Direct-coupled or capacitor-coupled inputs?

jeanseb86

I am thinking of getting a Ps Audio 200c and I was wondering weither I should use the direct-coupled or capacitor-coupled inputs? (I will use the preamp outputs on my AVR-335...)

Thanks
JSR
_____________
RTi10, FXi3, CSi3, PSW505
Harman Kardon AVR-335
Sony HDTV KP-46WT520 (46")

steveinaz

If your preamp has DC blocking at it's outputs use "direct". If it doesn't, use the capacitor coupled inputs.

If in doubt, use the capacitor coupled inputs.

unc2701

Yeah, I'd use the capacitor coupled inputs... it'll degrade your sound quality slightly, but on the upside, there's less of a chance of melting your speakers.

heiney9

jeanseb86 wrote:

I am thinking of getting a Ps Audio 200c and I was wondering weither I should use the direct-coupled or capacitor-coupled inputs? (I will use the preamp outputs on my AVR-335...)

Thanks
JSR
_____________
RTi10, FXi3, CSi3, PSW505
Harman Kardon AVR-335
Sony HDTV KP-46WT520 (46")

Each has it's own advantage and disadvantage. In this case I really don't think it will make a difference. Actually it's kind of strange question. I wasn't aware of any amplifier designs that have both types of inputs. Usually an amp designer will pick one or the other and then design for it. I'm assuming the way you posed the question that the coupling is taking place between the input stage and output stage of the amplifier as opposed to coupling the output to a load (speaker).

This is a big concept, almost impossible to understand without a bit of background in electronics. I'll have to get back to you as I can't simplify the idea enough in a short explanation. Plus I'm still having a hard trying to figure out why the designer of your amp has both input stage to choose from. From a design and implementaion POV these are two completely different types of inputs and I can't understand why a designer would take the time and effort to offer both in the same chassis or if it's even possible to have both incorporated in the same design, and still be effective.

Let me think awhile an if no one else explains it I'll try to tackle the question.

H9

mhardy6647

When it comes to amplification, DC is not your friend. There is nothing wrong with quality capacitors in the signal path (despite many claims to the contrary). The best designs will bypass the caps in the signal path so that impedance for all audio frequencies are low.

Without knowing anything else, I would definitely encourage some DC blocking somewhere between your source and your speakers.

heiney9

Here's a quick explanation of the basic differences between the two types. Not sure if they answer your question.

Whether an amplifier is one of a series in a device or a single stage connected between two other devices, there must be some way for the signal to enter and leave the amplifier. The process of transferring energy between circuits is known as COUPLING. There are various ways of coupling signals into and out of amplifier circuits. The following is a description of some of the more common methods of amplifier coupling.

Direct Coupling

The method of coupling that uses the least number of circuit elements and that is, perhaps, the easiest to understand is direct coupling. In direct coupling the output of one stage is connected directly to the input of the following stage.

Direct coupling provides a good Frequency response since no frequency-sensitive components (inductors and capacitors) are used. The Frequency response of a circuit using direct coupling is affected only by the amplifying device itself.

Direct coupling has several disadvantages, however. The major problem is the power supply requirements for direct-coupled amplifiers. Each succeeding stage requires a higher voltage. The load and voltage divider resistors use a large amount of power and the biasing can become very complicated. In addition, it is difficult to match the impedance from stage to stage with direct coupling. (Impedance matching is covered a little later in this chapter.)

The direct-coupled amplifier is not very efficient and the losses increase as the number of stages increase. Because of the disadvantages, direct coupling is not used very often.

RC Coupling or Capacitor Coupling

The most commonly used coupling in amplifiers is RC coupling. R1 acts as a load resistor for Q1 (the first stage) and develops the output signal of that stage. Do you remember how a capacitor reacts to ac and dc? The capacitor, C1, "blocks" the dc of Q1's collector, but "passes" the ac output signal. R2 develops this passed, or coupled, signal as the input signal to Q2 (the second stage). This arrangement allows the coupling of the signal while it isolates the biasing of each stage.

This solves many of the problems associated with direct coupling.

RC coupling does have a few disadvantages. The resistors use dc power and so the amplifier has low efficiency. The capacitor tends to limit the low-Frequency response of the amplifier and the amplifying device itself limits the high-Frequency response. For audio amplifiers this is usually not a problem; techniques for overcoming these frequency limitations will be covered later in this module.

Before you move on to the next type of coupling, consider the capacitor in the RC coupling. You probably remember that capacitive reactance (XC) is determined by the following formula: This explains why the low frequencies are limited by the capacitor. As frequency decreases, XC increases. This causes more of the signal to be "lost" in the capacitor.


This is about as simple as I can get it. Disadvantages of each type can be overcome based on the design. I personally prefer direct coupled over capacitor coupled, but it really depends on the entire design and can't be generalized. The fewer the gain stages the better Direct Coupled designs perform. The frequency limitations of Capacitor Coupled designs can be overcome. Again I don't understand how PS Audio was able to incorporate both types in one unit. Unless those terms are simply a bit of marketing to differentiate their product to make it stand out.

FWIW

H9

madmax

And all this time I've just tried the direct mode, made sure the speakers didn't pull in or push out upon power up and checked to see which sounded best...
madmax

jeanseb86

Thanks heiney9! Your posts were very helpful! I still don't know which one to use but at least now I know that I can use both! I just hope that I won't brake my RTi10 when trying it DC!

JSR
_____________
RTi10, FXi3, CSi3, PSW505
Harman Kardon AVR-335
Sony HDTV KP-46WT520 (46")

heiney9

jeanseb86 wrote:

Thanks heiney9! Your posts were very helpful! I still don't know which one to use but at least now I know that I can use both! I just hope that I won't brake my RTi10 when trying it DC!

JSR
_____________
RTi10, FXi3, CSi3, PSW505
Harman Kardon AVR-335
Sony HDTV KP-46WT520 (46")

Use whatever sounds better to you. There probably won't be much, if any difference. DC should never be present (well a little bit is possible/acceptable) at the amplifier outputs. Capacitor coupled won't let DC pass to the outputs should something fail in the amplifier. Direct coupled has safe guards in place as well depending on the design. Use either with confidence. However, if you are going to use the direct coupled turn your pre-amp (AVR receiver) first and then the amp as an extra precaution. Depending on the amplifiers design, a spike of DC can be present at power up and having the pre on first can reduce this. This is what I've been told. I don't always follow this sequence and I've been fine for 20 years with my Adcom which is direct coupled and has no current limiting protection circuits or muting relays.

H9