Folks this is my first Audio amp primer, "Bad, Good to Great". I wrote this at a level that most will enjoy without getting into too much technical minutia. It may contain grammatical and technical mistakes which will need correcting. I hope you all enjoy it.
Low Voltage Mini-Amps; Bad, Good, and Great
Mini amps (those that we generally use for arcade use) are generally run from a low voltage DC power source; 24V or lower. Their power ranges from 2 watts to almost 50. Some claim to be Hi-Fi which has turned into an ambiguous claim of sound quality. The term hi-fi was coined back in the midcentury to define high fidelity audio, void of audible hiss, pops and distortion. With the advent of digital music pop and hiss have all but disappeared. This leaves us with Distortion which technically comes in a few flavors. For this essay we will focus on True Harmonic Distortion (THD), as it is the most common. For the sake of simplicity in this primer a THD at rated power <1% is inaudible to most of us. I do know of people that claim they can hear down to .1%, but I have never challenged them. So for simplicity let say a good mini amp needs to produce a specific power that produces THD below a certain threshold, let say again 1%THD.
What limits these mini-amps from producing good sound? I have identified three elements that create these limitations: supply power (voltage and current), load impedance (8 or 4 ohms?), amplifier IC specs (all amps use ICs).
Before I dive into this deeper, allow me to offer a primer in ohms law which dictates the absolute power that can be delivered to a loudspeaker or exciter. Power is defined as supply voltage squared divided by the speaker impedance. Example 1: 12V squared / 8 ohms = 18W. Example 2:12V squared / 4 ohms = 36W. Example 3: 24V squared / 8 ohms = 72W. Example 4: 24V squared / 4 ohms = 144W.
From these four examples we can assume that a higher supply voltage and lower speaker impedance will produce the highest output wattage. Remember that these are absolute power assumptions that are chiefly dependent on supply current and the type of audio amp used. Therefore, it is unlikely that any amplifier can reach an RMS power value that is 60% of the absolute power.
General Requirements for a Virtual Pinball Audio Amplifier
These amps need to faithfully and realistically produce high transient sounds such as knocks, clicks, dings, and hits. To follow this requirement we need an amplifier that produces >4 Watts RMS per channel with a 15 Watt peak reserve with a THD <5%.
Other Parts to the Equation
The power source needs to provide ample current. Typical 2.1 system needs at least 2 amps at 12VDC. If using the CPU power source, the line input ground shields should only be terminated on one end (either from sound source or amplifier, not both). Ignoring this will produce ground loops where you might hear the 50/60 Hz hum or worse yet, CPU noise which will drive you nuts. Last most 2.1 systems use four amplifiers; two for the left/right channels and two in a bridge configuration for driving the sub-woofer.
Active Pre Amp tone/volume controls are always good to have, some are better than others. For a 2.1 system, a cross-over filter is included. This filter passes the mid bass to high audio to the left/right channels, and the low frequencies to the sub-woofer. This is all a personal preference and I am leaving it out of the equation for now with the exception of a volume control which I think is requisite.
The Bad Mini-Amp
Speaking in generalizations, a bad mini-amp is limited to 12V or lower and produces <4W at >5% THD @4 ohms. These amplifiers have little to no dynamic headroom and produce compressed audio that lacks depth. These amplifiers also have small power filter capacitors used as a temporary source of power during transient loading.
The Good Mini-Amp
As we progress up the food chain, we have amplifiers systems that range from 4 to 10 watt with a THD below ~2%. These amps can be supplied with a power source above 12VDC and have filter caps rated at >25V with a value > 1,000 uFd. Again, these requirements are generations.
The Great Mini-Amp
At the top, we have the heavyweights with a nominal supply voltage of 24V. This higher voltage allows them to drive higher impedance loads at power levels rivaling those of average mini-amps with 4 ohm loads. Expect power levels exceeding 12 Watts and THD down to academic levels. Great mini amps normally are class D amplifiers. Although scorned by many audio aficionados, these are power efficient devices that are small, cheap and make full use of a limited power source. Likewise, great linear amplifiers normally cost more, but are capable of driving lower impedance loads.
Bad to Great
There is a large contrast between Bad and Good Mini-Amps that is obvious to almost anyone that is not tone deaf. The mushy lows emanating from the woofer to the garbled sound coming from the midrange at rated power are signs of a bad amplifier. In most cases, bad amplifiers can sound acceptable at low volume.
As a Six Sigma Green Belt, I have been repeatedly told that ‘Good’ is the Enemy of ‘Great’. The exception is Mini-amps. The difference between good and great in this case is academic, with value being the factor. The examples in my research range from $5 to $25 USD and cost does not always define what is great.
Conclusion: Good is Great
These points are desired:
· Amplifiers with >16VDC power source
· Tone controls including treble, bass, volume
· Class D amplifier
· THD @ < 3% for all channels
· Power output 5W RMS, 20Watts Peak
· Output impedance down to 4 ohms (most will do this)
-Rick Horwitz
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