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previous: David Manson Anyone wanna take a crack a explain... -- 11/6/1999 8:58 PM

Re: Advanced feedback loop questions..

11/8/1999 1:25 PM
GFR	Re: Advanced feedback loop questions.. The type of feedback used in most guitar amps samples output voltage and subtracts a voltage from the input voltage, so it's called voltage-voltage or parallel-series (you connect the loop in parallel with the output, to sample the voltage - when you want to read a voltage in a load you just put your voltmeter in parallel with that load - and in series with the input, to subtract from the input voltage - voltages in a loop add together). When you sample the output voltage, the feedback loop tries to make the amp behave more like a constant voltag source. Say the impedance of the load changes and the output voltage tries to increase. The amount of signal that is fed back also increases and so the output voltage is kept more constant than without feedback. Making the output voltage less dependant of the load is decreasing the output impedance or increasing the damping. If you sample the output current the amp behaves more like a constant current source. Say the impedance of the load changes and the output current tries to increase. The amount of signal that is fed back also increases and so the output current is kept more constant than without feedback. Making the output current less dependant of the load is increasing the output impedance. You can sample the output current with a resistor in series with the output - when you want to read a current into a load you just put you DMM in series with the load. The kind of feedback used in the Valvstate is called "current-voltage" or "series-series". The way you mix the feedback signal with the input signal affects the input impedance of the amp. When you have voltages being subtracted in a series connection, this increases the input impedance. This is the case of both the "classic tube amps and of the valvstate. When you subtract a feedback current from the input current with a parallel connection - the currents into a node add -the input impedance is lowered. This is the kind of feedback used in a Fuzz Face.

11/8/1999 1:25 PM

GFR

Re: Advanced feedback loop questions..

The type of feedback used in most guitar amps samples output voltage and subtracts a voltage from the input voltage, so it's called voltage-voltage or parallel-series (you connect the loop in parallel with the output, to sample the voltage - when you want to read a voltage in a load you just put your voltmeter in parallel with that load - and in series with the input, to subtract from the input voltage - voltages in a loop add together).

When you sample the output voltage, the feedback loop tries to make the amp behave more like a constant voltag source. Say the impedance of the load changes and the output voltage tries to increase. The amount of signal that is fed back also increases and so the output voltage is kept more constant than without feedback. Making the output voltage less dependant of the load is decreasing the output impedance or increasing the damping.

If you sample the output current the amp behaves more like a constant current source. Say the impedance of the load changes and the output current tries to increase. The amount of signal that is fed back also increases and so the output current is kept more constant than without feedback. Making the output current less dependant of the load is increasing the output impedance. You can sample the output current with a resistor in series with the output - when you want to read a current into a load you just put you DMM in series with the load. The kind of feedback used in the Valvstate is called "current-voltage" or "series-series".

The way you mix the feedback signal with the input signal affects the input impedance of the amp. When you have voltages being subtracted in a series connection, this increases the input impedance. This is the case of both the "classic tube amps and of the valvstate.

When you subtract a feedback current from the input current with a parallel connection - the currents into a node add -the input impedance is lowered. This is the kind of feedback used in a Fuzz Face.