A friend of mine who gigs for a living, reports an issue with all of his Blues Junior's reverb function. So I'm deep diving on understanding more about how the reverb works, especially on the drive side of the pan. The problem he is describing seems like perhaps the reverb drive opamp may be latching or going into short-circuit protection under some conditions.
Turns out, I haven't found much hard data on how reverb pans should be driven. How much current, voltage, bandwidth, etc, should be directed to the drive coil. All I've dug up so far is "as hard as possible without coil saturation and a 6db/octave high pass slope filter, and no DC to avoid saturation, and current source driver"
Lots of interesting info at this link:
http://www.electricalfun.com/workbenchf ... b_tank.pdf
Some clues, but not a, "hey you dumb engineer, do this!"
The particulars of this solid state reverb implementation involve:
Tank: 3EB2C1B ungrounded 800-ohm input, grounded 2250-ohm output (from the reverb tank decoder ring)
Op-Amp: Rohm BA4560 dual bipolar opamp. There are several makers of 4560, but Fender sticks with the Rohm version, not TI. They used TL072 in earlier versions, but switched to the BA4560 a long time ago.
Here's a snippet of schematic showing the signal path. The voltages on the Fender schematic are with a 1khz, 10mv signal at the input jack and all pots at mid-rotation.
that's the intro. Discussion of the drive-side circuit to come in post 2....
Deep dive, reverb drive circuits and parameters
Moderators: pompeiisneaks, Colossal
Deep dive, reverb drive circuits and parameters
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Re: Deep dive, reverb drive circuits and parameters
So, how the heck does the op-amp drive this thing. Since we're driving this reverb pan's inductive coil to wiggle the springs, that means putting current into it.
Looking at the circuit below is pretty straightforward:
C17, R37, R38 form a simple high-pass filter and attenuation network, dropping the 4.2VAC from the third preamp tube down to 237mVAC at pin 5 of the opamp's non-inverting input.
The reverb pan has a nominal input impedance of 800-ohms, in parallel with R40, 4.7k resistor, putting the load at about 680 ohms. Note that the reverb tank input is not-grounded to the pan, so no ground there.
R39 is a 47-ohm resistor that "senses" the current across the 680-ohm load comprised of the 4.7k resistor and the 800-ohm tank.
The schematic indicates we should see 3.44VAC on the tank input, and 237mVAC at the opamp input.
If we plug the numbers into a little math:
3.44VAC/680-ohms = 5ma AC
And using the op-amp formula that I-out = Vin/R-sense
0.234VAC/47 = 5 ma (4.978ma).
C18 is there to kill the DC off the output. It's a high-pass, in the 100hz and up range, basically a short to ground.
I'm not entirely sure why D14 is there. It would conduct away any negative impulse or kickback from the tank that exceeds -15v (the opamp is powered by +/- 15v regulated rails). Why there isn't a similar clamping diode to +15v, I don't know.
Simple circuit, math seems to checkout.
Looking at the circuit below is pretty straightforward:
C17, R37, R38 form a simple high-pass filter and attenuation network, dropping the 4.2VAC from the third preamp tube down to 237mVAC at pin 5 of the opamp's non-inverting input.
The reverb pan has a nominal input impedance of 800-ohms, in parallel with R40, 4.7k resistor, putting the load at about 680 ohms. Note that the reverb tank input is not-grounded to the pan, so no ground there.
R39 is a 47-ohm resistor that "senses" the current across the 680-ohm load comprised of the 4.7k resistor and the 800-ohm tank.
The schematic indicates we should see 3.44VAC on the tank input, and 237mVAC at the opamp input.
If we plug the numbers into a little math:
3.44VAC/680-ohms = 5ma AC
And using the op-amp formula that I-out = Vin/R-sense
0.234VAC/47 = 5 ma (4.978ma).
C18 is there to kill the DC off the output. It's a high-pass, in the 100hz and up range, basically a short to ground.
I'm not entirely sure why D14 is there. It would conduct away any negative impulse or kickback from the tank that exceeds -15v (the opamp is powered by +/- 15v regulated rails). Why there isn't a similar clamping diode to +15v, I don't know.
Simple circuit, math seems to checkout.
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Re: Deep dive, reverb drive circuits and parameters
What about that ROHM RA4560...
Datasheet here:
https://fscdn.rohm.com/en/products/data ... 0xxx-e.pdf
Digging through the many pages and graphs, it seems to indicate that this opamp can swing +/- 12v and maybe +/- 12ma into a 700-ohm load.
That tells me, this amp is pushing that opamp to about half its capability with a 10mv guitar signal and all the knobs at mid-rotation.
As we know, your typical Tele/Strat is around 200mv and humbuckers ~400mv, maybe more.
