spring reverb unit

[birt]

I've got an old reverb spring i want to use but it has no code on it and doesn't look like an accutronics. the only tekst on it is "150 A-B".
i want to get an idea of input impedance but i don't vene know what side the input is. any clues?


bert

[George Giblet]

Measure the DC resistance of the two coils, take the lower value then compare that with the DC resistance of the accutronics tanks then find the impedance.

Alternatively you could measure it.

[Mark Hammer]

I may be talking through my hat here, but my guess is that if you drive it with a couple of NE5534s or an NE5532 or LM833, and stick a 100R resistor in series with the input, you should be in good shape.  The issue with reverb pans is that they can often present loads which are uncomfortably low for the driving circuitry.  All of those chips noted are completely at ease delivering lots of current to the sorts of loads characterized by reverb pans.  The use of a parallel pair, and the use of a minimum load provided by the 100R resistor, should permit the driving circuitry to function well.  "Sonically perfect"?  Well, that's another question.

[birt]

i've seen a schem that uses a transformer with high input impedance in front of the reverb tank to compensate that.

so basicly a reverb unit is just a buffer, amplifier, signal that goes trough the spring and revovery amp? so if i use a 386 in front of the spring i should be ok too since that's capable of driving low loads (speakers) without a problem? and then a recovery amp with a tonestack maybe and a wet/dry mix.. am i right?

[Gus]

I would do what George posted

Find the accutronics(google ask etc) site all kinds of cool stuff schematics etc.

[Mark Hammer]

i've seen a schem that uses a transformer with high input impedance in front of the reverb tank to compensate that.

so basicly a reverb unit is just a buffer, amplifier, signal that goes trough the spring and revovery amp? so if i use a 386 in front of the spring i should be ok too since that's capable of driving low loads (speakers) without a problem? and then a recovery amp with a tonestack maybe and a wet/dry mix.. am i right?

You're mostly right.  In the general picture, you want to drive the load with the configuration that is most comfortable doing.  It can be the case that an op-amp is not great at delivering current to a load in the range of speakers or headphones (under 150 ohms), and it can also be the case that a power-amp chip is not that great at delivering current to loads in the range of several kilohms.

In this regard, I agree with Gus and George that knowing a little more about the reverb pan in question is a better place to start than a simple shot in the dark.  The accutronics site has a handy chart showing nominal impedance and corresponding DC resistance (http://www.accutronicsreverb.com/ioic.htm).  That seems like a useful reference to have some idea of what you are dealing with.  Better choices can then be made about a suitable driving/recovery circuit.

If there is a rule of thumb to follow, though, it is that input impedance of a reverb pan is generally higher than output impedance.

[Peter Snowberg]

I know some older tanks were driven by a capacitor coupled amplifier stage, but I don't know anything about the impedance you would see in measuring.

[birt]

i measured DC resistance. one side has 22 ohm and the other 256 ohm. i can't really find a matching accutronics spring

[fikri]

I have an accutronics tank from an old fender montreux (forgot the impedance) I'm sure that it is driven by a solid state driver (since the amp is pure solidstate) But i dont know what is the best driver circuit for it.

[Mark Hammer]

i measured DC resistance. one side has 22 ohm and the other 256 ohm. i can't really find a matching accutronics spring

Maybe not, but the chart suggests that the input impedance is somewhere in the 150 ohm range, and the output impedance is something around 2.5k.  So, you can feel free to use a pretty normal op-amp for the recovery circuit, and something with tolerance for low loads (LM833, NE5532, NE5534) as the driver.