Reverb springs

[akis]

Has anyone got experience with using/driving reverb springs units?

I have been experimenting with an accutronics 9eb2c1b model and it is weird. The energy at the input drives the springs so that they physically move and generate resonances all over the place, a few Hz up or down and you can get 10x the output voltage, huge spikes and troughs a few Hz apart. It is much more pronounced at low frequencies (close to 500 Hz and below) and at high input voltages.

A way to avoid this is to lower the driving voltages but then you get almost nothing at the output. Another way I have found is to use a sponge to dumpen the springs and level the output at all frequencies (from 200Hz to 8000 Hz) but I cannot ensure a proper mechanical fit for it to be a proper solution.

This is not related to trying to increase the driving voltages with frequency.

Any comments appreciated.

[Enzo]

Look at an old Fender schematic for example. They may feed the audio througha 500pf cap to reduce the bottom. Low frequencies turn to mud in a reverb. That is why you see them on guitar amps but not bass amps. SO I don;t doubt you have trouble with lower freqs. Keep them out of it.

How much output are you expecting? If you are expecting line level outputs, you may be overdriving the input to get them. The reverb recovery circuit in guitar amops is a high gain stage.

There should be some application notes on the Accutronics web site, they may be helpful.

They pack a sponge in the works when shipping to prevent them flopping around, but the springs should remain free in the air when in use. Nothing should touch the springs.

[akis]

I have looked at Fender schematics (I own a Princeton Chorus) and also have written to the new owners of Accutronics, now in Korea.

For low frequency filtering, Fender use a 150 Hz high pass filter, so I am only looking at 200Hz and above.

The manufacturer suggests a driving current of 3.1mA for my model, which is almost absurd, as you can see on the table below I'd need 25V rms at 8KHz. At 200Hz, 330 and 500Hz, I'd need 806 - 1806 mV RMS which would make the whole tank ... shake, and produce all thiose weird resonances.

Accutronics reverb tank		9eb2c1b	
I(ma)	3.100	max output	
F Hz	R Ohm	V RMS mV	V in peak mV
20	70	217	307
100	140	434	614
200	260	806	1,140
330	418	1,296	1,833
500	612	1,897	2,683
1,000	1,220	3,782	5,349
2,000	2,330	7,223	10,215
5,000	5,200	16,120	22,797
8,000	8,300	25,730	36,388
10,000	10,200	31,620	44,717

I have decided to go for a more modest 1mA, which then produces this table:

Accutronics reverb tank		9eb2c1b	
I(ma)	1.000	max output	
F Hz	R Ohm	V RMS mV	V in peak mV
20	70	70	99
100	140	140	198
200	260	260	368
330	418	418	591
500	612	612	865
1,000	1,220	1,220	1,725
2,000	2,330	2,330	3,295
5,000	5,200	5,200	7,354
8,000	8,300	8,300	11,738
10,000	10,200	10,200	14,425

Now, 8KHz needs 8V rms, which is achievable from a +/- 15V supply and ordinary op-amp (buffered). I will try it today to see what sort of output I get out and what, if any resonances are still present.

[Enzo]

Not sure what your application will be, but 8kHz just doesn;t matter in a guitar amp. Most guitar speakers are well riolled off by 5k. This is why you don;t see tweeters on guitar amps.

If you already have a Fender amp with reverb, perhaps running test signals through it and monitoring signal levels would be illuminating.

Where did those resistance numbers come from? In the 9 series, an E input is 800 ohms of impedance, 58 ohms DC. I've seen freq response curves for these pans, but I have not seen an impedance curve.

I have to admit I am unsure what the problem is you are encountering with resonances. The whole point of a spring reverb is to generate signal artifacts not present in the original.

[akis]

The Fender circuitry has a high-pass filter at 150Hz and at 220 Hz, and a (variable) low-pass at about 5.5KHz on the recovery stage.

I have measured the impedances again as follows (first time my tone generator's control knob was loose and I was measuring slightly wrong frequencies)

F Hz	R Ohm
20	123
100	152
200	240
330	375
500	554
1,000	1,123
2,000	2,150
5,000	4,710
6,000	5,570
8,000	7,290
10,000	9,130

I am still experimenting, I have built a driver and a recovery circuit and am getting good results.

[Enzo]

My advice in sum is: don;t over-analyze this. Millions of guitar amps work just fine with no special design circuits. The fact that nominal impedance for one of these is not flat at all, is no surprise. We design amps for speakers labelled as "8 ohms" and we ignore that each speaker has an impedance curve that is all over the map. Yet the amp works just fine.

The sample drive circuit Accutronics publishes is simple and it works. Look for example at the reverb drive circuit in just about any Peavey amplifier. One 4558 or similar dual op amp drives and recovers the reverb.