Editor’s Note: This DI is a two-part series.
In Part 1, Nick Cornford deliberately oscillates the TDA7052A audio power amplifier to produce a siren-like sound and, given the device’s distortion characteristics, a functional Wien bridge oscillator.
In Part 2, Cornford minimizes this distortion and adds amplitude control to the circuit.
When audio power amplifiers oscillate, the result is often smoke, perhaps with a well-cooked PCB and a side order of fried tweeter. This two-part Design Idea (DI) shows some interesting ways of (mis-)using a common power amp to produce deliberate oscillations of varying qualities.
That device is the TDA7052A, a neat 8-pin device with a high, voltage-controllable gain, capable of driving up to a watt or so into a bridge-tied load from its balanced outputs. The TDA7056A is a better-heatsinked (-heatsunk?) 5-W version. (That “A” on the part number is critical; the straight TDA7052 has slightly more gain, but no control over it.) The TDA7052B is an uprated device with a very similar spec, and the TDA7056B is the 5-W counterpart of that. But now the bad news: they are no longer manufactured. Some good news: they can easily be found online, and there is also a Taiwanese second source (or clone) from Unisonic Technologies Ltd.
A simple circuit’s siren song
For the best results, we’ll need to check out some things that don’t appear on the data sheets, but let’s cut straight to something more practical: a working circuit. Figure 1 shows how the balanced, anti-phase outputs help us build a simple oscillator based on the integrator-with-thresholds architecture.
Figure 1 A minimalist power oscillator, with typical waveforms.
This circuit has just three advantages: it’s very simple, reasonably efficient, and, with a connected speaker, very loud. Apart from those, it has problems. Because of the amp’s input loading (nominally 20k) and the variation of drive levels with different loads, it’s hard to calculate the frequency precisely. (The frequency-versus-R1 values shown are measured ones.) R2 is needed to reduce loading on the timing network, but must leave enough gain for steady operation. (A series capacitor here proved unnecessary, as the internally biased input pin is being over-driven.) Its efficiency is due to the amp’s output devices being run in saturation: with no extra heatsinking, the (DIL-8) package warms by ~15°C when driving into an 8 Ω speaker. The square wave produced is somewhat asymmetrical, though good enough for alarm use.
Figure 1 shows a 5-V supply. Raising that to 12 V made only one change to the performance: the output became very, very loud. And it drew around an amp with a 10 Ω load. And it could do with a heatsink. And a TDA7056A/B rather than a ’52.
The Vcon input on pin 4 is not used. Left open, it floats at ~1.14 V, giving the device a measured gain of around 25 dB. Taking it close to ground inhibits operation, so a bare-drain MOSFET hooked on here can give on/off control. Taking it higher gives full gain, with a shift in frequency. If that is not important (and, in this context, why should it be?), logic control through a 22k resistor works fine. When inhibited, the device still draws 8–10 mA.
Feeding Vcon with varying analog signals of up to a few tens of hertz can produce interesting siren effects because changes in gain affect the oscillation frequency. But for a proper siren, it would be better to generate everything inside a small micro and use an H-bridge of (less lossy) MOSFETs to drive the speaker with proper square waves. (We’ve all heard something like that on nearby streets, though hopefully not in our own.)
Fancy sound effects apart, any power amp with a suitable input structure, enough gain, and balanced (BTL) outputs should work well in this simplest of circuits.
Determining distortion
So much for simplicity and raw grunt. Now let’s take a look at some of the device’s subtleties and see how we can use those to good effect. Distortion will be critical, but the data sheet merely quotes 0.3 to 1% under load, which is scarcely hi-fi. If we remove the load, things look much healthier. Figure 2 shows the unloaded output spectrum when the input was driven from an ultra-low-distortion oscillator, at levels trimmed to give 0 dBu (2.83 V pk-pk) and -20 dBu at the output with a device gain fixed at around 25 dB (Vcon was left open, but decoupled).
Figure 2 The TDA7052A’s output spectra for high and low output levels, taken under ideal conditions and with no output load.
Further tests with various combinations of input level and device gain showed that distortion is least for the highest gains—or smallest gain-reductions—and lowest levels. With outputs less than ~300 mVpk–pk (~-18 dBu) and gains more than 10 dB, distortion is buried in the noise.
That’s unloaded. Put a 10 Ω resistive load across the outputs, and the result is Figure 3.
Figure 3 Similar spectra to Figure 2, but with a 10-Ω output load.
That looks like around -38 dB THD for each trace, compared with better than -60 and -70 dB for the unloaded cases. All this confirms that the distortion comes mainly from the output stages, and then only when they are loaded.
A working one-chip sine-wave oscillator—
This means that we have a chance to build a one-chip Wien bridge audio oscillator, which could even drive a power load directly while still having lower distortion than the average loudspeaker. Let’s try adding a Wien frequency-selective network and a simple gain-control loop, which uses Zener diodes to sense and stabilize the operating level, as in Figure 4.
Figure 4 A simple gain control loop helps maintain a constant output amplitude in a basic Wien bridge oscillator.
The Wien network is R1 to R4 with C1 and C2. This has both minimum loss (~10 dB) and minimum phase shift (~0°) at f = 1 / 2π C2 (R2 + R4), which gives the oscillation frequency when just enough positive feedback is added. When the amplitude is large enough, Zeners D1 and D2 start to conduct on the peaks, progressively turning Q1 on, thus pulling U1’s Vcon pin lower to reduce its gain enough to maintain clean oscillation.
C3 smooths out the inevitable ripple and determines the control loop’s time-constant. R5 minimizes U1’s loading of the Wien network while C3 blocks DC, and R6 sets the output level. The unloaded spectra for outputs of 0 and -10 dBV are shown in Figure 5.
Figure 5. The spectra of Figure 4’s oscillator for 0 and -10 dBV outputs with no load.
—that has problems
While those spectra are half-decent, with THDs of around -45 and -60 dB (or ~0.1% distortion), they are only valid for a given temperature and with no extra load on the output. Increasing the temperature by 25°C halves the output amplitude—no surprise, given the tempcos of the diodes and the transistor. And those 3.3-V Zeners have very soft knees, especially at low operating currents, so they are better regarded as non-linear resistors than as sharp level-sensors.
Adding a 10-Ω resistor as a load—and tweaking R6 to readjust the levels—gives Figure 6.
Figure 6 Similar spectra to Figure 5’s but with the output loaded with 10 Ω.
THD is now around -30 dB, or 3%. Unimpressive, but comparable with many speakers’ distortions, and actually worse than the data-sheet figures for a loaded device.
So, we must conclude that while a one-chip sinusoidal oscillator based on this is doable, it isn’t very usable, and further tweaks won’t help much. We need better amplitude control, which means adding another chip, perhaps a dual op-amp, and that is what we will do in Part 2.
—Nick Cornford built his first crystal set at 10, and since then has designed professional audio equipment, many datacomm products, and technical security kit. He has at last retired. Mostly. Sort of.
Related Content
- Ultra-low distortion oscillator, part 1: how not to do it.
- Ultra-low distortion oscillator, part 2: the real deal
- Distortion in power amplifiers, Part IV: the power amplifier stages
- A pitch-linear VCO, part 1: Getting it going
- A pitch-linear VCO, part 2: taking it further
The post Power amplifiers that oscillate—deliberately. Part 1: A simple start. appeared first on EDN.
