(again, +/- xxV means +...V and -...V (pos. + neg. voltage))
The auxiliary power for an audio system is typically below 20Vdc volts and up to a few hundred mA current. Some examples are:
- ICs running on 5V
- 3.3V is becoming more and more common for digital circuits
- +/- 15V was and is common for audio op-amps for pre-amplifiers, active filters etc
- Gate drive voltage for power mosfets "VN10", typically 10 Volts above the negative rail and typically 50-200mA
- Power for relay coils, fan, lamps.
The Tripath chip and auxiliary power
It can be said at once that many of the Tripath power amp chips have internal regulators/generators for some of the voltages mentioned above.
- Many Tripath chips can generate the gate drive current/voltage needed to drive the MOSFETs and some also have a 5V regulator built in
- Lower power chips usually have a capacitive switching sub-circuit that may require a few diodes and capacitors to work
- Some, like the TA2022, TK2350 have an internal small switch mode power supply requiring an external diode and inductor
- The TA3020 which requires an external gate drive power supply
- The lower power chip usually have a linear regulator built-in for it's 5V section. All the chips targeting 12V applications have internal 5V regulators.
- The 5V section needs clean/well regulated power. A typical 78XX or 317 regulator performance is adequate.
- The VN10 does not need very clean voltage: a regulated SMPS is adequate.
The 41Hz Audio amp models and auxiliary power
The TA2020, TA2021B, TAA4100A, used for AMP3X, AMP6X and AMP9X have internal circuits for both 5V and gate drive so these do not need any external auxiliary power.
- The TA2022 used for AMP1x, AMP5, AMP10x need external 5V while the gate drive voltage is generated by a small built-in SMPS. The AMP5 and AMP10x have an on-board linear regulators, based on the LM317 on the PCB so they need no external auxillary power while AMP1x needs an external 5V supply.
- The TK2350 used for AMP2 and AMP8 have an internal SMPS for the gate drive but both amplifier models need an external 5V supply.
- The TK2050 has a built-in gate drive regulator but needs 5V. On AMP4 and AMP11x, a 5V linear regulator is on the amp board so no external 5V is needed.
- The TA3020 used for Truepath, AMP7 and AMP15x needs both gate drive (VN10) and 5V supplies. The Truepath and AMP7 needs external supplies for both while AMP15x has on-board regulators for both.
The 5V section
All Tripath chips have an analogue input section running on 5V plus some low power digital section also requiring 5V. This needs to be a well stabilized, quiet supply. The smallest models typically have a 5V regulator integrated, which only needs some small capacitors added near the chip. This voltage is not intended for other use.
Other Tripath chips need a 5V supply. Typically around 50mA. Many 41Hz Audio-amp models have a 5V regulator integrated on the main PCB, near the main chip. On many models there is a separate jumper / connector for the 5V, where you can either connect and external 5V regulator if you for some reason want that, or you can also use the on-board 5V to supply other 5V boards. Just take care that whatever external circuit you connect does not introduce noise on the 5V supply.
With other high end digital/analogue audio chips it is not unusual to split the supply for the analogue section and the digital section, even if they both run on 5V. However, the Tripath chips work very well running on the same supply and all our models are connected and operated that way. Splitting the two has no advantage at all in real life tests and may cause complications at start-up.
Gate drive voltage/VN10
All the Tripath based amps have MOSFET based output stages which need a gate drive voltage. N-channel MOSFETs typically need a gate voltage of around 10V above the source (negative side for N-channel MOSFETs) to conduct fully. In amplifier circuits, the 10V would be supplied relative to the negative rail. This supply voltage is usually referred to as VN10. The current needed depends on the MOSFET gate charge and the number of MOSFETs. Smaller amps may need less than 50mA on this supply while the larger ones may need 200mA or even more. VN10 must track the negative rail, staying at 10V above the negative supply side at all times.
A positive regulator is therefore required, using a negative regulator is NOT a good solution. The gate drive current/voltage for both low side and high side FETs are supplied via the negative rail. The negative FETs are driven straight from the VN10 while the high side FETs are supplied via a bootstrap circuit diode and capacitor. In order to charge the high side capacitor, a few switches to the low side are done when the chip powers up. The quality in terms of stability is not very critical for the VN10 supply, as long as the voltage stays within its specifications which is usually 9-11V. Lower and higher voltages may cause damage to the ICs and MOSFETs.
Auxiliary power for other ICs
Modern OPamps are often very tolerant to power supply noise, with power supply rejections ratios often exceeding 100dB. Using super quiet supplies for these, may be a waste of money. Other circuits like DAC and ADC may however require very clean supplies. Each circuit may have different requirements so reading data-sheets and testing may be required.
In large electronic systems it is common practice that a few raw supply voltages are distributed from a main power supply, while each board or even each/all major ICs have their own close-up regulators near where the power is actually needed. Normal voltage regulators are quite efficient at damping out supply line fluctuations/noise in the audio band. The method of each sub-board having its own regulators, minimizes the risk of noise being picked up along the supply lines and decreases the risk of different circuits contaminating the supply with noise.
The source for auxiliary power
Obviously, we can source auxiliary power along a few different lines.
From the main power supply with linear regulators
The advantage with this solution is simplicity, low cost and low output noise. It would involve a resistive element which could be as simple as a resistor, a transistor based circuit and some zener reference or a voltage regulator IC. The downside is that efficiency is low, some heat will be generated.
