Hissing noise in active mode, no hiss when passive

[drplanet]

I recently installed some Bartolini pickups (humbucking) and an NTMB+F preamp, with a passive bypass option. The pickup and control cavities are all shielded with copper foil connected to ground, and the bridge, strings, and even pickup screws are all grounded as well. In passive mode it's dead quiet when I mute the strings, even with the amp at high volume. But in active mode there's a background hiss that's mild but annoying when the EQ is flat, and unbearable when the treble is boosted significantly. It's a static-like, white-noise kind of sound, not like the buzz I was getting before I shielded the bass.

I don't see any gaps in the shielding or possible shorts in the wiring. And I tried a few different amps and power outlets, and shut off the light dimmers and a few other potential noise sources.

I can't imagine this is the baseline level of noise I should expect from the preamp, given other people's experience with that model, so it seems like noise must be sneaking in somehow. Has anyone had a similar experience and found a solution, or the source of the problem?

 

[Crater]

in active mode there's a background hiss that's mild but annoying when the EQ is flat, and unbearable when the treble is boosted significantly.

It sounds like the preamp is defective; I would contact Bartolini for a possible warranty replacement of your preamp, as that's most likely the source of the problem. Active components like op-amps and transistors can fail in a way that causes them to generate excess levels of white noise (hiss).

 

[byacey]

It could be the improper impedance loading presented by the pickups to the preamp input. This is very important to minimize the first preamp stage noise, which typically dominates the noise floor. It comes down to what impedance the designer was aiming for, and your pickups may be considerably off from that value.

Or it might be just a poor design.

 

[drplanet]

Thanks for the input! I have been in touch with Bartolini, we're trying to rule out other sources of noise before any warranty replacement of the preamp itself. Since both the preamp and pickups are Bartolini, I don't think it's likely to be an impedance mismatch issue. The resistance readings of the pickups and blend pot leading to the preamp match the spec values.

 

[byacey]

The noise of the first preamp stage usually dominates the overall signal to noise ratio of the entire device. Optimizing the preamp for least noise is good engineering practice.

 

[rockersunny95]

Thanks for the input! I have been in touch with Bartolini, we're trying to rule out other sources of noise before any warranty replacement of the preamp itself. Since both the preamp and pickups are Bartolini, I don't think it's likely to be an impedance mismatch issue. The resistance readings of the pickups and blend pot leading to the preamp match the spec values.

Hi, I am facing similar issue with my bartolini NTMB+F preamp. Would like to know if your problem got solved and how? The noise is irritating tf out of me!!

 

[wilco68]

I recently installed some Bartolini pickups (humbucking) and an NTMB+F preamp, with a passive bypass option. The pickup and control cavities are all shielded with copper foil connected to ground, and the bridge, strings, and even pickup screws are all grounded as well. In passive mode it's dead quiet when I mute the strings, even with the amp at high volume. But in active mode there's a background hiss that's mild but annoying when the EQ is flat, and unbearable when the treble is boosted significantly. It's a static-like, white-noise kind of sound, not like the buzz I was getting before I shielded the bass.

I don't see any gaps in the shielding or possible shorts in the wiring. And I tried a few different amps and power outlets, and shut off the light dimmers and a few other potential noise sources.

I can't imagine this is the baseline level of noise I should expect from the preamp, given other people's experience with that model, so it seems like noise must be sneaking in somehow. Has anyone had a similar experience and found a solution, or the source of the problem?

I just returned an MTD Kingston AG5 for all the same noise issues you mention.

 

[luchorojas]

maybe the problem is the master pot, if it is 250k, it´ll have hiss, change the master pot value for 25k

 

[iplay5strings]

maybe the problem is the master pot, if it is 250k, it´ll have hiss, change the master pot value for 25k

I would not advise this with an active/passive setup, a 25k pot would kill the top end of the pickups because of treble bleeding to ground. On an active only setup (where the volume pot is isolated from the pickups), what you say is accurate. Where do you have the internal input trim set on the NTMB? I would start with it all the way down so you have a quieter input gain stage going into the NTMB which should allow you to use the EQ controls with less noise as well.

 

[Tritone]

I Just saw this thread. IMHO, the newer single-chassis units do seem to have an inordinate amount of background hiss compared to the older two-part system. My opinion is based on the two newer units I purchased and had installed by trusted luthiers. I wound up replacing both with Nordstrand preamps (which are dead quiet), because the hiss was quite pronounced, even with the EQ set flat and the internal gain reduced. I've played active instruments exclusively for well over 30 years (and I'm a Bartolini guy), so I'm used to a bit of background hiss. However, the amount produced by both of these units was totally unacceptable. It could be that I just happened to get two bad units within 6 months of each other, but I have a hunch it has something to do with the new design and/or cheaper components used in their manufacture. IMHO.

