Two mic inputs
Many amps have two mic inputs that are described as "Input #1 is your high gain or normal input and #2 is the low gain input" I know that there are XLR to 1/4" cables and there are impedance matching transformers for connecting low impedance mics to high impedance inputs.
What is the purpose of the 2 inputs - each being 1/4" types yet describing them as high gain and low gain? Could you connect a low impedance mic to the low gain input using an XLR to 1/4" cable or do you still need to use a impedance transformer?
And I guess finally, I could just 'mess around' with cables and inputs and mics and find out what the result is BUT not really understanding the science behind it all, I'm not wanting to find out that one of the results could be a blown tube, amp or circuit board. Hope the question makes sense and thanks in advance.
Thanks Skip - a good explanation. I'm guessing then that any other amp that has the two inputs (e.g. HarpGear, Kalamazoo) is based on the same idea??
Also - and now I'm thinking its safe to 'experiment' with mics into either and even both at the same time- ????
Again, many thanks for the explanation. Much appreciated
Hello Hank- Although I am not familiar with the HarpGear amps, most vintage amps with two inputs for a given channel use this set-up. It is absolutely fine to experiment with using one or both of them. There are photos of Little Walter using two small stick mikes taped together.... I'll bet he was plugging in to two amps or two inputs at the same time.
Skip Simmons
Thank you Skip! Away I go. Best wishes for a Healthy and rewarding 2012.
Hello Hank- A good question! Fender began using this set-up way back in fifties and it is deceptively simple, like many of Leo's designs. To understand the reason for the high and low inputs, you have to remember that Fender expected people to use their amps as a PA system as well as an instrument amp, that's why there is more than one jack on even a little Champ. The "number two" inputs were intended for microphones and were deliberately made with less gain in order to help with feedback.
Now, how does it work? First realize that the input signal from either jack travels down the same little piece of wire to the first preamp tube. There really is no separate tube circuitry for the individual jacks on a given channel.
Next, a normal 1/4" jack has just two connections, - plus and minus. A Fender input jack has a third connection which contacts the "hot" lug when NO plug is inserted and has no contact to either lug when a plug IS inserted. Fender uses these three-contact "shorting" jacks and just three resistors in a simple passive circuit (BEFORE your mike signal even gets to the first tube) to accomplish three things: first is to silence the amp when no cord is plugged in, next is to make input number two have less gain than input number one, and the third is to establish the "input impedance" to ground for both the preamp tube and the guitar or microphone.
Check out the schematic of any classic Fender and you will see that the shorting contact on the number one jack is connected so as to lower the input impedance of input number two, which reduces the gain on that jack.
Whew!! I hope I explained it so everyone can understand it! That was a tough one.
Skip Simmons
Skip Simmons Vintage Amps