[ECM]

No idea what the cap values are on the output jack, but I would venture to say either 10nF or 2.83nF because that's what the PH-1 uses and if it were me building these, I would use what I had on hand.  It's just a hunch as I haven't done any math to determine frequency response of a 32ohm load with 10nF or 2.83nF across it.
 
EDIT:  Just took a quick look at the impedance of both values.  At 20Hz 10nF is 800Kohm and 2.83nF is 2.8Mohm.  At 20Khz 10nF is 795 ohm and 2.83nF is 2811ohms.
 
Keep in mind this is only the reactance of the cap, not taking into account the output impedance of the op amp or parallel impedance of the headphones.
 
EDIT 2:  Comparing photos of the HPA-2 and PH-1, the size of the caps across the output jack are the same size as the 2.83nF caps in the PH-1.

 

[speedking]

i will try to find some more info but i was wondering if there is any difference on the sound? Smoother and corrected highs maybe ..??

 

[speedking]

Can someone recognize this capacitor?
 

 
..wish i could fit and RK40 in my RA-1! I guess this would be a great improvement!

 

[ECM]

I haven't had a chance to look into this any further.
 
I've ordered most of the parts, just need to get it assembled.  And even then, I'm not so sure how it will sound compared to the original.

 

[speedking]

I did tried a few polystyrene capacitors last week . I tried from some 50pf to 10nf with really NO difference on the output sound on low and hi volume !
So the only purpose comes to my mind is that they use it for eliminating any "pop sound" from the "off-stand by-phones" switch!

 

[speedking]

can someone let me know if the front switch is 3-position (off-standby-phones) or 2-position (off,standby- phones)..
 
i'm just wonder what switch did they used and about the connection...
 

 

[ECM]

Were you able to figure out the type and value of the large capacitors?

 

[ECM]

I believe the switch is a three way that charges the batteries when in standby.

 

[devouringone3]

Yup, three way.
 
I thought Standby was to check charge indicator LEDs, which fade as the life of their respective battery goes down.
 
Phones = LEDs + Music; Standby = LEDs; Off = No function and silent
 
If Standby worked as a battery charger when on the external power supply, then I didn't know, nor can verify.

 

[speedking]

Thanks for the info!
Is it possible you can post a picture of swich connection?

 

[speedking]

  Yup, three way.
 
I thought Standby was to check charge indicator LEDs, which fade as the life of their respective battery goes down.
 
Phones = LEDs + Music; Standby = LEDs; Off = No function and silent
 
If Standby worked as a battery charger when on the external power supply, then I didn't know, nor can verify.

Does the HPA1 has the "pop" sound while On/Off the amp as the RA1 has?
I think in "Stand by" mode the amp is working and only the phones are out to save them from an unwanted dc offset while the caps are charging(while On) and discharging (while Off).
Its is very know that the RA1 has a similar design but the bypass caps have small value (0.1uf instead of the 330uf that HPA1 has).

 

[devouringone3]

Little sound from flicking the switch, maybe a faint pop, maybe not. The mechanical sound of flicking the switch masks it.
 
I don't have any HPA with me at the moment; they are in my bunker at mother's place.
 
They are considered mediocre amps, used more for hype factor and brand loyalty. I don't use mines because of their background hiss.

 

[speedking]

  They are considered mediocre amps, used more for hype factor and brand loyalty. I don't use mines because of their background hiss.

Really?!? i didn't know that they hiss! Is there any fault or they all just hiss? I was willing sometime ago to get one at a reasonable price (even a defect one)!

 

[devouringone3]

In 1991 it was ground-breaking, as headphone amplifiers didn't even exist.
 
To a degree it still is effective and functional, as a semi-portable wire with gain... but so is a (smaller and more affordable) CMoy.
 
RA-1 internals

 
 
Headphone amplifier topology evolved and enriched since then.
 
Personally I question the whole paradox of a dedicated headphone amplifier. It's not mandatory in the same way a power amplifier is to a pair of speakers. Every headphone jack in every device is already amplified for (usually) enough volume. If the quintessence of our devices is to get out of the way and disappear before the music, then why stick an “impurity” like a headphone amplifier in our circuit (provided that our headphones are sensitive and we don't need the extra loudness).
 
I just make sure my source is decent: I recently purchased an external DAC and there's a significant positive difference over my laptop's integrated audio.
 
 
My HPA-1 and -2 have subtle but annoying hiss with all my Grados, and Head-Fier PCF's too.

 

[speedking]

  In 1991 it was ground-breaking, as headphone amplifiers didn't even exist.
 
To a degree it still is effective and functional, as a semi-portable wire with gain... but so is a Chu Moy (being smaller and more affordable).
 
RA-1 internals

 
 
Headphone amplifier topology evolved and enriched since then.
 
Personally I question the whole paradox of a dedicated headphone amplifier. It's not mandatory in the same way a power amplifier is to a pair of speakers. Every headphone jack in every device is already amplified for (usually) enough volume. If the quintessence of our devices is to get out of the way and disappear before the music, then why stick an “impurity” like a headphone amplifier in our circuit (provided that our headphones are sensitive and we don't need the extra loudness).
 
I just make sure my source is decent; I recently purchased an external DAC and there's a significant positive difference over my laptop's integrated audio.
 
 
My HPA-1 and -2 have subtle but annoying hiss with all my Grados, and Head-Fier PCF's too.

I know the internal as I already own the RA1 amp! I have also built my own RA1 replica!
I never noticed any hiss on my battery amp...
I would like to know if you're willing to sell you HPA2?