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recommended power supply for a Bugle is [email protected].It is
+/- 15V. This is old shorthand for "two 15V supplies tied together so as you can get positive OR negative voltages".
As jh implied with "How do you make split supplies? You need both + and - about ground."
A "+/-" power supply has three terminals. Internally it may be two power supplies with four terminals with two terminals strapped together. Indeed that is how the stock battery array is connected: four snaps, but if you peek the connections, "+" on one batt connects to "-" on the other batt and to circuit ground/common.
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I'm using an older Coleco power supply that is labeled 12v - and 12v + and connecting to the Bugle via alligator clips to the 9v battery connectors - positive on the left side of the board and negative on the right side of the board.Does it also have a third wire?
I've seen some odd things on old game supplies. But this is most likely a single 12V supply, polarity marked, but no ground and no midpoint.
Shorn of distractions like most of the audio path, this is how the Bugle is wired:
Each supply rail is solidly set 9V up or down from the output jack shell/common. The opamp can swing maybe 6V up or 6V down relative to common and output jack shell.
This is what SamA seems to have tried:
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about 20 minutes or so, the sound begins to distort.According to the over-simplified drawing, the mystery is why it ever worked at all. The output jack shell is not connected to anything!
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Very odd, that it takes maybe 10 minutes for the problem to show up.That's the key clue.
Of course I've over-simplified. The real Bugle has capacitors each rail to ground. At power-up, equal capacitors will set the ground halfway between the rails. The output signal returns on the shell and signal current flows in the rail caps. It "works".
But that initial halfway charge is not "solid". Opamp input bias current starts to pull the common off-center, which is not a problem, until it is so off-center that it can't swing the peaks of one side of the signal. SamA hears distortion. His off-wait-on process re-starts with a new halfway charge.
I can't make out the rail-caps from the published info and am too lazy to look up bias current. You might run those numbers and see if time constant is minutes.
It's a good preamp and I dunno why someone would want to power it with a game-console supply. True, the Bugle will reject a lot of trash; you still have your very weak phono signal intimately connected with every vacuum cleaner and fluorescent lamp in the neighborhood. (At least a digi-game supply will reject RF on the powerlines.) Battery power floats far-away from mundane wall-power trash/hash. The Bugle wall power supply does a lot to damp the hash.
Here's a hasty-hack. Since we only need 30ma and most bricks have 100mA or more, "waste" power to fake a split rail. Two 1K will bleed around 10mA, chip bias currents just vanish in the flood. The output shell sees 500R, so with 50K amp input the crosstalk is 40dB down. I do not like it, do not advise it, but SamA showed that the preamp "works" with "infinite" power supply center-tap resistance, for a while, and this would swamp the lingering drift.
