SD-1 vs 808: bias at (+)input tone opamp

[owlexifry]

there’s something i’ve been pondering that i can’t quite reconcile:

let’s have a look at the (+)/non-inverting input of the tone control opamp of both circuits:




- 808 has a 10K resistor to Vref/+4.5V bias
- SD-1 does not have a bias resistor here.
(there is an 18n cap, but this would block any DC, so kinda irrelevant)

so what is the significance here?
how is it that the SD-1 can get away with this?

i don’t understand and i’m not ok with this

would really appreciate some pointers/discussion, cheers

 

[EGRENIER]

Ok, first I’m a newbie at this so after looking at it I probably get it wrong

My conclusion are:
- 808: the 10k is only there to reduce the current a bit lower of the bias 4.5V
- SD-1: the cap will not block the 4.5 DC, it will regulate it in case of voltage fluctuation.

None of these will be part of the signal path and affect how the tone stack works.

 

[jesuscrisp]

The 2 stages are directly coupled without a cap inbetween. The 18n cap in the SD-1 but also the 220n cap in the TS form a lowpass filter together with the respective 10K/1K resistor between the 2 IC stages. The 10K resistor in the TS is actually not needed, but it does sort of form a voltage divider together with the 1K resistor that, however, doesn't have a major impact on tone or volume with the values chosen.