Spring Reverb Circuit Problems

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#22
Tell that to Guitar Amps makers all over the World 😉
Also cheesy PA head makers, organs, etc. 😲

Serious, it all boils down to impedance (and lowered expectations 🙄)
TL072/MC1458 driven tanks use roughly 600 ohm input tanks, which is on the edge but still usable, sort of.

A few over the top, no-expense-is-too-much brave souls (such as Marshall) 🙄, splurge and use 2 or even 3 TL072 sections in parallel driving 150-250 ohm tanks, go figure 😄
 
#23
Gonecat said:
What's odd is that I am getting input signal at the tank input,
Click to expand...
I bet you are measuring from tank input Hot terminal to ground.
Remeasure across the input coil, I bet you have a wiring problem there.
BSST said:
Maybe connect driving opamp to inout coil via a blocking cap?
Click to expand...
No, you need a full DC path here or Op Amps gets unbiased.

BSST said:
Since the input coil is isolated, I suggest grounding its lower end. You'll get a bit more power by not driving through the 47 ohm resistor. Maybe make the 910k a jumper instead. The 330pF may be too small...
Click to expand...
Please don't, driving Op Amp will become open loop, then clip-distort-oscillate-slam against a rail-whatever.

NFB gain is [tank impedance]/47 ohm.
We all know what happens when you divide by Zero 😲

IMG_20240129_091326.jpg


That reverb circuit works fine, is used in a Million amps and in fact is recommended by Accutronics-Belton themselves on their site.
There must be a wiring or PCB error.
 
#24
There's a fine reason why the basic drive circuit is designed this way: A reverb tank needs to be driven by current (high source impedance), not by voltage (low impedance). And as JMFahey stated, a TL07x is too whimpy to drive a coil of the given impedance. You'd need a small power output stage, consisting of a properly biased complementary transistor pair. Or you'd replace the opamp by a NE553x, which is more up to task.

Unfortunately Accutronicsreverb.com have removed their drive and recovery circuit suggestions from their website.

Best regards!
 
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#26
A TL072 is NOT a suitable reverb tank driver!
"Tell that to Guitar Amps makers all over the World 😉Also cheesy PA head makers, organs, etc. 😲Serious, it all boils down to impedance (and lowered expectations 🙄) TL072/MC1458 driven tanks use roughly 600 ohm input tanks, which is on the edge but still usable, sort of."
OK. I'll tell them: A TL072 is NOT a suitable reverb tank driver! Take a look at the TL072 datasheet from Texas Instruments, page 21 Figure 6----the TL072 struggles when driving ANY load below 10K ohms, and at 600 ohms, it will only go to 2/3 of the supply voltage! That's terrible performance.
 
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#28
dotneck335 said:
A TL072 is NOT a suitable reverb tank driver!
"Tell that to Guitar Amps makers all over the World 😉Also cheesy PA head makers, organs, etc. 😲Serious, it all boils down to impedance (and lowered expectations 🙄) TL072/MC1458 driven tanks use roughly 600 ohm input tanks, which is on the edge but still usable, sort of."
OK. I'll tell them: A TL072 is NOT a suitable reverb tank driver! Take a look at the TL072 datasheet from Texas Instruments, page 21 Figure 6----the TL072 struggles when driving ANY load below 10K ohms, and at 600 ohms, it will only go to 2/3 of the supply voltage! That's terrible performance.
Click to expand...
Sure enough!
But that 2/3 drive must somehow be "enough" for many, since so many use that circuit.
I am not talking technical excellence on which I fully agree with you, but what I see looking around.

Music Man:
Screenshot_20240129-222027.png


Randall:
IMG_20240129_223418.jpg


Laney, which splurges and uses two halves of TL072 🤣
Their stock value will drop at NYSE 😄
IMG_20240129_222720.jpg


Marshall has been rumored to be using three Op Amps to drive a tank in their top line models 😱

Of course, a few do it right.
Enter the King of SS amps: Moog designed Lab L5, used by BB King:

labl5reverb-png.1003868


No, that is not the power stage, it's just the reverb driver.

The real power stage uses eight TO3 metallic transistors for some 200W RMS
 
#32
You have to remember this is cutting edge 1970's tech, the NE5534A would have been too expensive, and the 741 doesn't invert phase on overload (unlike the TL072), but then again the 741 has an abysmal slew-rate so neither of them are much cop for high level signals. Seems careless to not have wrapped the feedback loop round the discrete driver stages though...
 
#34
dotneck335 said:
Is that really a 741 opamp feeding the push-pull drivers? I most certainly would NOT call THAT the right way to drive a reverb tank..........
Click to expand...
What should they use then? 🙄
A Crown DC300?
A 9kW Lab Gruppen amo? 🤣

In 1970 LM741 was the only poker game in town, period.
TL072 or NE553x weren't even on their Father's scrotum yet, loosely translating an old Argentine expression. 🤷
Lots of high quality equipment, including Recording Studio equipment, used them.
The first Audio dedicated dual Op Amo was RC4739, and it went into a dead end, then "improved dual 741" RC4558 with much better Audio specs was created, and took the World by storm.
Used even today, 50 years later.

741 was a Godsend the time, I used them since Day 1, starting with individually boxed round metallic TO5 case, 8 gold plated legs, they were expensive.
IMG_20240130_161307.jpg
 
#35
“What should they use then?”

The AD815 DSL line driver. Gotta get that slew rate into audiophile territory.


Its driving metal spring, for Crissakes. Damn 741 is fast enough. Just needs a little more kick to deal with limited driver coil inductance.
 
#36
I didn't realize we were limiting the discussion to 1970 designs. Yes, at that time, the 741 WAS the only opamp available; and, even in the late 70s, NE5534s were about $5 a pop, so they would have been too expensive. And it is true that high fidelity performance is not necessary to drive a reverb spring.
But, with the availability today of pretty low cost, high performance opamps, I'd still not use a TL072 (or a 741). A 59¢ NE5532 will drive a reverb tank (≥ 600Ω) just fine, methinks.
 
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#37
I was 'out of the office' this past week and how cool to find all of this great discussion upon return!
You guys are great! I swapped out the TL072 for a NE5532 and bammo, i started to get signal! So my tank isn't shot, afterall.

At that point, I could trace the problem a bit further - a bad pot (replaced), and I don't think I need that 470k or .0033 cap (far right on the schematic, the output of the tank's Recovery circuit), the volume is pretty darn low even when I jumper those guys.

How can I bring the tank circuit higher up in level?
 
#38
Here's the updated schematic.

With regards to getting more level, I'm going to try messing with the resistors (2k and 330k in the recovery circuit) and see if that gets my wet level up.
Might try increasing R2 (in the top part of the schematic), as well. I'm just guessin' around at this point!

I've put the (DC) resistance levels I got from the tank on this schematic. Since they are very different from the original schematic, any suggestions for impedance handling? Is that 4.7k the right value going into the tank? I'm definitely getting a lesson on this one!
 

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#39
In contrast to your first posting it appears you've got an 8 Ω input impedance reverb tank. No opamp ever will suffice to drive it alone. You definitively need a more powerful drive circuit (add a »power« stage, consisting of a complementary transistor pair etc.). No use to mess with the recovery.

Best regards!
 
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