output ceiling ???

[djp8djp]

Hi. I probably should have posted here in the first place  . I tried another forum first, and got very little help there. I'll know better next time. 

I'm still a newbie at all this, but I've learned a lot by building a few guitar effects pedals. I'm hoping someone with more experience can suggest what might be wrong. This project is for a guitar onboard preamp, and I have encountered a strange problem. It's for the Bass-Treble module. It seems to have an output "ceiling". What do I mean? Well first, here's an intro to my project.

I pulled the filter design from an old application note (can't recall which one, I'll try to find it). But I have laid it out in LTspice, and tweaked the values to suit my needs. It's a "hinging" control, effectively a combination of two opposite-gain shelving filters, both having the same cutoff. It connects to a single linear pot, which in the middle position produces null effect (flat response). But turning it one way or the other will boost Treble and cut Bass, or vice versa. I wanted more Treble effect than Bass, and have balanced the circuit as such. So I finally got it to work fine in the sim (I know, that doesn't mean it'll work in the physical world, but it's a start).

To prevent interaction with the inductance of the guitar pickups, I have an initial unity-gain buffer that is fed directly from the guitar output. The buffer is implemented in one half of my dual op amp, and I used the other half to drive the filter. I added a resistor and capacitor between these stages - the resistor to insulate the op amp output from the load of the capacitor, and the capacitor for DC-blocking. The op amp is fed from a bipolar power supply that I built ... linear regulated +/- 15V. I connect its ground to the ground seen in the filter circuit on the op amp + input, which is also shared with the guitar signal ground.

The output has another resistor-capacitor pair, serving the same purpose as those between the buffer and filter stages. From there, I can go straight into the guitar amplifier. The actual design is to incorporate another stage (a midrange boost/cut circuit), and then an AMZ MOSFET booster (adjusted for unity-gain, serving as an output buffer). But for now, these are not at issue, and not included.

So what's the problem? I already said the Bass-Treble circuit works fine, so that's not it. As I mentioned earlier, the problem is that my output level seems to have a "ceiling". When I plug into an amplifier, everything is fine, up to about 3.5 on the dial. Then nothing more happens. No additional volume. From 3.5 to 10, I get the same volume. It's not the amp - I've plugged the guitar straight in, and with just the input buffer in between, and it works fine. I have noticed one interesting thing - if I leave the guitar alone and listen carefully, there seems to be at that point of the dial a little bit of extra hiss. Dunno if this has a bearing, but it's too much of a coincidence, so maybe that'll be a clue to someone.

What have I tried? All of the following:
- test just the input buffer... it works fine
- remove the input buffer... no difference
- remove the coupling resistor and cap between the stages... no difference
- remove the coupling resistor and cap after the Bass-Treble stage... no difference
- follow with another op amp buffer identical to the input buffer... no difference
- follow with an AMZ MOSFET booster (unity gain).. no difference
- pull and re-insert all components from the breadboard, in case it's mangled in there ... no difference
- pull all components and move sideways two holes, in case it's mangled in there... no difference
- try different op amps (so far TL072, NJM2068, JRC4562)... no difference
- try a different pot... no difference
- try different resistors and caps in the Bass-Treble circuit... no difference
- oh, and I've tried it without the resistors Rbt_7 and Rbt_8. They're just there to allow me to use a B100k center-detent pot, and they adjust the circuit gain and the pot taper. Whether included or not, there's no difference in the problem.

I probed the Bass-Treble circuit at different locations (see labels on circuit diagram), and got these results:
A1, A2, A3, A4 - normal (Good volume at amp from 0-10. Buffered levels are marginally greater)
B - low volume, is filtered with pot rotation
C - normal volume
D - low volume, definitely filtered with pot rotation
E - medium volume, definitely filters with pot rotation (higher level than at D !!! But still way too low)
F, G, and H - low volume
(for all of the above, "low volume" refers to the "ceiling" effect I mentioned, of no increase at amp past 3.5/10)

So, can anybody point out the problem? I'm stumped (which in itself is not unusual, but it has put me at a standstill)

(I hope I've linked to this image correctly...)

[seedlings]

Where is the volume pot in the schematic, what value, and what taper?  After the 'H'?

6dB isn't very much gain to our ears.  It could be that you're using a log pot were a linear taper would 'feel' better.

CHAD

[djp8djp]

Hi, thanks for replying.

The volume control isn't shown because I'm not using it. The guitar's volume pot for now stays wide open, so the input is static and not part of the equation. The issue is that when I turn up the volume on the AMP, it responds as usual from 0 to 3.5, but from 3.5 to 10, there's no additional volume - it's as if I'm not turning the amp up any more at all. This does NOT happen when I run straight from the guitar, or the input buffer, straight into the amp. Plenty of air outta the amp then! So the problem is not the guitar itself, not the input buffer itself, and not the amplifier. It's some darned thing in the Bass-Treble control that I just can't see.

And yeah, I know that +/- 3dB for bass and +/- 6dB for treble isn't much. But this is meant to be a tone control, a substitute for the guitar's regular passive RC filter, and it's supposed to be fairly subtle. Remember though that when it's topped out, it should yield 9db differential between bass and treble. And testing it as I can - low volume only, due to the issue at hand - it seems to be plenty. Of course, I can always twiddle the gain later by fiddling with pot size or those parallel caps... which I've already done to get it where it is now. Once I can crank up the amp, and have that do anything (!!), then I'll know if I need to change it.

[seedlings]

Hi, thanks for replying.

The volume control isn't shown because I'm not using it. The guitar's volume pot for now stays wide open, so the input is static and not part of the equation. The issue is that when I turn up the volume on the AMP, it responds as usual from 0 to 3.5, but from 3.5 to 10, there's no additional volume - it's as if I'm not turning the amp up any more at all. This does NOT happen when I run straight from the guitar, or the input buffer, straight into the amp. Plenty of air outta the amp then! So the problem is not the guitar itself, not the input buffer itself, and not the amplifier. It's some darned thing in the Bass-Treble control that I just can't see.

And yeah, I know that +/- 3dB for bass and +/- 6dB for treble isn't much. But this is meant to be a tone control, a substitute for the guitar's regular passive RC filter, and it's supposed to be fairly subtle. Remember though that when it's topped out, it should yield 9db differential between bass and treble. And testing it as I can - low volume only, due to the issue at hand - it seems to be plenty. Of course, I can always twiddle the gain later by fiddling with pot size or those parallel caps... which I've already done to get it where it is now. Once I can crank up the amp, and have that do anything (!!), then I'll know if I need to change it.

Aaah, I see how this is a mystery!  Could it be that you now have DC somehow at the output of your guitar?

CHAD

[djp8djp]

Hi

Well, thanks to all for your input. The crisis seems to be over - at least for this point in the project. The problem appears to have been in the crappy little amp I have in my workshop just for testing purposes. It's a little POS Squier amp I salvaged out of a package a couple years ago. Apparently its input stage has some pretty cheap stuff in there. Because when I plugged it into my Fender Champ and Mesa Mark V, both of them seem to have no issues whatsoever. The amp volume controls operated in their full range of travel. And to boot, the circuit works exactly as I intended.

Thanks all for your help. And I hope this thread lives on for others who are foolish enough to use a turd for a testing amp.