From the main power supply with a switch mode regulator
The advantage with this solution is high efficiency, low heat losses. A small switch mode regulator can be quite simple. Electrical noise can be of concern and some filtering on the input and output is usually required if the power is to be used for analogue circuitry. In the 41hz Audio amps, there are always additional filters when an SMPS is used for analogue circuits.
Additional transformer
The advantage with this solution is that the efficiency is high but requires additional mains power wiring and another secondary rectifier so the total cost is quite high. One advantage is that power from an additional axillary transformer can be used for start-up circuits for the main power transformer, for example for a soft start or remote start circuit.
Additional power supply
This would be a ready made power supply, based on one of the above techniques. It can be a simple solution but may be hard to integrate.
Regulators for the auxiliary power
There are a few considerations on generating these voltages and several ways to actually generate them.
Low voltage, typically 30Vdc or less
Here we have plenty of options. There are many linear regulators that can be used as long as the current and heat dissipation are acceptable. The old 7800 (positive) and 7900 (negative) fixed regulators are often fine, while the LM317 (positive) and LM337 (negative) have better specifications in many aspects so we usually use these. The LM317 is available for low currents up to several amps.
High input voltage, typically 30Vdc or more
Here, our options are dramatically fewer, especially if we need a negative regulator.
- There are a few positive linear regulator ICs available, for negative output, very few.
- For minimal loss, use switched mode regulators, SMPS. There have been many bad SMPSs out there so their reputation is yet to pick up in audio circuits. The output from an SMPS is more noisy than from a linear regulator by nature, so we may combine the two; an SMPS to lower the voltage with low loss and a linear regulator to clean up the output. SMPS can be positive, negative and even inverting.
Low currents, typically 50mA
It is an advantage to use a regulator that can also function as a current limiter if things go wrong. There are many 100mA rated regulators and we often use a 100mA TO-92 type regulator near the chip. These have very good noise levels in themselves and placing them near the main IC minimizes the risk of picking up stray noise. An advantage with these small regulators is the built-in current limiting to around 100mA, which usually allows connecting them without a fuse. However, the power dissipation capacity of the 100mA regulators limited, so they are usually combined with some kind of pre-regulator, taking the input voltage down to a few volts above the desired output voltage.
Higher currents, typically 100-200mA
If the input voltage is near the output voltage, we can easily handle a few hundred milli-amps with a TO-220 based regulator. However, as the power dissipation grows, we may be looking at other solutions. With the high power AMPs, running from 60V rails and needing 200mA VN10, a linear regulator from the main rail would generate around 10W of heat loss, so a small auxiliary SMPS or separate transformer is often used.
41Hz Audio auxiliary power boards and kits
Here is a summary of the auxiliary kits from 41Hz Audio
- PSU1-VR (#690) Dual positive adjustable regulators with rectifiers for axillary transformer
- PS2-P(#829) Single step-down SMPS combined with a linear regulator. Max 90V IN
- PS2-P+P(#779) Dual independent step-down SMPS combined with linear regulators. Max 2X90V IN
- PS2-P+N(#827) Positive and negative independent SMPS combined with linear regulators. Max +/-90V IN
- PS2-P+Inv(#828) Positive and inverting (positive in, positive and negative out) independent SMPS combined with linear regulators. Max +/-90V In
- PS317 (#688) Positive adjustable linear 100mA regulator circuit on TO-220-like footprint
- PS337 (#690) Negative adjustable linear 100mA regulator circuit on TO-220-like footprint
- PS4 (#317) SMPS Step-down, up to 90V in, 200mA out. TO-220-like footprint
- PS8 (#78) SMPS and linear regulators. +5V input, +/-12V 2x100mA output
- PS9-P (#2090) Positive linear transistor / pre-regulator. Input up to 100V. Output up to 500mA. For mounting on a heat sink
- PS9-N (#2091) Negative linear transistor / pre-regulator. Input up to -100V. Output up to 500mA. For mounting on a heat sink
Model |
Max Input |
Max current |
Type |
Voltage Set by |
PSU1-VR |
35V |
1A |
Transformer / rectifiers / linear |
Trimmers |
PS2-X |
90V |
2x250mA |
SMPS + linear |
Zener+resistors |
PS3x7 |
35V |
100mA |
Linear TO-92 on TO-220 sized PCB |
Resistors |
PS4 |
90V |
200mA |
SMPS on TO-220 sized PCB SMT |
Resistors |
PS8 |
5.5V |
2x100mA |
SMPS + Linear SMT |
Resistors |
PS9-X |
90V |
500mA |
Linear. PCB for mounting on heat sink |
Zener |
| Model | May refer to several versions (the 'x' part in model name) |
| Max. Input | Typical maximum input voltage. Many models allow higher voltage by simple modifications |
| Max. current | Max. allowable current load |
| Type | Summary of the circuit type |
| Voltage Set by |
Method for setting the output voltage: - Trimmer: very flexible, needs adjustment, a bit costly. - Resistors, compact and simple. - Zener, simple. Often part of the feedback loop. Requires a zener diode with the right value and these are only available at certain nominal values, and tolerance may not be the highest. Mostly for pre-regulators. |
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