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March 25, 2023, 07:34:05 AM

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Distorted sound only on "High" input

Started by solidstatularbells, December 02, 2022, 11:27:04 AM

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I bought a used Gear4Music house brand 35W solid state amp. 2 inputs (High & Low), TMB EQ stack, Master Volume. The PCB has "BELCAT CORP. SED-03" printed on it.

I have no schematic for the circuit.

On the Low input, it works fine, but on the high input it distorts when I play hard. Particularly low notes.

What could be the fault? I am not an electronics expert, but I can certainly manage to replace a capacitor or resistor. I assume the High input only has a few extra components so that it will be easy to replace just those.

Of course, if anyone has the schematic on hand, that would be useful.


From your description, I would say the amp is acting normally. What would you have it do differently ?


Agree, Mr Thud
If it has master volume then likely it has a gain pot as well
Try turning the master to full then turn down the gain pot.


Are you using two different guitars when you try the high and the low input ?
Does one of the guitars have active pickups and the other normal pickups ?
Some guitars have active pickups that have a battery and electronics installed in the guitars, they put out a larger signal then standard guitars with normal pickups.
That's the reason why some amps have a High and Low input (as far as I know).
One input is for the smaller signal of a normal guitar and the other input is for a stronger signal from an amplified active pickup guitar.
 This is not to be confused with amps that have more then one channel, usually one clean channel and one or more channels with distortion.

I think you will find that the amplifier is working like it should.
Does one input sound louder then the other with the controls set the same ?

There are no stupid questions.
There are only stupid mistakes.


Quote from: Loudthud on December 02, 2022, 07:45:55 PMFrom your description, I would say the amp is acting normally. What would you have it do differently ?

I always thought they worked the other way around.  High input attenuates the signal, so it should be more quiet and less distorted.  Low input is straight in.  Is this incorrect?
Life is what you make it.
Still rockin' the Dean Markley K-20X


Hi Joe, Yep can be confusing.
My understanding, it's about the Z (impedance) of the input.
A low Z input on guitar amps is for big signals or even line input.
While a Hi-Z input is very sensitive so as to pickup very small signals.
In the early days guitar PU's were low output and needed a very sensitive input to amplify well. (Hence the 1Meg grid resistor on the old Valve gear)
These days even the cheapest guitars have much bigger outputs than the 40's 50's era so the preamps will redline much earlier on the dial using the pu's around nowadays.

The old Fender circuits used 68k resistors on the Grids of V1 for LOW input. while the HIGH input switched the input Z to 1Meg. far more sensitive.

With super hot pickups you are likely better served by limiting the input Z to about 220k.
I've noticed this with some of the hotrod boutique Amps.
A Carvin Legacy is an Example of this where in put grid R is 220k.
With multi stage preamps this keeps thing from going stupid and also helps keep circuit noise lower.


There are some amps where the high and the low inputs are opposite to most normal amps, I can't remember which ones they are.

I think the most important thing to remember when choosing low or high input is to use the one that makes you smile, end of story.
There are no stupid questions.
There are only stupid mistakes.


I think the Lab Series schematic explains it.You cannot view this attachment.
There are no stupid questions.
There are only stupid mistakes.


Hi TassieV,
           I guess it depends on what the High/Low is referring to?
The size of the source signal or the sensitivity of the input, take your pick ;)

In the case of the Lab circuit
The LOW input would be much louder than the HIGH input for a given voltage.

But as the Lab uses *Inverting* inputs then yes the Hi input is HiZ  but with much less gain.

BTW The Lab input design is not an ideal setup, there are better ways.