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Line level power supply concepts

January 6th, 2019, 9:02 am

A line level power supply should provide several functions in order to be most effective.

First it must provide the signal section of the equipment with sufficient power to allow the least amount of disturbance of any audio signal regardless of frequency. To achieve that it must be able to provide sufficient power from the lowest to the highest frequencies. Regardless of the current demands of the circuit, sufficient current should be available for any time period to prevent voltage drops that could affect the linearity of the circuit.

Second, it should appear to the circuit as a source of power that is independent from any changes coming from the AC line. In other words, it should make the power supply appear to be an isolated battery.

The AC mains can cause problems to the line level circuitry in two main ways. The first is it can be unstable with respect to voltage and potentially current supply capability. Current capability is not at issue with the modest demands of line level equipment although it is at the power amplifier level. The second is the introduction into the circuit of spurious noise especially at high frequencies where the circuitry is most likely highly sensitive.

The power supply really consists of three sections. The first is the brute force conversion of the AC supply to DC with sufficient voltage and current to handle any load. The second section provides voltage regulation to ensure that long term voltage stability requirements are met. The third section provides audio and high frequency isolation from the basic regulated section. That third section needs to take into account the rate at which power is required from the circuit and the sensitivity of the circuit to rates of change of the power supply.

The first section of the supply is typically handled by either a transformer with rectification and filtration or a switch mode power supply. Both have advantages and disadvantages which are too involved to be handled here. For the rest of this discussion, it will be assumed that the power supply is a low voltage one designed for Op Amps because of the characteristics of those devices although the principals remain the same.

The second section provides the basic regulation of the DC voltage from the fundamental fluctuations that occur on AC lines. In addition some reduction of noise from the AC line occurs but that reduction is frequency dependent. Most modern regulators used in these supplies are optimized to reject fluctuations at line frequencies and below and do quite well in that range. Their ability to reject changes caused by line noise decreases with frequency. At the same time the ability of Op Amps to reject power supply changes also decreases as the frequency of those changes increases.

The third section provides an increasing ability to reject incoming noise as the noise frequency increases, offsetting the limitations in the regulators and amplifying circuitry. The best way to achieve that noise reduction is with a low pass filter consisting of a choke and filter capacitor. It is a passive solution that behaves well with minimal intrusion of the circuit. For best performance, the capacitor section of the filter should have several stages. The first should be large enough to provide sufficient current well beyond the lowest frequency signal demands of the circuit and also provide a turnover point of the filter below the frequency where the signal circuit begins to lose its ability to reject changes. That large capacitor section typically does not have the ability to provide sufficient current at a fast enough rate to handle the highest frequency demands. Paralleling that first capacitor section is a second consisting of a high frequency design intermediate value capacitor to increase the rate at which current can be supplied to the circuit. The third stage is local energy storage and bypassing of the power supply right at the power supply input leads of the Op Amp. Generally that consists of paralleled low value tantalum or HF electrolytic capacitors with monolithic ceramic capacitors. The tantalums provide current at the highest rate and both bypass any parasitic noise from the circuit that may try to enter the amp. For bypassing HF energy past the PS inputs, monolithic ceramic capacitors are best.

In summary, the three stage philosophy of power supply design allows tailoring of the power delivered to the signal circuit to minimize interference from AC line problems while providing whatever power is needed by the signal circuitry at any frequency.

Re: Line level power supply concepts

January 7th, 2019, 12:32 pm

As a follow up to Tom's comments, I want to make some general ones. These apply to regulated or unregulated supplies, and higher-voltage supplies for tubes as well.

The power supply for an audio application really is half the sound of the amplifier, and thus it is important to get it right. Basically, the most important thing is having low output impedance with lots of energy storage for meeting the demands of the load. Often regulation is not needed, but it often provides a lower cost way of removing line-frequency ripple. It also can be very advantageous for circuits with high gain at low frequencies, especially ones with somewhat less than stellar power-supply rejection such as tube phono preamps. If active regulation is used, it should be isolated from the feed from the audio stage with a series impedance such as a resistor or inductor, and then one or more effectively large shunt capacitors to ground that feed the audio circuit directly. One large electrolytic capacitor is usually sufficient for tube preamps, and several in parallel for tube power amps.

On the subject of bypassing these electrolytics one needs to look at the high-frequency requirements of the particular load or circuit. For most tube audio circuits the impedances are high and bypassing is not needed because the impedance of the typical electrolytic is much lower and dominates. For low-voltage high-speed digital loads, absolute care must be taken in bypassing as the impedance if the power supply must be kept low into the hundreds of megahertz region. This is the domain of surface-mount ceramics on transmission-line configurations. Extending back down into audio applications, high-speed op amps require bypassing as well, although not as rigorous. Here, a leaded ceramic capacitor or pair of capacitors is generally effective without transmission line requirements, but leads should be as short as possible for best performance. Inadequate bypassing can lead to oscillations that make the op amp perform improperly, and while it may seem to have the right output it may not sound as good as it could because something is going on that shouldn't.

One more note on the type of capacitor best suited for bypassing. All capacitors have a self resonance where the impedance falls as the frequency rises at a 6-dB slope, then takes a sharp dip towards zero at a faster rate (self resonance), and then rises as the inductive properties take over. The higher the Q of the capacitor, the sharper the dip. A strong self resonance can actually cause problems in the circuit, and it is usually better to have a capacitor that can absorb energy and dissipate it as heat rather than exciting an undamped resonance. So ceramic capacitors are generally the best choice here because they are somewhat lossy. High Q film capacitors are best for coupling in audio circuits, but not appropriate for bypassing, although in a small number of cases film caps may be used in high-voltage applications.

Hope this helps-David

Re: Line level power supply concepts

February 18th, 2019, 1:33 pm

David, for your point

"If active regulation is used, it should be isolated from the feed from the audio stage with a series impedance such as a resistor or inductor, and then one or more effectively large shunt capacitors to ground that feed the audio circuit directly."

What is the purpose of a series impedance in this case? It seems like this would increase the impedance of the power supply as seen by the amplifier, which would be a bad thing. That is, the lowest possible power supply impedance would be a direct connection to the active regulator output.


Re: Line level power supply concepts

February 18th, 2019, 11:31 pm

The capacitor across the output should present very low impedance to the audio circuit, and should be very large and of good quality. If there is no series impedance between the regulator and a capacitor to ground, there may be some instabilities in the regulator that result in oscillation or other phenomena that results in sonic degradation. If you omit the capacitor, the regulator's output impedance will rise at higher frequencies.


Re: Line level power supply concepts

February 18th, 2019, 11:40 pm

I am assuming in my previous post that we are using this for an audio-frequency circuit. If low impedance is required at dc, then a direction connection to the regulator may be required, or a sense signal sent back to the regulator to compensate for voltage drop in the series impedance. With the sense scheme you could use the large capacitor that would give the best sound and better performance at high frequency. With the direct connection the capacitor size may be limited as well as high-frequency performance. For audio, dc accuracy is generally not important.


Re: Line level power supply concepts

February 19th, 2019, 3:25 pm

That makes sense -- over the audio frequency range a large capacitor with a suitable decoupling resistor provides a lower power supply impedance compared to a conventional active regulator.

I'll have to get my soldering iron out and make a few changes!


Re: Line level power supply concepts

February 19th, 2019, 3:37 pm

The resistor decoupling of the output filter capacitor and the regulator works well. If the currents are higher the choke works better with less voltage drop and also some measure of energy storage.
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