This is a small Audio Amplifier with an AC coupled Instrumentation Amplifier at the Input to allow for troubleshooting audio circuits by helping to find, where a signal disappears or becomes distorted. It works by allowing a technician to inject a test signal and then follow it through each stage of the circuit, listening for where the sound cuts out. This helps isolate faults in multi-stage audio systems like amplifiers, preamps, and crossovers.

The high dynamic range of the human ear (> 100 dB) and the fact, that you don't have to turn your head towards a scope makes this a handy tool, when troubleshooting Audio or Low Frequency Applications.

The design is completely straightforward. An instrumentation amplifier at the input (AD8421) allows for a different GND connection. In order to compensate for probe attenuation, a resistor R2 mayst be assembled. The gain resistor can be calculated as R2 = 9.9 kΩ/(Gain - 1). This amplifier shall survive (Absolute Maximum Ratings) input voltages of 60 Vpp (not tested :-)

As the REF pin is held at GND, the output is symmetrical with respect to gnd. It is ac coupled to a voltage divider consisting of a fixed resistor (tbd) and a potentiometer, which's tap is connected to the input of a LM741. A power amplifier LME49600 (alternative BUF634F) drives a loudspeaker. Together with the LM741 it offers 20 dB of gain.

The circuit is powered by a RAC20-12DK, Bipolar Supply, ±12 V, by Recom.

The Photomodel is from Tektronix, but any other will work as well ...

The Device is best operated with any standard Scope 1:1 or 1:10 probe, as the instrumentation Amplifier at the input presents an input impedance of 1 MΩ. Absolutely no need to reinvent the wheel. As we are dealing with a spectrum up to 20 kHz, the compensation of the probe may be neglected. If you constantly use a 1:10 probe, a resistor of 1.1 kΩ my be installed at position R2. This will set the Gain of the AD8421 to Vu = 10, or 20 dB.

Of course, any tube video can be used as a signal source. Professionals use sinewaves with defined frequencies, such as 440 Hz or 1000 Hz. Those are usually generated by a Synthesiser like the FG2206, AD9833 Micro Waveform Generator or similiar.

The python script below let's you create wav-files with a sinewave of the frequency and amplitude of your choice. HAVE FUN !

ITEM	SUPPLIER
Loudspeaker	Pui Audio, AS05008MS-R from Farnell
Case	2 x KOH 04 A 160 ME, from fischer elektronik
Maybe useful	this website

The webmaster does not read these comments regularely. Urgent questions should be send via email. Ads or links to completely uncorrelated things will be removed.