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6AV6A for meter driver
6AV6A for meter driver
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Old 1st July 2008, 11:32 PM   #1
mashaffer is offline mashaffer  United States
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Default 6AV6A for meter driver

I am looking at doing a stereo preamp/processor. The case I intend to use includes a (2 actually) panel meter which I would like to use with a 1/4" mic input for response measurements.

The idea is to playback test signals such as warble tones or individual 1/3 octave pink noise signals and measure relative SPLs using the microphone. Would the 6AV6 be a suitable driver for the meter? Thinking of using the triode to amplify the output of a condenser mic and using the diode(s) and a cap to smooth the output to drive the meter through a series resistance.

Is the basic idea sound? Is there a concern for the miller capacitance of the high mu triode loading the microphone?
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Old 2nd July 2008, 09:35 PM   #2
Miles Prower is offline Miles Prower  United States
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^^^^

I would just solid state that whole meter drive circuit. It's much easier to implement with op-amps. As for the 6AV6, it's doubtful it could be used for this purpose. The triode section has one of the lowest Pd's I've ever seen: Pd= 500mW. You won't get much current from that.
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Old 2nd July 2008, 09:52 PM   #3
Michael Koster is offline Michael Koster  United States
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Interesting idea. A single tube VU meter drive circuit.

Typical Zo of a condenser mic is from 30 to 250 ohms. They have buffered outputs able to drive 100 feet of cable at 15pF/foot. The Cmiller will be 200pF at most, so I wouldn't expect a problem. Many mic amps have 12ax7 input stages and load the input with the grid.

The condenser mic will need phantom power unless you plan to use one of the few battery-powered ones (which are often electret types) so you'll need a little input circuit that will provide 48 volt phantom power through a pair of 6k8 resistors. In reality, a lot of condenser mics work fine a much lower voltages.

The next question is the VU meter impedance and sensitivity. Standard VU meters have 3900 ohms internal impedance and are meant to monitor a 600 ohm line through a 3K6 external series resistor. The load on the 600 ohm line is therefore 7k5 ohms. The meter should indicate zero vu at +4dbu which 1.228 volts RMS on the line. At this there is about 200 uA RMS=>DC signal current needed. I guess you could drive this using a high mu triode and a resistor divider. If you don't want the meter at anode potential there will need to be a load network with a DC blocking cap etc. You will probably need 10-20VRMS at the anode to drive the meter full scale through the divider.

Next is mic sensitivity and gain. Condenser mics typically put out 10-15 mV/Pa (1 Pa = 94db-SPL). So if you want 94db SPL to drive the meter to zero VU, you will need a gain of about 1000. At a gain of 100, you would need 114db SPL to get 0VU on the meter. Many condenser mics will handle about 120-130db before clipping and a typical noise floor of 20-30db(A). (BTW, this illustrates the basic engineering tradeoff with mic amps. Low noise and huge headroom...)

So a single stage mic-to-meter amp would only be useful for very sensitive mics and/or very high intensity sound sources. For speaker testing, room response, etc. you probably want a 2 stage amplifier.
You could use either external mic pres or build in some cheap mic pre.

The other issue is how to use the 6AV6 rectifier. I'm having trouble coming up with a circuit that would use the rectifier. These were used in AM radios where the rectifier could be cleverly interposed on the ground side of the IF transformer, with the signal side going to the grid. Sometimes a CT IF secondary fed a fullwave version. Maybe use a mic input transformer in step-up mode with a CT secondary to the grid and the diodes on the ends going to the grounded cathode. I guess a schematic is needed. The mic stepup transformer might put the gain into a useful range also. The problem with this might be too much voltage drop across the diodes. You would need to keep the loading very low 2M+ and here Mr Miller might be a problem as well as grid leak current.

Another wacky idea is use a dual triode or a triode+pentode. The pentode might be used for gain in the first stage and the triode could be used as a second stage biased at cutoff in class B so it also rectifies.

Bear in mind that getting musical sound out of the thing with rectification anywhere near the signal path is a compromise. If I needed music and metering I would split the signal.

In this context opamps look pretty simple, and you can add tricks like peak read/peak hold easily.

Enough for now...

Michael

PS half wave rectification is not the best for VU meters; they may wiggle a little on the low notes. Smoothing affects the time constant. Most people these days aren't as picky about this a sthey used to be.
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Old 2nd July 2008, 10:12 PM   #4
mashaffer is offline mashaffer  United States
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Will need to take a look at the meters. They are in an ADC Sound Shaper II equalizer. The unit had a SS circuit for just this purpose. I have the schematic so we should be able to analyse it to see what is needed.

The mic that I will use is a self powered (AA battery) electret with 600 ohm Zout. I will have to look up the output specs but I think that I do have them.

In the original circuit there is a pot for adjusting the sensitivity of the meter. Op amp is of course a possibility but if we can use the tube it would have a certain cool factor. One advantage we have is that we don't need enough gain for listening necessarily. All we need is enough current to driver the meter to full scale on fairly loud SPLs.

mike
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Old 2nd July 2008, 10:19 PM   #5
Michael Koster is offline Michael Koster  United States
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Some of the electrets are pretty hot. Some are pretty weak. The measurement omnis tend to put out good signal. Sounds like you may be able to use the tube for cool factor. One can research old schematics etc. for ideas also.

Since we have invoked opamp in this thread already, here is how I'm driving VU meters.

Michael
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Old 13th July 2008, 07:40 PM   #6
mashaffer is offline mashaffer  United States
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I can't seem to find the SSII schematic at the moment. The meter is graduated in dB (+/- 12) and has a DC resistance of 1.22K. The mic that I intend to use initially is one of these

Specifications are

Zout: 600 ohm +/- 30% @1KHz
Sensitivity: -70dB (0dB=1V/ubar) @1KHz
Freq Resp: 30-15KHz no tolerance specified.

Another thought that I had was to use a 12AX7 and solid state diode for rectification rather than the 6AV6A. It should provide more than enough gain and high fidelity is not really required for the meter.
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