That's some 20x-40x at the input jack and that would be true throughout the signal path of the amp, including what arrives at the opamp, assuming there's headroom enough that it doesn't clip.
My question is what happens to this opamp if it is driven to and past the limits? Doesn't really tell me in the datasheet what it happens, what "short circuit protection" does.
I will say, Fender has moved this exact reverb circuit to different spots in the Blues Junior, some with more or less drive to feed it. The schematic I posted is a Blues Junior III. The later IV moves the input one stage back in the amp's preamp section with no changes. However, it doesn't quite get as "wet" sounding there.
But I think the signal take-off point to the reverb drive in the Blues Junior III series (2003 and up to 2017) is too hot and may cause some undesirable side effects. That may be in line with what my friend is experiencing on the several of these he owns.
If R.G. or anyone else has some thoughts, I'd welcome your input.
Datasheet here:
https://fscdn.rohm.com/en/products/data ... 0xxx-e.pdf
Digging through the many pages and graphs, it seems to indicate that this opamp can swing +/- 12v and maybe +/- 12ma into a 700-ohm load.
That tells me, this amp is pushing that opamp to about half its capability with a 10mv guitar signal and all the knobs at mid-rotation.
As we know, your typical Tele/Strat is around 200mv and humbuckers ~400mv, maybe more.
That's some 20x-40x at the input jack and that would be true throughout the signal path of the amp, including what arrives at the opamp, assuming there's headroom enough that it doesn't clip.
My question is what happens to this opamp if it is driven to and past the limits? Doesn't really tell me in the datasheet what it happens, what "short circuit protection" does.
I will say, Fender has moved this exact reverb circuit to different spots in the Blues Junior, some with more or less drive to feed it. The schematic I posted is a Blues Junior III. The later IV moves the input one stage back in the amp's preamp section with no changes. However, it doesn't quite get as "wet" sounding there.
But I think the signal take-off point to the reverb drive in the Blues Junior III series (2003 and up to 2017) is too hot and may cause some undesirable side effects. That may be in line with what my friend is experiencing on the several of these he owns.
If R.G. or anyone else has some thoughts, I'd welcome your input.
Re: Deep dive, reverb drive circuits and parameters
R39 and C18 form a sloping HF filter in the feedback network so the output tilts, emphasizing highs to overcome the inductive nature of the coil. I'm not sure what "current sensing" is happening.
It looks like they saw overdriving caused latchup and moved the input to reduce the input level.
There are many opamps sharing that configuration. Maybe try a 5532 or a more stout opamp, as it's driving a a tough load.
R.G. will certainly have the best idea here.
It looks like they saw overdriving caused latchup and moved the input to reduce the input level.
There are many opamps sharing that configuration. Maybe try a 5532 or a more stout opamp, as it's driving a a tough load.
R.G. will certainly have the best idea here.
Tube junkie that aspires to become a tri-state bidirectional buss driver.
Re: Deep dive, reverb drive circuits and parameters
The circuit is interesting. It is designed to drive the reactive load of the coil with a constant amount of current for any input of V-in signal.
Consider this simplified schematic. Whatever current flows through R_Load develops a corresponding voltage across R_Shunt. R_Shunt is the "sensing" resistor in this current flow.
The voltage across R_Shunt is applied to the inverting input of the op-amp as negative feedback.
The op-amp's job in life is to make the inverting input the same voltage as the non-inverting input.
So the current flow into R_Load is determined by the formula:
I = Vin/R_Shunt
^^^ This is the defining equation.
Essentially regardless of the actual value in ohms of R_Load, up to the limits of the opamp and its supply voltage.
If you put 1 volt at V-Input, the opamp is going drive the output positive until the inverting input voltage is also 1-volt. That will happen when current through R_Load + 47-ohms = 2.12766ma, no matter what R_Load value is in ohms.
If we take the nominal 1khz value of 680-ohms of the tank coil and the resistor in parallel with it, we need the opamp to put 1.4468 volts into R-Load.
Hence we get constant-current drive of the reverb coil no matter the frequency and reactance of the coil at that frequency.
Consider this simplified schematic. Whatever current flows through R_Load develops a corresponding voltage across R_Shunt. R_Shunt is the "sensing" resistor in this current flow.
The voltage across R_Shunt is applied to the inverting input of the op-amp as negative feedback.
The op-amp's job in life is to make the inverting input the same voltage as the non-inverting input.
So the current flow into R_Load is determined by the formula:
I = Vin/R_Shunt
^^^ This is the defining equation.
Essentially regardless of the actual value in ohms of R_Load, up to the limits of the opamp and its supply voltage.
If you put 1 volt at V-Input, the opamp is going drive the output positive until the inverting input voltage is also 1-volt. That will happen when current through R_Load + 47-ohms = 2.12766ma, no matter what R_Load value is in ohms.
