Editor’s Be aware: This DI is a two-part collection.
In Half 1, Nick Cornford intentionally oscillates the TDA7052A audio energy amplifier to provide a siren-like sound and, given the system’s distortion traits, a useful Wien bridge oscillator.
In Half 2, Cornford minimizes this distortion and provides amplitude management to the circuit.
When audio energy amplifiers oscillate, the result’s typically smoke, maybe with a well-cooked PCB and a aspect order of fried tweeter. This two-part Design Concept (DI) exhibits some fascinating methods of (mis-)utilizing a typical energy amp to provide deliberate oscillations of various qualities.
That system is the TDA7052A, a neat 8-pin system with a excessive, voltage-controllable achieve, able to driving as much as a watt or so right into a bridge-tied load from its balanced outputs. The TDA7056A is a better-heatsinked (-heatsunk?) 5-W model. (That “A” on the half quantity is crucial; the straight TDA7052 has barely extra achieve, however no management over it.) The TDA7052B is an uprated system with a really comparable spec, and the TDA7056B is the 5-W counterpart of that. However now the dangerous information: they’re not manufactured. Some excellent news: they will simply be discovered on-line, and there’s additionally a Taiwanese second supply (or clone) from Unisonic Applied sciences Ltd.
A easy circuit’s siren music
For the perfect outcomes, we’ll want to take a look at some issues that don’t seem on the information sheets, however let’s reduce straight to one thing extra sensible: a working circuit. Determine 1 exhibits how the balanced, anti-phase outputs assist us construct a easy oscillator primarily based on the integrator-with-thresholds structure.
Determine 1 A minimalist energy oscillator, with typical waveforms.
This circuit has simply three benefits: it’s quite simple, moderately environment friendly, and, with a linked speaker, very loud. Other than these, it has issues. Due to the amp’s enter loading (nominally 20k) and the variation of drive ranges with totally different masses, it’s laborious to calculate the frequency exactly. (The frequency-versus-R1 values proven are measured ones.) R2 is required to cut back loading on the timing community, however should depart sufficient achieve for regular operation. (A collection capacitor right here proved pointless, because the internally biased enter pin is being over-driven.) Its effectivity is because of the amp’s output units being run in saturation: with no further heatsinking, the (DIL-8) package deal warms by ~15°C when driving into an 8 Ω speaker. The sq. wave produced is considerably asymmetrical, although adequate for alarm use.
Determine 1 exhibits a 5-V provide. Elevating that to 12 V made just one change to the efficiency: the output turned very, very loud. And it drew round an amp with a ten Ω load. And it may do with a heatsink. And a TDA7056A/B slightly than a ’52.
The Vcon enter on pin 4 is just not used. Left open, it floats at ~1.14 V, giving the system a measured achieve of round 25 dB. Taking it near floor inhibits operation, so a bare-drain MOSFET hooked on right here can provide on/off management. Taking it greater provides full achieve, with a shift in frequency. If that isn’t vital (and, on this context, why ought to or not it’s?), logic management by way of a 22k resistor works positive. When inhibited, the system nonetheless attracts 8–10 mA.
Feeding Vcon with various analog indicators of up to a couple tens of hertz can produce fascinating siren results as a result of modifications in achieve have an effect on the oscillation frequency. However for a correct siren, it could be higher to generate all the pieces inside a small micro and use an H-bridge of (much less lossy) MOSFETs to drive the speaker with correct sq. waves. (We’ve all heard one thing like that on close by streets, although hopefully not in our personal.)
Fancy sound results aside, any energy amp with an appropriate enter construction, sufficient achieve, and balanced (BTL) outputs ought to work properly on this easiest of circuits.
Figuring out distortion
A lot for simplicity and uncooked grunt. Now let’s check out among the system’s subtleties and see how we will use these to good impact. Distortion might be crucial, however the knowledge sheet merely quotes 0.3 to 1% underneath load, which is scarcely hi-fi. If we take away the load, issues look a lot more healthy. Determine 2 exhibits the unloaded output spectrum when the enter was pushed from an ultra-low-distortion oscillator, at ranges trimmed to provide 0 dBu (2.83 V pk-pk) and -20 dBu on the output with a tool achieve fastened at round 25 dB (Vcon was left open, however decoupled).
