Check my Opamp FX loop design for a tube amp!

[Morocotopo]

Hi guys, wanted to run this by you to get some opinions.
It´s an external FX loop for my tube amp, sort of a SS Dumblelator. The idea is that the thing will go into the pedalboard. The amp has pre out/power in jacks, just out directly from the last preamp´s triode cathode via blocking cap , and in into the PI, also via cap. Since it will be powered from 9V, I added a power upverter to get around 31V, to maximize headroom, with a few R.G. tricks thrown in. First a buffer to get low impedance, then a gain recovery module to get the level up for the power amp. Some values are yet to be defined, to define high and low bandwidth limits. The idea is, of course, that it doesn´t add any type of distortion or coloration to the signal.
Schem:



So, what do you think? Some values are still guesses (is that a word?). I could add a parallell/serial option, bright switches like the Dumblelator on the volumes...

Opinions welcome!

[defaced]

I have zero qualifications to comment on your circuit, but something did strike me. 

The amp has pre out/power in jacks, just out directly from the last preamp´s triode cathode via blocking cap , and in into the PI, also via cap.

That sounds an awful lot like an effects loop to me, less the send/return level controls.  What is the amp?  Have you tried just inserting a pedal between those jacks?

[Morocotopo]

Defaced, it IS sort of a primitive Fx loop as it is, but has some problems. First, the pre out is high-ish impedance and high level, no good for driving cables or for the input of pedals, that expect a gutar-like signal, otherwise they distort and not in a nice way. Also, the power in requires a high level signal, to let the power section deliver full power. So this thing deals with those problems.
The amp is a homemade one, sort of a mutated Deluxe.

[defaced]

Gotcha, makes perfect sense.  I'm in the process of designing/testing a high voltage MOSFET FX loop so yea, been down those roads. 

[Thomeeque]

 How complicated would be for you to place it directly into the amp, it would save you some trouble (driving long cable by weak pre-out, need for supply voltage upverter..)?

Another advice would be to drop output volume pot and use pot for output stage gain adjustment instead (to get the best dynamic range and strong output).

T.

[Morocotopo]

Thomeeque, the amp is arleady finished, I don´t feel like making another hole, rewiring, etc etc, an outside unit would make it simpler and also I could use it with other amps, or as an independent buffer, gain stage, etc, an utility box of sorts in the pedalboard.
I haven´t decided yet if the gain stage will be a fixed gain one plus vol control at the output or a variable gain one. You make a compelling argument, though (dynamic range/out level).
Thanks. I ´m gonna breadboard.
Anyone can comment on the in/out impedances as it is? Some values I stole from other circuits, some I figured out myself. For example:

- The in Z would be 1M, right? Good for a signal coming from a tube´s cathode?
- The send Z would be 100K, right? About the value I´ve seen in many pedal´s out...
- The return Z would be 51K, right? Good enough for a typical pedal´s out impedance?
- And the out Z would be 500K, right? Good for a LTPI´s input?

[DougH]

I was going to do something like that. Then I realized all I really needed was the recovery stage so I perfed that on a daughter card and installed it into the amp. So the old master vol is now the "send volume" and the  vol on the recovery stage is the new master volume. Powered it with the 24v relay supply and it works like a charm. My send was already buffered so I didn't need to do any work there. I used an idea from O'Connor's "The Ultimate Tone" vol 1 which has a pretty good chapter on fx loops.

[Thomeeque]

- And the out Z would be 500K, right? Good for a LTPI´s input?

Z1 is output impedance of op-amp (very small, hundreds of ohms max)
Z2+Z3 divider is your volume pot (let's consider decoupling capacitor big enough to be ignored)

Zout = (Z1 + Z2) parallel with Z3

Maximum is when Z1 + Z2 = Z3 ~ pot is set in the middle of it's value range (we can ignore Z1 here as it is much smaller then Z2 now).

So maximal Zout for 500k pot is 250k parallel with 250k = 125k (quarter of nominal pot value)

T.

[Morocotopo]

Tomas, very clear explanation of opamp output Z. Will keep it filed. Thanks!

Now, it´s sightly embarrasing to say, but...



While poking around my hard drive, I found a schem for the same thing I propose here. Ooooooooooops! Memory failing at such an early age!!! Once I saw it, a doubt assaulted me: Maybe I already made such a thing! I looked into the "PCB´s that never made it into a finished state with box and all" drawer, and sure enough there it was. Well, what can I say? Nothing, of course...

Well, I tried it and it works ok. Here´s the schem:



Of course this is a FET based one, a different animal. I don´t recall whether I put it together or I got it somewhere else. I still have to test it at high signal levels, to see how it behaves. A couple questions:

- The 33K R and 220p C in the input, are they needed? Radio freq filter perhaps?
- The 5K pot at the input I replaced it with a 100K pot, because I believe this defines the in Z in parallell with the 1M R. Am I right?
- The send needs a 1M or so R after the 470n cap I think.
- Regarding the difference between op amps and FETs, this could in theory, behave better in overload but have more inherent distortion than an op amp based one, right?

Well, the loop saga goes on. Any opinions welcome.

[defaced]

Would it be beneficial to have a buffer stage after the return amplifying stage FET? Or would this be similar to pedals where a buffer at the beginning of the signal chain buffers everything after it (within reason)?

[PRR]

> The 33K R and 220p C in the input, are they needed? Radio freq filter perhaps?
> The 5K pot at the input I replaced it with a 100K pot

Good ideas.

R10 could perhaps be 100K also.

> The send needs a 1M or so R after the 470n cap I think.

No. 10K-50K would be reasonable.

> this could in theory, behave better in overload but have more inherent distortion than an op amp based one

Given 30V(!!) supply, neither plan should ever overload.

However the zero-biased JFET buffer will swing (for J201) less than a volt negative. For utter overload-proofing, return R2 gate bias to something near half of supply voltage.

R9 may be excessive for 30V and J201 biased with 820 under it. _My_ suggestion is to fix R8+R9 at say 47K, then hand-select R7 in the range 47r-1K to get Q2 drain near half-way.

[Morocotopo]

Defaced, I don´t think a buffer would be needed after the return gain stage, it goes into the amp´s phase inverter, and that has a highish in Z, although I´m no expert. As it is, it works good as far as I can tell.

Paul, thanks for the suggestions. Regarding the buffer FET bias, I though about how it´s biased and it seemed strange to me comparing it to some textbook examples, but since it works... maybe because of the high supply voltage, the negative swing limitation is "hidden" or not so audible... or maybe the distortion sounds good (it doesn´t have obvious clipping) so I find it not objectionable!. Anyway I´ll rebias it. Adding another 1M R to V+ at that point would do it, right?. I´ll try your other suggestions.

[Morocotopo]

OK, the saga goes on. I´d like to edit the topic´s title, to better reflect  where it´s at now, but I can´t.
This is the schem as I modded it according to Paul´s suggestions:



Currently on the breadboard, it works good. Rebiased buffer, bigger in/out caps. Pot´s values are what feels better in each position to control volume. What else? Added bright switch at the ret pot, rebiased the return gain stage by changing the source R. I believe ´those are the main things. Still have to test it at "preamp on 11" volume, but that´s not a setting I´ll use too much. Now onto making the PCB.
Anyone sees anything wrong?

Thanks all for helping.