Editor’s Observe: This DI is a two-part sequence.
In Half 1, Nick Cornford intentionally oscillates the TDA7052A audio energy amplifier to provide a siren-like sound and, given the machine’s distortion traits, a practical Wien bridge oscillator.
In Half 2, Cornford minimizes this distortion and provides amplitude management to the circuit.
Within the first half of this Design Thought (DI), we noticed the best way to use the TDA7052A (or comparable) energy amp to construct a minimalist siren-type energy oscillator, and in addition, having checked out that machine’s distortion traits below numerous working situations, a easy however half-decent Wien bridge oscillator. On this second half, we’ll flip that semi-decency into one thing much more respectable, concentrating on minimizing distortion and ignoring the siren music of uncooked energy. We may also keep on with a 5-V provide, though the chip can deal with as much as 18 V. Determine 1 reveals the brand new circuit.
Determine 1 Including extra exact level-sensing permits significantly better amplitude management and in addition reduces distortion.
A lot of the modifications are within the management loop, however enter resistor R5 has been elevated as a result of we’ve extra achieve accessible now that Vcon could be pushed excessive in addition to simply being pulled low. In passing, the sequence mixture of R5 and U1’s nominally 20k enter resistance shunts R2/R4 just a little; a barely disturbing operation. Including 120k throughout R1/R3 may compensate for that, however the sensible distinction is tiny. The gangs on twin pots by no means fairly match anyway, so utilizing the lower-value half for R1 is nice sufficient.
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The management loop, detailed
Peak output ranges are actually sensed by comparability with the reference voltage outlined by R9 (or its wiper) and R10. (Watch out: pot R9 and glued resistor R10 ought to match, so the latter might should be trimmed.) The junction of R11 and R12 floats at ~650 mV till the height amplitude exceeds the reference, when D1 or D2 will briefly pull it decrease by ~450 mV. The ensuing attenuated pulse stream is filtered by R6/C6 and R7/C7 and level-shifted and buffered by Q1 into U1’s Vcon pin. That attenuation is important as a result of the efficient control-loop achieve would in any other case be extreme. U2 is an MCP6022, which has a low input-offset voltage and is quick sufficient to work properly as a comparator. The general temperature stability is nice, the output degree scarcely altering between 20°C and ~50°C.
Any residual ripple in a management loop like it will trigger distortion by modulating the sign. It’s minimized by the lead-lag filter R6/C6 and C7/R7, which additionally controls the settling time. The values used go away the loop considerably under-damped at low frequencies, however are a very good compromise.
The spectra with unloaded outputs at 0 dBV and -10 dBV are each proven in Determine 2.
Determine 2 The spectra for 1 kHz outputs at 0 dBV and -10 dBV, displaying THDs of round -60 and -73 dB respectively—say 0.1% and 0.02%.
Additional lowering the output ranges doesn’t enhance distortion a lot, partly as a result of this management scheme then turns into much less secure. Any offset of 1 output relative to the opposite may end up in one “part” of pulses being missed, resulting in elevated second-harmonic distortion, as implied by the relative ranges of second and third harmonics for the 2 ranges proven in Determine 2. Trimming the ability amp’s offset makes surprisingly little distinction, so it isn’t applied.
Attempting to derive the management voltage from a extra advanced full-wave rectifier gave just about the identical distortion figures, so we’ll keep on with Determine 1’s circuit, run it at -10 dBV or rather less, and settle for its THD.
Some ending touches
Swapping the capacitors C1 and C2 for 3n3 and 330n components gave the anticipated efficiency on increased and decrease ranges, although distortion was a number of dB increased on the low vary, and the extent was much less well-controlled on the highest frequencies. All that’s wanted to show this right into a helpful equipment is an output stage, for which the pretty standard circuit of Determine 3 works properly.
Determine 3 A easy output stage can ship as much as about +5 dBV, which is the rail-to-rail swing for a 5 V provide.
The additional stage provides virtually no distortion, and leaves a spare op-amp (U3b) which may very well be used to invert the sign from U3a to supply antiphase outputs. One ultimate spectrum, taken with the oscillator set for -10 dBV and the output for a swing of practically 5 V, is in Determine 4. That confirms an general THD of -73 dB, or 0.022%. Hardly earth-shattering, however nonetheless fairly first rate.
Determine 4 The spectrum on the output of Determine 3 for an output of ~+5 dBV.
No tweeters have been harmed within the making of this DI, although a 10-Ω, 1/4-W load resistor began to smoke. A 1-W half would have been more healthy, as it might solely have vaped.
—Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio gear, many datacomm merchandise, and technical safety equipment. He has finally retired. Principally. Kind of.
Associated Content material
- Energy amplifiers that oscillate—intentionally. Half 1: A easy begin.
- 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
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