Equalizer Project

Found a bare pcboard for a 6 band EQ at this URL.

http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=46&Itemid=26

Circuit uses thru-hole TL074 opamps, not SMD/SMT parts. I can set the band and center frequency controlled by each pot. Anything risky about this pcboard circuit?

Found this 8 band Stompbox EQ which looks promising at $30 plus
shipping.

http://www.musiciansfriend.com/product/Danelectro-DJ14-Fish-and-Chips-7Band-EQ-Pedal?sku=151873&src=3SOSWXXA

There is an old piece of software I used to illustrate the spectrum of
an unwanted background tone I heard in my hearing aids. Runs ok on WIN XP.

Spectrogram v5.17 (294kB)is the last SHAREWARE release of this program.

http://www.n0ss.net/index_k2.html

At same URL see tutorial "K2 crystal Filter Alignment using Spectrogram v5.1.7" K2 is a HAM radio kit.

I will be using these EQs in two ways to steepen band skirts.
1. In series with my multiband Radio Shack EQ

OR
2. Two of these EQs pcboards in series.

Dave_s

Standby Amplification

The passive loop described can be used for listening to audio from any source. However some users might wish to be able to listen to very low power "Line Level" audio signals that are very low current sources and are not meant to drive earphones (or loops).

To do this it might be useful to add a second jack to the loop's housing and add a stand-by amplifier for such uses. Normal cell phone use probably wouldn't ever make use of this jack and the battery would never be drained. Accordingly lithium batteries that can sit idle for 5 years without self-discharge would probably be chosen.

Battery power needed for low level "Line-Out" use would be about 1.5ma at 1.2 volts. An EverReady 1.5V lithium “Photo” AA primary cell (2900 mah) would be suitable and would probably last about 2.5 years even with daily use. There are some new 1V amplifier IC's with very low quiescent current that should do the job

A switch has been provided in the prototype below to mute the mic.

Prototype Photos

[Click on images for expanded view.]

This is a 40 turn loop that presents a strong signal without amplification. The distortion that accompanies amplification as an amplifier strains to supply the substantial current needed by a single-turn loop is eliminated by multi-turn designs of this type. It is very light and the loop itself could be described as a light "gooseneck" loop with memory that you can easily form to the shape you prefer.

The smooth nylon cover is a spiral with rounded edges so air can flow between it and you. I find it quite comfortable and configurable but others may prefer a full plastic jacket.

This unit is fully passive (no batteries) and is quite sensitive for low level signals. It is shown operating with a small cordless phone that happens to have much lower audio output level than our cell phones. My wife's cell phone's volume has to be set at mid volume when this loop is used with my 211 dSZ Savias.

The connecting cable is a 3.5mm stereo type plug on one end and 2.5mm stereo on the other. It is wired to properly connect the loop to the earphone signal and the loop's electret microphone to the cordless or cell phone's conventional input contacts contained in its 2.5mm jack.

The loop's housing supports a good cardiod unidirectional microphone made by Labtec. The mic's directional pattern results in a surprisingly good "close-talking" sound to the outgoing voice.

Neckloop project

The objective is to make a neckloop that works much better than what's available, has plenty of volume, doesn't need you to ever change its battery, and can be plugged into regular audio stuff that normally drives earbuds or can serve as a good sounding plug-in headset for standard 2.5 mm jacked cell phones.

In many cases the strength of the magnetic field at the hearing instrument’s telecoil turns out to be about 20% to 25% of what it would be If the telecoil were perfectly aligned in the center of the neckloop. So that means that you want to generate about 5x more field strength at the loop's center than you need at the telecoil.If you want 100 ma/meter at the telecoil you need 500 ma/meter in the loop.

The field strength at the neckloop's center equals the current times the number of turns divided by the loop diameter. If you want 500 ma/meter and the loop diameter is .25 meters then:

I*Turns = 500*.25 = 125 ma-turns.

If you try 8 turns then:
I = 125/8 = 15.6 ma

For example at 8 turns the loop has negligible impedance. Just driving the loop at this level with a 16 ohm source would require an open circuit rms voltage (audio) source of .253 vrms. If you put a 16 ohm resistor in series with the coil to match the source (strongly recommended), you would need a source with .5 vrms open circuit. In other words your radio would have to be able to drive a 16 ohm load at about a 5 mw level. Many sources may operate at levels far below this.

More turns give you greater output. A 40 turn loop gives you 20*log(40) or 32 db more gain than a one turn loop and 20*log(5) or 14 db more than an 8 turn loop. So for a 40 turn loop you would need a source with about .1 vrms open circuit voltage to produce an effective 100 ma/meter field at the telecoil.

Using a resistor to make the load impedance 16 ohms, the required output from the device would be (.1^2)/16 or .7 mw. This should be available from most devices that drive earphones.

Design approach and prototype details will follow.