Tight or Loose?

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sepulchre
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Tight or Loose?

Post by sepulchre »

I guess this is pretty duh question, especially from someone who has built a few amps. But there's a matter of terminology that I never quite got. When it is said that the bass, for instance, is "tight" or "loose" what exactly does that mean?

I appreciate your indulgence.
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rsalinger
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Re: Tight or Loose?

Post by rsalinger »

sepulchre wrote:I guess this is pretty duh question, especially from someone who has built a few amps. But there's a matter of terminology that I never quite got. When it is said that the bass, for instance, is "tight" or "loose" what exactly does that mean?

I appreciate your indulgence.
"Focused" or "Flabby"?
Music is an expression of the inexpressable ~ Vernon Reid, Musician.
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Reeltarded
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Re: Tight or Loose?

Post by Reeltarded »

Typically more about filtering than anything else. Thump or woof. Cleaner sounds might be better with woof and dirty might prefer thump.
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sliberty
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Re: Tight or Loose?

Post by sliberty »

Tight bass usually refers to a bass response that stays cleanEr, and doesn't sound like the speaker cone is about to flap off of the frame. Loose (often used to describe poorly filtered amps like some tweeds) will have that overly distorted, overly compressed bottom end that is not pleasing to some (and quite pleasing to Neil Young fans). Tight bass is also characterized by less bass in some cases, although modern (60's or later) well filtered amps can have plenty of tight bass.
Mant
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Re: Tight or Loose?

Post by Mant »

I always think of loose as raw tweedy touch sensitive no negative feedback
Kind of sound.
Tight is more controlled with negative feedback.
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sepulchre
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Re: Tight or Loose?

Post by sepulchre »

Thanks for the info, guys. That 'tightens' my understanding of it. :^)

I'm building my first amp with NFB and I've read, as Mant said, that the more there is, the tighter the bass becomes. And since 'tighter' doesn't necessarily mean less bass, as sliberty says, that's what I want and I'll use the 16 ohm tap to get the most feedback voltage.

It also has a hand in determining how quickly it breaks up:
"No feedback will give alot more gain(some more noise), a bit looser bass and a slower shift from clean to dirty, as you add feedback the opposite things take place. "
This seems a little counter intuitive. I mean, less gain but gets dirty faster? Is this how it works? If so then I definitely want more feedback. This amp should have Plenty of gain.
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Koop
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Re: Tight or Loose?

Post by Koop »

sepulchre wrote:
It also has a hand in determining how quickly it breaks up:
"No feedback will give alot more gain(some more noise), a bit looser bass and a slower shift from clean to dirty, as you add feedback the opposite things take place. "
This seems a little counter intuitive. I mean, less gain but gets dirty faster? Is this how it works? If so then I definitely want more feedback. This amp should have Plenty of gain.
This is a great question...I'd like to try to understand this as well. I'm certain there are people on this board that can explain...please.
Jana
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Re: Tight or Loose?

Post by Jana »

I'll take a shot at the question.

First, a word about gain. Yes, NFB does reduce the gain of the power amp section. But, what that means is that instead of the preamp volume/master volume being set on 6 to get a certain level of power from the power amp, it might have to be set on 7. Not a big deal.

Another factor with the loss of gain is the narrower band from clean to dirty. How? Think of how the NFB works, it takes a portion of the signal from the OT secondary and injects back into the beginning of the power amp (out of phase, hence negative feedback). This signal does not add to, but instead subtracts from, the overall signal gain. Put another way in simple terms, feed 10 volts into the power amp, capture 1 volt from the output that is out of phase and send it back to the input-- 10 - 1 = 9. In effect what it does is sort of a compression if you want to think of it in guitar effect box terms.

The NFB does other things too such as increase the bandwidth, reduce some distortion, etc.

I personally like a lot of NFB. I like the tight low end, the feeling of a very responsive, tight "Chunk" when I hit a chord. It seems like it is more precise and authoritative. I suppose it also stems from my past--I always played closed back cabinets with Marshall type heads--no reverb. People who grew up on fender's with the open back and reverb probably won't like as much NFB. Just a guess.
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sepulchre
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Re: Tight or Loose?

Post by sepulchre »

I know how NFB reduces the gain. Still not clear on how that narrows the band from clean to distortion. I'll do some more research. However, I'm certainly with you on the tone. I have used Fender open backs, though it's been awhile. But I do want a responsive feel. Right now I'm using AX84 1X12 boxes which do have a port on the back but I wouldn't exactly call them "open back". Plus I am hoping to see a 1960a box in my future, but who knows when.

So I'm going to try using the 16 ohm tap for the NFB and see how that goes. One question: I note that on the schematic I'm using the NFB resistor is different depending on the tap used, ie. 100K for 16 ohm, 56K for 8 ohm and so on. Is this to keep the NFB voltage about the same no matter which tap is used? Or does one end of the tap scale (4 or 16) yield more than the other, in spite of the resistor difference? Is this some thing that can be played with; tuned to taste?

Thanks for all the helpful responses!
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martin manning
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Re: Tight or Loose?

Post by martin manning »

The voltage ratio across the transformer is inversely proportional to the square root of the impedance ratio, so for a given primary voltage the voltage at the 16 ohm tap is 1.4x the voltage at the 8-ohm tap, and the feedback resistor will have to increase proportionally to maintain the same FB ratio. FB ratio is certainly something that can be adjusted to taste.

A more sudden break-up occurs with NFB because when the amp is driven hard enough for the power section begin to clip, the output amplitude is reduced relative to the input. The fixed FB network returns the same fraction of the output voltage, however, so when clipping occurs there is relatively less NFB, resulting in more gain, less damping, and a quick descent into distortion.

NFB tightens bass because it reduces the effective output impedance of the amplifier, which increases damping of the speaker cone motion.
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sepulchre
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Re: Tight or Loose?

Post by sepulchre »

Wow! Great explanation. Now I get it.

Thanks a million!
gingertube
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Re: Tight or Loose?

Post by gingertube »

To vary what Martin said above a little.

Feedback replies on sacrificing gain to achieve its results.

As gain reduces as you approach the clipping level the resulting reduced feedback will cause the amp to move even further toward clipping.

When a power amp clips the effective gain goes to ZERO and so does the feedback action.

So feedback will cause a power amp to go into clipping earlier but once you hit the clipping threshold there is effectively no feedback and response becomes loose, or at least, response to that part of the signal which is above the clip threshold becomes loose.

The effect of this is for clipping to become harder which produces more higher order harmonics which in moderation gives the amp "edge" but when overdone can make the amp sound "harsh".

If you look at what the engineer types call the transfer function, that is, the output voltage vs input voltage graph, you will see that as feedback is increased the change from linear response to saturated changes from a smooth curve to a more abrupt corner. The smootherc this is the lower order the harmonic content, and conversely, the more abrupt the "corner" the highr the order of harmonics produced.
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