Gain block question
Posted: Sat May 11, 2019 8:34 am
Here I go again, another wierd discrete component audio query.
Still in the audio reproduction mode (it's my main electronic interest
) and this time I'm trying to understand how the "volume control" on the PSAudio amps work. Paul (the P in PSAudio) says that they use gain blocks for their volume controls so that they have a constant impedance presented to the source, thereby not altering the source signal integrity usually caused by impedance mismatch.
I've put together a few sims using the most ubiquitous popcorn bjt, the 2n3904.
The first uses variable base voltage, which in turn produces variable low base current. There's a very miniscule range where the signal goes from a few nanovolts to full input. Setup is source into the collector, emitter to resistor that leads to gnd, and a variable resistor from +V to gnd, wiper to base.
The second uses a variable resistor in 1/2 of the gain resistor string to change the gain. Always amplifies somewhat from a few mv to way past the point of distortion. Setup is a classic Class A VAS with a same value VR paralleling the collector resistor, wiper tied to the collector end of the pair. Then I take the resultant signal off either the collector or emitter (inverted or non-inverted, respectively) into a resistor tied to gnd.
Both times I am reading the results from between the transistor and the load resistor.
Obviously I am having trouble getting it to work. AM I barking up the wrong tree with a bjt and I should use a MOSFET instead, or is there another way that I am missing.
CeaSaR
(I believe I am looking for a voltage controlled amplifier with serious negative gain [0 signal passage] to unity gain or just slightly over unity. Probably will require MOSFET instead of bjt. Would still like to understand how to do with bjt as I enjoy a good challenge.
)
Still in the audio reproduction mode (it's my main electronic interest
) and this time I'm trying to understand how the "volume control" on the PSAudio amps work. Paul (the P in PSAudio) says that they use gain blocks for their volume controls so that they have a constant impedance presented to the source, thereby not altering the source signal integrity usually caused by impedance mismatch.I've put together a few sims using the most ubiquitous popcorn bjt, the 2n3904.
The first uses variable base voltage, which in turn produces variable low base current. There's a very miniscule range where the signal goes from a few nanovolts to full input. Setup is source into the collector, emitter to resistor that leads to gnd, and a variable resistor from +V to gnd, wiper to base.
The second uses a variable resistor in 1/2 of the gain resistor string to change the gain. Always amplifies somewhat from a few mv to way past the point of distortion. Setup is a classic Class A VAS with a same value VR paralleling the collector resistor, wiper tied to the collector end of the pair. Then I take the resultant signal off either the collector or emitter (inverted or non-inverted, respectively) into a resistor tied to gnd.
Both times I am reading the results from between the transistor and the load resistor.
Obviously I am having trouble getting it to work. AM I barking up the wrong tree with a bjt and I should use a MOSFET instead, or is there another way that I am missing.
CeaSaR
(I believe I am looking for a voltage controlled amplifier with serious negative gain [0 signal passage] to unity gain or just slightly over unity. Probably will require MOSFET instead of bjt. Would still like to understand how to do with bjt as I enjoy a good challenge.
)