Vref Weirdness with Opamp... Help!

[Paul Marossy]

OK, so I got commissioned to build a Supa-Trem workalike the other day for a friend.  I used a verified PCB layout as a guide to build it on perf.

I have checked and double checked everything and I am pretty sure I don't have a mistake anywhere. I have the correct voltages on the board. The problem is that I have to feed it a VERY strong signal from an audio oscillator to get it to pass a signal at all, and it only works with a guitar if you strum it really hard, and it only makes a farting sound for half a second.

Now the only difference that I can see is that I used a Vactrol VTL5C7 I had lying around instead of a VTL5C1. Would that make a difference? There does seem to be some differences between the two, according to this data sheet here: http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf (see sheet 41)

I audio probed it, and I can hear a thumping noise at the dual opamp, so it appears that the LFO is working fine. But the output of the input opamp is very weak unless the input signal is very strong as well. I'm kind of stumped why it is acting like this.

So here is the really weird part: If I pull the opamp out, I measure approx. 1/2 the supply voltage at Pin 2, just like it should have. But as soon as I put an opamp in there, that voltage changes anywhere between about 1.5V and 6.8V, depending on what opamp I try. I've tried a CA3130, NE5534, LM741, TL071, TS271 - none of them work.

Is this circuit so that finicky that only the specifed opamp (TL061) will work? That's the first time I've ever encountered this sort of problem before. WTH?   Someone recommended that I use an input cap (the schematic didn't show one). Do you think that would fix the problem?

[CynicalMan]

OK, so I got commissioned to build a Supa-Trem workalike the other day for a friend.  I used a verified PCB layout as a guide to build it on perf.

I have checked and double checked everything and I am pretty sure I don't have a mistake anywhere. I have the correct voltages on the board. The problem is that I have to feed it a VERY strong signal from an audio oscillator to get it to pass a signal at all, and it only works with a guitar if you strum it really hard, and it only makes a farting sound for half a second.

Now the only difference that I can see is that I used a Vactrol VTL5C7 I had lying around instead of a VTL5C1. Would that make a difference? There does seem to be some differences between the two, according to this data sheet here: http://www.datasheetcatalog.org/datasheet/perkinelmer/VT500.pdf (see sheet 41)

I audio probed it, and I can hear a thumping noise at the dual opamp, so it appears that the LFO is working fine. But the output of the input opamp is very weak unless the input signal is very strong as well. I'm kind of stumped why it is acting like this.

So here is the really weird part: If I pull the opamp out, I measure approx. 1/2 the supply voltage at Pin 2, just like it should have. But as soon as I put an opamp in there, that voltage changes anywhere between about 1.5V and 6.8V, depending on what opamp I try. I've tried a CA3130, NE5534, LM741, TL071, TS271 - none of them work.

Is this circuit so that finicky that only the specifed opamp (TL061) will work? That's the first time I've ever encountered this sort of problem before. WTH?  Someone recommended that I use an input cap (the schematic didn't show one). Do you think that would fix the problem?

I don't have a schematic for this so I'm just guessing as to how this works. Try testing Vref with the input unplugged. If it's 4.5, an input cap would probably help. You might as well put one in anyways to prevent bias problems.

I found a thread on the other forum about a a demeter tremulator clone (which the supa-trem is supposedly based on) that mentioned "There is a missing 47n coupling capacitor at the input of the TL061 on the schematic, and the 10k resistor connected to the speed or rate control is shown as 1k in the parts layout picture." If that's the one you are talking about, that's probably your problem.

[Paul Marossy]

I don't have a schematic for this so I'm just guessing as to how this works. Try testing Vref with the input unplugged. If it's 4.5, an input cap would probably help. You might as well put one in anyways to prevent bias problems.

I found a thread on the other forum about a a demeter tremulator clone (which the supa-trem is supposedly based on) that mentioned "There is a missing 47n coupling capacitor at the input of the TL061 on the schematic, and the 10k resistor connected to the speed or rate control is shown as 1k in the parts layout picture." If that's the one you are talking about, that's probably your problem.

Yep, that is the circuit that I am building. This is good info to know, I didn't see that.

So my problem with biasing might be just due to not having an input cap? That's interesting. I didn't know that, I guess because I never ran into this problem before...

[puretube]

This?... :

[Paul Marossy]

Yeah, that's it. I made a mistake and used a 47 ohm resistor instead of a 0.047uF input cap, as the PCB layout doesn't make a real clear distinction between caps and resistors. Anyway, that input cap makes it work now! But now I have a problem, it doesn't have any tremolo effect at all. I think that it must be the optocoupler. I guess I need to get a new one, that will probably fix the problem as I've checked every other possibility and that's apparently the point where the LFO is supposed to modulate the output signal...

[puretube]

Well, it is a known fact that Bajaman (after years of experience in tube-amp-maintenance) only recently has found out
the difference between Anode and Cathode,
and the art of depicting these 2 electrodes or their designation...
(through the help of an other forum...)

so there`s a chance that you might have looked at one of his older, possibly reverse-polarized layout-indications
which may delude from the proper orientation of the LED?  

[Paul Marossy]

Well, it is a known fact that Bajaman (after years of experience in tube-amp-maintenance) only recently has found out
the difference between Anode and Cathode,
and the art of depicting these 2 electrodes or their designation...
(through the help of an other forum...)

so there`s a chance that you might have looked at one of his reverse-polarized layout-indications
which may delude from the proper orientation of the LED? 

I don't think that he got that wrong, it looks to be correct to me. Either the optoisolator I used is the wrong one for the job or it's faulty. 

[Johan]

the bias issue probably has to do with opamp bias current requirement...a 5532 is not getting enough current through a 1Meg resistor to bias properly with just 4.5volt...but a TL061/71/81  should..I'v seen it ( and used it) before...and not all opamps are happy at gain less than 2(unless connected as follower), something to consider
j

[Paul Marossy]

the bias issue probably has to do with opamp bias current requirement...a 5532 is not getting enough current through a 1Meg resistor to bias properly with just 4.5volt...but a TL061/71/81  should..I'v seen it ( and used it) before...and not all opamps are happy at gain less than 2(unless connected as follower), something to consider
j

Those are some good points. In any case, it simply refused to work at all until I put the input cap into the circuit. 

[Paul Marossy]

Update: OK, so I got the specified optoisolator and put it into the circuit. It works now, but I think something is still wrong. The problem I have now is that the level control when turned fully counterclockwise (no tremolo effect) is a lot louder than when the tremolo is full on (fully clockwise).

I thought maybe the 1K resistor before the trimpot was too big, so I used a 100 ohm there and it still doesn't work. Basically it's adjusted so that the 10K trimpot is effectively at zero ohms - that's the only way that it will work. So this has me a bit baffled. Could it have something to do with the opamp I am using? I am wondering if there is not enough current from a TL071 to properly "drive" the optoisolator.

Any second opinions?