Hello all,
looking for help from someone who has a lot of experience in designing reverb drive circuits. I found many variations of the classic TL072/74 circuits. I decided to take one and modify it for my purpose. I got it up and running. Reverberation works fine! However as soon as i increase the wet/dry pot (R4), i get horrible 60hz hum!
The signals from U1A and U1d are fine, and clean. but when the tank circuit is introduced into the mix, the hum happens. I have put bypass caps all over the place! the spec for the IC states to use .1uf as close to the power pins. Which I did.
So what could be wrong here?
see my attached schematic.
looking for help from someone who has a lot of experience in designing reverb drive circuits. I found many variations of the classic TL072/74 circuits. I decided to take one and modify it for my purpose. I got it up and running. Reverberation works fine! However as soon as i increase the wet/dry pot (R4), i get horrible 60hz hum!
The signals from U1A and U1d are fine, and clean. but when the tank circuit is introduced into the mix, the hum happens. I have put bypass caps all over the place! the spec for the IC states to use .1uf as close to the power pins. Which I did.
So what could be wrong here?
see my attached schematic.
Attachments
^^^^^ THAT.
use long wires and move reverbb tankn around, dpeshum chhange?
Pickup coil is VERY sensitive and will pickup transformer hum even 12" away.
Fender Twin Reverb had to mount reverbb tank inside a padded bag at the bottom of the combe, below the speakers, 14" OUTSIDE the chassis ... and even so...
use long wires and move reverbb tankn around, dpeshum chhange?
Pickup coil is VERY sensitive and will pickup transformer hum even 12" away.
Fender Twin Reverb had to mount reverbb tank inside a padded bag at the bottom of the combe, below the speakers, 14" OUTSIDE the chassis ... and even so...
okay, thanks for the suggestions. I think probably the biggest issue is that I have the tank sitting about 2" from the power transformer! Its a very small box I have it all mounted in. Dang! and I do remember the Fender amps always have the spring mounted to the bottom of the box. I will do some experimenting with moving the tank away and see what happens. and try those other suggestions. will let you know!
That.
You have many bass cutting treble peaking at work there
1) you are driving a pure inductor (input coil) with a constant current signal from U1b .
Which creates a 6db/oct rising frequency response, increasing treble nd cutting bass big way.
That said, THAT is the way to drive a reverb tank.
Anything else gives you muddy boingy sound, so don´t change that.
2) R9 + 10uF (C4 combined with C5) also cut bass boost treble big time, again suspect that is done on purpose.
You may increase capacitors to, say, a maximum of 100+100uF in series, but don´t overdo it.
Too much bass signal makes reverb sound like door closer or bed mattress springs
3) C6 and R10 cut bass below 160Hz but believe me, you don´t want those frequencies hitting the springs.
As is, it´s an almost Miracle cheesy springs provide useful sound, don´t try your luck too much
I make my own reverb tanks and use springs designed to hang toys from the ceiling, go figure!
> The TL072 isn't designed to drive loads much lower than 2k, yet here its driving 150 ohms. Is that going to be an issue?
Everybody does it. (Well, some beef-up.)
The tank only needs ~~30mA to saturate. Most older chip opamps would do 10mA very accurately but 30mA clean enough for this job. The reverb is NOT the main signal path, a little distortion phattens-up the reverb, it's fine for a basic system. If you want ULTIMATE REVERB then the sky is the limit. (Considering the quality of presently available tanks, I would not bust my butt over it.)
Everybody does it. (Well, some beef-up.)
The tank only needs ~~30mA to saturate. Most older chip opamps would do 10mA very accurately but 30mA clean enough for this job. The reverb is NOT the main signal path, a little distortion phattens-up the reverb, it's fine for a basic system. If you want ULTIMATE REVERB then the sky is the limit. (Considering the quality of presently available tanks, I would not bust my butt over it.)
thanks for the info. I will look into experimenting with the caps. First I still need to figure out where the HUMMMM is coming from and eliminate this, or none of the other fixes will be of any use.
I got to thinking, I have seen commercial reverb units that are in small boxes no more than the size of a Fender bassman head. They don't have 60hz hum issues? so I wonder how they isolate the tank from the xfmr? I wonder, could I create a faraday cage for the tank to isolate it from the mains 60hzs?
I got to thinking, I have seen commercial reverb units that are in small boxes no more than the size of a Fender bassman head. They don't have 60hz hum issues? so I wonder how they isolate the tank from the xfmr? I wonder, could I create a faraday cage for the tank to isolate it from the mains 60hzs?
@JmFahey. I moved the tank about 12" from the power xfmr and the signal cleaned right up! sounds great! So brings me back to my last question. is there some way I can shield the xfmer from the tank? My goal was to make this fit in a small shoebox size enclosure. I am wondering if I make a faraday cage around the xfmer would it be shielded enough to block the flux waves?
okay, I fixed it. I was able to orientate (rotate) the spring tank 180 deg so that the input side was a far from the xfmer, and the hum dissipated. There is a slight hiss. but very minor. So now the only issue I have to deal with is to figure out how to get it so the gain is the same with and without the reverb engaged. Right now when I switch in the reverb, I loose some gain. and as mentioned before, I also loose some low end freq.
....10uF (C4 combined with C5) also cut bass boost treble big time, again suspect that is done on purpose.
You may increase capacitors to, say, a maximum of 100+100uF in series, but don´t overdo it.
Too much bass signal makes reverb sound like door closer or bed mattress springs
Spring Reverb "has" to be cut below 400Hz. I had a box that tried to hit 100Hz, and on full-range material it just sounded sick.
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