Pass HPA-1, what do we know?

[generg]

All the best Jeff! I did this 5 years ago and it is like a new life!

Bonne chance!

 

[avinunca]

Best Wishes for your op.

 

[generg]

Zachik, are you aware that the gain is only 8dB or x2.5.......?

 

[Dennis Hui]

Jeff, wishing you a smooth operation and speedy recovery.

 

[Zen Mod]

Major caveat

 

[Pars]

Best wishes on your surgery Jeff. And a speedy recovery as well.

 

[Zachik]

I believe the current mirrors and the feedback resistor control the gain. So double-check the resistor values R13 and R14, and R27.

Will do, and I will report back.

Did you zero the offset with the OPAMP out of the circuit? (There's a jumper for that in the later schematic; in the earlier one you have to remove the OPAMP from its socket.). If the circuit is out-of-balance and the OPAMP is working hard to bring the offset back down, that might affect the gain? (I'm not sure here.)

I did not try that, yet.

What's the voltage of your input signal? (If you're using something like Roon you might have the volume attenuated on the software side.)

Been using PS Audio's Directstream DAC MK1 - according to their specs:
1.414 VRMS (Single Ended)
I have tested with another amp, and it was VERY loud fast. HPA-1 Clone could not get loud with HD600 even at max volume 🙁

(Major caveat: I go in for open-heart surgery next week, so I'm scatter-brained as all hell. Take everything with a grain of salt....)

Jeff, I am sorry to hear about it. Fingers crossed you'll be as good as new shortly after the surgery!!! 🙏

 

[Zachik]

Zachik, are you aware that the gain is only 8dB or x2.5.......?

Yes, but that does not mean that it cannot drive HD600 even at full volume above VERY quiet levels. Right?
I heard a different HPA-1 driven by the same DAC a couple months ago, and it got VERY loud easily. Definitely something wrong with my build...

 

[generg]

Did you make for a try a direct connection after the 0R47 at the output to your headphone, hoping that no relay grounds your output?

 

[Zachik]

I believe the current mirrors and the feedback resistor control the gain. So double-check the resistor values R13 and R14, and R27.

R13 = 220 ohm
R14 = 100 ohm
R27 = 223 ohm

All above are actual measured.

 

[generg]

R27 is 681R in Jeffs circuit

 

[generg]

With 223R you get 0.8V out when you put 0.5V in, according to Spice simulation.

Maybe your low value is due to the parallel servo? Disable the servo jumper and look if you get 680R for R27 then.

 

[Zachik]

Here is the photo of the actual resistor:

That is not 681 ohms?
(I am not a EE, and totally forgot the art of deciphering resistor value by its colors that I studied in high-school MANY MANY years ago)

 

[Zachik]

Maybe your low value is due to the parallel servo? Disable the servo jumper and look if you get 680R for R27 then.

I believe my servo is disabled. (or is it disabled when the servo jumper is shorted?) see:

 

[generg]

At first look it is, can you write the colors down please?

I do not understand why you measure 223R, there must be a reason. I can go to basement and measure my 680R ....
:--))

 

[Zachik]

At first look it is, can you write the colors down please?

So I wonder why it measures (in-circuit) 223 ohms?
Colors : blue, grey, brown, black, brown
not 100% sure about 2nd and 3rd colors (right after blue)...

 

[Zachik]

I do not understand why you measure 223R, there must be a reason. I can go to basement and measure my 680R ....

No idea. I am measuring when amp is off.

Edit: maybe some resistor in parallel affecting my measurements???

 

[generg]

It is 681R, I measure now mine....

 

[generg]

my 680R measures also as 223R, so this cannot be the reason, no matter if the jumper is closed or open.

You cannot give in with a sound card and the REW app a defined voltage in and look what you get out?

Meanwhile I would look if not your relays do something funny. If I were you I would isolate the output from the relays and look if I get there a good voltage.
Or go to the pre out and look if there is more juice.....
At any case funny, maybe someone can come in and help too...

 

[generg]

We have left from the 680R the 332 going to GRD and right from 680R we have the 300R at the output to GRD. So parallel to the 680 we have 300+332 = 632 connected by the GRD.
Correct?