serf music – lab jc

master and surf… serf and turf? wall of verb

today there will be lots of pix to explain this… but i want to talk about reverb. not natural reverb. no. spring reverb! the “wall of sound”!!! i love it. dick dale… (bigot rock). Surfaris! the Beach Boys! ZOMBIES, Mary Weiss and the Shangrilas, Phil motherfucking Spector! Ronnie Spector (swoon!), Diana Ross, the jesus and mary chain, my bloody valentine. you get the idea. big wet luscious pillowy jangly spring REVERB, motherfuckers!

i did mention that i like it… of course it has an important relationship with 20th century popular music. and the electric guitar! but it was added as an effect to recordings and live performances with prejudice beginning in the american 1960s. spring reverb is created by driving an electromechanical transducer with a signal, that vibrates a long ferrous string (a steel wire). variations in the magnetic field near the transducer caused by the signal, move the wire. and if you coil up the wire nice and tight, into a spring, it can be much much shorter! which is important in a practical gadget. some distance away, another transducer senses the motion of the wire in it’s own magnetic field, which induces a current, in proportion to the motion (a signal), which is like the original but delayed by TIME. the time it takes for the motion to travel down the wire… of course there are reflections, and resonances and distortion and slewing… but it’s all kind of similar (and yet not at all) to what happens to air in a bounded field (a room). and it is pleasant to experience!

i will get to spring reverb tanks in a moment, and i have made one for this project, although you can simply buy one. they are a part of the guitar amp industry. the internet will provide more info. let’s just move on to what to do with it once you have one! the focus here will be for guitar but clearly adaptable to the larger context of sound effects for music.

i don’t just love spring reverb… i have strong opinions about how it oughta be, in order to be loved. the classic Fender style with the signal for the spring tank taken from an early part of the signal chain in a guitar amp, and then fed back in before the output stage is just disappointing, although still way way better than no spring reverb. i also want a LONG spring, and that means length! short springs are cleaner, and less resonant… and more dull and boring. all the weird splashy ringing space wave of a long multi-spring tank is the only thing to focus on if you are rolling your own. if you want middle of the road, you can just go buy it. also, it is WAY more interesting to drive the tank from the speaker end of things. in fact, i like to drive the tank directly from the output of the amp itself. and that means we can simply use another small power amp with it’s own dedicated loudspeaker… and let the main amp cut clear through without any muddyness from the verb… crunch! (wait for it….), whoosh…. yeah.

some older Ampeg amps did it exactly like this… and the designer, Jess Oliver, was a pioneer of spring reverb, and in my opinion the one to study, because there are many sonic advantages to doing it this way. but there are also disadvantages, which is why most guitar manufacturers NEVER do it like Ampeg did. the biggest problem is that guitar amps are simple and economical (cheaply made). noise isn’t ever really seen as much of an issue… and if you consider who it is that uses the shit, you can see why. if there is noise and unpleasant debris in the output of the guitar amp, there will also be an amplified facsimile of it in the reverb. more so because it is added twice. in addition to this, it costs more to make a whole other amp just to do verb. especially a tubed amp. but i don’t care about that.

putting the effect at the end of the musical amplification chain has one definitive plus compared to anywhere else: it mimics what happens in the material world. reverb happens AFTER the stimulus. separating these jobs makes for clear goals that cannot interfere with one another. guitar happens… then comes reverb. there is no interaction between the amps or speakers.

