Spring reverb impedance toggle?

[rockgardenlove]

So, I'm building a spring reverb unit, and I'd like to be able to use it both with a guitar, but also on the line out of my Presonus Firebox.  I've looked at the Firebox spec sheet, and it's got a line out impedence of 51 ohms.  I know it's bad to have the impedence too low, because it will cause treble loss, but is it bad if the audio is going into a setup that has a much higher inpedence than the source signal? 

The reverb I'm using is the Stage Center from GGG.  Schematic here:
http://generalguitargadgets.com/diagrams/stage_center_reverb_sc.gif
It seems like it would be around 1M, I'm not sure how the much opamp(TL074) will change that number.  I tried looking at a datsheet, but I'm not quite sure what to make of it.  A datasheet is here:
http://www.ortodoxism.ro/datasheets/texasinstruments/tl074.pdf


So anyways, I'd like to add a toggle in to be able to switch the input impedence to around 5.1 ohms.
I did some math and figured out that a 5.11ohm resistor will come very close to 5.1 ohms, in parallel with the 1meg resistor in the schem.  Anyways, now that I know that, is it as simple as installing a toggle that will put that 5ohm resistor in parallel with the 1meg....


It can't be that simple.  So, how would I do this?  It's a pretty big jump impedence wise, but I'd really like to make this work.  I'd prefer not to add a whole new buffer section, but if I have to I will.


Thanks!

[Ardric]

is it bad if the audio is going into a setup that has a much higher inpedence than the source signal?

No, it's preferred to have a low impedance output into a high impedence input.  I think line inputs typically have an input impedance around 10k; somebody please correct me if I'm wrong.

R8 also forms a high-pass filter with the input capacitor C3.  If you drop R8, say by switching in a 10k resistor in parallel, then I'd also switch a 2.2uF bipolar cap in parallel with C3 to preserve the bass response.  Or just leave C3 bigger all the time.  I think I'd leave it at 1M/0.022 myself.

Don't load your line out with only 5 ohms!  That's less than a lot of speakers!

BTW, what's up with Craig's "dwell" control?  It appears to control the output level of the recovery amp, while the "mix" controls the dry level.  I thought dwell usually referred to the level or gain on the input side of the tank, not the recovery gain.  That seems more useful to me.  Maybe like this: http://sound.westhost.com/project34.htm.  That one could still use an input buffer though.

I'm also not sure about the bipolar power switching using both jacks, or the idea of battery power at all.  A reverb tank is like a speaker; it's low impedence and needs lots of current to move it.  9V batteries won't last long, and one will die before the other.

[rockgardenlove]

Oh man, serious issue here.

5.1 should be 51!  That was smooth.


So, I don't need to change anything?  Is that what you're saying?

[d95err]

BTW, what's up with Craig's "dwell" control?  It appears to control the output level of the recovery amp, while the "mix" controls the dry level.  I thought dwell usually referred to the level or gain on the input side of the tank, not the recovery gain. 

Yes, the controls on the Stage Center Reverb seem mislabeled. The "Mix" knob should be called "Dry", the "Dwell" should be called "Wet". I suppose you could add a Dwell control by making R7 variable.

I don't know if the circuit is designed for guitar or line level. If it's for guitar, you really want to lower the gain of the reverb driver when you use it with a line level signal, otherwise you might get a lot of distortion there. If the circuit is designed for line level, you probably want to increase the gain to use it with a guitar level signal. Perhaps an 1M pot for R7 would work.

[rockgardenlove]

Right, good idea to lower the gain of the driver. 
Any other possible mods?