Hifi amp as signal tracer

[diy didi]

Hi all.
I would like to know if it is advisable to use my hifi bench test amplifier as a signal tracer, or should i rather get a dedicated amplifier/ speaker for this. What type of circuitry will I need front- end to add this to my hifi amp? I want to be able to probe valve amps etc.
Any advice please.

 

[counter culture]

These work pretty good for signal tracing...

Ceramic Earphone for Crystal Radio Sets High Impedence Earpiece Condenser Type | eBay

 

[Ian Finch]

For many years, I just used a cheap AM pocket radio which gives an isolated instrument with low hum problems. Mind you, you need a HV coupling capacitor, suitable shielded lead and attenuation before connecting to low voltage radio circuits to safely probe tube amplifiers. A cheap, switchable X10 'scope probe is a good start toward this.

 

[amptech]

I would agree with Ian here.. it doesn't have to be very loud to hear music signals when tracing out circuits. for many years I used a small pair of pc powered speakers of 1W for field work etc, I then built a two channel test amplifier using a TDA2050 ic and parts from the junk box that worked a treat.

 

[diy didi]

Can I use a little practice amp such a Fender 15R amp modified for signal tracing? What type of input impedance will I need for signal tracing?

 

[Conrad Hoffman]

I don't think it matters much what you use, but the probe is certainly important. I'd want about 1 meg in series, followed by a coupling cap and then paralleled reversed diodes to ground, or maybe back to back zeners if you want more signal. The resistor needs to be large enough to limit the current to the diodes for the highest voltage you might accidentally encounter, say the B+ in a tube amp. I'd also put some high value resistors from the cap to ground to discharge it and prevent pops. That's a minimum- no doubt there are other niceties that could be added.