No sound from Thor....very sad.....

[danielzink]

Hey all,

Just buttoned up a Thor project today (etched board from ROG schematic) and to my dismay......no sound......

This is actually the first time a build did not fire up on the first shot.....

I'm gettin' nothin' - I mean nothin' from this thing.

Here's the rundown:

Q1 2N5457

D=5.08
S=.31
G=0

Q2 J201

D=8.24
S=.19
G=0

Q3 2N5457

D=8.23
S=4.49
G=2.05

Q4 2N5457

D=4.11
S=.35
G=0

IC1 TL071ACP
1=.14
2=4.11
3=3.79
4=0
5=.14
6=4.12
7=8.24
8=0

The 20k trimpot is maxed out there is no adjustment up left in it - I can go back down - but it's pegged at it's max right now. The 5K still has some adjustment left.

there's only the two polarized caps - and I've checked and double checked those....I've been over the board with a magnifying glass and there's no solder bridges.

Anyone have any ideas based on these notes ?

Thanks, Dan

[Darkness, Darkness]

Hello,

I would suggest you test each stage with an audio probe (basically put a capacitor right after a test point and plug it to the amp. Don't forget to plug the ground too.) Doing so you should be able to spot in which stage the problem is located. Debugging should be easier. Personally I always use an audio probe when building a distorsion box so I can immediately know if something is wrong or not and this also helps to figure out how the sound is shaped.

Good luck !

[soulsonic]

The Drain voltage on Q2 is way way off. This is why there is no sound; it's not biased and effectively turned-off.
Ditch the 20K trimmer and replace it with a 15K resistor. Replace Q2's Source resistor with a 5K trimmer. You will now be able to bias it correctly.

[danielzink]

The Drain voltage on Q2 is way way off. This is why there is no sound; it's not biased and effectively turned-off.
Ditch the 20K trimmer and replace it with a 15K resistor. Replace Q2's Source resistor with a 5K trimmer. You will now be able to bias it correctly.

Will try this later this evening.

Thanks for the input !

Dan

[danielzink]

The Drain voltage on Q2 is way way off. This is why there is no sound; it's not biased and effectively turned-off.
Ditch the 20K trimmer and replace it with a 15K resistor. Replace Q2's Source resistor with a 5K trimmer. You will now be able to bias it correctly.

Will try this later this evening.

Thanks for the input !

Dan

I jumped the gun here....

So Q2's source (adjustment) - which is a 20k (trimpot) right now - should be swapped with a  5K Trimpot ?

Just making sure.

Thanks, Dan


<edit>

It sounds like you're saying that my trimpots are/could be reversed ?

[stm]

Q2 J201

D=8.24
S=.19
G=0

Q3 2N5457

D=8.23
S=4.49
G=2.05

Q4 2N5457

D=4.11
S=.35
G=0

IC1 TL071ACP
1=.14
2=4.11
3=3.79
4=0
5=.14
6=4.12
7=8.24
8=0

The 20k trimpot is maxed out there is no adjustment up left in it - I can go back down - but it's pegged at it's max right now. The 5K still has some adjustment left.

Watch out!

You have a problem either with your 20k trimpot on Q2 or Q2's drain is short circuited to V+ or VCC, which is 8.24V, as indicated by the reading on IC1-pin7.

How can I tell this?  Simple: Q2-source is at 0.19V, which are across a 1K resistor, which gives a source current of 0.19V / 1000ohm = 0.19 mA.  Drain current should be the same, so voltage drop across the 20K trimpot resistor should be 0.00019A * 20000ohm = 3.8V.  Subtracting 3.8V from Vcc this should give the drain voltage: 8.24V-3.8V = 4.44V.  As this is not happening, the only conclusion is that Q2-drain is shorted to Vcc either by a short, by a bad wiring, or by means of a defective 20k trimpot.

Based on the calculation above, you should be able to bias Q2 properly after the problem is solved by setting the trimpot between 70% and 100% its full rotation.  Notice that even though the schematic calls for a 4.5V bias, you may want to experiment with a higher drain voltage, particularly 5.5V, as this produces a more open sound (at least to my ears) and a more symmetric clipping in this stage, as the drain can go down only to about 1.5V.

[danielzink]

Welp.....no love from my Thor today........

I swapped out the 20k trimmer and ended up with all the exact same readings (from 4.78 to a max of 8.31). I have been over this board with the double magnifier and can not find a crossed trace a solder bridge....anything...I went over the board 3 times with a flux cleaner pen...no luck.

really cranks me too....I wanted this thing to be perfect.....I have got to get it sorted out.






Dan

[soulsonic]

NO NO NO!!!
REMOVE, the 20K trimmer. REPLACE it with a 15K resistor (or something close, 10K would probably be fine). Then, replace the 1K Source resistor (of that same transistor, Q2) with a 5K trimmer.
As it is now, the 20K trimmer is not on the Source, it is on the Drain.
You may also want to swap a new JFET in Q2's place. It is possible to fry them with static; it doesn't happen often, but it can happen - or maybe you just got a defective one. Barring any other mistakes in the build, that's my next choice for explaining the problem.
Great looking build, BTW! I hope you can get it to make sound soon - the Thor is really a great sounding circuit.

