Tube screamer problem

[seanm]

I have a generic problem with tube screamer type pedals. When I play my bass into a tube screamer with the gain maxed out, I get a very hollow sound with an echo in the background.

I am working on isolating the problem, but was hoping for some suggestions. This is playing fingerstyle on a passive bass with flats. I cannot reproduce it with my cheap guitar. It may just be the high gain is picking up my bad technique.

The problem shows up with both my mixer and my amp. Both are very clean and solid state (no tubes). If I slam a SansAmp Bass Driver DI after the tube screamer, it sounds great! So it looks like the massive EQ in the SansAmp cleans up the problem.

I have the problem with: Fulltone Bassdrive, VooDoo Labs Sparkle Drive, TS-808 clone and other pedals. I don't remember having this problem with the SD-1.

Do any other bass players out there have this problem?

[seanm]

Ok more information. I built the following standalone clip stage.



According to the excellent article at GEO, the bias resistor to +4.5v does not affect the sound, so I upped it to 1M. This lets me drop the input buffer but still get a high Z input. I also fixed the gain at 470k just to make things easier. With this circuit I can reproduce the problem.

So I first removed the 47pF cap since it is just there to smooth out the distortion. No change. When I say no change, I mean to the fundamental hollow echo problem.

I then removed the two diodes. Problem goes away. So it is something to do with the diodes and/or their interaction with the other components.

Put the diodes back. Removed the 47nF cap. Problem goes away *BUT* gain goes down. Since the problem on occurs at high gain, this is inconclusive.

Replaced the 47nF cap with 1uF to lower the cutoff from about 720Hz to about 30Hz. Problem seems to go away. I only say seems because the distortion goes way up, as expected, and that could be masking the problem.

I am testing the problem with the opening to "Hey hey my my". So on the A string: A ABCD. So the fundamentals are in the 50 - 100 Hz range. Note on a basses low notes, the second and third harmonics are generally more important than the fundamentals.

Any ideas?

[seanm]

Ok, I tried it on the P bass, tried it on the Musicmaster. Decided to get out my gig bass to see if strings where an issue.

One thing I have not mentioned is that there is a lot of background hum with the test circuit. I just assumed it was the breadboard layout and ignored it.

Plugged in the gig bass. Still a problem. Rolled of the tone. Wow, it was like I hit a switch. All the hum went away. So did the problem. Turn the tone up a smidge, hum comes back.

So I placed a 0.1uF cap from input to ground. Solves the problem, even on the original P bass which has no tone control. Of course, this solution seriously changes the tone

[cd]

I haven't played bass into a TS circuit in a long time, but I would hazard a guess that since the ghosting is not there with the clipping diodes removed, the behaviour is normal.  The clipping diodes do just that - clip the signal - but in the process, the clipped signal unleashes a bunch of harmonics.  In the TS, it's mostly 3rd order.

When you rolled off the tone on your bass, less guitar frequency signals getting through = less clipping, less harmonics, no ghosting.

Anyway, I would lower the value of the 1M bias resistor (for noise purposes) - go as low as possible until the loading becomes too apparent.  470k is safe, 100k may be safe as well, depending on your pickups.  I'd also replace the clipping diodes with something with a higher threshold, maybe red LEDs.  I usually don't like messing with the cap on the - input; 720Hz sounds best to me (w/guitar), lower than that and it gets messy/farty.

[seanm]

I haven't played bass into a TS circuit in a long time, but I would hazard a guess that since the ghosting is not there with the clipping diodes removed, the behaviour is normal.  The clipping diodes do just that - clip the signal - but in the process, the clipped signal unleashes a bunch of harmonics.  In the TS, it's mostly 3rd order.

When you rolled off the tone on your bass, less guitar frequency signals getting through = less clipping, less harmonics, no ghosting.

Hmmm... that makes sense. Maybe I could get rid of them with a notch filter?

So I tested it with my amp. If I put a -18dB cut at roughly 1.5kHz, the hollow echo goes away. It does affect the sound though.

Thanks.

[seanm]

Bump for the work week crowd.