Amps with multiple inputs

andy__woods · October 11th, 2006, 10:45 PM

Hey guys. Whats the deal with multiple inputs on an amp? The Hot Rod Deluxe I got last week has 2 inputs, and if I recall correct, a Bassman has 4 inputs. Whats the reasoning for this? Do different inputs provide a different sound? I can understand the VOX AC30, it has a normal, and then top boost, but I don't know about the others.

Educate me! This is a great place to learn, and I know you guys will give plenty of good answers

Dacious · October 12th, 2006, 01:55 AM

Because electric guitars or other instruments have different 'impedances' on their pickups, so do inputs. Normally you have a 'high' and 'low' impedance. Many people think that is high and low volume but it isn't.

Your guitar has a coil of wire wound thousands of times around a magnet/s or polepieces which contact a magnet. You can measure an 'impedance' or resistance to an AC signal, which is really a tiny oscillating voltage cycling through a sine wave.

In order to maxmimise this voltage, the other end of the cable needs to meet a similar 'impedance'. Most passive guitar pickups are high impedance. So you would normally go into the 'hi' input, as this would give the most gain (volume) and dynamic details.

If you have low impedance pickups (few windings, driven by a preamp) like say active pickups or a keyboard which normally have line-level outputs, you put it in the low. Putting a high impedance pickup in the low will drop response somewhat therefore volume and character will seem somewhat more muted. Sometimes, some effects units will work better into the 'low' as well.

Your amp has only one channel - the 'drive' is actually cascading more gain into the one signal path, not an actual second channel. Many 4- or 6- input amps like the Bassman have a separate preamp intput each with a low and hi (half a 12AX7 which is really two small gain amps in one).

People don't normally do it, but there's no reason you can't plug two guitars in one amp with multi-channels. Plugging two guitars into the low and high inputs on one channel probably won't give good results. It is not going to damage anything, but won't sound flash.

JohnnyCrash · October 12th, 2006, 02:55 AM

In some cases the reasoning was very pragmatic. In others it was merely a bright cap or some simple tweak to make it a "different" input.

Some older Fenders for instance had a mic and instrument input (impedance).

Also, some "high and low" input circuits vary in the way they actually function.

Some inputs function, tonally, exactly the same. They're just there for more input sources. Some are different (bright channels for instance, then there's top boost, high/low, etc). Sometimes a different circuit had little to do with impedance, it just was a different tonality (a bright input for instance sounded, uh, brighter). Other times it acted different, whether it was impedance or an entirely different preamp circuit.

This kind of design mentality is where some folks jumper channels together. In most cases it is pretty much running two halves of a 12AX7 in parallel, which, to me, doesn't have much of a "wow" factor.

It all depends on the circuit and what each input is intended/designed for.

macduff · October 12th, 2006, 08:44 PM

Also, back in the "old" days, sometime, all the instruments in the band, and the vocals would all be plugged into ONE amp... I've seen pix of bands with the guitars, accordion and mike all in one amp...

Oh, and by the way, when you plug more than one guitar into the inputs of some older Fender amps, both guitars take on the tonal characteristics of the "Brite" channel.

October 11th, 2006, 10:45 PM	#1 (permalink)
andy__woods Tele-Afflicted Join Date: Sep 2005 Location: Michigan Age: 23 Posts: 1,169 60 Photos	Amps with multiple inputs Hey guys. Whats the deal with multiple inputs on an amp? The Hot Rod Deluxe I got last week has 2 inputs, and if I recall correct, a Bassman has 4 inputs. Whats the reasoning for this? Do different inputs provide a different sound? I can understand the VOX AC30, it has a normal, and then top boost, but I don't know about the others. Educate me! This is a great place to learn, and I know you guys will give plenty of good answers

October 12th, 2006, 01:55 AM	#2 (permalink)
Dacious Friend of Leo's Join Date: Mar 2003 Location: Godzone Posts: 2,612 35 Photos	Because electric guitars or other instruments have different 'impedances' on their pickups, so do inputs. Normally you have a 'high' and 'low' impedance. Many people think that is high and low volume but it isn't. Your guitar has a coil of wire wound thousands of times around a magnet/s or polepieces which contact a magnet. You can measure an 'impedance' or resistance to an AC signal, which is really a tiny oscillating voltage cycling through a sine wave. In order to maxmimise this voltage, the other end of the cable needs to meet a similar 'impedance'. Most passive guitar pickups are high impedance. So you would normally go into the 'hi' input, as this would give the most gain (volume) and dynamic details. If you have low impedance pickups (few windings, driven by a preamp) like say active pickups or a keyboard which normally have line-level outputs, you put it in the low. Putting a high impedance pickup in the low will drop response somewhat therefore volume and character will seem somewhat more muted. Sometimes, some effects units will work better into the 'low' as well. Your amp has only one channel - the 'drive' is actually cascading more gain into the one signal path, not an actual second channel. Many 4- or 6- input amps like the Bassman have a separate preamp intput each with a low and hi (half a 12AX7 which is really two small gain amps in one). People don't normally do it, but there's no reason you can't plug two guitars in one amp with multi-channels. Plugging two guitars into the low and high inputs on one channel probably won't give good results. It is not going to damage anything, but won't sound flash. __________________ My other Telecaster is a Thinline The Tele Bible, Ch 1, v 10 Love thy Telecaster, covet not thy neighbour's Strat!

October 12th, 2006, 02:55 AM	#3 (permalink)
JohnnyCrash Poster Extraordinaire Join Date: Mar 2005 Location: Lost Angeles and Orange County Posts: 7,128	In some cases the reasoning was very pragmatic. In others it was merely a bright cap or some simple tweak to make it a "different" input. Some older Fenders for instance had a mic and instrument input (impedance). Also, some "high and low" input circuits vary in the way they actually function. Some inputs function, tonally, exactly the same. They're just there for more input sources. Some are different (bright channels for instance, then there's top boost, high/low, etc). Sometimes a different circuit had little to do with impedance, it just was a different tonality (a bright input for instance sounded, uh, brighter). Other times it acted different, whether it was impedance or an entirely different preamp circuit. This kind of design mentality is where some folks jumper channels together. In most cases it is pretty much running two halves of a 12AX7 in parallel, which, to me, doesn't have much of a "wow" factor. It all depends on the circuit and what each input is intended/designed for.

October 12th, 2006, 08:44 PM	#4 (permalink)
macduff NEW MEMBER! Join Date: Sep 2006 Location: Lower Michigan Posts: 4	Also, back in the "old" days, sometime, all the instruments in the band, and the vocals would all be plugged into ONE amp... I've seen pix of bands with the guitars, accordion and mike all in one amp... Oh, and by the way, when you plug more than one guitar into the inputs of some older Fender amps, both guitars take on the tonal characteristics of the "Brite" channel.