AUDIO WATTMETER

Here’s an easy way to measure an amplifier’s output power without trying to convert voltage to power measurements. Resistor R1 provides the load for your amplifier and should be rated at least twice the maximum amplifier power output; for example, if your amp puts out 25 watts, R1 should be rated at least 50 watts. The meter scale must be hand calibrated, and will take some time and effort, but once done it’s done for good. Remove the scale cover from meter M1 and borrow an AC variable auto-transformer, or connect a 1000 Hz signal generator to the amplifier output. Connect the output of the auto-transformer (or amplifier) to binding posts BP1 and BP2, and connect an AC voltmeter (VOM) across the binding posts. Set R2 to off – full counter-clockwise if correctly wired. Adjust the auto-transformer (or amplifier) output until the AC meter indicates 20 V rms – the voltage for 50 watts across 8 ohms. Adjust potentiometer R2 for a full scale indication on meter M1. Seal R2’s shaft with a drop of Glyptol or nail polish. Reduce the voltage across the binding posts in accordance with the table shown and mark the meter scale accordingly.

Parts List For An Audio Wattmeter

BP1, BP2 – Insulated binding posts

C1, C2 – 100 uF, 50 VDC

D1, D2, D3, D4 – Diode, HEP-134

M1 – 0.1 mA DC Meter

R1 – 8 ohm, 100 watt resistor, see text

R2 – 1500 ohm linear taper potentiometer

LOAD MATCHER

Most audio circuits transfer their maximum power at minimum distortion only when the output impedance is matched to the load impedance. But it is often necessary to connect equipment of differing impedances. For example, how do you correct an amplifier with a 600 ohm output into an amplifier with a 50 ohm input? Usually, if the 50 ohm input is connected across the amplifier with a 600 ohm output, the excessive loading caused by 50 ohms will sharply reduce the output of the 600 ohm amplifier, and will generally increase the distortion sharply.

A minimum loss pad is the device used to match a high impedance to a low impedance. Though there is always a signal level loss through a pad, the circuit shown provides the absolute minimum loss that can be obtained while providing a precise match. If the resistance values work out to odd values, such as 134 ohms, use the closest standard value. Though 5 percent tolerance resistors are suggested, almost as good performance will be obtained with 10 percent resistors.

REMOTE SPEAKER SETUP

Even if your hi-fi amplifier does not have output terminals for remote speakers, it is easy enough to add them without complex switching equipment. With few exceptions, modern solid-state amplifiers have no output transformers and automatically match any speaker impedance between 4 and 16 ohms. The only important consideration is that the total impedance connected to the left and/or right speaker output is never less than 4 ohms, or the amplifier will attempt to deliver so much power output, the output transistors will self-destruct. If your main speakers have an impedance of 8 or 16 ohms, simply add remote speakers as shown :

Switch S1 turns the remote speaker on and off. Since transistor amplifiers usually put out more power at 4 ohms than at 8 or 16 ohms, adding the extra speakers does not substantially reduce the volume at the main speakers because the amplifier sees a lower impedance load and attempts to drive more power output into the combined speaker load. If your speakers are 4 ohms, and you plan to use 4 ohm remote speakers use the circuit modification shown. Switching in the remote speaker will result in the main and remote speakers being series connected for a total load of 8 ohms.

STEREO SHUTOFF

It happens to just about everyone. One minute you’re listening to the hi-fi, the next you’re called away to answer the doorbell or a phone call. You forget all about the music, the record plays through, the automatic turntable shuts off — but the amplifier stays on until you happen to pass by and notice the glow from the pilot lamps. Yet, this simple circuit, which you can throw together in less than an hour, will automatically turn off the amplifier when the turntable shuts off. The relay coil voltage is taken from across the phonomotor; when the turntable motor is on, relay K1 closes and applies power to AC socket SO1; When the turntable shuts off, removing voltage from the motor, K1 opens, disconnecting power from outlet. Because the turntable automatic shutoff switch might not be able to carry the amplifier load, the AC power for SO1 is taken off before the automatic shutoff switch. Switch S1 bypasses the relay contacts and applies power to the socket even when the turntable is off.

