Analog current drive amplifier manual

Analog current drive amplifier user manual

Pinout and layout

Powering on

Amplifier power on/off is controlled by onboard J2 or by external control over J4.

If J2 is on, then external control is disabled and amplifier is always on. Power supply controls if amplifier is on or off.
If J2 is off, then external control is controlling amplifier. Active hight signal. 3.3V to 12V control input. Galvanically isolated.

If amplifier is powered and in ON state. Green LED will be on.

Fault output

There is fault output signal on J4 connector Control OUT pin. This pin can be used to inform external MCU about amp state. If no protection is active this signal is open collector output. If some protection is active fault signal pulls down to GND isolated pin. You have to limit current to be below 40mA max.

Any MCU input with pullup will work. This output is isolated over opto isolator.

Amplifier protections

Amplifier has protections:

Overtemperature protection
Clip protection
Overcurrent protection

All these protections will output state over green LED. If some protection is active LED will start blinking every second 500ms pulse followed by 2 seconds pause. Counting blink code tells which protection is active.

If protection kicks in amplifier goes to off state. And stays there until reset. Reset is done by repowering amp by power supply or J2 or J4. Power suplly resets instantly and J2 with J4 resets after full error blinking code is blinked out.

Clip protection is a little different. It is more a quality feature than a safety feature. So if clip is detected the amp mutes for 3 seconds only. But if it happens more than 5 times over 30s period, then the amp switches off and blink code appears. And reset needed. This feature is to inform the user that he is entering into clip zone, and input signal has to be reduced. Clip protection see which amp is clipping and which polarity it is happening at. Positive or negative.
Power loss protection works as silent turn off/on feature and can detect if short voltage drop occurs on power rails. If it drops more than 4.5V but voltage stays above 20V the amp switches off and blinks out error code.
Overcurrent protection kicks in if current >5.4A. Reaction time 30uS.
Overtemperature protection uses thermistor mounted on power chips and are set at 80 deg C. If heatsink is not adequate or has restricted airflow this protection will switch off the amp for safety reasons. The blink code will output.

Additionally second layer of protections is on power chip itself. Overcurrent and overtemperature protections just at higher current and temperature.

Speaker outputs

Speaker output terminals J3 and J17 has two + pins. They are internally connected. - pins are not grounded and cannot be grounded in any way ! Speaker both terminals should be left floating and cannot be paralleled or bridged left with right channel. - pin also has some voltage. So it is very tricky to measure amplifier performance if you want to do so.

Stability

This amp is not inherently stable with all loads. In other words it is not unity gain stable. It is stable with resistive loads 3.3Ohm and more. Below 3.3Ohm oscillations can start. This is only if resistive load connected and in current mode.

This is because the lower resistance load makes effective gain of current amp lower. Phase margin is degraded and amp can oscillate. But if a real electrodynamic driver is connected, the inductance isolates the resistive part and this is not a real problem.

Even for resistive load if oscillations will start they will rise until overcurrent protection kicks in. This can be tested easily by shorting outputs on current drive mode. Amp switches off and blink code appears.

Frontend filter (for advanced users)

This section is for advanced DIY'er it requires electronics knowledge.

Amplifier input sensitivity Pic 7. could be decreased from the default one if needed. R25, R22, R32, R28 are 0R resistors, so input sensitivity is at max. Replacing these with larger resistance makes resistor divider. In example placing 10k resistors input sensitivity is lowered. In other words amp gain reduced 6dB.

Pic 7. Input sensitivity

EQ filter adjustments also done by soldering resistors. By design they must strictly stay between min and max values in Table 1. If you don't understand the idea or want to do filter in DSP then you can skip end of this section. And if you understand what you are doing here is Tina-Ti design file for filter response calculation.

Using this filter when in mixed mode 1 it is possible to adjust response to within 1dB variation compared to response in voltage mode. Overall adjustment better to do in DSP. And absolute best option is to lower high frequency boost with first and second cut filters with this analog filter and resonance bump EQ to do in DSP. In such way SNR of the amp is improved. For this reason default filter configuration uses slight downward slope as this amp is meant to be used in mixed mode. Not voltage mode by default. Amplifier uses two RC filters in series to create desired slope.

Another most problematic part of current drive amplifier is that at resonance output is increased. Good thing that in reality the SPL peak at resonance is a lot lower than expected from impedance audio driver graph. Second thing that in mixed mode amplifiers output impedance is pretty low at resonance and peak is low in SPL. So simple filter fixes this.

table 1. Filter adjustment table

Output impedance

Amplifier has 3 modes of operation:

Voltage
Mixed mode 1
Mixed mode 2

Output impedance of an amplifier is 0.5 Ohm at voltage mode. In special case if really needed output impedance can be reduced to ~0 Ohm. Just by disconnecting speaker negative return wire from J3, J17 minus terminals and connecting negative return to J13 Supply GND terminal. Warning ! By doing so you loose overcurrent/oscillation protection.

Mode 1 and mode 2 changes impedance as a function of frequency. Pic. 8

Mode 2 is more of a current drive, but at the expense of larger output impedance at drivers resonance. Anyway if output impedance is more than 10 times higher the load impedance it could be considered as current drive. Which is around 100 Ohm. Above that it is nice to have, but makes little difference.

Mixed mode 1is probably the preferred way if working at drivers resonance.
But mixed mode 2 is more of a current drive and can be favourite in example multichannel system where you are working above drivers resonance, and the peak is not a concern.

Pic.8 Mixed mode impedances

If amp is used in mixed mode and without DSP resistors R14 and R61 might need change. It depends on min impedance of the connected driver. Find minima of connected drivers impedance and multiply by 2. It is because amplifier gain in current mode depends on drivers impedance. Different gains has to be matched in transition from voltage to current if flat response is required. For better understanding see Pic. 9

Pic.9 Mixed mode gains

Power supply and heatsink

Amplifier board is designed for SMPS supply and best to use with it. Transformer based supply might work well, but better to avoid. Voltage variation of such supply could create some small issues. Or at least reduced usable power output. SMPS doesn't degrade sound quality if amp is done right. In example CONNEX SMPS300REh power supply will work well. - +36V looks optimal without knowing other requirements. As lower voltage will limit output capability and higher voltage increases heat at idle. But everything depends on application.

Heatsink definitely depends on application. But 0.4C/W is nice to have. Something like Dissipante 3U 300mm used in this enclosure.

Dimensions

Dimensions are in mm. Bottom picture shows four holes to mount amplifier to a heatsink. Amplifier comes with L shape brackets. M3 bolts should be used.

Specifications