ROG Tube Reamer help

[wbajzek]

Hello. I'm breadboarding a runoffgroove.com Tube Reamer right now, but am running into a problem... I assume it's a totally rookie mistake, but the output seems to be identical to when I'm plugged straight into the amp. Disconnecting various parts of the circuit, including the battery, make the sound go away, so I am pretty sure I haven't unwittingly bypassed it... So maybe both halves of the opamp are producing unity gain, and therefore there's not enough signal for the diodes to clip?

I have only 2 intentional deviations from the schematic:
1. I am temporarily just using a single resistor instead of the gain control. The value of this seems to have no bearing on the sound.
2. I have temporarily omitted the volume pot.

OpAmp is a JRC4558DD but I've tried a few with no difference
Thanks!

[mth5044]

It is helpful to post a link to a project.

http://www.runoffgroove.com/tubereamer.html

Also, post some voltages and possible a picture of your breadboard.

[wbajzek]

Here are some photos. My battery is 9v and Vref is 4.5. Pin 3 (+in) reads 2.29v.The opamp outputs both read 4.5v. Anything else in particular?



Large version: https://dl.dropboxusercontent.com/u/597433/breadboard/1.jpg



Large version: https://dl.dropboxusercontent.com/u/597433/breadboard/2.jpg



Large version: https://dl.dropboxusercontent.com/u/597433/breadboard/3.jpg

[bluebunny]

Unity gain would suggest a short from the output of an op-amp to its inverting input.  But I can't see a short on your board.  Unless one of your diodes is hosed and has become a short circuit.  It's a long shot, but lift the diode legs in turn to see if the volume jumps up.  Also try substituting that 51K resistor for something bigger.  Your "fixed" gain control is at its minimum setting.

[wbajzek]

I've tried a 220k resistor in the feedback loop to no effect. I've removed the diodes and the cap so there is *only* a resistor between the output and - input, to no effect. And I've tried different op amps to be sure it wasn't the op amp.

To back up a bit... I think I have an incomplete understanding of how it's supposed to work.

1. the op amp tries to match the levels of the + and - inputs, so feeding the output directly to the - input of the op amp lets it see the same level at both inputs, so it does not increase gain.
2. the capacitor is acting as a low pass filter and allows low frequencies to pass through from the output to the - input, causing the high frequencies to be amplified more than the low
3. (getting hazy here) so low frequencies can pass through unaffected, and the resistance impedes the signal's ability to bypass the diodes, so more signal goes through the diodes and gets clipped, distorting the - input and therefore affecting the output waveform
4. diodes pass signal through in one direction, and above a certain voltage they will clip.

does that sound right? your suggestion that output to - makes sense in my understanding... I just don't see it unless my breadboard is bad somehow.

what do the resistor/capacitor going to ground do? I seem to remember disconnecting them at some point to no effect.

I'm on my way to work now so will have to get back to debugging tonight. thanks for the help!

[antonis]

what do the resistor/capacitor going to ground do? I seem to remember disconnecting them at some point to no effect.

They form a low-pass filter but if you've disconnected the resistor with no effect, something is going totally WRONG..!!
(you should have "infinite" gain..)

My estimation is that you have some wrong wiring/connection concerning signal path so you allways get a "clean" signal..

[deadastronaut]

pull it all off, and start from scratch..its only a small circuit anyway..

go through the schematic bit by bit marking off each node connection 1 by 1...

its easy to get wrong connections when breadboarding, be methodical, and it,ll work.  

then you can start tweaking..lots of fun. 

[lopsided]

does that sound right? your suggestion that output to - makes sense in my understanding... I just don't see it unless my breadboard is bad somehow.

Hello,
do You have a Digital Multimeter? If so, have you tried measuring the resistance between the output and inverting input? If its the 220k - it would mean this part of the circuit is probably right and the problem is elsewhere and if its 0 - there is a hidden short somewhere (faulty breadboard maybe...)

Jakub

[wbajzek]

I think I'll keep poking at it until I get a chance to buy a bigger breadboard... I bet I'll find that my error is due to jumpering to a different buss in order to fit everything in the feedback loop.

lopsided, I will give that a try tonight. But I am failing to understand something... won't some signal pass through the capacitor and diodes regardless of the resistor in the feedback loop?

[wbajzek]

deadastronaut, tweaking is the goal. I am playing through an old gibson crestline skylark, which I love but is a little finicky for distorted/overdriven sounds. I have a hand-me-down Tube Screamer that sounds close but not quite right... A little dull and not as much gain as I would like. I'm hoping that by messing with the diodes and opamp in the Tube Reamer I can find a combination that works the way I want.

