Is there anything obviously wrong with my reverb franken-circuit?

[deadlyshart]

Hi everyone. I finally put together my reverb pedal and it works, but I'm getting some pretty nasty (unintended!) noise/buzz. I'm referring to this as a "franken-circuit" because I kind of put it together from two other schematics I've found: first, the very vague and sparse mono circuit from the BTDR-3 manual, here: http://www.tubeampdoctor.com/images/File/BTDR-3%20DIGI-LOG%20REVERB%20MODULE.pdf



the other is the circuit from 1776effects.com rub-a-dub deluxe (RADD) reverb cicuit:



So, in putting them together, I didn't change a ton. However, I think I changed/forgot just enough to make it crappy, and I'm wondering if anyone can see just from this what the main offenders are likely to be. It's still mostly based on the BTDR manual schematic, but I used the power supply section from the RADD schematic (a voltage-divider), as well as adding capacitors across the op amps as they did in the RADD circuit. I also changed the 10k dual pot from the BTDR manual schematic that controls the reverb depth to a 100k dual pot. Here's what I assembled:



If you compare the power supply section I assembled to the RADD circuit, I changed a couple things, mostly based on what I had at the time (I was really excited and impatient to put the circuit together...stupid, I know). I got rid of the 33 ohm resistor that was directly in series with the 9V supply since I didn't understand why it was there, and I changed the 100uF cap that was across the 9V and GND to 47uF. I also (stupidly) left out a capacitor that was across the regulated 5V and ground.

So I know probably at least a couple of these things I changed or left out are important and maybe the reason the circuit sucks. I was wondering if anyone could tell me about a few parts of this:

• Why did they have that 33 ohm resistor directly after the 9V input in the RADD circuit?
• I know that they often put caps to smooth out voltage fluctuations, so I get why they put C7 in the power supply section, but why did they also add C6? Do you really need a cap for each different bias line? I'm guessing I really shouldn't have skipped adding the cap (C8 in the RADD circuit) across the 5V output and GND?
• How critical are the exact values of those caps, like C6 and C7?
• In the original RADD circuit, they have caps C2 and C4 across the op amps. From what I understand, those essentially make it a low pass filter, right? Why do we want to be cutting off high freqs? Are the values of those caps very critical? I just used the values they did, but they also have different resistor values.
• My friend told me that those 1M resistors in the RADD circuit, R1 and R7, are "bleed resistors", to let charge buildup dissipate. Are they very necessary? How does charge buildup happen anyway here?
• I noticed that they had a cap in series with the output in the RADD circuit, C3. I realized that that's actually probably really crucial, right? Because I'm operating the op amp from 0-9V, so the signal is around 4.5V and the op amp output is going to have a large DC component without that cap...am I understanding that correctly?

Sorry for all the questions, I'm trying to get my head around this stuff. Any advice at all is appreciated, thank you.

[samhay]

The major problem you have at the moment is that the input to the output mixer (second op-amp) is DC-coupled to the input.
At a minimum, put a cap in series with the 10k feeding the second op-amp.

However, the input impedance of our design is very low for guitar work. Not a problem if there will always be a buffered pedal in front of it, but if driven directly by a guitar, it will tone suck.

Why don't you want to build the RADD?

[deadlyshart]

Ah, of course... is that just an error in their schematic, because it's not meant to be very detailed?

Sorry, I'm pretty new to this... why does the input impedance affect how it will be for use with a guitar? (my very naive guess is that the guitar has a high impedance, so if the input impedance is low, it won't be well matched and you'll lose some range of frequencies..?)

Oh! I didn't realize that you were the person who designed the 1776effects pedal! Now I feel bad for using your schematic without buying the kit 

Honestly, it mostly wasn't that conscious of a decision -- I just started with the BTDR manual one because it was simple and what I first had. Also, I kinda wanted a simpler thing: the RADD has 4 pots (unless two of them are combined, but I don't think so), while all I really wanted was a reverb depth, and dry/wet.

[thermionix]

No springs, no tubes. 

