Help me diagnose an op amp reverb driver circuit I built for belton "digilog" module

[nickb]

Are you refering to the following link?
Which of the several documents on the page teemuk linked do you refer to?

I was thinking of this one BTDR_CarrierBoard.zip.

You don't have to fight the opamp limitations and power supply issues as it uses tube drive/recovery. Of course, you need to add a tube...

[J M Fahey]

Not answering to any particular post, since many different, even contradicting options were posted.
Simply back to the original intention of interfacing the Belton Brick with a tube amp, there's 2 basic ways to add it to an amp which was not designed with reverb from the beginning.
That's an important point, because if you are converting an existing, spring based design to digital, (which
I guess is Belton's intention), 80% of the work was already done, but if you are adding it to, say, a JCM 800, you have a lot of work to do.
The first way, already suggested here, was to use it at the front end, like any floor pedal.
Fine, simplest, least requirements, may even work from rectified filaments, because signal level is low.
But it stinks of a "Digital Engineer" approach.
+5V single supply is great ... for digital.
It gives you even LESS headroom than a humble 9V battery .... which is already a handicap
And LM358 is probably the only "audio" Op Amp Digital Engineers are aware of.
Ludicrous performance, has built in crossover distortion (the output stage is *unbiased* pure class B), its only advantage is that it was *designed* to be happy with a single 5V supply.

What am I pointing at?
You NEED to use good, real audio Op Amps, TL072 is an Industry workhorse, and you NEED +/- 15 V supplies. Period.
Since you also seem to need +5V at 100mA , only solution to get both is to add a 12+12 to 16+16VAC small transformer, at least 250mA capable, and forget the original transformer for this use.

If you build such a supply, then you will be able to add Reverb *after* the MV and before the power amp input.

[elipsey]

I was thinking of this one BTDR_CarrierBoard.zip.

You don't have to fight the opamp limitations and power supply issues as it uses tube drive/recovery. Of course, you need to add a tube...

So there are several files in the archive, one of which is a nested archive:

elipse@ubuntu:~/Downloads/temp$ unzip BTDR_CarrierBoard.zip 
Archive:  BTDR_CarrierBoard.zip
  inflating: carrier_revc.zip        
  inflating: CarrierReferenceSchematic.pdf  
  inflating: integration manual.pdf  
elipse@ubuntu:~/Downloads/temp$ unzip carrier_revc.zip 
Archive:  carrier_revc.zip
 extracting: carrier revc.pcb        
  inflating: carrier_revc.pdf        
  inflating: BTDR1 CARRIER.xls

CarrierReferenceSchematic.pdf shows you how to integrate the carrier board into an amp without offboard op amps, but you still need the carrier board for that, right?


Here's a picture of it from "integration manual.pdf":


Oh wow!! carrier_revc.pdf is the schematic for the carrier board! My Bad. I hope you all can forgive me for missing this, since it was in an archive inside another archive at a link to a page full of other links. Looks like the carrier board has a bridge rectifier, and a linear regulator (sounds kinda familiar) but no op amps. Is that an N-channel MOSFET on the output? I'm a little fuzzy on the FET symbols. It looks like I would have to add at least one tube to do this. So it's interesting, but it's actually not very much like what I've been doing, and certainly wouldn't have helped with my op amp problems. Good to think outside the box though.

Is this the file you guys wanted me to look at?

[elipsey]

Finally a schematic!

The first stage could easily output 5 volts AC but we still don't know what we are dealing with do we, hint, hint?

OK, I finished the schematic. I know it's not beautiful, but it actually took me quite awhile to do all that photoshopping (or gimping, as it were). I needed to do it anyway since I'm bound to give the amp away sooner or later, and the recipient will need a schematic. I'm about 90% sure this is right, but if something looks really wacky, ask me about it. Maybe I made a mistake in the schematic, or maybe I made a mistake in the amp...

