I am in the process of building a combo amp based on a LM3886 chipamp. The power amp section and power supply are built so now I'm focusing on the preamp section. Based on various posts I've read on here I've settled on the JFET based Fender Blackface clone found here:http://www.redcircuits.com/Page120.htm Now I'm investigating the best way to add reverb to the design. I would like something integrated in to the amp rather than an external pedal/box of some kind. The Stage Center reverb circuit seems like a pretty simple design that gets good reviews so I'm leaning that way. I have read the section on spring reverb in teemu's book but,as usual, I still have a couple of questions.

-Where should the circuit be placed in the signal chain? Obviously the stage center was designed as an effect pedal so it was intended to be placed before the preamp stage. However after studying various amp schematics (which include reverb) it seems the reverb circuit is placed between the preamp and the power amp.

-Assuming the reverb should be placed between the preamp and power amp stages, what modifications (if any) should be made to the standard Stage Center schematic? Is it sufficient to simply build the schematic as-is and wire it into the signal chain? Should the input buffer circuitry be removed?

-What is a good way to control reverb level? It seems the stage center unit has effectively two volume pots feeding into a summing amplifier. One pot attenuates the "dry" signal reaching the summing amp while the other attenuates the output of the recovery stage. Is the first pot necessary?

-Matt

QuoteWhere should the circuit be placed in the signal chain?

Where will you want it? If it's first in chain, all signal processing within the amp will also affect the reverberated signal. If your amp adds a distortion effect it will distort the clean signal plus the reverbs sound within it, which will sound very different from applying reverb to a distorted signal. Usually people prefer all these time-based effects (reverb, echo, delay, chorus etc.) as last ones in effect chain. Hence placement to about the last preamp stages makes sense. Usually the signal makes a parallel loop to reverb just before the volume control.

Ask yourself these questions:
- What is the impedance of the takeoff point? Will inserting a parallel path to reverb load this point extensively? Will you need a buffer?
- What is the impedance of the tank you want to be driving? Will there be a need for a semi power amp circuit to drive the reverb or is the circuit / device sufficient to drive the tank on its own?
- What is the input sensitivity of the tank (input signal needed to produce the maximum output from the tank)? This will vary a lot depending on the tank. Will there be a need to boost the signal from reverb takeoff point, and if so how much? Will this mean that you need to use higher supply rail voltage?
- Do you want the tank to have a brighter response? Reverb transducer is inductive, so when you drive it with a conventional voltage amplifier you loose power at higher frequencies due to higher impedance load, hence the reverb signal sounds darker. You can compensate this by turning the reverb driver stage to a current amplifier. What other voicing does the reverb circuit need?
- What is the output signal amplitude of the tank? Will there be a need to boost it to make it match the amplitude at mixing point, and if so how much?

You see, while reverb circuits are often pretty basic they need to be tailored to match your amp. Stage Center Reverb is a good design for a pedal (and for a certain type of tank), not so much a good design to be implemented to an amplifier or to be compatible with the numerous different types of reverb tanks out there.

For lowest noise and minimum interference, you want to drive the reverb tank with a fairly high signal and only use an amplitude control for the output signal of the recovery stage. You could also add tone controls there but most reverbs seem to be just fine without.

This amp (http://www.fender.com/support/amp_schematics/pdfs/Frontman_25R_Schematic_653.pdf), for example, has a pretty generic reverb circuit. Low-impedance signal takeoff point for the parallel reverb loop, then the actual circuit: current amplifier –type reverb driver amp and a basic non-inverting OpAmp recovery gain stage. Then a standard, volume control –type control for the reverb level at the output of the recovery stage and finally reverberated and clean signals mixed together with a simple resistive mixer. You'll find a similar circuit from plenty of other amps, the only differences being in component values or perhaps some slight variations in voicing the circuit. Some may also have discrete transistors at the output of the driver stage to boost the signal current. Some tanks need this. You could even replace the OpAmps with discrete circuits if you wish but I don't see much point in that, though.

I simply don't see any reason why to re-invent the wheel when you can just take a simple circuit like this and use it with modified component values that suit your preamp circuit.

In the circuit you posted you basically have two typical options for the reverb take off / mixing point, one after the tone control circuit but before the volume control, or one at the output. Both have benefits and drawbacks, when you think about it. I'd be inclined to turn the "volume control" into a "gain" control (no changes involved whatsoever unless you want to put some series resistance to P4 to limit the range from going to total zero), then put another potentiometer, "master volume", to the output, which is preceded by the parallel reverb loop. The 30V supply rail can be split to half for symmetric 15V power supplies for OpAmps.

Thank you for the feedback! Excellent info as always.
I was already planning to add a master volume control in addition to the volume already in the circuit. I'm not sure what the output impedance of that blackface clone preamp circuit is though. The description mentions that it is fairly high, but I'm not sure how to calculate it for an SRPP circuit. Presumably a buffer will be required at the take-off point for the parallel reverb circuit. Here are some follow-up questions I have:

-Should I buffer the signal before the junction where the parallel reverb circuit splits off, or should I just buffer the reverb branch of the circuit? The schematic you linked to seems to buffer before the junction (U3-B).

-Assuming the buffer is placed before the point where the two parallel circuits (dry and reverb) diverge can I simply tie the output of the blackface preamp to the input section of U3-B in the fender schematic? (including C22 and R36 in the feedback loop of U3-B) and then tie the output of U4-A to the master volume pot? Will U4A have a low enough output impedance to maximize signal transfer to the power amp? Or will an additional buffer be required?

-I can't seem to find any reference on the fender schematic regarding what type of reverb tank to use. A quick search online hasn't netted me anything but I'll keep looking. If anyone knows the model (presumably an accutronic?) I would love to know.


Sorry for all the questions. I'm still trying to get off the ground with all this stuff
-Matt

ok, I spent some time studying the schematic you sent and would like to get your feedback on an idea.
I've attached a schematic to show what I'm thinking... Please excuse any gross errors as I'm still learning 

Here's what I did:

• Copied the reverb section from the Fender Frontman 25 schematic from U3-B to U4-A.
• Converted U3-B to a buffer rather than an inverting gain stage. It seems fender is using this (along with U4-A ??) to recover losses from the tone stack. In my case the preamp already has a gain stage to recover those losses so no gain should be required (??)
• Converted U4-A to a buffer as well. For the same reasons stated above. It seems this was used as a buffer to combine the dry and wet channels as well as to provide some additional gain to the combined signal. I don't think I need that additional gain due to the gain provided by the preamp.
• Added a master volume control after the final buffer.

So, that's about it... does the circuit seem reasonable or am I missing the boat with regard to the purpose of U4-A and U3-B in the fender schematic? One concern I have is that I may need to adjust the gain of the reverb tank driver to avoid saturating the input coil but I'm not really sure if that's needed. The gain currently seems to only be 2x... Can I simply prevent saturating the tank input by choosing the impedance of the input coil carefully?

Edit: One more question.. is there any particular reason fender would choose to use two different types of op amp in the driver circuit? Can I get away with a pair of NE5532 op amps or a single TL074?

-Matt