Parts List For Stereo Shutoff

K1 – 117V AC relay with contacts rated at least 5 amperes at 117V AC (Radio Shack 275-207)

S1 – Switch, SPST (Shutoff bypass)

SO1 – AC Socket

CB SNIFFER PROBE

It’s often difficult if not impossible to detect RF in mini-power RF circuits such as used in walkie-talkies; generally, service grade test equipment just isn’t sensitive enough. Next time you are working on a CB walkie-talkie and can’t tell if a lower power RF amplifier is working, just throw together a CB Sniffer Probe from remains of the old junk box. Better yet, why not be prepared in advance because all new components will cost less than $10. A small plastic rod about 6 inches long, cemented to L1, will allow you to se the sniffer as a probe. To align, place the sniffer near the antenna of a known good walkie-talkie, key the transmitter, and using an insulated alignment screwdriver adjust trimmer capacitor C1 for maximum brilliance of neon lamp l1.

Parts List For CB Sniffer Probe

C1 – 5 to 30 pF trimmer capacitor

l1 – NE-2 neon lamp

L1 - RF choke, ohmite Z-144 or equiv.

MAKE YOUR OWN EEG DEVICE-BIOMEDICAL PROJECTS

Overview

An EEG signal is usually acquired through silver-chloride covered electrodes, though sometimes other materials like pure silver, tin, steel or gold are used. The signal amplitude is only a few microvolt and needs to be amplified several thousand times before it can be captured. Because it is faint, the signal can very easily drown in noise, particularily 50/60Hz hum from the mains which is transmitted capacitively (i.e by an electric field) from the wiring in your house.

To handle this, the signal is first amplified by a high quality instrumentation amplifier, which measures the voltage difference between two locations on the scalp. In the example in the previous section, we used C3 and P3. This ensures that a large percentage of the mains hum never enters the system, because the level of the mains hum on those two locations is essentially the same.

Afterwards the signal strength is increased further by normal amplifiers, and passed through a low-pass filter which minimizes distortion caused by so-called aliasing that may occur when the signal is converted to digital samples.

Below is the block diagram of one EEG amplifier channel, and the Right-leg driver (DRL-circuit).

Simplified block diagram of the ModularEEG amplifier

Some parts are not included here. The schematic gives you all the details if you are interested.

The EEG signal is picked up by the two topmost electrodes and passed through the protection circut. It serves two purposes: First, it protects the circuitry from electrostatic discharge (ESD) and second it protects the user from failing circuitry. In theory at least.

Leaving the protection circuit, the signal enters the instrumentation amplifier where it is amplified 12 times. After that, the signal is amplified about 40 times in a second amplifier stage. You can't see it in the diagram, but there is a reason for splitting the amplification into two steps like this. Between the two stages there is a high-pass filter which removes DC-voltage offsets.

Some electrode materials, such as gold or steel, are polarizable. This means that electric charge can accumulate on the surface of the electrode, building up a relatively large DC-voltage, sometimes several hundred millivolts if you are unlucky. In theory, you would amplify a 200mV signal 480 and get a 96 volt output. In reality, the circuitry can handle about 2.5V so the output signal would be stuck at at a maximally high or low level, usually +/- 2.5V and not contain any EEG. The highpass filter tries to solve this problem.

Finally, the signal is amplified 16 times more and lowpass filtered. The filtering is done to prevent aliasing effects later on, when the signal is digitized.

Below the signal amplifiers, and the filter, sits a third amplifier pointing the other way, seemingly sending a signal to the user. This is the right-leg driver. It is named like this for historical reasons. The driver is, and was, previously only used by ECG meters, which measures the electrical activity in the heart. During ECG sessions, the driver (also abbreviated DRL, for Driven Right Leg) is attached to the right leg, as far away from the heart as possible.

The purpose of the DRL is to reduce common-mode signals such as 50/60Hz mains hum, by cancelling them out. It replaces a ground electrode which older EEG designs use, and can attenuate mains hum up to 100 times more than the instrumentation amplifier can do by itself.

After the filtering, the signal is ready for acquisition by the analog-to-digital converter which in our case is located inside a microcontroller. The microcontroller sends the digitized EEG to a PC via a standard serial cable. To protect the user from electrical faults, the EEG device is electrically isolated from the PC and external power sources. The block diagram below shows this.DOWNLOAD MORE INFORMATION ABOUT THE PROJECT FROM THE LINKS BELOW

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