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Mendelt

    the Netherlands
Spring reverb driver
« on: April 05, 2005, 05:45:15 AM »
I recently built a schematic from generalguitargadgets to drive an accutronics spring reverb. A friend of mine had a reverb-tank lying around so he wanted one too. But when I tried it out it sounded all distorted. It seems like his reverb-tank is a different version with an imput impedance of 8ohms and the tl082 opamps i used don't seem to happy driving this.
I looked on the accutronics site and came up with the following schematic

http://www.accutronicsreverb.com/drive1.pdf

But I was wondering what transistors i should use. The schematic says D40D8 and D41D8 for an 8ohm reverb but i can't find these anywhere.

I think that it should be possible to use this without the feedback-loop. My output-opamp allready has his own feedback-loop and the two transistors are connected as emitter-followers so they don't really need feedback right?


gyraf

    Aarhus, Denmark
Spring reverb driver
« Reply #1 on: April 05, 2005, 05:47:22 AM »
You could use BD139/140 for this. And you need to get the feedback from the output.

Jakob E.
..note to self: don't let Harman run your company..

Mendelt

    the Netherlands
Spring reverb driver
« Reply #2 on: April 05, 2005, 05:59:54 AM »
Wow! you're fast!

Could you explain why i need the feedback from the output? (not that i don't believe you but i'd like to understand a bit more of this stuff)

To me it doesn't look like i'd need feedback to set the gain. The gain from the output stage is 1 cause it's just two emitter followers so i could just set the gain with the feedback loop around the opamp...

And the output doesn't need to be very clean because it goes right into a spring reverb so reducing distortion doesn't look like my biggest concern.

Do i really need the two transistors here? I recently built a one-transistor-guitar-buffer from generalguitargadgets
(this one
http://www.generalguitargadgets.com/diagrams/buffer_bip_sc.gif)
Couldn't i just use something like this with a beefier transistor? This would eliminate any crossover distortion from the two transistors.

Mendelt

    the Netherlands
Spring reverb driver
« Reply #3 on: April 05, 2005, 08:04:59 AM »
Never mind my stupid questions. I just looked a bit better and you're right.

I'll just use the schematics from the accutronics site with the transistors you recommended.

thanks

PRR

    Maine USA
Spring reverb driver
« Reply #4 on: April 05, 2005, 11:54:47 AM »
> I think that it should be possible to use this without the feedback-loop.

Sure, but it will suck. The driver is heavy and has a falling frequency response. It also has a rising impedance. The classic way to fix the response is to drive with a high impedance, 5 times the coil nominal ompedance. This is natural with tube drivers. Opamps normally give a low impedance output. The $0.06 trick is to put in a resistor. But now 5/6ths of your opamp output power is wasted. Since the coil needs very little power to get muddy, this may be acceptable. The other trick is a small series resistor, and take feedback from there. The opamp is set to hold the resistor voltage constant, resistor current is constant, coil current is constant at all frequencies.

The beefier opamps "should" drive the 8 ohm coil with >40 ohms dead series resistance and conventional feedback (before the resistor). 100-200 ohms may be needed to keep the opamp from straining too bad.

Dang near any NPN-PNP pair will work. If you went back in time to when those Accutronic schematics were prepared, I bet you would find that those were the cheapest 30V parts around. The PN3904/3906(?) jellybean pair may be easier for US-folks to find today.

Mendelt

    the Netherlands
Spring reverb driver
« Reply #5 on: April 05, 2005, 03:17:30 PM »
Thanks for all the usefull information.

This way i've got a couple of sollutions i can try out. I'll report back which one I like best.


 

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