[deadastronaut]

Cool,  nothing like a "not quite sound" to get you motivated..

[wbajzek]

It works! I pulled out the first feedback loop circuit and realized that the LPF to ground was connected to pin 1, not pin 2.

[bluebunny]

Hey, so it is!      That'll be the last time you ask us myopic lot for help eyeballing your board.  Which way to DIY-eyeglasses.com? 

[deadastronaut]

It works! I pulled out the first feedback loop circuit and realized that the LPF to ground was connected to pin 1, not pin 2.

yay!!!!..

@marc, aww man, tell me about it, i have 2 different sets of glasses now,,.pita.

[wbajzek]

I'm sort of struggling with this circuit. Since I hadn't hooked up the volume pot, I temporarily bypassed the second gain stage so I could hear the signal without overdriving my amp. I found that by omitting the capacitor in the first gain stage and adding one as an LFP at the end of the circuit, the tone was right on the money, but of course the volume fell far short of the guitar direct into the amp. I have played around with reintroducing the second gain stage, but can't get the tone quite right... Although I think I am probably overdriving my amp again and I suspect the second gain stage is clipping, which I would like to avoid.

It could be that the volume pot is all I need, I just have a very limited set of components right now and am out of jumpers. I ordered a bigger breadboard and jumper/alligator cable sets that will make experimentation much easier. In the meantime, I made modified schematics and some notes so I can get back to where I was with it.

Bluebunny, when you say that the feedback loop is at its minimum gain setting with the 51k resistor, do you mean that if I used a smaller resistor, the gain would not decrease? Or just that it is the minimum gain setting for the circuit because it is a fixed component? I'm wondering if I could reduce the gain of the second stage by using a smaller resistor, since the unattenuated output currently is very far beyond a useful value. In my ideal world, I could set the second gain stage to boost up the signal to a reasonable level without affecting the tone.

Thank you all for your help!

[bluebunny]

Bluebunny, when you say that the feedback loop is at its minimum gain setting with the 51k resistor, do you mean that if I used a smaller resistor, the gain would not decrease?

It's at its minimum gain with the components you have wired up (and effectively the minimum gain for the standard gain setup - the gain pot is shorted-out).  If you reduce the value of the 51K resistor further (like, short it out!), the gain will go down to unity.  An op-amp with maximum negative feedback (i.e. a short circuit from output to inverting input) is a buffer.  Conversely, if you reduce the negative feedback (more resistance), more of that famous "infiinite" op-amp gain becomes available.  Same goes for the other stage, if you need to tinker with that too.

[wbajzek]

got it. thanks!

I'll experiment with it after work tonight and see if I can get that second gainstage in line. I misinterpreted what you said and thought I was already running that stage at its minimum, but now it makes more sense.

[wbajzek]

I had a little time to play around with it more and can better describe.

Version 1: If I bypass the second gainstage and run U1a output through the 1k resistor to the output jack, with a 100n capacitor running to ground between the resistor and output jack I get the sound I want but at a low volume. If I'm not mistaken, this means I'm just running through a low pass filter, and I understand that filtering will reduce the volume but I'm surprised it is doing so that much.

Version 2: If I bypass the second gainstage and run U1a output through the 1k resistor and the 100n capacitor in series to the output jack, similar to the original schematic, I get earsplitting volume and an unassessable tone because I assume I am hearing only my amp's preamp tube distorting at that point.

What I would like to do is accomplish the same filtering as in version 1, but without such a drastic drop in overall volume. I'm guessing there's probably something simple I'm missing. Any thoughts?

[Kipper4]

As you can see theres a 1k ,22nf low pass filter (LPF)between the op amp stages.
Removing the secong op amp takes away the make up stage after the filter. this stage has a gain of around 6.

Have you tried tweeking the LPF between stages instead? Maybe leave the 1k and swap the cap out until you get the sound you want.
Leave the final 100nf decoupling cap in too before the output pot.
Just a thought.

[wbajzek]

Bullseye! Thank you, Kipper4. I am now back to the original schematic, with a few substituted caps; I got the sound I wanted and the maximum volume is now in the "booster" range rather than the "I'm afraid I'll hurt my amp and ears" range.

And thank you again to all of you, because not only am I happy with my tweaked circuit, but I think I actually understand it all now, too!