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Small (but nice) amp: low-freq "phase beating"?


PhilMan99

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I've got a Fender Bassman 25 (not my only amp, but I like it for practice), but this question should apply to small amps in general.

 

I got the Bassman 25 so I could "crank-up" the low EQ (bass knob). Most other small amps for same or less $$$ didn't do nearly as well (clipping, rattling, other nasties). It generally works great, but I've started noticing something - sort-of a "phase-shifting beating". With the low EQ (bass) turned-up, on the low notes I hear sort of "beating" like you get when two notes at a VERY slightly different frequencies collide - remember the "fat" synthesizer sound with 2-3 oscillators? It differs, depending on what note I play, but is limited mostly to the E string between 3rd & 7th frets.

 

I went to the store to hear another Bassman, and it does the same thing, so I don't think I've got a bad unit. This problem is most noticeable with no background noise. When jamming with others, or playing with background noise (stereo) I can't really hear the "beating", but in a noisy environment, sounds a bit like distortion.

 

I'm thinking this is related to trying to get such a low sound out of a small cabinet (Bassman 25, 33lbs), but I don't know much about speaker design. I will observe that the Bassman 25 cabinet is "ported" (2 small ports on front, different tube lengths).

 

Anybody notice anything similar?

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Well, I had the same problem with one of my little amps....

 

'til I turned off the ceiling fan. You got a fan running?

"Let's raise the level of this conversation" -- Jeremy Cohen, in the Picasso Thread.

 

Still spendin' that political capital far faster than I can earn it...stretched way out on a limb here and looking for a better interest rate.

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Originally posted by davebrownbass:

Well, I had the same problem with one of my little amps....

 

'til I turned off the ceiling fan. You got a fan running?

No fan, either at home (my Bassman 25) or music store (their Bassman 25, different amp). At this point I'm not clear if it is:

* Fundamental "design issue" with Bassman 25 pre-amp or power-amp

* So much low EQ (both bass and amp), making more obvious the small cabinet dual-ported design of the bassman 25.

* A/C-line interference (although I discount this, since the same thing happens at music store)

 

I bought the Bassman 25 to get "big amp" sound out of a small amp, although at low volume. I learned on a 2x15" Acoustic cabinet years ago, so I'm used to a lot of "bottom-end". Maybe I'm just hearing the trade-offs of a smaller cabinet...

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Hmmn, sounds like maybe it's just a matter of pushing a smallish amp a bit hard. Not the cab, but the amplifier itself. What's the rated power of this amp? I would bet that the power supply is sweating it a little on some low notes with the low-eq turned up like you describe.

 

I assume that this is a solid-state amp; some tube amps will suffer from a poorly matched driver-tube, even with well matched output-tubes, with some frequency cancellation and reinforcement problems. Perhaps a similar phenomenon is occurring here.

 

Power supply shortcomings are often especially apparent in the low frequencies. It doesn't necesarily mean that there's something wrong with the amp, it might just be a little strained at some frequencies when you put a lot of demand on it. I've had some similar experiences with a little Danelectro "Nifty-Fifty" model guitar amp; it sounds surprisingly good for a clean jazzy sound, but I can't turn up the bass quite as high as I would like without a similar problem. -k

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Man, aren't you expecting a lot from the little guy? I bet that if you roll back the bass just a little, and roll back the volume just a little too, just enough on both to only hear a slight overall reduction in low-end and volume, that you'll hear a disproportionate drop in the "phase-beating" phenom. Maybe it won't completely go away, but it'll probably smooth out some.

 

If not, if I'm completely wrong, then let me know, and we'll both have learned something here! -k

 

P.S.- Try it through another, bigger cab sometime, and see if this still happens, with all the controls set the same way. Then, you'll know for sure if it's the cab design or not! -k

Ask yourself- What Would Ren and Stimpy Do?

 

~ Caevan James-Michael Miller-O'Shite ~

_ ___ _ Leprechaun, Esquire _ ___ _

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Originally posted by CaevanO'Shite:

Man, aren't you expecting a lot from the little guy? I bet that if you roll back the bass just a little, and roll back the volume just a little too, just enough on both to only hear a slight overall reduction in low-end and volume, that you'll hear a disproportionate drop in the "phase-beating" phenom.

It does it with volume way down - the Bassman 25 is just for low volume practice, and it does it with volume at 1/4. Turning the bass to 1/2 on both amp & bass make it go away, but any more bass EQ and I can hear the "beating". I don't expect miracles, just trying to understand sound, cabinet design and amps a bit better.

 

Note: my Bassman 200 does not exhibit the "beating" at all.

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