Need help for troubleshooting a modification to a reverb circuit on a guitar amp to an FX loop circuit

Question

I have a Fender Frontman 25R and I would like to use the reverb send and return as an effect loop instead. I don't mind getting rid of the reverb tank of the amp. So I found the schematic of the amp and here is the area of interest:

As you can see, R42 adjusts the reverb level and I would like to keep it for my general effect level. Note that P13, 14, 15 and 16 are not connected. On P17 and 18, I used the wire going to the point with a dash on top as the sleeve of a 1/4 jack plug, P17 for the send jack and P18 for the return jack.

I made a little bit of research and found an article discussing the topic. The writer gives the following schematic and explanation:

Simple, right? So, I was happy and thought my problem was solved. I made the little circuit, correcting what looks to me like a mistake (J1 sleeve goes to J2 tip in figure 2, which does not make sense to me but feel free to correct me if I am wrong) and it ... kind of worked.

I can plug a pedal in the then created loop but I have to push the level on the pedal and even then, the effect is not that much present even with R42 on 10. Plus, it changes the sound in an unpleasant way (thin and trebly) and adds quite a bit of noise to it as well. If I unplug the pedal, the sound goes back to normal.

Does anyone have an idea on how to fix this? I was thinking about an impedance or a signal level issue. If needed, I also have the reverb unit model number and specification document.

Answer 1

The things I would try:

1. Take the FX send between P13 and ground (not between P13 and P14)
2. Remove C22
3. Increase the values of C26 and C28; they make the effect sound thin.
4. You may actually want a serial FX loop: remove the connection between U3-B and U4-A. Of course when the FX return is unplugged you want the FX loop to be shortened. Something like this:

In that case you should bypass R42 and make R43 smaller.

If you want to keep it as a parallel FX loop, decrease the value of R43 to have the effect louder in the mix (something between 1k and 10k).

Answer 2

Op-amp U5a is driving the reverb spring input transducer as a current source. You have a couple of options: either convert the stage to a voltage source or add an appropriate value of load resistor so as to ensure the op-amp doesn't saturate.

The easiest might be to just add an appropriate load resistor. The schematic that shows one person's mods shows a 100R resistor - try that. Connect that 100R resistor between nodes P13 & P14. Take your send output from P13 (tip) and Ground (sleeve). Add a 100R resistor in series between P13 and the output jack if you are concerned about preventing damage to the op-amp if the output is shorted.

The input return has far too much gain: it is currently about 67.7. Far too much gain for a standard effects device. The easiest way to bring the gain way down is to simply lift one end of resistor R40. Still may have too much gain but give that a try first.

Answer 3

Let’s first try to understand what Fender is doing in their reverb circuit.

There is a large loss of high end signal going through the mechanical reverb springs so the high end needs to be greatly boosted on the driver side. This is done in the output driver stage which is a high pass filter with a lot of gain.

But Fender goes even further, again amplifying the high end in the reverb return circuit with another high pass gain due to R40 and C26. C28 then cuts some low end. All this to make up for the loss of high end in the reverb springs.

But in an FX loop we don’t want any of that. We just want unity gain and a flat frequency response.

The reverb send has an interesting twist, in that the “ground” pin (P14) isn’t tied directly to ground but to the negative input of the opamp. I suspect this is a trick to reduce (cancel) hum picked up by the cable to the reverb element. But you shouldn’t need that if you’re just driving a standard guitar cable to an fx box.

The output driver U5 is just a basic opamp that happens to be able to drive higher currents than a standard opamp but certainly doesn’t have to. It can be used directly to drive the fx send but it’s a good idea to put a small resistor on the output to provide some short circuit protection.

There’s also a light low pass filter (7kHz) in the previous stage. That has fairly negligible effect but you can reduce C22 to 47pF or eliminate it entirely if it bothers you.

The small C25 on the receiver is just for RF immunity and should be left in place – it won’t affect audio at all.

C28 (.015uF) is providing an unwanted cut in low end, but it also is providing a desired block of any unwanted DC offset imparted by the fx loop. So we want to increase it, not eliminate it.

I would suggest the following:

1. Remove R45 (or C29). This eliminates the high pass amplification on the send.
2. Add a 1K resistor to P13, take send’s hot output (tip) from other side of 1K.
3. Attach send’s shield to ground, not to P14.
4. Remove R40 (or C26). This eliminates the high pass amplification on the receiver.
5. Increase C28 to 0.1uF. This provides full audio bandwidth on the receiver.

Edit (after your comment that the signal levels are still too small):

There’s not really enough information in the schematic snippet to really know what the absolute levels are through the system so we’re left with some trial and error.

Before adjusting the gain on the return path we need to make sure the level we’re sending to your fx box is correct. You can check that by listening to your fx box output directly and/or looking at its input meter if it has one. If it’s too soft then we need to boost the send level.

There are two ways to do that. One is to increase R35 in the previous stage, but it's already pretty big so I'd opt to put the gain in the final stage. Do that by putting back R45, but connecting it straight to ground rather than through C29. The gain is 1 + (R44/R45). With R45 not present (infinity) the gain is one. R45 = 22K then gain = 2 = +6dB. Find the gain that makes your fx box happy.

Note: if R45 ends up being bigger than about 500 Ohms (likely) then you can actually put C29 back in: the rolloff won't be in the audio band and you'll leave your Frontman a little closer to original condition.

Your fx box should have a nominal gain of one, so now you can adjust the gain if necessary on the return path. Again, there are two places to do that. Decreasing R43 will work but only to a point: if it gets too small it will affect the linearity of R42 and you might have to make C28 even bigger. Ideally it stays bigger than the 50K pot.

The better place to add gain is in the opamp. If you restore R40, connecting it straight to ground instead of through C26, the gain of that stage is 1 + R41/R40. Making both 3.3K will give you +6dB and is a good starting point.