Boss FA-1 Circuit Analysis

[redbagy]

Hi, with reference to this schematic (https://aionfx.com/app/files/docs/prism_documentation.pdf) - can anyone let me know the purpose of C4 please?

[FSFX]

Hi, with reference to this schematic (https://aionfx.com/app/files/docs/prism_documentation.pdf) - can anyone let me know the purpose of C4 please?

It is not really needed - then also theoretically C5 isn't really needed either as both op amps are biased to the same reference voltage.

[PRR]

Both these caps reduce "scratch" from the pots. Omit them if you wish.

[FSFX]

Both these caps reduce "scratch" from the pots. Omit them if you wish.

That is not likely to be the case. As long as the offset voltage of the op amps is very small, there should be no DC across the pots to create a 'scratch'. The feedback pot even has the wiper connected to one end which is good practice when pots are used as variable resistors. The tone pot should not be affected by any 'scratch' unless the track is worn out and the wiper is dirty.

[Rob Strand]

That is not likely to be the case.

It's highly likely the intent is to prevent scratch - whatever the cause.   Some designs throw everything they have at a problem other draw the line a little below.   The downside of lots of AC coupling caps is more caps to fail.

What's a little obscure is taking the output from C4 instead of the opamp, since C5 is present.

[FSFX]

It's highly likely the intent is to prevent scratch - whatever the cause.

It is a bit surprising for a Boss design as it looks more like a simple design done by an intern rather than one of the more experienced Boss engineers.
Surprising as well is that from a production engineering and costing point of view it uses two extra 10uF capacitors that could be eliminated from the design. Even the Baxendall design is a bit unconventional with the equal capacitor values rather that the 10:1 usual arrangement.

[Rob Strand]

It is a bit surprising for a Boss design as it looks more like a simple design done by an intern rather than one of the more experienced Boss engineers.
Surprising as well is that from a production engineering and costing point of view it uses two extra 10uF capacitors that could be eliminated from the design.

I actually see it as being the opposite!  I see an experience guy throwing everything at it and the extra caps is the price for doing that.    Things like this are almost arbitrary choices.

Even the Baxendall design is a bit unconventional with the equal capacitor values rather that the 10:1 usual arrangement.

All normal for a feedback Baxandall circuit.  (Technically the non-feedback version is the James circuit.)

Original Baxandall paper from Wireless World 1952,
http://douglas-self.com/ampins/wwarchive/wwarchive.htm#baxtc

[redbagy]

All normal for a feedback Baxandall circuit.

Any reason for other Baxandall circuits to use a 10:1 ratio? Also could you name a few other circuits/pedals that use this tone control circuit? Personally I found it quite nice.

[FSFX]

All normal for a feedback Baxandall circuit.

Any reason for other Baxandall circuits to use a 10:1 ratio? Also could you name a few other circuits/pedals that use this tone control circuit? Personally I found it quite nice.

I think it is just that a lot of implementations use log or audio taper pots and so to get the right control range it needs 10:1 capacitors.
Baxendall originally used linear pots (and one with a centre tap as well) so he used equal value capacitors.

The popular Duncan Amplification Tone Stack Calculator application has 10:1 capacitors and log pots in its James tone stack simulation.

[Rob Strand]

Any reason for other Baxandall circuits to use a 10:1 ratio? Also could you name a few other circuits/pedals that use this tone control circuit? Personally I found it quite nice.

The passive circuit requires log taper pots.   That sets a (roughly) 10:1 ratio of impedances.   Since you (normally) want the response of the tone control to be flat when the pots are set mid-position that forces a 10:1 ratio onto the whole circuit.

The 10:1 ratio also sets the maximum amount of boosts.    The circuit works by having a base-line attenuation and unity gain is the actually the level of the boost.

What you find with the James passive tone control is it can be a little tricky getting a nice symmetric response from the controls - not such a bit deal for guitar amps etc.

There's a number of mods you can do the James circuit, sometimes done for HiFi circuits because you can't trust the accuracy of the log pot tapers (anything from 10% to 20% at 50% rotation).   The first mod is adding resistors in parallel with the pots and another is putting a resistor in series with the lower treble cap. 

The resistor between the bass and treble sections affects things as well.