I just prepared a schematic and quickly tested a line amplifier made by Trimax in 1970 in Australia. So far there are no known details on this amp module, or the transformers it uses, or the output stage transistors (previous thread I started Fairchild AX6126 TO-5 transistor data ?). So this thread is likely a first foray in to its performance and design.
Photos of unit are below, along with a link to my draft schematic, and a frequency spectrum plot.
E-cap rating of 25V sort of suggests a -24Vdc regulated supply, and the dc bias levels measured indicate that is reasonable (compared to a lower rail voltage). The output stage npn's idle at about 70mA and 16-17V (ie. 1.2W) on a common heatsink which settles to about +25C above ambient. 1971 magazine data for Australian Fairchild TO-5 bi-mesa power bjts indicate package thermal resistance could be down at 17.5C/W, so die temp is likely circa 70C on my bench. The output stage devices idle with a little difference in current (circa 65 and 73mA) which could be trimmed for balance, but not sure if there is any benefit in increasing/decreasing. Apart from the output stage feed at -23.3V (as 24V supply is fed through 6R8), the other circuitry has a -17.4V rail (about 3mA consumption).
The output level noticeably starts clipping at about 13Vrms in to a 560R load, and distortion shoots up above 12Vrms (250mW output). Distortion rises linearly from 0.012% at 1Vrms to 0.075% at 10Vrms, and the 2HD dominates. The voltage sensitivity with gain pot at max is 55x, so 220mVrms input for 12V out.
A frequency spectrum plot at 5.3Vrms output (50mW) shows the design is quite wideband and within 1dB from a few Hz to 45kHz. The testing loop uses a 100x scope probe and loopback response is calibrated flat across that bandwidth.
It is assumed that the amp is designed for 600 ohm input and output circuits. As I don't typically have an application for this type of line amp I may end up using the module for its signal gain and its isolation between input, power supply and output circuits, which may come in handy for soundcard type testing where the soundcard test signal output is commonly limited to 1-2Vrms, and DUT input and output may not allow probing to use a single common ground point.
Ciao, Tim
https://www.dalmura.com.au/static/Trimax A71 Schematic.pdf
Photos of unit are below, along with a link to my draft schematic, and a frequency spectrum plot.
E-cap rating of 25V sort of suggests a -24Vdc regulated supply, and the dc bias levels measured indicate that is reasonable (compared to a lower rail voltage). The output stage npn's idle at about 70mA and 16-17V (ie. 1.2W) on a common heatsink which settles to about +25C above ambient. 1971 magazine data for Australian Fairchild TO-5 bi-mesa power bjts indicate package thermal resistance could be down at 17.5C/W, so die temp is likely circa 70C on my bench. The output stage devices idle with a little difference in current (circa 65 and 73mA) which could be trimmed for balance, but not sure if there is any benefit in increasing/decreasing. Apart from the output stage feed at -23.3V (as 24V supply is fed through 6R8), the other circuitry has a -17.4V rail (about 3mA consumption).
The output level noticeably starts clipping at about 13Vrms in to a 560R load, and distortion shoots up above 12Vrms (250mW output). Distortion rises linearly from 0.012% at 1Vrms to 0.075% at 10Vrms, and the 2HD dominates. The voltage sensitivity with gain pot at max is 55x, so 220mVrms input for 12V out.
A frequency spectrum plot at 5.3Vrms output (50mW) shows the design is quite wideband and within 1dB from a few Hz to 45kHz. The testing loop uses a 100x scope probe and loopback response is calibrated flat across that bandwidth.
It is assumed that the amp is designed for 600 ohm input and output circuits. As I don't typically have an application for this type of line amp I may end up using the module for its signal gain and its isolation between input, power supply and output circuits, which may come in handy for soundcard type testing where the soundcard test signal output is commonly limited to 1-2Vrms, and DUT input and output may not allow probing to use a single common ground point.
Ciao, Tim
https://www.dalmura.com.au/static/Trimax A71 Schematic.pdf