[anotherjim]

• Why did they have that 33 ohm resistor directly after the 9V input in the RADD circuit?
• I know that they often put caps to smooth out voltage fluctuations, so I get why they put C7 in the power supply section, but why did they also add C6? Do you really need a cap for each different bias line? I'm guessing I really shouldn't have skipped adding the cap (C8 in the RADD circuit) across the 5V output and GND?
• How critical are the exact values of those caps, like C6 and C7?
• In the original RADD circuit, they have caps C2 and C4 across the op amps. From what I understand, those essentially make it a low pass filter, right? Why do we want to be cutting off high freqs? Are the values of those caps very critical? I just used the values they did, but they also have different resistor values.
• My friend told me that those 1M resistors in the RADD circuit, R1 and R7, are "bleed resistors", to let charge buildup dissipate. Are they very necessary? How does charge buildup happen anyway here?
• I noticed that they had a cap in series with the output in the RADD circuit, C3. I realized that that's actually probably really crucial, right? Because I'm operating the op amp from 0-9V, so the signal is around 4.5V and the op amp output is going to have a large DC component without that cap...am I understanding that correctly?

Sorry for all the questions, I'm trying to get my head around this stuff. Any advice at all is appreciated, thank you.

Ok,
1: 33R in series with power feed makes the input cap a better noise/ripple filter. It can also act as a sacrificial fuse if it's power rating is just enough to stay cool in normal operation. It also reduces inrush current when first powered up -  a discharged capacitor is a short circuit!
2: The circuit returns signal via Vb, so Vb also needs to be a low impedance return path for the signal. Signal is AC which passes right thru C6 "bypass" cap rather than being restricted by the resistors in the divider.
3: Bypass caps are usually oversize. We don't always know how big they need to be, particularly the power input one. Most designers seem to fit big but not silly big and 10u -470uF are typically found. Too big on Vb takes a long time to charge up. Too big on +9 is a large surge current when the PSU first gets connected to it - but that 33R eases that some. And really silly too-big won't fit in the box.
By the book, those regulators should have 100nF ceramic cap to ground on in and out pins. These guard against the regulator oscillating. I've never seen that happen but I've never not fitted them.
YOu do need to add a large cap on the brick 5v input as the original schemes both show.
4:Those feedback caps on opamps are low pass filters -  combined with the feedback resistance that sets the corner frequency. When you see pF cap values, unless the feedback resistance is high <100k, the filter is only intended to stop radio frequency interference getting in or out (the delay chips in the reverb brick run at radio frequency). You can usually get away without them, but commercial designers tend to include them.
5: 1M anti-pop resistors. The charge your friend knows about is possible if the input caps are allowed to "float" when nothing's connected. The 1M tie to ground (aka pull-down) keeps the input side of the caps at ground whatever and what is connected won't change that provided it too is referenced" at ground -  which it should be.
6: Related to 5. You have it right, the "coupling" or "DC blocking" cap removes the circuit DC allowing the final output to be ground referenced with a pull-down resistor.

[samhay]

>is that just an error in their schematic, because it's not meant to be very detailed?

No, in non-guitar land, most op-amps are run off bipolar supplies. The data sheet example is following this approach - BTDR bricks are probably mostly sold to Karaoke manufacturers rather than stomp box builders.

>Sorry, I'm pretty new to this... why does the input impedance affect how it will be for use with a guitar? (my very naive guess is that the guitar has a high impedance, so if the input impedance is low, it won't be well matched and you'll lose some range of frequencies..?)

Short answer is that you will lose treble. Longer answer e.g. here: http://www.geofex.com/article_folders/bypass/bypass.htm

>Oh! I didn't realize that you were the person who designed the 1776effects pedal! Now I feel bad for using your schematic without buying the kit

Not me. I have designed other stuff, but not this one.

>Honestly, it mostly wasn't that conscious of a decision -- I just started with the BTDR manual one because it was simple and what I first had. Also, I kinda wanted a simpler thing: the RADD has 4 pots (unless two of them are combined, but I don't think so), while all I really wanted was a reverb depth, and dry/wet.

As an alternative, you could remove the volume control in the RADD and make R5 = 22k, and replace the tone control with a 4k7 resistor and remove the 220n cap...

[deadlyshart]

Thank you both so much for all the really detailed help. It makes a lot of sense now. Samhay, I'm actually doing what you suggested; I realized that what you said totally makes the most sense to do.

I have a really, embarrassingly basic question about this circuit, though... If the goal of the circuit isn't to amplify, why do we even have op amps? Why not just go directly into and out of the reverb chip?

Is it so you can have dry/wet control, because it wouldn't be able to mix the signal that goes through the BTDR with the signal that doesn't, without op amps?

[anotherjim]

I think the brick was intended to be supported by the product it is used in.  It relies on the rest to keep it safe. The brick is a sealed module which you would struggle to repair. I would rather some cheap op-amps got blown up by a high voltage zap from the outside than have to replace the module.
The module itself is not very hi-fi. It's a good plan to let the product give the dry signal a good quality path, so that part is left to you.