L´Andratté
Well-known member
Check out the new TapeOp for his DIY spring reverb!
I like his style!
And I like to discuss this, but have to read first ;D
I like his style!
And I like to discuss this, but have to read first ;D
ruffrecords said:X chooses an inverting op amp stage with a 100K input resistor. This resistor is in series with the output impedance of the tank as far as noise is concerned so the noise from the 100K is added to the noise from the tank. This very easy ti fix, Just change it to a non-inverting stage with 1ooK from the + input to ground. The 100K is now in parallel with the tank output impedance and does not increase noise. While you are at it you can change the feedback network from 1M and 100K to 10K and 1K which will reduce the noise generated by this network by 20dB.
buildafriend said:Is this in reference to the johnson noise that is produced by the resistors?
buildafriend said:Is this in reference to the johnson noise that is produced by the resistors?
ruffrecords said:.......This very easy ti fix, Just change it to a non-inverting stage with 1ooK from the + input to ground. The 100K is now in parallel with the tank output impedance and does not increase noise. While you are at it you can change the feedback network from 1M and 100K to 10K and 1K which will reduce the noise generated by this network by 20dB.
The mixer G2 sums the wet and dry signals. Here he uses a non-inverting stage when a virtual earth mixer would have been better. The signals are line level so noise is not a problem so you can safely use the inverting stage a virtual earth mixer requires. He uses a 5.23K feed resistor for the wet and 47K for the dry. This appears to be to get 20dB gain for the wet and 0dB gain for the dry but it also causes serious interaction between the wet and dry controls. I am not sure if this is deliberate but it is certainly odd. A VE mixer would do the job without the interaction
Lastly, a minoe point, he uses an inverting satge for the tank drive circuit. Nothing wrong with this except he adds EQ to the input arm which means the input impedance varies with frequency, The data sheet for the part recommend a non-inverting set up and again it is easy to convert it and avoid a variable input impedance.
Yes you can. He has an inverting preamp followed by a non-inverting mixer so overall on phase flip. I suggested a non-inverting stage followed by an (inverting) virtual earth mixer so also overall on phase flip.buildafriend said:Couldn't one flip the phase pretty easily somewhere else to make up for the inverted signal while not ending up with higher "noise?"
There is a great chart in Douglas Self's small signal design book that charts out Johnson noise in resistors by R value. It looks as Ian explained, the higher the R value the more Johnson noise. This is a fairly serious thing to look at when working with low THD+N specs like that of modern op amps. Maybe not as much in a tube amp..? But overall it's noteworthy theory.
ruffrecords said:Yes you can. He has an inverting preamp followed by a non-inverting mixer so overall on phase flip. I suggested a non-inverting stage followed by an (inverting) virtual earth mixer so also overall on phase flip.
My gut reaction is the inherent noise floor of a spring tank system will swamp any contribution from the amps.
Or maybe he intentionally made it lower fidelity than his full price version...emrr said:In this case it's clearly described as a DIY simplicity reduction of a product he sells, which he's lived with and bench tested extensively.
moamps said:As I said, this way direct signal will be mixed with out of phase reverb signal, no matter which type of summer is used.
For sure.
Then why make the make-up amp inverting, where high impedance/low noise and no mixing is needed?PRR said:Reverb is inherently random phase.
I think driving the tank with a 20watt amp is something I haven´t thought off (LM386=1watt max.). Curious about the benefits. Someone ( ;D) has to build it and report!PRR said:The reverb tank is not so very hissy; it is indeed an 800 Ohm resistor. The usual reverb hiss is hasty implementation: insufficient drive, and high-hiss recovery amp or mix-stage.
L´Andratté said:Then why make the make-up amp inverting, where high impedance and no mixing is needed?I think driving the tank with a 20watt amp is something I haven´t thought off (LM386=1watt max.). Curious about the benefits. Someone ( ;D) has to build it and report!
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