speaker driven reverb

[Chuck H]

Hello all

I'm building a speaker driven reverb. Thanks to all who responded to my last thread about this project. This is a refinement of the input for the tank. I've never designed with a panel meter or such a low voltage limiter before. I'm hoping for some critique or suggestions if any is in order.

The unit has to work with amps of different wattages so it must have an adjustable input. The unusual input adjustment keeps the impedance fairly even so the input capacitor frequency knee stays the same. The resistor marked ? is to adjust the needle position on the meter. having never designed with a meter before I'm not sure which meter to get. For this app I'm guessing a mV or uA meter would work. I know that if I choose a DC meter that I'll need to use a diode or possibly a diode rectifier for the meter input. But that's as much as I know about it. The meter is there so the user can set the input level. The diodes are circuit protection for the meter and tank transducers in case the user fails to adjust the input correctly.

The 10r resistor at the tank input adjusts the voltage so the clip level of the diodes is in relation to the voltage needed at the tank.

Hooked to an amp it will change an 8 ohm load to a 7.65 ohm load at 1000hz. But I don't think this is a big problem.

So far it all works on paper as far as I can tell. Any advice?

Thanks

Chuck

[booj]

If this device is connected to a speaker is it not being driven by the amp directly, with the speaker being in parallel with the input? Just another angle on what the input looks like

[Chuck H]

Yes, this is a seperate device that would run parallel to an amps speaker and then to a recovery amp and speaker.

Chuck

[Chuck H]

bump

[gmoon]

Chuck, I stumbled on this recently-- a Goldtone dual 30 watter with the foldback reverb schema.

The schematic, linked from this page.

The preamp stage is the recovery amp...

[Chuck H]

Thanks.

Chuck

[J M Fahey]

Hi Chuck. It does work, and has been used before. There was an old Gibson add-on reverb, which was in fact a Hammond tank driven by the original amp. and with its own external amplifier and speaker. The Fender Super Champ 12 even drives the reverb with its own power amp and re.injects the signal into the preamp.

[Chuck H]

Originally Posted by J M Fahey

Hi Chuck. It does work, and has been used before. There was an old Gibson add-on reverb, which was in fact a Hammond tank driven by the original amp.

I know it works to drive a tank in this way, and I know about the Hammond and Gibson systems for doing it. My Q was about critique on my "variable" design, panel meter implementation and circuit protection. Since this needs to work with amps of several wattages it needs to be adjustable and not damage the tank input transducers if it's improperly adjusted.

Originally Posted by J M Fahey

The Fender Super Champ 12 even drives the reverb with its own power amp and re.injects the signal into the preamp.

That sounds interesting. I'm going to look into that. But part of the point to this rig is being able to OD the power amp of the host amp and then add reverb. Since the power tubes would be clipping hard, I don't think trying to inject a second signal through the amps own power section would sound very good.

Chuck

[imaradiostar]

didn't the original hammond circuit use a lightbulb as a current limiter? dig up the ao35 circuit and take a look- I swear it was a pretty simple little thing.

jamie

[Chuck H]

The light bulb limiter is something I really wanted to use, but it was limited in range. This will be a "reverb only" amp that has to operate with "host" amps from 5 watts to 100 watts. Some light bulbs may yet sneak into this circuit.

Thanks

Chuck

[imaradiostar]

Maybe you could follow the airbrake school of thought and have a stepped attenuator with a multi position switch. You could have steps that would match commonly available amp output levels- say 5, 10, 20, 30, 50 and 100 watts. This allows the use of an inexpensive switch and a multi-tap wirewound resistor. I haven't done the math on the voltage steps recently but I recall it being pretty simple when I designed an attenuator for a friend a little while back.

You could still incorporate the lightbulb as extra protection for the verb and some sort of rheostat as a "dwell" knob. You could use zeners to clamp the voltage in the event that you accidentally used the 5 watt setting with a 100 watt amp.

jamie

[Chuck H]

Originally Posted by imaradiostar

Maybe you could follow the airbrake school of thought and have a stepped attenuator with a multi position switch. You could have steps that would match commonly available amp output levels- say 5, 10, 20, 30, 50 and 100 watts. This allows the use of an inexpensive switch and a multi-tap wirewound resistor.

If you look at the diagram above, that's similar to what I've done. The drive level control IS an attenuator. The reason I'm doing it this way is so that the impedance of the circuit doesn't change with adjustment. The panel meter will tell the user if it's adjusted correctly. A simple switch would work fine for me, or any other tweak, but I fear the average non tech guitar player would be frustrated trying to "set" the unit correctly without a visual indicator and the resulting distortion of misadjustment would be confusing. I thought about a switch but I think the single pot adjustment will make this easier to build and operate.

Originally Posted by imaradiostar

You could still incorporate the lightbulb as extra protection for the verb and some sort of rheostat as a "dwell" knob.

Well, I have most of the voltage division dealt with before the adjustment. So I should be able to use a plain ol' five watt wire wound pot instead of a full blown rheostat. And the light bulb may still make it into the design.

Originally Posted by imaradiostar

You could use zeners to clamp the voltage in the event that you accidentally used the 5 watt setting with a 100 watt amp.

The amount of drive the tank requres is really quite small. I can't get zeners with a low enough zv. I thought to employ them upstream from the input, but that could start effecting the dry signal from the host amp. The diode clipper was suggested in my first thread about this project. I thought it was a good idea so it's in the revised diagram above.

Thanks

Chuck

[Steve A.]

Just a thought- what if you were to use several colored LED's to let someone know if the controls were set properly? I think that most users would prefer LED's over meters. (I have no idea how that would be done.)

Steve Ahola

[Chuck H]

Originally Posted by Steve A.

Just a thought- what if you were to use several colored LED's to let someone know if the controls were set properly? I think that most users would prefer LED's over meters. (I have no idea how that would be done.)

Steve Ahola

+1 on all counts.

I wanted LED's in the first place. But the voltages at the tank input are too low to light them. Upstream from the tank, I can't remember why I couldn't do it upstream but I did go down that road. I have made other changes since though so I may re examine it. or not. I know my friend will like the meter. I'll set it up so that ctr on the meter means "correct". That's about as easy as it gets. I don't want to complicate the circuit with an LED string array so if I were to use LED's it would be a green one that lights when adequate signal is reached and a red one that lights when clipping starts. That may be harder to adjust (for me anyway) trying to negotiate blinking lights and guessing the half point between where the green lights and the clipping starts. Just 'getting the needle to read in the middle' seemed simpler.

Chuck