but this puts demands on the reverb amp that are much more extreme than on the guitar amp. guitar amps employ sloth. a reverb amp is like a really high end hifi amp, that adds it’s effect as precisely as possible, no matter what the source is doing. which brings us to the effect itself: reverb is what? it is more of the original, but later. it can have echos but is more about the first repeat than anything else, and a reaction to the stimulus of the first period. a long period: FAR longer than the periods of the musical program. seconds long. also the spectrum is towards the “warm” end of the range. that is, WAY more bandwidth limited than most guitar players have any idea about. think of the last time (if ever) you were in a church. there are almost NO high frequencies in reverb. but a lot of low-mids. some upper mids and low treble. there is some variation in the industry, but generally a range of 100 Hz to 6 KHz is more than enough. 3KHz to the top is completely workable and much like a church response. my own personal opinion is that it is also sometimes interesting to play with the “hall sound” of a really large space… and that means going all the way down. but that isn’t necessary for guitar. for synths and drums, it very much could be! in any case we need to drive the spring and amplify the delayed signal.

but the amp doesn’t need to be powerful… good efficient midrange drivers are what we are driving. and a watt or two is enough for most cases. the amount of reverb that is too much, is still far below the level of the main signal. we can parallel loudspeakers for better efficiency and put them near the floor to get some boundary reinforcement. more low end.

above is a stab at what i believe is appropriate. a couple of watts. octal tubes. a sharp eyed circuit maven will see Blöbaum’s screen cascode in Q1. crazy low distortion and, most importantly, noise. i like musical noise, don’t get me wrong! but not in my reverb. as far as parts go, this is not a demanding circuit. the Fender Champ type output transformer is perfect for this application. widely available and affordable. i am not going to put any feedback around it so i can use a shitload in front of it to accomplish the other goals i have. old radio trannies would also work fine! DC heaters remove a large source of hum from the output… and a low parts count compared with a push pull type amp. the one part to lavish are the power resistors in the buffer circuit. they have 100 volts across them and some mA of current. i used Caddock power metal film. good wirewounds will not hurt either. layout?

well, it has to fit in my box, so a horizontal mounting is smart.

and a chunk of aluminum makes an easy mount for the current sources, buffer and power resistors… and it’s easy to add perf board for mounting the circuit. there’s room for modifications that i will probably try out. one thing that comes to mind is a line level input. that will require a buffer to drive the tank… a nice extra! it could be solid state or hybrid to save space. then the reverb could be added on to all sorts of inputs: keyboards, synths, or mixes from a desk.

this is a clear beginning, with room on the left for the input stage, and room on the right for power supply.

separate transformers for heater and B+, mounted on neoprene and wrapped with mu metal keeps emf low and the vibration damped.

and don’t forget to ground that shield! this is also an opportunity to ensure some duct tape has a ceremonious place of honor. respect your peers! i really like the black “super sticky” type for this… i’m not a fan of glue inside a chassis. but duct tape? ooooooh!

boom! wire that shit up!

about two watts with 4 volts peak to peak input… it’s enough.

10 KHz is pretty at 2 watts.

clipping. classic single ended, and you can see the Champ tranny, is not bad. not great… but not bad.

Marconi – RCA 5693 and a 6V6… beautiful.

6SJ7Y (military) and metal 6V6. rugged, and no different from the previous tubes, measurement wise.

5693 and RCA 6F6G. my fave, but no significant measured difference from the others. there is so much error correction used in this circuit that there really shouldn’t be.

okay, so i need 4 clean peak to peak volts of signal to get to 2 watts. and the tank puts out about 50 millivolts pp with the level i get from the guitar amp.

below is the output of my tank with 22 ohms in series with the power amp and 14 Volts pp driving. this is a workable signal level for the make up gain stage. I think 50 mV peak to peak is enough to work with and still not have much pressure on the post reverb tank circuit for gain or noise. A bit more signal wouldn’t hurt but I don’t want to burn the input coil.

i have been pushing tetrodes and pentodes as a high performance solution for all things small signal. and to get from 20 – 50 mV to 4 Volts, a gain of 80 ought to do. that’s +36 dB. and that is full tilt wall of sound,,, that isn’t what we will normally require. we could actually probably get away with a gain of 50.

above is my prototype transconductance stage with a buffer and feedback back to the grid. my typical application. R12 can be increased to get the gain i need, and i bet 470k will work fine.