Simple hi-pass filter for Roland JC amp?

[Rodgre]

Hey all. I have been using a JC-55 in stereo with a Fender Vibroverb and I have had issues with the bottom end on the JC. I have gone through many speakers in it, not by driving them too loud, but because the amp is so clean and hi-fi that it sends a lot more deep bass to the speakers than most other amps. What happens is that some real low-end stuff from, say an analog delay sweeping around with infinite feedback, hits the amp, I can hear the speakers trying to reproduce the real sub harmonic stuff and flubbing out. I believe this is what has killed all the newer Jensens that I've put in it. I currently have some nice 8" Celestions in it and I really don't want to blow them.

The EQ on the amp leaves a lot to be desired, as even turning the Bass knob down doesn't filter out the deep bottom end from making its way to the little 8" speakers!

How might one go about either building a pedal before the amp input or else tweak the input of the amp to roll off below 100Hz? Would it be as simple as putting a cap in series? How would one compute the right cap value to have Thr right cutoff frequency?

Thank you
Roger

[garcho]

Do a search for an 'active HP filter'. You can build one with an op amp and a couple resistors and capacitors. You can hard wire the tuning for the cutoff you want in this particular case, or just find a tunable filter and use a pot.

[Seljer]

Or maybe just lower the value of the coupling capacitors all over the circuit (or at least at the very start of the preamp)

Just glancing over the Roland JC schematics available on the net, the input stages all seem to have a 82nF coupling capacitor and 1Megaoohm input impedance, the gives you HPF with a cutoff frequency of 2Hz . Try lowering that to 3.3nF or 4.7nF.

edit: heres the math for the simple first order RC filter http://en.wikipedia.org/wiki/High_pass_filter#First-order_continuous-time_implementation

[Mark Hammer]

It might be simpler to simply cut some of the low end of the delay signal in the delay pedal itself, perhaps making that cut a toggle-selectable option.

[PRR]

It's only 25 watts per speaker. While Eights are too darn small for bass, there ought to be a tough efficient Eight which will survive "any"thing. I would not be looking in Jensen and Celesion's older designs.

A series-cap to speaker is very effective in shaving subsonics. Size the cap to equal nominal speaker impedance (8 ohms) at speaker resonant frequency. For guesstimated 90Hz of a loud Eight, I get 231uFd, call it 220u. This needs to be NON-polar and at least 50V. Such caps are best found as "crossover caps", and PartsExpress.com is a major source. Don't fool with platinum and papyrus ultra-caps priced higher than your whole rig. Two 220uFd NP 50V should be under $10. You can find them far cheaper than PE, but mystery merchants may sell you crap, go with a good vendor.

If cheaper by the bushel, get a 10-pack of 100-150uFd caps so you can parallel-up 100 200 400uFd combinations to taste.

On the board: their funky power-amp does not lend itself to a simple 2nd HPF. Instead, try clobbering bass <60Hz at the first two stages, C2=0.003uFd and R9=68K