Playing with solid state reverb

[hesamadman]

Upon the arrival of some parts that I  have on backorder, I am going to toy with adding reverb to my quad el84 build. I want to keep it simple and cost down so I am trying a solid state reverb. Using an 8 pin IC Op Amp and a MOD 8EB2C1B reverb tank, I hope to achieve a working reverb in parallel with my signal just before the PI.


I haven't found anyone who has tried such a reverb but I would like any input any of you may have 

[2deaf]

Is it a dual op amp?

Any particular op amp? 

Are you going to drive the tank with one op amp and recover with the other or use both to drive?

How are you getting the +/- power?

[hesamadman]

Is it a dual op amp?

Any particular op amp? 

Are you going to drive the tank with one op amp and recover with the other or use both to drive?

How are you getting the +/- power?

It's a dual op amp. Driver and recovery. Small 20v transformer rectified, filtered, and dropped to 15vdc.

I ordered a TL072 and a couple more to try.

[hesamadman]

I am using this schematic for my power supply. Using an 18v transformer, not a 20v.




For my first test application, I am going to try to imitate a blues jr reverb circuit. Then see what I can do from there.

[hesamadman]

I changed my mind on power supply. Im going a different rout on the schematic.


The 15 power supply is just to power the OPamp correct? I was going to try a 24v 20va xfmr. Would this be sufficient?

[2deaf]

That regulated power supply is way overkill for your application.  20va is also way overkill.

I use a 36v transformer @ 65ma. 

[smackoj]

Hi; I guess I don't understand why you don't just use one of the hundreds of schematics avail. for SS amps with reverb circuits? To me that would be a sure way to know you have a working model. I have been messing with my Kustom K200B amp lately and the reverb  on it is quite lush. Just a suggestion. I also agree with prior post that you should use a simple set up for your PS. You could search for small filament transformers and find a good one for 15 bucks or so. That should power one or two op amps or transistors without breaking a sweat.

 

[hesamadman]

The only reason I would be using these transformers, though overkill, I have many of these lying around from low voltage applications at my job.


I am using an available schematic for this reverb. I am using a fender blues jr setup since the amp I am using it in is el84 driven, like the blues jr.


Im completely open to suggestions as I am building this not for the sake of having reverb in my amp but for the sake of learning reverb design into amp builds in general. I would love it if you shared a link to the model xfmr you guys would use so that I may research.


Thanks a ton for all your input.

[2deaf]

I use a Tamura 3FS-336 which is in stock at Mouser for $5.66.  If you don't like PC mount, Hammond has a 187A36 which Hammond says is unavailable but Mouser has them in stock.

You can use the transformers you mentioned, but you might have to go with a voltage doubler circuit.

[hesamadman]

I use a Tamura 3FS-336 which is in stock at Mouser for $5.66.  If you don't like PC mount, Hammond has a 187A36 which Hammond says is unavailable but Mouser has them in stock.

You can use the transformers you mentioned, but you might have to go with a voltage doubler circuit.

Thanks 2deaf.
Since this is my first reverb circuit I am going to go with what has been proven to work so I will use what you recommend.

[2deaf]

The op amp driver in the Blues Jr. is the same idea you see all over the place, including other Fender models.  It has the tank input as part of the feedback loop for the non-inverting op amp that is driving the tank.  When the impedance of the tank input is small compared to the resistor in parallel with it, the current through the tank input is fairly constant as frequency changes because the feedback is proportional to the impedance of the tank input which is proportional to the frequency.  Overall, this setup can do an impressive imitation of a tube/transformer driver.

There are a couple of things to start with that are probably obvious, but I'll mention them anyway.  I use sockets for chips because I can't solder IC's without ruining them.  The TL072 can be replaced by a huge number of other op amps without affecting the performance of your circuit.  You can't ground the tank input because it is part of the feedback loop.  You can use a two conductor with shield that has the shield connected to ground at the amp, but not at the tank.  Or you could use a single conductor with the shield hooked to the junction of the resistors in the feedback loop.  Whatever you do, the phono plug cannot touch the tank housing.  On older tanks, I had to cut a special little insulator to keep the plug from touching.  Newer tanks have jacks that are already insulated.

