Opamp for fx loop

[MarkusBass]

The loop circuit is build using two LND150 which is less than $1 each. So the price is almost the same as the price of two opamps. You can either buy the circuit from Mojotone or build it on your own. I don't have the schematic but it wouldn't be difficult to draw one.

Mark

[Chuck H]

Re: DC elevated filaments. There are plenty of circuits that do raise Vck above the filament voltage. In these cases elevating the filament string for hum reduction makes sense, and I've done it with good results. Another situation where elevating the filaments is useful would be when circuits with a high Vck, like cathode followers are used. Modern history has demonstrated that some new era tubes aren't as robust as those from the golden age WRT filament to cathode differential. And some circuits with cathode follower circuits have failed because of this. Elevating the filaments reduces that differential.

[Zozobra]

First my apologies for the necrobump. I'm currently trying to design the most compact loop board I can (sockets off board) that I can slot whenever I'm space limited or just use as an output buffer for preamps. I've kicked a few HV loop designs around but always got caught out by some weirdness somewhere so I thought I'd try out an opamp loop using a HV voltage clamp as a supply which is a pretty tried and tested approach. As ever, I like to sim stuff first if I can to get a ballpark idea of what is going on which leads me to a bit of a spice headscratcher.

If I use a fixed +30V supply and a +15V rail for a virtual ground/VREF then the circuit behaves as expected and can swing up to 30Vpp before clipping which is good enough for me.

When I try and use the voltage clamp/Zener regulator it goes to hell. When it's not connected to the opamp circuit I get the expected rails but when it is hooked it the +30V rail drops to +8.5V and the +15V rail drops to less than 1V

Have I made a noob error that I can't see or is this just some spice oddness?

[pdf64]

Weird!
Perhaps c3 and c8 values are a lot higher value than necessary / beneficial.
Maybe try c8 to 0V rather than Vref.

[Zozobra]

Weird!
Perhaps c3 and c8 values are a lot higher value than necessary / beneficial.
Maybe try c8 to 0V rather than Vref.

I've just tried c3 & c8 at 100n and c8 to 0V and still the voltage rails are screwy!

[strato56]

According to spice, how much current does a TL072 need ?
Maybe there's too much voltage drop over the 47K

[nickb]

If you are using the (texas insts) TL071.301 model it comes up with over 8ma per amp at +/-15V. The datasheet says 2.5mA max. Caveat utilitor!

[Zozobra]

According to spice, how much current does a TL072 need ?
Maybe there's too much voltage drop over the 47K

That did it. I'm sure I tried lowering that too!
Thanks

[J M Fahey]

Capacitors do not affect DC.
You have a couple schematic errors, Iīll download the image and edit it.

[J M Fahey]

Check and post voltages 1 / 2 / 3 / 4 , both with and without load (Op Amp circuit) , open indicated connection and I guess you should add the missing loop jacks in the empty rectangle.

[J M Fahey]

[pdf64]

Wow, how was it possible to miss that short?

[Helmholtz]

I guess you should add the missing loop jacks in the empty rectangle.

^^^That, and make sure you use the same "ground" for send and return.

[Zozobra]

Ah damn it, that was the wrong schematic. I'm sure I uploaded the one with that corrected :/

Edit: the inline one doesn't have that short but the attachment does. I tried deleting it but it showed up anyway!

[J M Fahey]

Not that a big deal but it will screw your simulation for sure, try to erase the wire and components conected to it and redraw that area from zero.
And add the jacks too.

FWIW I design and build using TL072 all day long since itīs my favorite building brick, and can confirm they "eat" between 3 and 3.5mA per DIP8 package, meaning 2 op amps (which you canīt separate and will sink full supply current even if 1 is unused).

IF you want to be even "greener", you can use a TL062 which uses around 1/5th of that.
Not too hot "audiophile" specs but ample for Guitar duty, in fact itīs very popular in Active Bass and Guitar circuits, for long battery life.