How to put a tone defeat mod to my amp?

[baskiria]

Hi,

I have a Sony TA-F235R, and there is no tone defeat switch on it. I see that the tone control is using the negative feedback to modify the response. I'm looking for some help on where should I either install a switch to go around the tone control or how to modify the opamp section to remove the tone control capability... The first option I'd like better...

Schematic attached!

Thank you for your help in advance.

 

[currentflow]

Hi,

I have a Sony TA-F235R, and there is no tone defeat switch on it. I see that the tone control is using the negative feedback to modify the response. I'm looking for some help on where should I either install a switch to go around the tone control or how to modify the opamp section to remove the tone control capability... The first option I'd like better...

Schematic attached!

Thank you for your help in advance.

Hi baskiria,

One way of achieving this would be with a switch (DPDT for stereo) as shown. Although this would defeat the tone function, you would still need the op-amp IC301 1/2 to act as the input buffer. Whether this arrangement would offer much of an advantage is probably debatable.

 

[baskiria]

Thank you for the quick and detailed answer.

I'd keep the opamp for sure, as I want to do the least modifications to the current circuit. The main problem is with the tone control in circuit the whole amp has a 1-2dB rise at midbass frequency that for sure alters phase also to some extent, and I'm looking for a ruler flat response, as otherwise this amp measures well in regards of distortion. To my great surprise there is only a small amount of 2nd order distortion and a somewhat taller 3rd order peak measured at 1kHz 1-10W, higher orders look completely canceled or way below my measuring equipment range. Not something I can tell of some other amps I measured.

Thanks again!

 

[currentflow]

Thank you for the quick and detailed answer.

I'd keep the opamp for sure, as I want to do the least modifications to the current circuit. The main problem is with the tone control in circuit the whole amp has a 1-2dB rise at midbass frequency that for sure alters phase also to some extent, and I'm looking for a ruler flat response, as otherwise this amp measures well in regards of distortion. To my great surprise there is only a small amount of 2nd order distortion and a somewhat taller 3rd order peak measured at 1kHz 1-10W, higher orders look completely canceled or way below my measuring equipment range. Not something I can tell of some other amps I measured.

Thanks again!

My pleasure. The modification would certainly result in a flat response as far as the tone controls network is concerned. However, there may be slight roll-off effects at the lower (sub) bass frequencies due to C302, C304 and C306. These components should not significantly affect the mid-bass range though.