Subwoofer pre-amp

[nbergsma]

Hi,

I am trying to design subwoofer pre-amp. I don't have a lot of experience (read: no experience 😉 ) with analog / audio designs and I would like your opinion on the design I made.

The requirements for this project are:
- stereo balanced line ins
- unbalanced line out
- high quality (quality over cost)
- variable cut-off frequency (~150 ... ~350Hz)
- VU-meters for both input signals and the output signal

Things I'd like know:
- are there any (big) mistakes in the design ?
- are there resistors / caps / etc. missing ?
- is the combination of opamps possible ?
- does anyone have tips on the designing the pcb ? for example should I use SMT ?

Thank you in advance.

 

[MJL21193]

I think there is something wrong with your zip file, I can see anything when I unpack and open it.

 

[454Casull]

I am able to open the PDF file inside.

 

[nbergsma]

mmm strange... here it is again!

 

[AndrewT]

go to doug Self's site and to ESP's site and read up on balanced signals and balanced inputs.
Then read the expert, Walt Jung.

Your schematic has unbalanced impedances, 10k & 20k on the hot and cold inputs.

 

[nbergsma]

@AndrewT, thanks you for your response. The 'hints' you gave were very useful.

I updated the schematic so that the impedances on hot and cold pins are the same.

Do you, or anyone else, have additional comments on this circuit ?

Regards,
Niels Bergsma

 

[AndrewT]

I updated the schematic so that the impedances on hot and cold pins are the same.

where?

 

[nbergsma]

I added two op-amps (one for Audio IN L, one for Audio IN R). They are located on the left side of the schematic, horizontal mirrored (next time I'll resolve the component numbering)

I used the circuit from Doug self's site, figure 12 (
'Superbal'-configuration)