Line Transformer Question

Discussion in 'General Electronics Chat' started by DF113, Oct 5, 2007.

  1. DF113

    Thread Starter New Member

    Oct 5, 2007
    5
    0
    Hey,

    I am working with an audio amplifier. It has a rather low gain and needs an additional 5x or 14dB. It also has a rather low input impedance ~10K per phase. It is a differential input.

    I really want to use a transformer input for the complete isolation of grounds and the CMR capabilities.

    Is it possible to keep the overall input impedance high, as well as to get the additional voltage to properly drive the amplifier? I am thinking not and I will have to design an active gain stage to go in-between with high input impedance and low output impedance.

    Thanks!
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Hard to answer that without a schematic. Ordinarily, changing the feedback elements will give the extra gain.
     
  3. ixisuprflyixi

    Active Member

    Sep 16, 2007
    52
    1
    Problem is if you change the feedback components you may lose some of the gain stability the circuit has built in. You want to use a transformer at the input but a pair of FETs would give you an extremely high input impedance but unfortunately adds no gain. Like beenthere said the schematic or even a detailed description of the amplifier would be helpful.
     
  4. DF113

    Thread Starter New Member

    Oct 5, 2007
    5
    0
    Each side of the differential amplifier has a 1.8K input resistor. Then an 8.2 that wraps around to the output side. The - side input goes to the grounded output. The + side goes to the + speaker output, so it sees the speaker before it sees ground.

    I can not change these values, therefore can not change the gain of this section of the amplifier, which unfortunately has a rather low input impedance.

    Hope that helps you to help me.

    Thanks for the fast responses guys.

    -Paul
     
  5. DF113

    Thread Starter New Member

    Oct 5, 2007
    5
    0
    Anyone?

    I should clarify, the 8.2K's are the feedback resistors.

    -Paul
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well, all you've really told us about is the value of the items that you can't (or won't) change! We have no clue as to any of the other unknowns that your dealing with.

    How about making up a schematic of the thing? You could download something like Circuitmaker Student or the Eagle Layout Editor (the latter has a much larger components library, but no built-in Spice)

    Either that, or tack another amp on the output of this one.

    Your power supply could be marginal, and attempting to boost it's output could lead to severe distortion - or the amplifier components may not handle the added signal levels well. It's possible that some components of your amp have drastically changed value over time. I've had that happen to mil-spec carbon resistors that were sitting in a sealed package for 30 years; what were supposed to be 670 Ohm resistors measured anywhere from 420 to 1.6K Ohms! If I hadn't seen that with my own eyes, and known the history behind those resistors, I would've called someone who claimed such a thing a liar.

    Other than that, I don't have a clue, since you haven't posted a schematic yet.
     
  7. DF113

    Thread Starter New Member

    Oct 5, 2007
    5
    0
    I am not sure what hasnt been said yet.

    I have an audio amplifier that has a gain of 5x (~14dB). Not nearly the normal 10x or so. Instead of using an active FET or Opamp gain stage in front of it, I am wondering if it is possible to use a transformer to increase the voltage gain. I am pretty sure that a 600//15000 ohm transformer will have the perfect 5x voltage gain. My concern is that this will also increase the current requirement from the preamp. This would mean that the input impedance of the amplifier would go down (probably by a factor of 5).

    I am not sure what the point of drawing the thing up would be, it is a fairly simple amplifier. Just think typical amplifier with 10K input impedance per differential input, but a low gain of 5x. It was designed to have an active gain stage before it, but I am thinking that for the absolute best CMRR and complete isolation of grounds, the ideal solution is a transformer based input.

    So, I am thinking out loud, and I think that this is correct, I am just looking for confirmation.

    -Paul
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    There is more than one design possible for an amplifier (or differential amp, for that matter). That's why we keep asking for the schematic.
     
  9. Xray

    Well-Known Member

    Nov 21, 2004
    58
    1
    Maybe yes, maybe no. Keep in mind that a transformer located at the input of an audio amplifier carries with it some potential problems such as common mode noise and hum. Also, a transformer may severely skew or limit the overall frequency response curve especially at the low end of the audio spectrum. Personally, I would not choose to stick a transformer at the input of an amplifier unless absolutely necessary for some special design, and where the above mentioned problems are of no concern.
     
  10. DF113

    Thread Starter New Member

    Oct 5, 2007
    5
    0
    Here it is:

    [​IMG]

    The first resistor on in+ and in- is 1.8K. The primary feedback resistor is 8.2K on each one. The lower feedback path is cropped off of the schematic.

    -Paul
     
Loading...