Determine 2 The TDA7052A’s output spectra for top and low output ranges, taken underneath ideally suited circumstances and with no output load.
Additional checks with numerous mixtures of enter degree and system achieve confirmed that distortion is least for the best good points—or smallest gain-reductions—and lowest ranges. With outputs lower than ~300 mVpk–pk (~-18 dBu) and good points greater than 10 dB, distortion is buried within the noise.
That’s unloaded. Put a ten Ω resistive load throughout the outputs, and the result’s Determine 3.
Determine 3 Related spectra to Determine 2, however with a 10-Ω output load.
That appears like round -38 dB THD for every hint, in contrast with higher than -60 and -70 dB for the unloaded circumstances. All this confirms that the distortion comes primarily from the output levels, after which solely when they’re loaded.
A working one-chip sine-wave oscillator—
Because of this now we have an opportunity to construct a one-chip Wien bridge audio oscillator, which may even drive an influence load straight whereas nonetheless having decrease distortion than the typical loudspeaker. Let’s strive including a Wien frequency-selective community and a easy gain-control loop, which makes use of Zener diodes to sense and stabilize the working degree, as in Determine 4.
Determine 4 A easy achieve management loop helps keep a relentless output amplitude in a primary Wien bridge oscillator.
The Wien community is R1 to R4 with C1 and C2. This has each minimal loss (~10 dB) and minimal part shift (~0°) at f = 1 / 2π C2 (R2 + R4), which supplies the oscillation frequency when simply sufficient constructive suggestions is added. When the amplitude is giant sufficient, Zeners D1 and D2 begin to conduct on the peaks, progressively turning Q1 on, thus pulling U1’s Vcon pin decrease to cut back its achieve sufficient to keep up clear oscillation.
C3 smooths out the inevitable ripple and determines the management loop’s time-constant. R5 minimizes U1’s loading of the Wien community whereas C3 blocks DC, and R6 units the output degree. The unloaded spectra for outputs of 0 and -10 dBV are proven in Determine 5.
Determine 5. The spectra of Determine 4’s oscillator for 0 and -10 dBV outputs with no load.
—that has issues
Whereas these spectra are half-decent, with THDs of round -45 and -60 dB (or ~0.1% distortion), they’re solely legitimate for a given temperature and with no further load on the output. Rising the temperature by 25°C halves the output amplitude—no shock, given the tempcos of the diodes and the transistor. And people 3.3-V Zeners have very delicate knees, particularly at low working currents, so they’re higher considered non-linear resistors than as sharp level-sensors.
Including a 10-Ω resistor as a load—and tweaking R6 to readjust the degrees—provides Determine 6.
Determine 6 Related spectra to Determine 5’s however with the output loaded with 10 Ω.
THD is now round -30 dB, or 3%. Unimpressive, however comparable with many audio system’ distortions, and truly worse than the data-sheet figures for a loaded system.
So, we should conclude that whereas a one-chip sinusoidal oscillator primarily based on that is doable, it isn’t very usable, and additional tweaks received’t assist a lot. We want higher amplitude management, which suggests including one other chip, maybe a twin op-amp, and that’s what we are going to do in Half 2.
—Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio gear, many datacomm merchandise, and technical safety package. He has ultimately retired. Principally. Form of.
Associated Content material
- Extremely-low distortion oscillator, half 1: how to not do it.
- Extremely-low distortion oscillator, half 2: the actual deal
- Distortion in energy amplifiers, Half IV: the ability amplifier levels
- A pitch-linear VCO, half 1: Getting it going
- A pitch-linear VCO, half 2: taking it additional
The put up Energy amplifiers that oscillate—intentionally. Half 1: A easy begin. appeared first on EDN.
