Brunetti Mercury

[motterpaul]

Okay, I have already checked with continuity tester (just bought  a new one) but it has to be something, so I will keep looking. I hate the problems I just can't see. Seems odd that I would have that + an opamp that doesn't seem to work (but I have tried 3 of them now).

I also ordered lt1054 - but might take awhile.

[Frank_NH]

Keep checking and reflowing solder joints.  If you lose the signal early, there has to be a reason.

If you can't get this to work, I'll build another copy of the layout just to double check everything (my original has carbon film resistors, but metal film would be better).  And I can always reuse most of the parts, including the FETs and IC!

[motterpaul]

Thanks Frank. I really don't mean to "bust yer balls" but I have been over this build 100 times. I have re-flowed all joints, pulled on all leads looking for loose solder joints, double-checked continuity, layout, voltage, audio flow, etc.

I replaced all of the caps around the two ICs with electrolytics. I even took the cuts & links picture and flipped it horizontally so I could double-check my cuts.

It could boil down to the charge pump chip, I don't know. I do know I get that voltage drop at the 220R, and that I get no pin 6 audio output from the Tl071.

I guess the last thing I could do is post more pics just to make sure I did not miss something.

It is also my fault that I am not better at troubleshooting layouts to see if I see an error there. I say that because in your post you ask people to build it to verify it, so I am not sure if you have built this specific version yourself.

Can you link to the schematic?

[motterpaul]

Here is another option - I have offboard working charge pumps for 18v. What could I take off of your build and just insert +18v to the Tl071?. Somewhere around row J?

[Frank_NH]

The schematic is at FSB (search for Brunetti Mercury) - look at the last page for the schematic.

Injecting the voltage directly may work, but I'd be cautious and wait for the charge pump.  The thing that puzzles me is that you are losing the signal completely.  It could be a bad component.  But without your actual build in hand, I can't know for sure.  I don't mind rebuilding it (it DOES sound great!) with metal film resistors.  

For the record this was originally posted in unverified layouts but was moved here.  Perhaps it should be moved back.

Sorry about the troubles...

[motterpaul]

Yeah, I know it sounds great in the demos. When I plug it to my test box I can hear some hum that goes up & down with the volume. It wants to work - it is some little thing.

I was looking at a different layout with trimmers and they connect all the trimmer legs to something. Here...



I just got some LM833s in the mail - now what were those for...? (geeze)

[Beaker]

Just a quick one Paul - are you absolutely 100% sure that your JFETs are genuine ones. That fake batch of J201s I reported a couple of months ago were printed with an "F" for Fairchild.

Is it possible you have some too?

[motterpaul]

It's a reasonable question. I am using 2n5457s, and I also had some fake ones, but they did not even look like Fairchilds. Most of the Fairchild ones I have do test out  as Okay (I think as exactly 5.0v forward). But I should test the ones I am using right now to make sure.

However. the problem does not seem to be around the Qs, although you never know. I will test them again with my Atlas DCA..

[motterpaul]

SUCCESS.  I finally got this to work using a 741 instead of a TL071. Nothing else changed. I should have known the OpAmp was the problem because I had no pin 6 output - but I was using different chips from a "brand new" tube - never opened, that I must have ordered way back when I started.

I tried three different ones - so either the whole batch I had is bad (possible, but not something I have personally seen before) or else it is possible that changing the charge pump to a 7660s made it so the Tl071 just wasn't happy.

And by the way - this is by far the BEST 741 circuit I have ever heard. Nice changes Frank (to 5457, and to lower gain pot) this pedal rocks. It is loud and ballsy but almost no hair at all (with a 741!).

[motterpaul]

FYI: One of my 5457s was a lower output (marked fairchild) so I did replace it, but that alone did not solve the problem. But it probably helped it sound better.

Thanks Beaker and especially thanks to Frank for all the thought & hard work you put in.

[Frank_NH]

Hey Paul,

Good to hear!  I actually did rebuild this last night with the posted layout (and all metal film resistors) but haven't had time to check it out.

Some notes:

* The op amp can be any single op amp - I actually used an LF351 (because that's what I had on hand).  But this pedal might benefit from a rail-to-rail op amp since the op amp can potentially clip here (though there's high head room with an 18V supply).  But given that the 741 is used in circuits like the distortion+ without feedback diodes at 9V, it's a good choice for this circuit.  I'm going to try it out myself.

* For Q1 and Q2, you can use J201, 2N5457, 2N5458 or other N-channel JFETs you have on hand.  However, for better bias control, you can change the trimmer values for your JFET as follows:

Q1: J201=50K, J202, 2N5457=20K, 2N5458=10K
Q2: J201=50K, J202, 2N5457=10K, 2N5458=5K

Note that the gains will change with the JFETs.  For example, my spreadsheet calculations show that for Q1 with a source bypass cap, the J201 gets a gain of 32, the 2N5457 = 23, and the 2N5458 = 18.  This is an area which is ripe for experimentation based on your tastes.  I like lower gains when they're cascaded like this, but that's me.

