First of all,
A HUGE THANK YOU to the 24 forum members who appreciated my post.
Keith regarding your 2 queries:
1. "In a stereo power amplifier, the current drawn by 1 channel from the
Shared power supply,
will leave an imprint on the power received by other channel."
All Power Supplies (PSU) in amplifiers are non-ideal to some extent. When power is drained from the PSU, its voltage falls. This is technically called regulation. The drop in PSU voltage on load can be as high as 10% to 15% of its No Load voltage.
As an example, sudden loud music on the left channel, can cause the Power Supply voltage to fall. The Right channel even without that loud music passage on it, will be fed a lower voltage from the shared PSU.
The Power that can be delivered into the speaker, depends on the PSU Voltage.
As an example, if the amplifier is driven by a +30 VDC & -30 VDC PSU ( Total 60 VDC) it can deliver a theoretical max power of 56 Watts RMS into 8 ohms.
If due to a music transient on the left channel, the power supply voltage falls to +25 VDC & -25 VDC, the Right Channel will receive only +25 VDC & -25VDC ie a total of 50 VDC, allowing the Right Channel to deliver only 39 Watts max !
The above is an example of a huge music transient dropping the PSU voltage. In regular use, where channels are Not driven to their full power (normal usage), each channel creates a continuous ripple voltage on the power supply rails, which gets fed to the other channel, via the common PSU.
Of course, distortion is measured with a constant Power Supply voltage. If the Power Supply voltage itself keeps fluctuating, the distortion will increase. The Extent to which an amplifier can ensure fluctuations in its PSU voltage is referred to as Power Supply Rejection Ratio (PSRR). It can vary substantially based on the amplifier design, and the margin of extra PSU voltage available.
Insight: Amplifiers where the voltage gain stage is fed via a constant current source provide much better PSRR.
If each audio power amplifier has its own PSU (as in Mono Blocks), transient music passages in one channel will not affect the other channel.
Some amplifiers deploy Very tightly
Regulated Power Supplies. These alleviate channel Cross talk via the PSU, but add to the cost and heat dissipated. Regulated PSUs also are believed to lend a particular sonic signature ... "Iron Fist Grip on the music" though it
could be at the cost of restricted music transients & flow.
2. "Capacitor and
Transformer characteristics (and differences) "
Transformers are elaborate devices with several hundred meters of wire in their windings. The Gauge (thickness) of the wire, how its wound, the air space between the wire windings and the slot in the core, the material of the core, all tend to change (Very measurably) the parameters and performance of the transformer.
The Transformer has its
Self-Inductance (also called leakage inductance) and interwinding capacitance, which influences not only the transformer's regulation, but also its transient response, and power delivery at different frequencies. Large windings create large leakage inductance, which hampers transient power delivery. Interleaving windings reduces self-inductance, but increases
interwinding capacitance that has a negative effect on transient and power delivery of the transformer, to High Frequency Music signals. There are, as always, engineering trade-offs.
A great example was the
mains transformer used in the classic NAD 3020 amplifier. This was a budget amplifier, with a very Punchy sound, yet price very attractively. Till then, getting Low Frequency slam from an amplifier meant a Huge (read Expensive) Transformer. But the NAD 3020 had a modest transformer & unregulated PSU that gave its best in the lower mid bass region.. 50 Hz to 100 Hz. It would not sustain its power into 20 Hz testing. The manufacturer (NAD) instead spoke of the amplifier's
“Dynamic Headroom” .... How loud it would play transients. Priced for mid-fi setups, and mid fi speakers that mainly played Mid bass, not deep bass, the Amplifier was a commercial hit.
Dan Agostino admitted in an interview that he had tried more than 10 transformer designs before finalising the transformer for his Momentum integrated amplifier. The challenge was to have a low profile (height) transformer that would fit in the case and yet provide the required transient & steady state power delivery.
Then there are different construction types - E I core Transformers, R Core Transformers & Toroidal Transformers. Each of these have differing power delivery characteristics.
Regarding
Capacitor Characteristics (and differences):
Many scorn the concept of audiophile capacitors, but lets look at the construction of large (Electrolytic) Capacitors, used in an amplifier PSU...
The Electrolytic capacitor is made by rolling a Large (several meters) of thin conductive (often aluminum) foil. The foil is kept thin to be able to squeeze in more layers of the foil. An electrolytic paste is spread on the foil before rolling it.
Thin foil = Higher electrical Resistance... called ESR or (Equivalent Series Resistance) of a capacitor.
Higher resistance creates more heat in the capacitor during use. If a High ESR capacitor (e.g. the ones with wire terminations are used with high currents, they can actually explode. Exploding Caps are quite dramatic! Material is easily propelled with enough force to stick on the ceiling.
For high current applications, low ESR resistors MUST be used. These are also called PG (Professional Grade) capacitors and have SCREW TERMINALS.
Incidentally, a 10 deg C increase in the operating temp of an electrolytic capacitor HALVES its operating life ! A 40 deg C increase in core temperature will reduce capacitor life from say 100,000 Hours to just 6,250 Hours!
PG capacitors are rated for higher operating temperatures.
An alternate method often used is to use a bank of 10 or 20 ordinary (non-PG) capacitors in parallel of smaller value. This reduces the ESR by a factor of 20 since the ripple current is shared by 10 or 20 capacitors. Such capacitor banks look neat and impressive in photographs
Long Foil (for higher capacitance values) = Higher Electrical Inductance
The long length of foil in an electrolytic capacitor also has a substantial inductance, which comes into play at higher frequencies of ripple current on the power supply (Created when the amplifier plays HF signals).
Interesting Fact: Most large electrolytic capacitors turn into inductors above 30 KHz to 50 KHz !
So, there is much that needs to be considered when designing the 'simple' power supply of an audio amplifier
(Apologies for this long post, but you ‘provoked’ me
)