[danielzink]

NO NO NO!!!
REMOVE, the 20K trimmer. REPLACE it with a 15K resistor (or something close, 10K would probably be fine). Then, replace the 1K Source resistor (of that same transistor, Q2) with a 5K trimmer.
As it is now, the 20K trimmer is not on the Source, it is on the Drain.
You may also want to swap a new JFET in Q2's place. It is possible to fry them with static; it doesn't happen often, but it can happen - or maybe you just got a defective one. Barring any other mistakes in the build, that's my next choice for explaining the problem.
Great looking build, BTW! I hope you can get it to make sound soon - the Thor is really a great sounding circuit.

Okay,Okay,Okay......

Gimme some time here.....

[B Tremblay]

I'd try a different FET in Q2 before changing the biasing setup.  Many have built the circuit without needing to make such modifications.

[stm]

Let me start saying that your box and PCB look very neat.

Just buttoned up a Thor project today (etched board from ROG schematic) and to my dismay......no sound......
...
I'm gettin' nothin' - I mean nothin' from this thing.

Regarding your problem, you started saying you had no sound at all, and voltage readings indicated a possible short on Q2 drain to VCC.

Welp.....no love from my Thor today........
I swapped out the 20k trimmer and ended up with all the exact same readings (from 4.78 to a max of 8.31).

Now, you mention you do have a voltage adjustment range from 4.78 to 8.31.  At 4.78 you should definitely have sound.  Moreover, I suggested before that 5.5V was a preferred voltage reading for Q2, rather than 4.5V. Please confirm if you have sound now with bias at 5.5V.

If your concern is actually getting down to 4.5V, there are at least four actions possible (choose just one at a time):

1) Follow Soulsonic's advice. It should work, but you'll need to hack your PCB to fit the 5k trimmer on the source.

2) As Brian suggests, try replacing the J201 on Q2 with another J201. If you have several J201's try them until you get a one that biases across full range.  This should be by far your first course of action, as it is the least invasive.

3) The reason we opted for the 20k trimpot is that the traditional 100k allowed biasing in a wide range, however adjustment was more concentrated in a small region of the pot's travel.  If you have a 50k trimpot you can replace it instead of the 20k and you'll be able to do the adjustment.

4) The reason you are not getting proper biasing is because your device has lower VP, so you need a smaller source resistor as well.  Just change the source resistor with 820 or 680 ohms and you'll be able to obtain the adjustment with the 20k trimmer.

Good luck, and don't forget to report back how it goes.

[danielzink]

I would like to take the time to say thank you to everyone who took the time to post regarding my problems.

It's been said so many times on this forums and others -that when you're most frustrated - you need to take a step back - breathe deep and sometimes even walk away......

I'm actually embarrassed to say that I got the circuit working.

It sounds awesome....it sounds unlike any other pedal that I've built so far - I'm very impressed.

I had forgotten a ground wire.........

'nuff said.


Dan

[soulsonic]

Ha, awesome! Trust me, we've all done that one before!
I'm glad you like the sound; it's definitely one of my favorite overdrive/distortions!

[danielzink]

Ha, awesome! Trust me, we've all done that one before!
I'm glad you like the sound; it's definitely one of my favorite overdrive/distortions!

With that all being said.....hve you - or have you heard of trying any other opamps in the circuit ? or would it comprimise the inegrity of the unique voicing of the Thor.

I've got a few hanging around - TL072's, 5532, OPA2134 etc.

Thanks, Dan

[B Tremblay]

Thor uses a single op-amp IC and the others you listed are dual op-amps.  Due to the different pinouts, they cannot be interchanged.

[stm]

Nothing like a cup of coffee and a happy user report in the morning!
Great to hear you like it.  Your build quality looks so neat you HAD to have it working :-)

Regarding the opamps, you might like to try an NE5534 (a single version of the NE5532) or an LF351 or an LM741.
I believe the TL071 is up to the task, nevertheless 5534 is more hi-fi and could make a difference. In the end your ears should guide you. The 741 might suffer from slew-rate limiting; whether this might be better or worse is something that needs verification.  As the IC is on a socket feel free to experiment.

[MadMac]

The 741 might suffer from slew-rate limiting; whether this might be better or worse is something that needs verification.  As the IC is on a socket feel free to experiment.

This slew-rate limiting you are referring to could this be the cause of my Thor sounding gated? 
I've have used a TL071 op amp and been unsuccessful at attempting to remove a gated sound over the top of the distortion even though I can bias Q1 and Q2 to the specified voltages.
An audio probe seems to tell me that the gating is happening around the Mu-Amp section. 
It seems to sound correct at the gate of Q4 with the audio probe but then on the drain I get the gating sound either that or the problem is just amplified at this point in the circuit.

Could it be that perhaps I need to adjust source and or drain trimpots / resistors to allow for component / voltage tolerances?

[soulsonic]

I used an LF411 in mine; sounds killer.

[grolschie]

NO NO NO!!!
REMOVE, the 20K trimmer. REPLACE it with a 15K resistor (or something close, 10K would probably be fine). Then, replace the 1K Source resistor (of that same transistor, Q2) with a 5K trimmer.
As it is now, the 20K trimmer is not on the Source, it is on the Drain.

Just out of interest Soulsonic, would this kind of biasing change work in the Eighteen?

[danielzink]

I used an LF411 in mine; sounds killer.

Any biasing changes needed if trying this ?

I mean heck...opamps are a buck and a half from Mouser.....try 'em all ! 

Dan