If we take the nominal 1khz value of 680-ohms of the tank coil and the resistor in parallel with it, we need the opamp to put 1.4468 volts into R-Load.
Hence we get constant-current drive of the reverb coil no matter the frequency and reactance of the coil at that frequency.
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Re: Deep dive, reverb drive circuits and parameters
I wish I could be more help with specifics. Reverb tanks are odd speakers driving strange air (the springs) and picked up by odd, low output pickups. There is a lot of variation in the units, even ones that are supposedly compatible.
I think you're right, the driver IC is not doing a good job. I've had many issues with this in the past. The best all-solid-state driver I found was the one from the Thomas Vox V11x3 series. This was one gain transistor driving an AB pair of TO220 transistors from a 24V supply so voltage and current were not problems. I think you're going to find that opamp drivers are going to hit their limits in most configurations. Maybe one of the baby power amps, bigger brothers to the LM386, would work. A hybrid with an opamp driving an NPN/PNP output pair might cure the limiting problems.
I quit using tanks some time back. Got tired of the issues. I worked up a semi-universal reverb tank replacement using a Belton reverb module and a couple of opamps with attenuators and variable gain that can match most drive/recovery signal levels on real tanks. I put a soft clipper on the input to the module so it won't be overdriven on any signal level.
I think you're right, the driver IC is not doing a good job. I've had many issues with this in the past. The best all-solid-state driver I found was the one from the Thomas Vox V11x3 series. This was one gain transistor driving an AB pair of TO220 transistors from a 24V supply so voltage and current were not problems. I think you're going to find that opamp drivers are going to hit their limits in most configurations. Maybe one of the baby power amps, bigger brothers to the LM386, would work. A hybrid with an opamp driving an NPN/PNP output pair might cure the limiting problems.
I quit using tanks some time back. Got tired of the issues. I worked up a semi-universal reverb tank replacement using a Belton reverb module and a couple of opamps with attenuators and variable gain that can match most drive/recovery signal levels on real tanks. I put a soft clipper on the input to the module so it won't be overdriven on any signal level.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
Re: Deep dive, reverb drive circuits and parameters
Less is more, they say.
I think I figured it out, the amp is just hitting the reverb op-amp just way, way too hard.
The schematic says the DC and AC voltages are with all knobs in the middle, fat-off, reverb all the way down, and 10mv, 1khz at the guitar input.
What I found is increasing the input signal to 50mv was just enough to push the drive side of the op-amp into clipping. It did pretty much exactly what the ROHM datasheet says. Powered with +15/-15v and into a 700 ohm load, it will do 25vpp (+12.5v to -12.5v).
50mv isn't much of a guitar signal, and pushing it towards more normal levels of 100mv to 250mv, the opamp is far into hard clipping, basically turning a clean sine wave into a square-ish mess. That drives the reverb tank with mostly noise which really doesn't sound good and gives the perception of the "reverb cutting out".
The preamp node where the reverb send comes from is the 3rd gain stage. With only 10mv of guitar input, it's already producing 4.3vac. If you crank it up with the fat switch and the volume dimed with all that great tube distortion, the preamp is producing about 160vpp at that node. Way too much for the reverb section.
The fix I came up with was to reduce the drive into the opamp by halving the value of R38 from 68k to 33k, at the bottom leg of the 1m/68k voltage divider.
In practice it works with normal guitar and gain settings much better now, with more reverb, by driving it with less signal.
I think I figured it out, the amp is just hitting the reverb op-amp just way, way too hard.
The schematic says the DC and AC voltages are with all knobs in the middle, fat-off, reverb all the way down, and 10mv, 1khz at the guitar input.
What I found is increasing the input signal to 50mv was just enough to push the drive side of the op-amp into clipping. It did pretty much exactly what the ROHM datasheet says. Powered with +15/-15v and into a 700 ohm load, it will do 25vpp (+12.5v to -12.5v).
50mv isn't much of a guitar signal, and pushing it towards more normal levels of 100mv to 250mv, the opamp is far into hard clipping, basically turning a clean sine wave into a square-ish mess. That drives the reverb tank with mostly noise which really doesn't sound good and gives the perception of the "reverb cutting out".
The preamp node where the reverb send comes from is the 3rd gain stage. With only 10mv of guitar input, it's already producing 4.3vac. If you crank it up with the fat switch and the volume dimed with all that great tube distortion, the preamp is producing about 160vpp at that node. Way too much for the reverb section.
The fix I came up with was to reduce the drive into the opamp by halving the value of R38 from 68k to 33k, at the bottom leg of the 1m/68k voltage divider.
In practice it works with normal guitar and gain settings much better now, with more reverb, by driving it with less signal.