Preamp for lm384

[fossilshark]

In my electronics class we are building a stereo lm384 amplifier and i feel the need to take it up a notch. Using only parts ive scavenged and tested to work i want to add a single 12ax7 preamp to the lm384s (with a pot to control how much power from the preamp goes to the chips). Can anyone suggest a circuit using just one triode in the 12ax7 so i can use one triode for one chip and the other for the other chip. This is for a guitar of course. Any help is appreciated

[fossilshark]

*update* lm384 require a signal input of .5v rms. Am i correct when i say i would need an impedance matching transformer between the 12ax7 and lm384s?

[PRR]

> my electronics class

What level?  (A EE Grad student should be able to work this out with minor pointers. A Sophmore would need more guidance.)

> input of .5v rms. ...impedance matching transformer

You start with Voltage (current not specified), then move to an impedance question implying a very costly part, with no explanation.

What power for the tube? (How many different power supplies is this part-Watt beast gonna need?)

FWIW, a single tube on a tube-like power supply, from guitar, has nearly no "flavor". The output level of the first stage in, say, a Champ is well below where the signal gets bent-up. We find "tube sound" in later stages.

[PRR]

> lm384 require a signal input of .5v rms

I do not think this is correct. No such "0.5Vrms" number appears on the LM384 datasheet. It does give Gain and Output Power. Max Out Power obviously varies with supply voltage, but it should only take a few seconds to work out the extremes.

[fossilshark]

Drew up a schematic and had it confirmed by one of my teachers. Yes i know there is absolutely no benefits to adding a tube stage but hey, i get bored. I think i figured it out, thanks for the repies

[2deaf]

In my electronics class we are building a stereo lm384 amplifier and i feel the need to take it up a notch. Using only parts ive scavenged and tested to work i want to add a single 12ax7 preamp to the lm384s (with a pot to control how much power from the preamp goes to the chips). Can anyone suggest a circuit using just one triode in the 12ax7 so i can use one triode for one chip and the other for the other chip. This is for a guitar of course. Any help is appreciated

Are you the guy who is in the 11th grade?

It is possible to make a single power supply that would power the LM384's, the 12AX7 plates, and the 12AX7 heaters.  Those LM384's can take an astonishing bite of current and the heaters can take even more, so I'm sure it would require a higher VA transformer than whatever you are using in your class.

A humbucker can overdrive a 12AX7 with a 9 volt power supply.  In order to overdrive it at higher supply voltages, you could put a jFET in front of it.

lm384 require a signal input of .5v rms.

The LM384 has an absolute maximum input voltage of +/- 0.5V.  This would be peak voltage.  The output would have long since clipped by the time you got up to 0.5V_p input with a gain of 50.  I interpret that spec. to mean you will damage the device if you exceed 0.5V_p at the input.       

[PRR]

> LM384 has an absolute maximum input voltage of +/- 0.5V.

By golly, it does!

As you imply, we'd never get near there in "normal" use. The output would be clipped, totally distorted.

A peek at the internals suggests that at -0.6V the input base-collector junction will forward-bias and conduct big. If unlimited, it would blow. 0.5V gives a little safety margin.

Not stated on LM384 sheet: "nearly all" analog pins can take 10mA without damage. They need to be this rugged to shrug-off static electric discharge in handling.

So we could wire to 100V as long as we put 10K in series. And 10K against the internal 150K is hardly-any signal loss in normal use.

A "proper" 12AX7 stage runs on 300V. We could go 33K series with trivial loss of signal. However a 12AX7 stage only eats about 1.5mA. It's all 100K load and 60K internal impedance. A worst-case could be 300V/60K or 5mA. A 12AX7 stage is not likely to melt the input of an LM384.

300V supply on top of 6/12V for heater and 9V-18V supply for LM384 is getting to be more supplies than amplifier-stuff. Also 300V in classrooms sounds dangerous. And we don't really need "maximum performance" to flog a little hi-gain chip. It is tempting to starve the tube with perhaps one 12V supply for LM384, heater, and plate.

But tubes do poorly at low voltages. Their output declines much faster than the voltage, and their happy-bias becomes unpredictable and erratic. An old rule of thumb is to select Mu < Vsupply. 12AX7 Mu=100 can work dandy with 100V supply (table lists 90V conditions for ACDC radio use), but may punk-out at lower supplies. 12AU7 Mu=20 is a better bet. Happy 12AU7 gives gain of 15, which may decline to gain of 10 at 12V supply. LM384 at 12V supply is 4Vrms max output, gain of 50. Gain of 10*50 is 500. 4V/500 is 8mV input sensitivity. This is plenty of gain for guitar, we more often aim at 20mV for all-around use. We can take that difference in a volume control between the tube and the chip.

[kagliostro]

For classroom  experiments a low voltage tube preamp can give some happy (?) experimentation time


http://www.tube-town.net/cms/?DIY/LoV-Projekte/Banana_Booster_%28engl%29

http://www.aronnelson.com/DIYFiles/up/12AU7-6111_Valve_Caster_Summary_Rev002.pdf






Franco

[fossilshark]

Wow thanks, i will use this schematic. Yes im the guy in 11th grade, and i did get that weird a$$ amp circuit working (if im the same guy your talking about). We are buildin 15v power supplys to go with the lm384s, how many amps would i need from the power transformer?

[fossilshark]

Also, i could use 12ax7s instead of 12au7s right?

[shooter]

how many amps would i need from the power transformer?

12XX7 typically runs ~1.5 - 3mA @ ~150 - 300VDC + .3 to .6A @6.3VAC

i could use 12ax7s instead of 12au7s right?

Yes,  if the sand part fry's at .5volt in, starting with an AU gives you less gain, and less chance to toast your sand

[fossilshark]

I dont have a 12au7 on hand, and the nearest electronics store is 2 hours away. I will take my chances. Im a noob so i dont know what you mean exactly by "sand" component. Is that the chip?

[PRR]

Figure a power amp, 8 ohm load, as "like" a 50 Ohm resistor. 12V/50r = 0.24 Amps.

The 12A_7 heater wants 0.15 Amps at 12 Volts.

The tube plate circuit demand at 12V is super-teeny compared to the above. As a worst-case, assume a 47K plate resistor and the 12A_7 lays there like a dead short. (It won't amplify this way, just a worst-case for power estimates.) 12V/47K = 0.24mA = 0.000,24 Amps.

Half-Amp is ample.

[shooter]

Is that the chip?

sorry, ya,