Basic layout/functions of section in amplifiers

Discussion in 'General Electronics Chat' started by chunkmartinez, Jan 10, 2013.

  1. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
    I am wanting to really get into amplifer design...or atleast, repair. There are some books and resources I have in mind for it and I have studdied the very basics of transistors like biasing BJTs, MOSFETs and the differences between those two transistors and diff amp class topologies.

    I wanted to start this thread so I can have a quick basic outline of amplifer section functions. I understand I need to studdy for myself to learn it but I know there are alot of people who could give some quick basics so I know what keywords and concepts to search for, it's the best way for me to learn.

    So I have some basic questions:

    Power supply capacitors - I have a 300w amp kit I put together that has two large capacitors and also see them on all amplifiers. My amp is a class D amp. What exactly do the caps do? I know in a rectifier circuit (AC - DC) they smooth ripple to a flat DC voltage, is the concept sort of similar to amps? I realize that music is not justa smooth DC so it's obviously not the same thing so what do they do in amps? I think they help to balance voltage or current when the power supply is beeing "stressed"?

    Power supply transformer of amplifier - This pertains to large caraudio amplifers, most of these questions actually do. In a caraudio amp, the vehicles voltage is 12-14.4v, do these amps require different power supply then indoor amplifers(a radio connected to an AC socket inside). Is the rail voltage of the amp output taken from the power supply? I have gotten into rail splitting with OP amps a little but I'm curious how the rails are "created" in caraudio amplfiers? I see and hear of the large custom toroid transformers in car audio amps and I always see two of them, how does using two work? Are they combined?

    Are power supply toroid transformers tough to custom make? I have heard about handwinding them yourself and beefy ones are reaaaly expensive so it would be nice to be able to wind my own.

    About transformers - The basics I know about them is that they invert voltage and current by a ratio. I don't understand how a transformer can step down from 110v to 12v, and the current is supposed to be inversely steped up by the same ratio if current drawn depending on load. Also, I understand that a certain current draw requires a large enough guage wire to handle it safely, but if I am on only using thin wire, with 110v on primary, drawing little current on Pri, the primary will convert the high voltage to a larger current on the secondary side where the primary wire couldn't handle the current, and does it mean the whole transformer is drawing little current in a step down transformer and still allowing a good amount of current drawn from secondary side(if the design allows it).

    There are plenty of other questions but i'm writing too much already. Any pointers about how the different stages work, etc would be awesome!
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
    A DC supply is separate from the AC signal being amplified. Transistors, IC's, etc. require DC supply.

    A transformer converts an AC voltage to another AC voltage, while preserving the power (ideally, in reality there are losses). Since Power = Voltage x Current, this means a lower voltage with the same power will have a higher current. The device being supplied determines the amount of current drawn from the supply and thus the transformer secondary, and in a step-down transformer, the current going into the primary will be lower.

    Here's a table of current capabilities of various wire sizes:
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    An amplifier which supplies a real 300w RMS into a 4 ohm load (for example) requires P=E*E /R voltage. ~ 35V RMS or ~97V PP.
    That means the amp needs a supply of about +/- 50 VDC at about 8.6 amps.
    Edit: You can tell a lot about what an amplifiers real power capability is by multiplying the PS fuse size*Voltage supply.
    Nowadays amp ratings appear to be somewhat 'Optimistic'....

    When working from 120VAC a large 50/60hz center-tapped power transformer, diodes and filter capacitors suffice as a basic supply.
    12 VDC needs to be boosted by various methods. For high voltage and current a transformer with push/pull or a flyback topology can be used, but more components are required.

    Google push-pull and flyback converters.

    It would be interesting to see the schematic of your 300w amp.
    Last edited: Jan 11, 2013
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Audio amps don't need massive amounts of continuous power, but they do need signal swing range which is why car amps which are limited to 12V input often have a switcher inside to boost the supply voltage rails way up.

    You don't need lots of continuous power unless you just want to shake the ground with a subwoofer.
  5. chunkmartinez

    Thread Starter Senior Member

    Jan 6, 2007
    Today, I just started reading bcae1. Has anyone heard of the site? It seems to be giving me some decent basics on amp power supplies and how the switching power supply works..