How to add balanced input to single ended amp?
Quite a basic question, and I have read a few posts on it - but I am a bit of an elctronics novice (to say the least!).
I intend to buy myself two mono Hifimediy T3 amps soon and package them into two separate cases to give me two nice monoblock amps. The amps come with single ended inputs. I would very much like to have both balanced and single ended inputs to them. Firstly because I am not yet sure whether to place the amps near the speakers (requiring long runs of wire that will pass the 220v AC power cables that power my tv/digibox/etc) and also because I intend to use the balanced version of the minidsp. There is also the possibility I will get a DCX2496 instead of two minidsp's, or indeed a different DSP altogether to act as an active crossover.
Is there a product or simple way that I can add both inputs to the chassis, possibly with a switch to select which one to use? I have seen examples of this on other peoples amps, but am not sure how they have done it.
I know I can wire the minidsp to run single ended output, and probably will do that to get myself started.
If it is completely pointless I guess I wont bother, so look forward to your input..
The simplest method is to use a transformer (LUNDAHL for example). Another technique is to use integrated circuit INA134. Cheaper but more complicated.
As jej points out, a transformer is a low-tech but ungodly expensive option. Lundahl, Cinemag, Jensen make good transformers. The Jensen JT-11P-1 or Cinemag CMLI-15/15B would be good options. I'm not familiar with Lundahl's product line so I can't point to a part number there. But you'll find that those transformers will set you back a good $70~100 each.
A differential amplifier using an audio grade opamp (LME49990, LME49710 would be my choices) is a lower cost option. However, note that the performance of the differential amp is tightly linked to the matching between the resistors. Get 0.1 % resistors (they're not that expensive anymore) or add a potentiometer in series with Rg and adjust for maximum CMRR. This is done by applying the same AC signal to the two inputs and adjusting for minimum AC output voltage.
The instrumentation amps like INA134 have precision resistors built in. However, they are not optimized for low distortion.
If I have a power source (15-0-15v) I use Rod Elliot's project 87. If not, I use Nuetrik NTE1 transformers. They are reasonably priced and good quality.
Balanced Transmitter and Receiver II
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