Hi all, I'm going to be building a 12AX7 preamp for musical instrument purposes to match up to a Leslie speaker amplifier. The circuit I'm building is based on this, but without the pre/post tone controls (only the "output warming" control): HammondWiki - Kons 12 AX7 based EQ&Overdrive Circuit What I would like to do is create a way to light an LED (or neon lamp, anything really) when the AC signal level AFTER the "master volume" control reaches +12 dBu or so, which is theoretically the "design" input level for the Leslie amplifier.
Note that in my case, the output will be connected directly to the Leslie amp, so there's no additional gain stage after the "master volume" control. I'm thinking I'll have to use an op amp or something to drive an LED, which is OK with me. I was thinking of using a simple circuit connected to the 6.3V filament winding with a half-wave rectifier and voltage regulator to generate +12V DC or something to drive the op amp.
I'd appreciate your thoughts and ideas. If I can avoid adding the op amp I will definitely be happy but I'm not holding my breath. Thanks!
Note that in my case, the output will be connected directly to the Leslie amp, so there's no additional gain stage after the "master volume" control. I'm thinking I'll have to use an op amp or something to drive an LED, which is OK with me. I was thinking of using a simple circuit connected to the 6.3V filament winding with a half-wave rectifier and voltage regulator to generate +12V DC or something to drive the op amp.
I'd appreciate your thoughts and ideas. If I can avoid adding the op amp I will definitely be happy but I'm not holding my breath. Thanks!
The LED wants more power current than the whole preamp. The opamp wants much less than 250V(?).
The best fit may be a Magic Eye tube.
The best fit may be a Magic Eye tube.
There are tons of parts like this one on ebay. 20seg 57mm LED VU meter module with Peak Hold Function DC5V Power Supply-10G6Y4R | eBay
That is a thought, but it's way more elaborate than what I want (a single LED/indicator at a fixed dBu/voltage level). Plus that thing operates at preset conventional levels, I'm not even sure it would show +12 dBu, and if it did it would be far into the red zone.
You'd just need an attenuator at its input, probably a series resistor would do. A buffer, a precision full wave rectifier, and a window comparator would work as a DIY circuit. The filament supply could power the circuit.
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This will get you started, you want a 4.36V peak to trigger a peak hold indicator. Compare that with a reference voltage.
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You do not need, nor really want, a Precision Rectifier as a peak flasher.
You are not interested in 1mV-10V dynamic range, only 4V. For music purposes, it does not even have to be temperature stable (within reason).
Attack/decay timings are more important. Do we want to see a 1mS clip that nobody will hear? Do we want a 10mS decay that the eye can miss? Timings like 10mS-30mS catch and 1 Sec hold are probably more useful.
The world is FULL of schematics for mixers with peak lights.
Yes, a +4dDu meter becomes a +12dBu detector with a pot or pad in front. Audio level control is a basic soundperson skill. Cutting high level to fit low windows is easiest of all.
You are not interested in 1mV-10V dynamic range, only 4V. For music purposes, it does not even have to be temperature stable (within reason).
Attack/decay timings are more important. Do we want to see a 1mS clip that nobody will hear? Do we want a 10mS decay that the eye can miss? Timings like 10mS-30mS catch and 1 Sec hold are probably more useful.
The world is FULL of schematics for mixers with peak lights.
Yes, a +4dDu meter becomes a +12dBu detector with a pot or pad in front. Audio level control is a basic soundperson skill. Cutting high level to fit low windows is easiest of all.
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