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Author Topic: What's the Difference Between Hi and Lo input jacks on some amps?  (Read 5576 times)

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Dante

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I thought I'd ask. What's the diff?

Is the HI for a stronger signal? Like humbuckers? Whereas LO would be for single coils?

Or, is the HI for plugging my rackmount preamp in (because it's a hotter signal and I have no FX loop), and the LO for standard signals like a plain ol' guitar?

Or...something else entirely?
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PrimalScream91

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I believe that the Hi input attenuates the signal, the low input does not.

That is a criminally simple way to put it, but it's what I've always understood. Some will tell you that your pickup type (active/passive/single/hum) determines what input you should go into, but I've never bought into that.
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MarshallJMP

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Normally the Hi means HIgher sensitivity, so in guitar player language more gain  >:D
And lo ,well LOwer sensitivity.In the old marshall heads the difference between both is 6dB (voltage ratio of 2)
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van Sinn

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One is High, the other is Low.
You use the Hi when you wanna get High, you choose the Lo when you wanna chill, no?  :dunno:

Oh well, one is a high impedance input, the other has a lower impedance.
They're used for matching different sources with different impedance.
Depending on how you source behaves, you may see/hear not too much difference, or one may sound/work better than the other.
Or as MJMP just stated it (annoying how often those university folks tends to right so often :lol: )

EDIT: Oh, I was sure I'd seen different impedance levels too; actually pretty sure my Sunn Model-T had it this way, but schemos are packed down, so won't be checking up on this.
« Last Edit: Time Format by vansinn »
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MarshallJMP

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The impedance doesn't change much.
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rnolan

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It very much depends on the amp (preamp) and the era, it can mean hi and low sensitivity (initial input gain) as MJMP said or it can also mean low and hi impedance (though this is not common these days).
Early amps had multiple inputs to handle the whole band, guitar, mic, bass .. as PA's were still relatively rare, reasonably primitive and very expensive, so their inputs were designed to take the common signals from all the band.
When more specific PA style amps came out (with their associated column speakers) they accommodated (sometimes) hi and lo impedance mics.  This changed when mixers and multicores came along, hi impedance looses top end after 25 feet (ish) and mics (mostly) went to 600ohms and balanced (came a bit later ?).
The hi and lo inputs on guitar and more commonly bass amps came with the uptake of active PUs which generally had higher output than passive PUs which overloaded the preamp input (in not a nice way). So it's very much a read the manual thing.  These days I see it mostly on bass amps to accommodate the hi output signal from active bass PUs.
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MarshallJMP

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Well I was talking about guitar amps in general but yes R has a point.
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Dante

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I see that every post has a different answer. Each is logical enough to believe

I thought the Hi was on top and the Lo was on bottom, but these are side-by-side, so that got me wondering  ::)
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rabidgerry

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the guys have already explained this, but I hit upon it when talking about my bass players bass amp on another thread which has a high and low input.

My basic understanding of it has always been, the "Low" input is some DB lower volume and therefore more suitable say a guitar with active pickups.

The "High" input is for guitars with passive pickups and therefore is a little louder.

So the low is lower signal volume to compensate for the active pickups extra volume.

So since I have no active pickups I always use high inputs on any equipment that has it, such as my Peavey Rockmaster preamps which have high and low input.

I got my definition for these inputs from the manuals for certain gear, and the have always said, "active pups for low inputs" "passive pups for high inputs".

If this is wrong someone correct me. :facepalm:
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rnolan

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The terms Hi and Lo get used for a few things in different ways (e.g. my MB1s have Hi and Lo or Full outputs for bi Amping or not).  To be sure what it means for a particular unit, check the manual/specs.
RG what you are describing is (I think) the most common but I'd think of it as Hi gain (coz lower output PUs so need more input gain) and Lo gain as the active PUs are generally (not always) higher output.  In the input circuit the difference is probably just some "pad" resistors between the Lo jack and where it meets the input circuit.
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rabidgerry

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well in the context of what Dante is asking Richard, the MB1 info isn't to do with the hi and lo inputs on guitar amp

RG what you are describing is (I think)

uh yes, so one is more suitable for passive pickups and the other active pickups (without nit picking and making things convoluted - he wants a basic understanding of what these are for unless I'm mistaken)

I've never viewed the hi and lo inputs as one has more gain than the other, but I guess gain is volume and not distortion as is generally thought so in a round about way I AM thinking one has more gain than the other so that's a good way of looking at it Rn.

My actual experience of using hi and lo inputs is always been, the guitar sounds quieter when using the lo.  My first amp (which I still own) had the hi and lo inputs and I just used the hi always because I didn't have to turn the volume knob up so much as I did when I used the lo input.  There was no extra distortion using the hi as opposed to the lo, only a difference in volume.
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rnolan

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Hey RG, I used the MB1 just as an example of other places those terms/words are used. Obviously they have to be taken in context.  Although even then I've seen examples (not often) of them used the other way around e.g. Lo means the inst has low output so boost more, Hi it has high output so boost less...

In the end it's like a simplistic "fixed" version of the gain control on a mixer channel, Lo = knob turned down (as plenty of signal), Hi = knob turned up as signal needs boosting, so pick 2 ballpark points (which cater to most insts') and fix/set them in the circuit.
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rabidgerry

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Hey RG, I used the MB1 just as an example of other places those terms/words are used. Obviously they have to be taken in context.  Although even then I've seen examples (not often) of them used the other way around e.g. Lo means the inst has low output so boost more, Hi it has high output so boost less...

In the end it's like a simplistic "fixed" version of the gain control on a mixer channel, Lo = knob turned down (as plenty of signal), Hi = knob turned up as signal needs boosting, so pick 2 ballpark points (which cater to most insts') and fix/set them in the circuit.
having the lo and hi the other way around would totally confuse me, mainly because I have only ever come across the examples that have the Hi input for passive/quieter pups and lo for actives.  But I'm not surprised there is someone somewhere who has varied the usual meaning of each input.
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Kim

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The QuadTube has an Input Sensitivity Switch on the front panel next to the Input jack that may or may not do what other amps use separate Hi and Lo Input jacks for.

So, according to that QuadTube manual:  "Input Sensitivity Switch -  Sets the input sensitivity of the QuadTube 150M.  If you see the Input Clip LED coming on when you play hard, then switch to Low."

I notice a change in the actual sound when I switch this to Low:  seems like there is a bit less Gain and also a bit less of the higher EQ freqs in that Low setting than what I hear in High. 

FWIW, plenty of amps have had the option of using High and Low Inputs long before active pickups were invented, and it's also entirely possible that each amp manufacturer had their own different ideas in mind when designing the Input jack circuitry.   The Input jacks on say a Supro, Sunn, HIwatt, etc. may not have the same intentions as say a Marshall, Fender, Dumble, etc.
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MarshallJMP

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Well now I think of it,with a JCM800 master volume (or the older JMP) 2203 and 2204 models the lo input bypasses one half tube.
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