Frequency response in reverb circuit

[überfuzz]

Hi

I'm planning to add variable high pass filter on the input to a reverb send circuit. Looking at standard reverb send circuits there's often a high pass filter cutting the bass frequencies.

The cut frequency should be proportional to 1/R, i.e. f=1/(2*pi*R*C where R is given by the potentiometer.

I think it would be cool feature as when I record guitars I find myself I tinker with and fine tuning the frequency response of the signal I send to the reverb. This would give me the possibility to tune the reverb when I'm on stage, so to speak.

Has anyone here tried something like this? Is it worth it? It should be a fairly simple mod, swapping a resistor for a pot.

Cheers!

[Chuck H]

Hi

I'm planning to add variable high pass filter on the input to a reverb send circuit. Looking at standard reverb send circuits there's often a high pass filter cutting the bass frequencies.

The cut frequency should be proportional to 1/R, i.e. f=1/(2*pi*R*C where R is given by the potentiometer.

I think it would be cool feature as when I record guitars I find myself I tinker with and fine tuning the frequency response of the signal I send to the reverb. This would give me the possibility to tune the reverb when I'm on stage, so to speak.

Has anyone here tried something like this? Is it worth it? It should be a fairly simple mod, swapping a resistor for a pot.

Cheers!

First, please post a schematic of the circuit you propose.

And B) I like a little more low end depth in my reverb. It sounds good. But it takes A LOT of fussing to make it work without acoustic feedback. I've gone to no end of trouble to achieve it and there have been two occasions when I almost gave up. It's that problematic. So proceed at your own peril

[überfuzz]

The design is fairly minimal at this stage, I'm at one capacitor one pot. The capacitor is 0.033uF and the pot is 1MOhm. This will feed the reverb driver with a signal that I can control fairly good, in terms of frequency response. However, I'm not sure of the proper level of the signal hitting the grid of the driver tube.

I have build the circuit, but didn't have time to start testing it. I'll update here when I have.

[überfuzz]

I gave it a though during lunch break. The impedance of the reverb tank is not likely to be linear, i.e. linear proportional to the frequency. If the pot+cap set up doesn't cut the cheese I might have to add some shelving filters to the reverb circuit...

[Mick Bailey]

I experimented with input filtering but did it with a rotary switch and fixed-value caps. I did this because my original build had too much low-end. I really liked the sound but couldn't tame the feedback because I couldn't isolate the tank enough from external vibration. I found though that for a given setup there's a lower cutoff frequency, below which problems begin to stack up. If you have a dwell control you may find an increase in dwell means raising the cutoff frequency to prevent feedback. Conversely, as you raise the cutoff frequency you lessen the signal drive to the coil and need to compensate for this by increasing the dwell. What I arrived at was pretty much a HPF setting that worked for any situation so I used this as a fixed-value setting and used the vacated space for tone control on the output of the recovery stage. I like that even better and it got away from the need to adjust the controls as a pair. Now I have dwell/mix/tone/output level. I have my mix so that it goes from 100% dry to 100% wet.

What is interesting is to parallel different value caps across the output coil. It creates a response peak that can be shifted and behaves like a Varitone control. The effect is fairly pronounced and I though that in another build I'd incorporate this into (say) a 6 position rotary control.

[Helmholtz]

The impedance of the reverb tank is not likely to be linear, i.e. linear proportional to the frequency.

The reverb transducer is essentially an inductor with a high Q value. Wiring capacitors in series or parallel will produce a resonant frequency response. Without knowing the inductance results are not exactly predictable.

The impedance of the transducer rises fairly proportional to frequency. As a result transducer current falls with increasing frequency and so does the "power" delivered to the springs. Consequently bass drive is overemphasized. Results should improve with increased output impedance of the drive circuit, striving for constant current drive.
But also a capacitor across the output transducer (as mentioned by Mick Bailey) can help to regain treble.

[Mick Bailey]

I didn't wire my HPF input filter directly to the tray, but on the input to the drive circuit, so avoiding resonance on the input. This is perhaps similar to what überfuzz mentions.
Constant current drive does give a very balanced output. My last reverb build uses tubes, but the actual drive is taken from a constant-current MOSFET arrangement. This preserves the drive at higher frequencies and aligns more with the optimal drive required (according to the Accutronics specification).

[überfuzz]

I tried the adjustable high pass filter set up and it works. My cut frequencies are a bit off though, specially the low end. The driver stage doesn't have power enough to drive the tank at lower frequencies.

I'll use the pot of the high pass filter to try and find a sweet spot. The I can use this to recalculate suitable values for the reverb driver's high pass filter.