Traynor DynaGain 30 Bad Cap?

[bobloblaws]

Again, I struggle with english. I am not trying to be an ass or confuse you.

Your english is good, no worries.

Grounded = connected to ground. Ground referenced =/= grounded; it means X volts from ground; with op amps in split supplies, it means very close to ground dc wise at idle.

Still not sure I follow, and I'd like to understand it. Let's use a practical example, again, nevetslab wrote "That stage is ground referenced via 470k to the (+) Input terminal". So if I was the designer of that circuit and I said "I need this stage to have a certain voltage (or voltage range), so I will reference it to ground with a 470K resistor between the input terminal and ground" with the result being that the voltage at the input terminal is something other than 0V (assuming ground is 0V), is that how the term is used?

[kscience]

Your english is good, no worries.


Still not sure I follow, and I'd like to understand it. Let's use a practical example, again, nevetslab wrote "That stage is ground referenced via 470k to the (+) Input terminal". So if I was the designer of that circuit and I said "I need this stage to have a certain voltage (or voltage range), so I will reference it to ground with a 470K resistor between the input terminal and ground" with the result being that the voltage at the input terminal is something other than 0V (assuming ground is 0V), is that how the term is used?

I mis spoke. ground reference is just that; ground is considered the zero point around which supply and signal voltages are measured.

In op amp circuits, most of them- they use a balanced split power supply, like +/-12 or 15 volts dc. This allows them to get inputs that are dc coupled; no cap is needed to break a dc bias from the last stage. With single supplies & discrete devices, you need capacitor coupling between stages to block the dc from the previous stage. Also with bipolar transistors the base has to be at least .7v above ground to bias the device on, so inputs here are also ac coupled. For sure with tubes! the plates can be a couple hundred volts above ground.

jfets & tubes are easier to work with, they self bias and can be fed directly off of a dc coupled guitar pickup [passive] or eq or volume controls.

But you put dc on the input of an op amp that runs at 0v in and out*, it will be multiplied by the gain of the stage. fine for lab use, doing math with the op amps but not so much in audio. pretty important in bench power supplies & battery chargers though. To say nothing of voltage regulators.

*op amps have output offset voltages due to the imperfection of discrete devices but for audio it is usually trivial. But ac coupling will minimize the muddiness of excessive low frequencies that will even modulate the detail; this often sounds bad. well, tremolo is one exception, but we quibble.