Trimax A71 Programme Line Amplifier circa 1970

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trobbins

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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

Trimax A71 front.jpg Trimax A71 side b.jpg Trimax A71 side.jpg
https://www.dalmura.com.au/static/Trimax A71 Schematic.pdf
Frequency spectrum 50mW 560R load.png
 

emrr

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34-35dB if I am running that right. Interesting to see the various philosophies on where gain lived, many of the same era are 70+. 1W output was somewhat of a standard range for these sort of amps. +/-6dB, depending on the company.

Have you measured transformer resistance and/or ratio/gain?
 

trobbins

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Soz, I forgot to push an update of the schematic on-line - please refresh your download cache. Transformer winding TR's and DCR's are in the schematic. Yes +35db gain, and likely a +24dBm rated output level. Perhaps a mix of issues were at play for the seemingly low +24dBm rating, such as user required output level, bandwidth, npn part availability/heatsinking and class A low distortion, output transformer availability, 24V rail, and enclosure size.

In Australia, government owned entities typically had to tender for procurement of equipment, and the results of tenders were published in the government gazette so are searchable. It appears that this serial 238/30 procurement was for the Bureau of Meteorology (BoM), as L.M.Ericsson (the owner of Trimax) won a tender in Feb 1969 for 'supply of programme line amplifiers', and some parts have datecodes from 1969-70, but there is some doubt as to datecodes and whether these amps were for that BoM tender award. The BoM operated diverse communications facilities around Australia, although most line amp applications are often associated with radio/TV stations and broadcasters like the ABC. The bandwidth performance is pretty wide, and I'm not too sure if that aligns particular audio-related comms requirements.
 

emrr

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Got it. Looks to me like semi-bridging input for 600, could be 150 source. Output pri looks like 600, sec like 150 or lower. There are definitely plenty of things over here that were +24 max out.
 

trobbins

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Messages
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Afaik, most line comms was 600 ohm based here. I'll see if additional info comes in, especially wrt technical details - one input so far is that the A71 was used in commercial radio stations in South Australia. There is also a 1967 AWA studio sound equipment doc (http://www.preservationsound.com/wp-content/uploads/2012/02/ABCstudioEquipment.pdf) that indicates the use of mainly 600 ohm lines in Australia, and also line amps with 250mW output were not uncommon.

Wrt the input transformer impedances, the 100k pot load does indicate spec impedances higher than 600 ohm.

I also tracked down what I thought was a current imbalance concern with the output stage, but that was related to resistor value drift, and also made me replace 4 resistors that had noticeably drifted - so caps were good but resistors are more worrying!
 

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