[MisterX]

I seriously doubt the trimmer is at fault.

Check the transistors and make sure you have the right ones in the right spots (you may have to bend them over to read the numbers)

 

[dbfreak]

Correct about the trimmer. I have 8 other BB627's so I removed the one in the right channel for another and viola, was able to set the R channel like the others. Wierd, guess one of my OPAMPS arrived defective. Now, to case the beast and hear how it sounds...

 

[MisterX]

That is weird, hope the seller offers a refund for that op-amp because those are not cheap (especially now that they are difficult to find).

 

[dbfreak]

I'm not even going to bother seeing if he'll refund it. I've learned to swallow my losses and carry on. Got a decent deal on them anyway although they are not the BP's.

 

[dbfreak]

Whip me with a wet noodle if this is a dumb question. Is S2R also supposed to be jumpered in PPA's preliminary setup? I found the instructions vague in some areas (and yes, I know there is a silk screened diagram for jumpering
S2R on the board). I know it's not me, I did score in the top 1% in reading comprehension (nation-wide) on several of the standardized tests (before they were dumbed down).

 

[n_maher]

Yes, jumper S2R assuming that you don't have bass boost installed.

Here's a picture of a completed PPA for your use.

 

[tangent]

Step 2 is "Jumper S2". Are you asking whether this means both S2 positions, or is there some other confusion here?

 

[dbfreak]

Thanks for the pic, it does explain everything now! Yes, there was a bit of confusion but now it's straightened out. I guess, it's my having to deal in facts and specifics everyday that gets me wound up even in my electronics hobby. Again, thanks for the replies.

Edit: I'm going to switch to unleaded solder now...

 

[dbfreak]

Wow and thank you very much! Flipped the switch and she sounds like heaven. Love that STEPS power supply. She compliments the PPA quite well. Now on to going into class A and adding the bass boost.

 

[Xyrax]

What affect does C2 have on the PPA if they aren't exactly what's specified in the parts selection guide? I realized after I finished building my PPA that I got my C2's backwards. It's specified in the parts selection guide that the ideal set up is 2x .01uf-.022uf and 3x .1uf-.22uf. I must have gotten my part numbers mixed up and used 2x .1uf-.22uf and 3x .01uf-.022uf caps in the C2 positions. Will this cause any problems?

 

[TzeYang]

nah, they're going to work like how it was suppose to be at the first place.

The thing is to sandwich the lowest capacitance values in between three other higher capacitance values.

You get better bypassing results this way.

 

[dbfreak]

PPA completed (all voltages within spec and sounds very nice).

Now on to my question...

I would like to bias into class A and I understand that each voltage drop across R10 (at 1K) means 1mA. So, I measure across R10G and get 21V, 16V on R10R and 22V R10L. What do I dial the trim pots at? All three channels have BB627's, STEPS power supply at 24V and will be using Grado SR225 headphones. Any help appreciated!

Added: Is the voltage drop based on the output of the power supply at 24V?

 

[MisterX]

Quote:

Added: Is the voltage drop based on the output of the power supply at 24V?

The JFET cascode is a constant current source, how effective would it be if it's ouptut depended on a specific PSU voltage?

Quote:

So, I measure across R10G and get 21V, 16V on R10R and 22V R10L. What do I dial the trim pots at?

Adjust the appropriate trimmer until the voltage across the appropriate R10 = 1 volt, this = 1mA of current.

(1mA is just a good place to start, feel free to experiment with the "settings" and see how much difference it makes)

 

[dbfreak]

Thanks again MisterX!

 

[d-cee]

I just received a PPAv2 + STEPS stack.

Here are the essential measurements:
Gain: ~9
Output buffer bias: ~45mV (measured across all buffer output resistors)
Opamp bias: ~1mV (measured across R10)
Output transistors: BD139 and JE253

STEPS is putting out 24v.

However I experienced some strange things.

Only when using my DT880s I got some wierd funky as distortion/white noise when I turned up the volume knob and it got to a loud part of the a song it would go out of control and then even if I turned down the knob the crazy noise stayed. Is this oscillation?

It happened in a couple of opamp combinations:
2 x OPA637 + 1 x OPA627
and
3 x AD843

It only happened with the above opamps and with the 250 ohm DT880, does anyone know what's happening?

Other headphones/opamps used without issue were K501, K701, DT531, 3 x AD8610, 3 x OPA627, 2 x AD744 (using comp pin hack) + AD829.

I've ruled out oscillation due to low gain, unless I'm reading/measuring the gain resistors wrong (possibly) here's a picture of the left channel gain resistors, right is identical:



I plugged the values of R3, R4, R5 and R6 into tangent's gain calculator Electronics Calculators and got ~8.9 if anyone would like to confirm... (I can't figure out how to read resistors, I just measured across their leads with my multimetre)

What else might cause oscillation or whatever's happening?