op amp-based effects loop circuit for guitar amplifier

Thanks for the reply and comments, Martin.

Power is taken from the point W1 on this schematic:

Filtration & regulation circuit are on shown on the first .pdf above the main schematic. It's the zener's in that circuit (D5 & D6) I was thinking about replacing with regulators. D1-D4 prevent the opamps from getting zapped, correct?

Also, I will stick with a TL072 before trying the Analog Devices chip; no sense blowing-up a $3 op amp!

Regards, Eric

To adapt this circuit for a split supply, +15 would go to pin 8, -15 to pin 4, and VREF would essentially become ground, correct? If so, are C10 and C3 necessary?

Martin & YD: Great! Thanks very much for the help! Regards, Eric

I am trying to avoid electrolytic caps in the signal path your point about safety is valid. Out of curiousity, is it common for op amps to leak DC when they fail? Regards, Eric.

Yes, that's correct. In case you take this route, recommended, you can replace all capacitors by shorts, they're there only to block DC levels from getting in each other's way. VREF and GRND should be connected together. It might be an idea to keep C1 and C8 to block external DC levels from getting into the circuit.

Whatever you do, DO NOT try to use the +B supply for this circuit.

- YD.

Yep, looks reasonable at first sight. But you are totally wrong in replacing the zeners. These are here to prevent high voltage signals from zapping the opamps.

I'd just use 8 pin ic sockets on a homebrew, so you can easily change the opamp. you may blow up a few prototyping. Stick with the TL072 until you have it working.

What the circuit doesn't show is the power supply. what are you going to use?

martin

Oops misread your post about the power supply.

I would personally not use 380V dropped down. This circuit needs split rail supplies, say +12V and -12V. Also there is a safety factor while testing to be considered. try

martin

Hello Eric,

Just be careful and mind situations like when one of the supplies fails but not the other. After all, you don't want to fry any expensive gear that is connected. Maybe you could at least leave the output caps in.

Regards, Joerg

Hello Eric,

Take film caps. Worth the few extra pennies.

They contain output transistors on the chip. One to the positive and one to the negative rail. If one decides to short out then yes, they can dump a whole lot of current into whatever is connected. Or whatever the current limit on the power rail provides.

Regards, Joerg

Joerg, Thanks very much for your help. I doubt a 47uf film cap (C10) exists that will fit in the space available so Black Gate electrolytics will have to do... Eric

Now that you mention it....

A 47uf coupling cap results in a low frequency response of 1.4 Hz with a 10kohm load (average for studio rack gear), based on this calculator:

Given the lowest note on a guitar is about 70Hz, a 0.47uf (film) cap would be fine since low frequency response would be around 44hz with the same load... or am I missing somthing?

Eric

Hello Eric,

If it really needs to be this high you won't find a film cap. But I try not to use electrolytics of any kind in high-end audio designs. Considering that there is a 470 Ohms in series I don't know why 47uF is needed, even at low frequencies.

Regards, Joerg

try the pansonic or nitai brands of bipolar caps. they are designed for AC coupling.

martin

"Good enough for Marshall" is what I'm afraid of... Their recent products (1990 and newer) are not known for their reliability hence the original question about the quality of the design. Furthermore, B+ drops >5% at times (when cranked) and no voltage regulator is perfect. I'll stick with a separate, split supply.

Hello Eric,

The Z of a 0.47uF cap at 70Hz is around 5kOhm. You might want to shoot for a larger cap here. I am not a musician but AFAIK guitars generate lower frequency resonances.

You can get film caps up to several uF. Except for SMT where realistically the limit would be around 1uF.

Regards, Joerg

Hello Graham,

Yes, but ... I have repaired a few over the years. When this stuff is used outdoors a lot it gets hot. Really hot (it is about 107F here right now). Some of the electrolytics I have seen in there were in deplorable shape. Dried gunk clumps on the outside, others with an oily goo around the bottom. In one unit I found about five of those. Plus a few cigarette butts.

From a performance point of view electrolytics are probably ok. But film caps aren't that expensive anymore and they last a long time.

Regards, Joerg

Well, of course it can be done, but unless you're sure of what your doing can be a bit dangerous. If any of the bias zeners happen to fail open there'll be much merriment.

To Eric (OP): If you want to try it, you'll need to feed the supply circuit with about 6 mA, the TL072 draws 4 and the rest goes through the zeners. You'll need to drop 350 volts from the +B over a 56k 5W resistor connected between +B and the +30V point of the supply.

- YD.

It's kinda OK.

The use of zeners as shown to provide the supply volts and centre tap is a shunt regulator configuration. You can't use a 78xx type device that way. It's not stunning but I'd leave well alone.

Graham

Why do you say that ? It's good enough for Marshall and if there's no other volts readily available inside the box......

Graham