ACP+ Pre: lack of bass on pre out

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MattHauck

Member
Joined 2019
I just built an ACP+ tonight, and hooked it up to my ACA and DIY MarkAudio fullrangers. Compared to the previous pre amp, and the sound straight from source (my computer), there is a very noticeable lack of bass. Like the low notes are inaudible.

However, when I plug in headphones (66ohm Audio-Technicas), the bass comes back.

Anyone have any idea about what I may have done wrong? I checked the resistors in the pre out path, I am pretty sure they are the correct spec.
 
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MattHauck

Member
Joined 2019
I did have to scrounge the caps for c1 off another project, They are the spec called for, but there is a chance I damaged them while pulling them while desoldering. I just can't wrap my head around why it sounds fine through headphones and then lacks bass when used as a pre.
 
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MattHauck

Member
Joined 2019
Bump- I have tried using the headphone jack to send signal to my ACA, and the bass comes back. So I think I'll bypass R16 and R17, but still seems very odd. Any insights would be much appreciated!
 
Andersonix

Andersonix

Member
Joined 2004
Paid Member
Odd!
What schematic are you using? Mine doesn't have R16, R17...
 

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rayma

rayma

Moderator
Joined 2011
Did you use 1000uF for C3?
If C3 is too small, the 33.2R load resistor on the RCA OUT jack will cut off the bass.

The circuit is designed for 33.2R loads, so the RCA OUT jack, being used for higher impedance loads,
has its own 33.2R load resistor added when that output is selected.
 
Last edited:
M

MattHauck

Member
Joined 2019
rayma said:
Did you use 1000uF for C3?
If C3 is too small, the 33.2R load resistor on the RCA OUT jack will cut off the bass.

The circuit is designed for 33.2R loads, so the RCA OUT jack, being used for higher impedance loads,
has its own 33.2R load resistor added when that output is selected.
Click to expand...
Well, there it is. Got shipped 10 uf instead of 1000 uf on the gold caps for c3. Will replace and report back! Thanks for the help!
 
rayma

rayma

Moderator
Joined 2011
Good work. It's an unusual circuit that is optimized for a 33 ohm load, so you have to switch one in,
if using a high impedance load.
 
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