Joe Rasmussen "Trans-Amp" - 40 Watt Transconductance "Current Amplifier"
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PLEASE BUILD THIS WITH DUE CAUTION!
[Click on it to enlarge]
There are good reasons why there are no "Current Amplifiers" on the market today, with exception of the Nelson Pass F1 and F1J amplifiers. This DIY project is about building a low cost 40 Watt Transconductance Amplifier using National Semiconductor's LM3875T.
WHY BUILD ONE?
Before going ahead: READ THIS BY NELSON PASS: Current Source Amplifiers / Full Range Drivers
Conventional amplifiers are of course "Voltage" types, where the amplifier outputs Voltage similarly to what we see on an oscilloscope with X-Y axis' representing Voltage (vertically) versus Time (laterally). The current is dependant on the load.
With a "Current" type amplifier, this is far less intuitive to understand, but basically it is the opposite, here the voltage is dependent on the load.
It becomes then clear that the Voltage Amplifier needs to have a low output impedance, so that the amplifier can control the Volts without any sagging. Effectively, the Voltage is 'regulated.'
What is not so clear is that a Current Output Amplifier needs to have a VERY HIGH OUTPUT IMPEDANCE!
So a Voltage Amplifier is defined by its Low Impedance.
A Current Amplifier is defined by its High Impedance.
The notion that there is any such thing as "Damping Factor" - with a Current Amplifier - forget it!
Indeed Damping Factor is largely a myth, what really happens, as Richard Small told me way back in 1975 (he is the S in T-S), that any series impedance/resistance with the Voice Coil erodes Qes and if high enough, destroys the speakers Bass Alignment - so that the Electrical Damping Qes within the speaker is non-existent.
If the DCR of the Voice Coil is 6 Ohm and Qes = 0.5, then an additional series of 6 Ohm will double the Qes = 1.0... and so on. Your Bass Alignment no longer works.
SO THIS AMPLIFIER CAN DAMAGE THE WRONG SPEAKERS!
For the technically minded: The Qes and Qms of the driver that determines its part of the bass alignment, these two are in parallel. If the Qes is eroded completely away, then take note of the Qms as the real Q of the alignment. That way we can plot what that particular alignment does in software. Enter T-S Parameters and Box details, but modify Qes to 1000, enter correct Qms value, then it will plot the actual 2Pi response when driven by a current amplifier.
Also, the Crossover (High Pass) to the Tweeter could also damage it in a very short time.
Yes, the use of Current Output Amplifiers are suitable for Full Range Drivers and also speaker systems that are designed to work with current.
Such as the latest Elsinore Mk6 Loudspeaker Project.
Cheers, Joe R.
.
.
PLEASE BUILD THIS WITH DUE CAUTION!
[Click on it to enlarge]
There are good reasons why there are no "Current Amplifiers" on the market today, with exception of the Nelson Pass F1 and F1J amplifiers. This DIY project is about building a low cost 40 Watt Transconductance Amplifier using National Semiconductor's LM3875T.
WHY BUILD ONE?
Before going ahead: READ THIS BY NELSON PASS: Current Source Amplifiers / Full Range Drivers
Conventional amplifiers are of course "Voltage" types, where the amplifier outputs Voltage similarly to what we see on an oscilloscope with X-Y axis' representing Voltage (vertically) versus Time (laterally). The current is dependant on the load.
With a "Current" type amplifier, this is far less intuitive to understand, but basically it is the opposite, here the voltage is dependent on the load.
It becomes then clear that the Voltage Amplifier needs to have a low output impedance, so that the amplifier can control the Volts without any sagging. Effectively, the Voltage is 'regulated.'
What is not so clear is that a Current Output Amplifier needs to have a VERY HIGH OUTPUT IMPEDANCE!
So a Voltage Amplifier is defined by its Low Impedance.
A Current Amplifier is defined by its High Impedance.
The notion that there is any such thing as "Damping Factor" - with a Current Amplifier - forget it!
Indeed Damping Factor is largely a myth, what really happens, as Richard Small told me way back in 1975 (he is the S in T-S), that any series impedance/resistance with the Voice Coil erodes Qes and if high enough, destroys the speakers Bass Alignment - so that the Electrical Damping Qes within the speaker is non-existent.
If the DCR of the Voice Coil is 6 Ohm and Qes = 0.5, then an additional series of 6 Ohm will double the Qes = 1.0... and so on. Your Bass Alignment no longer works.
SO THIS AMPLIFIER CAN DAMAGE THE WRONG SPEAKERS!
For the technically minded: The Qes and Qms of the driver that determines its part of the bass alignment, these two are in parallel. If the Qes is eroded completely away, then take note of the Qms as the real Q of the alignment. That way we can plot what that particular alignment does in software. Enter T-S Parameters and Box details, but modify Qes to 1000, enter correct Qms value, then it will plot the actual 2Pi response when driven by a current amplifier.
Also, the Crossover (High Pass) to the Tweeter could also damage it in a very short time.
Yes, the use of Current Output Amplifiers are suitable for Full Range Drivers and also speaker systems that are designed to work with current.
Such as the latest Elsinore Mk6 Loudspeaker Project.
Cheers, Joe R.
.
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