Tone mods Music Man RD-50

[lowell]

I'm not too familiar w/ tweaking tone in solid state preamps. Anyone know what my options are in this amp as far as reducing treble and increasing bass? I have played a bit w/ the tone stack with some results but would like to know if there are other options surrounding the ICs... kinda like tweaking a common cathode stage of a tube preamp.

[loudthud]

I couldn't help wondering if a bad capacitor isn't causing a bass rolloff somewhere. Does it seem to lack bass with and without the limiter switched on? Check C1, C3, C24, C25, C26. There are two versions of this amp, have you looked at both?

[lowell]

Loudthud,
Thanks for the tip... The amp actually sounds quite normal to me and I don't think there is anyting wrong... but my friend would like to reduce the treble and increase the bass... so I thought this would be a good opportunity to learn about tone-shaping in solid-state preamps. Say those caps are good and the amp is operating like new, could you give some tips on tweaking the preamp?

[jrfrond]

You can apply standard tone stack mods and shunt caps to that circuit. The way the tone stack works in this and most other SS amps with passive networks is that the cap values are multiplied by a factor of 10 (decimal one place to the right), while the pot and resistor values are divided by a factor of 10 (decimal one place to the left), so that 25KA pot would correspond to a 250KA pot in a tube amp, while the .22uF cap corresponds to a .022uF in the same amp. The 820 and 1.5K midset resistors on the bottom of the stack would correspond to 8.2K and 15K. The division/multiplication of values keeps the time constant of the RC networks identical vs. their tube-based brethren.

[lowell]

Huh... VERY interesting. thanks for that. why is that? ... cause of different circuit impedance?

[loudthud]

Since these amps are build on printed circuit boards, experience and the proper tools are required to perform any mods. Finding parts that will fit on the circuit board is another challenge. Some of the small ceramic caps you can find won't be the best sounding.

In the tone stack(s) you can increase the low mids by changing C5 or C13 to 0.1uF. This won't be easy because of all the wires going to the circuit board but it looks like you can tilt the board up without having to unsolder any wires. Reducing C4 or C12 will reduce mids and highs.

To push the highs down without killing them all together I would suggest a series RC network across R2 and/or R44. Start with a resistor equal to or half the value of the resistor to be shunted and a cap equal to C = 1/(2*PI*R*F) where R is the original resistor and F is the frequency where the highs start to reduce (somewhere 300Hz to 1KHz). Adjust R and C as desired. I like to use a trim pot and a cap substitution box to find what sounds right, then install fixed components.