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what is "Transformer-Like-Amplifier" ?
Old 16th January 2009
Lives for gear

what is "Transformer-Like-Amplifier" ?

i was looking at the portico pre and i saw this:
Input: Balanced, using "Transformer-Like-Amplifier" (T.L.A.) with toroidal Common Mode Rejection Low Pass Filter

so, this is NOT a transformer. just a circuit to sound 'like" a transformer?

Old 16th January 2009
Gear Maniac

Just a guess since I haven't seen the insides of a Portico...
but based on the description, it sounds like they're using THAT chips, the Ingenious line?

from the THAT website
Developed by Bill Whitlock of Jensen Transformers, the patented InGenius input stage uses clever bootstrapping to raise its common-mode input impedance into the megohm range without the noise penalty from the obvious solution of using high-valued resistors. Like transformers, InGenius line receivers maintain their high CMRR over a wide range of source impedance imbalances — even when fed from single-ended sources. But unlike transformers, these wide bandwidth solid state devices offer dc-coupling, low distortion, and transparent sound in a small package at reasonable cost.
Old 16th January 2009
Gear Maniac
Radioman's Avatar

TLA is a pre stage with some input transistors including (i think) a costant current stage, like many other hybrid (transistor + opamp) schematics. He "acts" like a transformer but he can't sounds like a transformer simply because is not a transformer...
The toroid into the input wires should acts like the Common Mode Rejection Filter fitted inside many AC mains RFI filters.
Old 16th January 2009
Lives for gear

The expression Transformer Like Amplifier was used by Mr Neve to describe SOME of the attributes of the input stage he worked on.
The main consideration is that it should have a high 'common mode' rejection (cancelling of signals applied in phase to the 2 input terminals) and that like a transformer, if either 'side' of the input were to be disconnected then it appears to have a very high impedance when measured from a ground (again like a disconnected transformer winding) which is 'open circuit' except for circuit stray capacitances.
Of course the TLA input has no transformer and so will not have any of the hysteresis and saturation 'qualities' of a real transformer.
The TLA action is only maintained up to the point where the input signal approaches the supply rails (usually around +-17 Volts) wheras a real transformer would be able to withstand at least 100 Volts or more depending on design quality. The 'InGenius' 'chip' available from THAT corporation is quite similar to Mr Neve's design with the aim of achieving the same goals but in an easy to use plastic chip.
There are also several similar TLA output stages which also try to emulate the ideal of a 'floating' transformer secondary without the cost and distortion incurred by a real transformer.
All types have some advantages and disadvantages although the real transformer is pretty much 'idiot proof' it carries the penalty of cost and distortion. The 'TLA' designs usually work well unless the 'common mode' potentials exceed the supply available to the (chip).
Matt S
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