Amp Camp Pre+Headphone Amp - ACP+

[peterpaan89]

I don’t see any big issues on the board, though I’m no expert. Seems you went with an alternative way of mounting the RCA jacks. Might be worth verifying they’re hooked up correctly. For instance, a few of the ‘G’s’ (ground) in the silkscreen were inadvertently bumped down when the artwork was sent off (or at least I assume that was the case).

Thanks for your advice, I don't think the RCA jacks are mounted falsly.

 

[peterpaan89]

I notice that you seem to be using Nichicon “Gold” caps for both input and output coupling duties. I did the same, and had the same issue. Swapping the input cap for an Elna Silmic II solved this issue for me, though it’s a tad too smooth for my liking now. I’ll have to see how the sound changes as the cap burns in. Not that I believed in capacitors changing the sound or capacitor burning in until now, but I guess I was wrong.

Thank you very much! That is indeed helpful I will definitely check that. Which ones are the input caps, would you tell me please?

 

[peterpaan89]

Peterpaan89 - Please measure the DC voltage across R4 on both channels. It should be within a couple tenths of 1.2V

Across R4 I get 0.7/0.8V. That seems to be wrong?! What can be the problem here, do you have a suggestion?

 

[Zen Mod]

did you re-checked all resistor values and position?

 

[peterpaan89]

Yes, checked all resistors for right position and value

 

[peterpaan89]

after 2sec more of thinking , there is a question - do you know how to test/match JFets ?

see enclosed sketch - you can use 9V battery instead of supply marked as 12V




DC jack , right close to it - pads V+ and GND

set your DVM to Vdc ( range more than 24V) , black probe to GND , red probe to V+, measure , write here

Very late reply but here it is: 24V PSU voltage

 

[peterpaan89]

put proper size resistor in R13 position - 0207 size, same as you use for R12, for example

there is substantial dissipation for that small one

hum-buzz - confirm PSU voltage

screw thin wire with pot nut and solder wire to closest GND pad, that way you'll get pot case silent

do you have additional under-plate pcb, for shielding?

also, compare voltages across R12 in both channels, write here

Voltage across R12 = 325mV on both sides


underplate pcb => yes, I have one


"screwing thin wire to pot nut and solder it to gnd" by pot nut you mean from one of the 4 nuts that hold the screws in place that connect the pcb to the ground plate => screw wire in one and solder to gnd? Did that but doesn't help with the humming

 

[Zen Mod]

volume pot nut - it's somewhere 9mm of inner Diameter ...... that's nut for sleeve around pot shaft

that way you're connecting volume pot metal case part to GND , so there can't be buzzing when you touch the shaft

325mV across R12 sez that there is proper 150mA though OS

what is voltage at plus of C3, ref. to GND?

 

[peterpaan89]

Ok got it. Will do. Gnd is on all the non used empty holes? Or do I need to go up to the bigger one that says GND?


Voltage of C3 is 11.5V

 

[peterpaan89]

Humming is gone!!! thank you Zen Mod. Sooo good.

 

[6L6]

Your front end is starved for current. (due to wide range of variations in J113, yours are low Idss) That may be the issue with your sound.

Place 33R resistor in parallel with R4 (to make ~27R) or replace R4 with 27R.

 

[Zen Mod]

use ohmmeter and little thinking to find appropriate GND point

you confirmed two relevant points (Iq, C3+) and they're OK

voltage across R4 is not logical , it's too low

what is origin of your JFets?

 

[peterpaan89]

Your front end is starved for current. (due to wide range of variations in J113, yours are low Idss) That may be the issue with your sound.

Place 33R resistor in parallel with R4 (to make ~27R) or replace R4 with 27R.

Thanks for the reply. So in case I have to order some resistors anyways I shall replace R4 with a 27 uF resistor? Or what does R mean here?

use ohmmeter and little thinking to find appropriate GND point

you confirmed two relevant points (Iq, C3+) and they're OK

voltage across R4 is not logical , it's too low

what is origin of your JFets?

JFets are from diyaudio.com, should be fine.

 

[peterpaan89]

So my JFets might be the problem?

 

[codyt]

27 ohms. R being an abbreviation for ohms (resistance)

 

[peterpaan89]

Thanks!

 

[codyt]

You have a Jfet (J113) providing constant current to your signal/gain jfets (J74’s). 6L6 is suggesting that your constant current jfet (j113) is not providing adequate current to the J74s. Remedy is to lower the resistance from the J113, thus providing more current.

 

[peterpaan89]

Though I do not understand why this would be I understand what to do now. Thanks again for explaining. So I will get different resistors and see if R4 current then will be closer to 1.2V.


Thanks to all of you for your quick support! It's much appreciated. Once I learn a bit more, I'm sure I will build more Pass stuff already super happy with my ACA mono blocks. Next step might be a F5 one day

 

[codyt]

It’s not really anything of your doing. The idss range for the J113 part is wide, so without knowing what that value is, the value of R4 will be a bit of trial and error.

 

[peterpaan89]

Oh okay! Wasn't aware of this possibility. Good to know for the future builds!