BTW Q3 is just a voltage follower (JFET buffer) so a 2N5457 works well there (no sense wasting a good J201 for a buffer!).

* Definitely use the LT1054 charge pump here.  I basically modded the original schematic to employ this chip as it is capable of higher currents and seems more robust overall.  

---

I'll post a slightly modded layout later which only has some tweaks to part positions for easier fabrication.  Still, somehow I manage to place big 47uF, 100uF electrolytic caps next to crowded parts of the layout!  

Finally, one of the reasons I like this circuit (and chose to make my own layout for it) is that it roughly simulates an IC boost/distortion circuit overdriving an amp (as simulated by the JFET stages).  With the high headroom from 18V, it's not as compressed as your standard tube screamer, and it uses NO diodes for clipping.  Quite cool!

 

[motterpaul]

frank wrote

Finally, one of the reasons I like this circuit (and chose to make my own layout for it) is that it roughly simulates an IC boost/distortion circuit overdriving an amp (as simulated by the JFET stages).  With the high headroom from 18V, it's not as compressed as your standard tube screamer, and it uses NO diodes for clipping.  Quite cool!

Those are all the things I like in overdrive as well - gain in the pre-stages & plenty of headroom.

I don't think I would change a thing.

BTW: it is odd, I was on a streak where everything I built worked for me, but suddenly even circuits I already built and had close to working are now just pooping out on me. Frustrating, and who would guess it was my opamp? I have never had that prob before.

[Frank_NH]

Just updated the layout.  Very minor edits, all verified now!  

[motterpaul]

hey Frank - all i see is the gain pot is now 250k. is there anything else you changed?

Can I ask why?  Here is a weird thing, once i boxed the circuit up it kind of sounded different. It is likely my fault, i think i may have put the input lead too close to the opamp. But it also squeals now (which it never did when in my test box).

Then when i mounted it to my pedal board it sounds even more different - very bright and gainy. I don't know what could have happened (maybe the bias got moved?). i have not yet opened it up again to check it, i need to do that.

anyway - did you happen to notice changing when you boxed it up or put it on your board?

[Frank_NH]

Hey Paul,

I changed it back to the original 250K as that sounded fine when I tested it last.  But feel free to reduce it based on your tastes.  I mentioned all of my mods in the original post.

As for squealing, I haven't noticed that at all but I didn't try to box it.  Do all the usual things like using shielded input/output wiring.  You can also try a different op amp - it seems to me a rail-to-rail op amp would work well here, but that may not cure the squealing.

Regarding the bias - I keep thinking that the perhaps the best way to handle the JFET bias issue is to use a socket where the pot goes, attach a pot temporarily to the circuit, then when you have it biased, measure the resistance and replace the pot with a resistor.  Much less to go wrong later on down the road.  And its not like the transistors will burn out like tubes!

[motterpaul]

I see, you were referring to your original changes. This: " There is a slight mod for the output capacitor (3.3nF --> 330 pF) which removes less high end " is interesting because my treble now is SO bright I have it turned almost all the way off.

The circuit sounded fantastic on its own, it i just weird that i have never had a circuit change like this one did when boxed up. Not your fault, just an odd happening. I will check the usual things when i open it up again (i have to dismount it from the board, i use strap-lock ties - not velcro).

[strassercaster]

Wow never heard of this pedal til today. Its a bad ass pedal. im worried about the squeal i still havent got my pinnacle to stop squealling at higher gain settings. I love the demos on you tube and I want to try it. Whats a good sub for the bat85?Nice Job Thanks

[motterpaul]

The squeal came from the circuit input being too close to the opamp - I moved it further away and all is good now.

I started going to some local jams and I saw that players here tend to like more gain - so I took the Wampler Ecstasy, Catalinbread Wiio and Nagy Supatone off my board. I now use the Brunetti for rhythm with the AMT B1 for leads - a great combination.

I think I could use just a tad more gain (its on full) - so I will probably change that gain pit to 250k, but otherwise still a great pedal.

[Battery Acid]

Oh boy, I finished one of those yesterday and took it to reheasal today, and I must say I'm very impressed! Sounds awesome, maybe the best distortion pedal I've ever heard.
Just a tip, forget about biasing the jfets in 9v, bias it using your ears, mine had much less gain and didn't sound very impressive with 9v feeding into j201's. I tuned it by ear, and now is a beast (and with a amazing tone!)

[Frank_NH]

Usually, biasing around the 1/2 supply voltage is recommended, but as you say biasing by ear is best.  In fact, biasing off of 1/2 supply introduces a more asymmetric clipping, which has lots of nice, even order harmonics.