Over the years there have been many different audio amplifier designs which have found favour for a while and then been supplanted by newer ideas. One of them has crossed the bench of [Jazzy Jane], it’s a current dumping amplifier from the mid-1980s. A nicely-done home-made project on stripboard mounted on a wooden base board, it sports a power supply, RIAA pre-amp board, and the amplifier itself.
The current dumping amplifier is one that combines a small class A amplifier with a hefty class B one, and through feedback trickery uses the combination to remove the crossover distortion of the class B stage. It’s a simple yet elegant circuit with fewer parts than an equivalent class AB amplifier, and there was a time back in the day when it was all the rage. This one has an op-amp providing the class A part and a complimentary pair of Darlington pairs as the class B.
The video below the break shows the process of bringing the amp back to life, a process mostly concerned with the power supply. There are a set of tantalum capacitors which have failed, and the replacements she’s using turn out to have problems too. They’re a period part for a project of this age, but we might have been tempted to go for another capacitor type here.
The result is an unusual amplifier, brought back to life. You may have seen [Jane] feature here before, with her 1950s signal generator.
Current dumping amplifier schematics always looked strange to me. I am considering building one, just to try one out.
Solid tantalum capacitors strike again. These capacitors deserve the poor reputation they have acquired.
Not to be confused with wet tantalum capacitors. Those are absurdly expensive, but allegedly very reliable.
I didn’t know there was a difference. Looked up pictures of wet tantalum caps and found out I have a bunch of old tubular metal axial wet tants I pulled. I always just assumed they were electrolytic. Definitely going to test them now. Thank you for the info.
I like Jazzy Jane’s channel and would like to see more original style videos like this one from her.
The more modern option is a tantalum polymer cap, which is also significantly more reliable. You’re actually lucky if tantalum caps only go short. They’re well known to catch fire.
It’s a fun demo for newbies: overvolt a tantalum (it only takes 20-30%), or reverse polarity. Do this over a fireproof surface, or you may leave more of an impression than you wanted to. (seriously: a big one will burn through a linoleum floor right through to the underlayment)
This also handily gets rid of your stock of flammable tantalum caps.
Good article.
“…This one has an op-amp providing the class A part and a complimentary pair of Darlington pairs as the class B…”
“complimentary” should be “complementary”.
Good article.
Curious: why did those type of amplifiers stop being ‘all the rage’?
I would guess class D becoming good enough for thin and light and Carver’s trickery around tuning class AB. I’d love to hear from someone in the know
Despite measuring amazingly well for what essentially had a Class B or C output stage, the audiophile types looked at current dumping designs with distrust. Eventually one current dumping amplifier design gained a good reputation, the Quad 405.
Better Class AB amplifier designs slightly outperformed the current dumping designs. The Leach amp by Marshall Leach is a good example.
Of course once Class D amplifiers started becoming good enough, they almost entirely took over.
“… why did those type of amplifiers stop being ‘all the rage’?”
In one short word/phrase: the Class-D amplifier.
One would suspect that the over-riiding reason is for the same reason as that which has caused the linear power supply to be almost completely displaced by the SMPS (switched-mode power supply); has (very happily) resulted in the very expensive high-power linear AC power inverter’s being replaced/displaced by a much-lower-cost switching equivalent; has resulted in almost all audio applications being replaced by “switching-and-filtering” techniques (think CDs; DVDs; digital synthesizers)…
If the application can utilize it and down-stream elements/applications are indifferent to it, digital switching techniques almost always result in lower power consumption.
Almost always.