[elipsey]

Not answering to any particular post, since many different, even contradicting options were posted.
Simply back to the original intention of interfacing the Belton Brick with a tube amp, there's 2 basic ways to add it to an amp which was not designed with reverb from the beginning.
That's an important point, because if you are converting an existing, spring based design to digital, (which
I guess is Belton's intention), 80% of the work was already done, but if you are adding it to, say, a JCM 800, you have a lot of work to do.

Haha, actually, it's quite a bit like a JCM800

The first way, already suggested here, was to use it at the front end, like any floor pedal.
Fine, simplest, least requirements, may even work from rectified filaments, because signal level is low.
But it stinks of a "Digital Engineer" approach.
+5V single supply is great ... for digital.
It gives you even LESS headroom than a humble 9V battery .... which is already a handicap
And LM358 is probably the only "audio" Op Amp Digital Engineers are aware of.
Ludicrous performance, has built in crossover distortion (the output stage is *unbiased* pure class B), its only advantage is that it was *designed* to be happy with a single 5V supply.

Actually, haha again, my only formal training in electronics was a digital design course in my senior year of college. But in my defense, my grade was pretty bad

The reasoning behind my choice of op amp was that the instructions said I need an ...Op? ...Amp?? um, oh I have one of those thingies left over from that pedal I built!

loudthud was saying earlier that the LM358 is too slow, and it sounds like there are lots of other problems with it. Since we're on the subject, it seems kind of weird to me that stuff made of transistors can be too slow for audio. I'm used to hearing people complain about things being to slow for RF!

What am I pointing at?
You NEED to use good, real audio Op Amps, TL072 is an Industry workhorse, and you NEED +/- 15 V supplies. Period.
Since you also seem to need +5V at 100mA , only solution to get both is to add a 12+12 to 16+16VAC small transformer, at least 250mA capable, and forget the original transformer for this use.

If you build such a supply, then you will be able to add Reverb *after* the MV and before the power amp input.

Actually, if it were small and didn't cost too much, maybe another transformer wouldn't be so bad. I'll shop around. Thanks for the input.

EDIT: how about a Hammond 262B24: 12/12V @ 0.2/0.2A ?? It's $30 though....hmmm

[g1]

On a side note, that FET circuit on the carrier board is just for muting of the reverb. Depending on you final design, you may not need it or may want to use a different method of turning off the reverb signal.

[clint]

Hey, Clint here from the telecaster world..
Looking at the data sheet of the reverb module, man, you don't need no damn op amps! You will need a buffer like a JFET or transistor to drive the 10k input impedance of the module, but that should be way simpler than the op amp and supplies and filters and such, the transistor can use the 5 v supply. A source follower or emitter follower driving the module, then you got to do some figuring about the return.
Since you have one or two tube stages before the module, so there should be plenty of input signal without having an op amp.
I'd try an npn transistor with the collector tied to 5 v and the emitter tied to ground via I dunno, whatever value gets you a nice current for the part you try, and bias the base to 3 volts or so with a voltage divider?
I'm rusty as heck with discreet transistors, I am more into JFETS and tubes, so if this is the slightest bit viable of an idea, I'm certain other more learned fellows will have more specific and correct ideas as to how to make a source follower or emitter follower to drive the 10k ohms input of the module.
I think for the level control, I'd do it on the input of the module, and use the output of the module to drive the cathode of the tube near the cathode follower.
In any case, I think there is a way to not need the op amp, seeing as you have tons of gain anyway overall..

[J M Fahey]

A Hammond is a luxury here, I bet you can get a 12+12V transformer for $10 or less.
Surplus is fine
Or recovered from dead equipment or from some local tech junk pile (literally) or ....
It's as generic as can be.
And you don't even *need* to regulate Op Amp rails.
12+12VAC>bridge+ 2 x 2200 to 4700uFx25V= +/-16V (raw and already directly usable by op amps without hum)
*If* you want to improve that, add to each rail 220 to 450 ohms 1/2W + another pair of filter caps.
And you can get +5V from raw +16V DC.