Or it could mean I just don't hear high treble any more.....
 
Still loving the fostex.
 
regards,
 
Giles

 

[ExpatinJapan]

Quote:

Thanks Gilles. 
I will chill on it.
I just hate loose ends and unanswered questions  because of my curious nature.

Still wish this question could get answered (sorry it still bugs me LOL), must be a head-fier out there that has the skills and means to measure it.
 
*Or else I may just sell it and buy the Hifi-m8. I like how Centrance (and Venturecraft) are transparant and open about their products specs.