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Stagg Upright Electronics

S

SUBASSMAN

Guest
Feb 16, 2015
5
0
71
#1
I just recently purchased a new Stagg Electric Upright Bass and am having the same problem posted by another TalkBass member some time ago. The output has a significant DC bias that takes some time to dissipate. For a high impedance amp this bias can remain to some degree. This causes significant popping when switching the Bass in and out for tuning and such. Recommendations were to install DC blocking capacitors and bleeder resistors however I never saw any final resolution on the TalkBass forum. My question is whether anyone has had similar problems and recommendations on how to resolve it. I would even consider an external device.

Regards
 
#2
Have you measured it? What is the DC voltage you have on the tip connector of the jack? Does the preamp have one battery or two? Does your amp pop when you plug it in, (with the volume and gain up)? Does your bass do this in every amp? Or are you talking about a pop when you hit the mute/tuner button on your amp? Do you have a link to the old discussion? Or are you talking about a mute switch or bypass switch that's actually on the bass?

Sorry about all the Q's, but without seeing the circuit of the bass, and possibly the amp, it's hard to be specific.

If you measure a DC voltage on the output, but it dissipates (slowly, through the meter), it may not need a cap, but simply a resistor to "bleed" this to earth faster. A 100k-220K will generally be fine for this. If your bass is permanantly active, you could wire this right onto the jack, one side to the tip and the other to earth. If it has some active/passive setup, the resistor will have to be wired to the switch I'd think, so it stays on the active side.

If you measure a DC at the output and it doesn't shift, then either the output cap is faulty or there isn't one for some reason. (Some preamps with bi-polar supplies don't use one. Their earth simply floats and one battery supplies -9V and the other +9V.) I'd run a 10uF electrolytic cap in line with the output, as well as the 100K resistor. If the cap is not bi-polar, make sure the positive side goes to the preamp, and the negative side goes to the bleed resistor. I'd use an axial cap and a short piece of heatshrink.

Again, this is all speculation, so if you aren't experienced, please take it to a reputable tech. Pops etc, can be caused by various things...
 
#3
RobbieK,

Thanks for the response. This is a single +9VDC sourced internal preamp located within the bass. The tip to earth readings are about 3.6 volts upon initial power up and bleed down to less than half a volt after about 12 seconds. There is an immediate voltage spike upon power removal with approximately the same voltage and bleed off rates of opposite polarity. There is no schematic available however attempts to read the output impedance is impossible as there is definitely a capacitor charging regardless of meter polarities.

Not sure if I can post an active link here but I have attached the URL of the same problem under a different thread.

Popping and DC output voltage on my bass | TalkBass.com

I am still looking for clues and will probably get a variety of 1/4 watt resistors to test out.
 
#4
I'm still not clear on when you actually get the pop. Is it when you actually plug in the bass to the amp? (I'd also check that there's no DC on the input of your amp as well.)

As I mentioned, if there is DC, and it dissipates slowly, then this means there is a cap there, but it is only really bleeding back down through your multimeter.

If it is a typical opamp type preamp, then a 100K resistor across the output will make no audible difference, but will drop that output cap quick smart. In fact, you could easily go as low as 5K or less before you hear any signal loss with most opamp type circuits (and typical pedal or amp input Z's).

If I'm correct and there's no bleed resistor, it makes me wonder why the designer omited it in the first place. Perhaps the circuit was originally going to have its volume control on the output and it was shifted to another part of the circuit. Who knows...
 
#5
RobbieK said:
I'm still not clear on when you actually get the pop. Is it when you actually plug in the bass to the amp? (I'd also check that there's no DC on the input of your amp as well.)

As I mentioned, if there is DC, and it dissipates slowly, then this means there is a cap there, but it is only really bleeding back down through your multimeter.

If it is a typical opamp type preamp, then a 100K resistor across the output will make no audible difference, but will drop that output cap quick smart. In fact, you could easily go as low as 5K or less before you hear any signal loss with most opamp type circuits (and typical pedal or amp input Z's).

If I'm correct and there's no bleed resistor, it makes me wonder why the designer omited it in the first place. Perhaps the circuit was originally going to have its volume control on the output and it was shifted to another part of the circuit. Who knows...
Click to expand...


Placing the live output through any switch in/out device such as a pedal tuner causes the loud pop. I will follow up and check if the amp has some type of DC bias that may be causing the problem.

The other thread I listed was from another person with the same model bass so its definitely not just my particular unit.

Again, many thanks for the recommendations.
 

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