[smackoj]

I did not know the Blues Jr. had a ss reverb circuit in it. Not surprised to see it though. Here is a link to an old school style ss Standel amp that has an easy to read transistor powered reverb circuit. Worth taking a look at if you prefer to work with transistors.

http://el34world.com/charts/Schematics/files/standel/standel_custom_studioguitar.pdf

 

[2deaf]

Here is another discrete transistor driver from my notes.

[hesamadman]

Thanks guys!!!


How do you go about installing your reverb components on your builds? I wouldnt think you could get an IC socket on turret lugs so I was trying to figure out the best way to assemble the reverb board.

[2deaf]

Now this is where Radio Shack folding really sucks.  They had a couple of boards that were perfect for this application.  Mouser has hundreds of boards.  Search under "prototyping", otherwise you'll never find them.  Ignore anything over $10.00.

[2deaf]

Try Mouser part #854-SB400.

[hesamadman]

I just placed an order for some of these components. Ill report back after I throw this reverb in the circuit 

[hesamadman]

That regulated power supply is way overkill for your application.  20va is also way overkill.

I use a 36v transformer @ 65ma.

I built this power suppply. I had one question...and maybe this is what you needed in your schematic but I need 15 volts DC. Youre power supply has a leg to +vcc and a leg to -vcc. Those two legs combined will give you around 30v. So my question is.....for me to get my 15vdc do i use the top leg and the center tap for my -vcc?

[2deaf]

TL072 uses a +/- power supply so +15V (+Vcc) connects to pin 8 and -15V (-Vcc) connects to pin 4.

[hesamadman]

TL072 uses a +/- power supply so +15V (+Vcc) connects to pin 8 and -15V (-Vcc) connects to pin 4.

Threw me for a loop. Thanks man. Reverb is working great.

[hesamadman]

Thanks for all your help on this project. I didn't realize that some op amps require a positive and negative voltage. This was a huge learning experience. Lots of fun.

[hesamadman]

I was doing some reading on the reverb tank that I purchased and it says this about the drive circuit


"This drive circuit requires a floating input coil (i.e. insulated input connector)"

I dont really know what that means. Was hoping someone could explain this a little more.

[2deaf]

Blues Jr. Reverb for Reference.

[2deaf]

The floating input coil just means that the input coil in the tank input transformer doesn't connect to your circuit ground (like we usually do).  The input coil forms part of the feedback loop for the op amp.  If you ground the input coil, it destroys the feedback loop and the op amp goes wild.  Reply #10, second paragraph gives two options for connecting the cable.  On the above attached schematic, the junction of R46 (4.7K ohms) and R45 (47 ohms) is where you can connect the shield with a single conductor and shield cable.

The op amp feedback loop consists of a parallel RL circuit (R46 and the reverb tank input coil) in series with a resistor (R45).  The ratio of the impedance of the RL circuit to the resistance of R45 determines the voltage gain of the op amp.  The impedance of the input coil (L) increases as the frequency increases and so does the voltage gain of the op amp because the ratio of the RL circuit to R45 increases.

If the voltage and impedance in the Ohm's Law equation increase the same amount, then the current remains constant.  Therefore, when the impedance of L is small compared to R46, the current through the input coil doesn't change very much with frequency, which is what we want with reverb drivers.

When the frequency gets high enough that the impedance of the RL circuit is significantly affected by R46, the current through the input coil begins to significantly decrease, which is also what we want.  R46 also puts an upper limit on the voltage gain so that the op amp is not pushed into clipping at higher frequencies. 

[hesamadman]

The reverb is working well. I appreciate all the help. Are there any things that I can do to modify the reverb at all?