 

[micguy]

I can't imagine this is the baseline level of noise I should expect from the preamp, given other people's experience with that model, so it seems like noise must be sneaking in somehow. Has anyone had a similar experience and found a solution, or the source of the problem?

One of the reasons I don't like active basses is that so few of them are really designed well when it comes to the preamp. I designed a lot of low noise audio circuits in my career, and most onboard preamps are not done to standards that I would accept. Noise is one of the most common things that is not done well. EQ that can be dialed to "painfully bright" is the second. That treble boost makes the noise problem even worse.

The noise of the first preamp stage usually dominates the overall signal to noise ratio of the entire device. Optimizing the preamp for least noise is good engineering practice.

It is, and it'd be nice of that was done, but....if you look at the parts that are being put into onboard preamps, most are optimized more for long battery life than for low noise. There is a tradeoff there that many designers make that I would never consider.

 

[Jerry Catanescu]

It is not hard to design an onboard pre that's essentially dead quiet for most practical purposes. You might just have to use a $2 or $3 chip instead of a $0.50 one, and also make some well-informed choices when it comes to surrounding resistor values. The theory behind it is (or should be) well known and abundant.

You can have low power, low noise and low cost -- but you have to choose two.

 

[kyral210]

Good luck with fixing the issue!

I had Epiphone Thunderbird Pro pickups that I thought the preamp was bad on. I stripped everything out and wired the pickup to the out jack. There was still some hissing. I bought a new set of pickups, hand wound with metal shielding covers, and the hiss was still there. It turned out that I was boosting too much in my signal chain.

Maybe your active circuit is adding a volume boost, which is amplifying inaudible noise into audible. If there is a volume boost when the active is engaged, run an EQ pedal flat with a level cut so you can have equal volume when engaging the EQ when the active circuit is on. Worth trying to see if the hiss is still there.

 

[Jefenator]

I'm wondering about the gain, too.
IME the boost on Bart pre's can be important for bringing softer passive pickups up to a normal level - and the internal trim pot is great for matching other instruments.
Hiss & hum only bug me when they are a distraction with a group. Magnified at home (with bad house wiring) I don't sweat it too much.

 

[byacey]

It is not hard to design an onboard pre that's essentially dead quiet for most practical purposes. You might just have to use a $2 or $3 chip instead of a $0.50 one, and also make some well-informed choices when it comes to surrounding resistor values. The theory behind it is (or should be) well known and abundant.

You can have low power, low noise and low cost -- but you have to choose two.

Matching the impedance of the pickups to the input device is critical. Bipolars are better suited to low impedance sources, whereas FETs are better for higher impedance sources , such as a typical bass or guitar pickup. If the input stage is discrete devices, it should be included within the feedback loop of the subsequent opamp stage.
I would control the levels after the opamp stage rather than at the front end between the pickups and the 1st preamp stage. This maintains a constant input impedance and allows the input to be balanced, instead of single ended.

 

[Lownote38]

I just installed this Bartolini preamp in an old '95 Fender MB-5 using the stock pickups, and it's dead silent. I don't have the passive/active option on mine. It's just active. Perhaps that push/pull pot is the issue. (??). Seriously, mine is dead silent no matter where the gain is set on the preamp.

 

[luchorojas]

I have a lakland SK-5DX with Bartolini MM+J pickups and NTMB+F, with 250k pot I had a very present hiss noise, even with treble pot in mid position, when I changed the pot to 25k, (with push pull) this fix it all, the hiss is very low, completely aceptable, the passive mode in my case is only for S.O.S cases, I really dont used it, but it´s possible yet. Also the sound in my ears is too much better with 25k pot (active mode).

 

[Derek Kay]

I have found I prefer passive PUs to active for frets. Best example: I added a bypass switch to my Stingray 5 to offer the PU out passive. The sound was quieter although still distinctly MusicMan, but sounded more natural, especially for recording.

My fretted basses are now all passive humbuckers, but I had the luxury of my Walter Woods and it's incredible preamps. These will eat any 9v onboard/pedal/box for quality, and are the first port-of-call for my PUs. Boxes & effects follow that in the chain.

It helps if the PUs, cavity and ALL the wiring is shielded, and the PU heights carefully configured for an equal response from each string. Then you can add more gain on the preamp and match active basses, but with a natural, studio quality sound.

I have contoured my vol & tone pots with various resistors & capacitors to get a range of onboard settings like an active bass does.

Conversely, I like my active EMGs on my fretless P-bass!
And long leads are not a problem...

 

[JakesAy]

Just ripped out the bartolini NTMB FL preamp from my tobias killer B bass for this same reason.. the floor hiss was unbearable. I installed a G&L MBEQ3 18v preamp and it is much better and quieter. The eq points are smoother and usable.