Adding a speaker to monitor dummy load

[CAS]

I am planning to build a variable impedance (4,8,16R) dummy load with voltage and scope ports for testing amplifiers.

I was wondering if anyone has any tips for safely integrating a small speaker for monitoring the audio. I'd. Also like to be able to add a pot to adjust the volume of this speaker and add a switch to take it out of the circuit completely.

My questions are:
- Does this monitoring speaker need to be a specific wattage (above the max output of the amps being tested?
- what would the circuit look like (with a switch and pot added to disconnect and attenuate the speaker?)

Thanks

[martin manning]

Something like this? You could replace the line out jack with a small speaker, or the line out jack could be a switching type if you want both. https://ampgarage.com/forum/viewtopic.p ... 43#p131643

[CAS]

Thanks Martin.

Looking at your schematic, would a 1/4watt resistor suffice for R2 and what wattage would you recommend for RV1? Also, can you explain the monitor select switch, please? Is that determining what impedance is expected to be connected to the line out (or speaker in my case). Or is it supposed to be matching the impedence that is chosen when plugging into one of the inputs.

I have a 8ohm .5watt 2" speaker that I was thinkin of using. Would this get fried?

[martin manning]

1/4W is fine for the 100k, and a standard 0.25W pot will also work. A 0.5W speaker should be ok, as you can use the level control.

The monitor switch is used to disconnect the monitor and to connect to the right place on the load resistor when the different impedance inputs are used.

[CAS]

A side question. I noticed in your Rev2 that J1 and J2 are connected to the same position on R1. Wouldn't J1 need to be connected to another point on the resistor, other than the 8 to achieve 4ohm?

Also, what impact does R2 and RV1 have on the impedance presented to the load inputs? Sorry for all the questions.

[martin manning]

J1 is a TRS jack, in which the ring contact is used to ground the other end of the load resistor, putting the center contact at 4 ohms to ground. J2 does not have that function, so the center contact is at 8 ohms to ground.

When you switch the monitor function in, the 100k plus the level pot, and whatever its output is connected to, and the scope’s 1M input impedance are all in parallel with the load. At worst, that would reduce the 8 ohm load to 1/(1/8+1/100000+1/1000000)=7.999, which is insignificant.

[CAS]

Thank you Martin. Is the purpose of R2 to protect the scope in case of shorting?

[martin manning]

No, it's just a divider to get the speaker voltage down to roughly line level. For example 50W into 8 ohms is 20V rms, and the 100k/10k brings that down to a maximum of 2V rms.

[CAS]

Would my 3" 8ohm .5 watt speaker prefer line level? Should I use a different value for R2, or will that suffice? Thank you

[nuke]

Just add a small radio speaker with its own amplifier and volume knob. The high input impedance of a simple active amplifier would not affect your measurements nor loading, and would be the easiest way to handle a variety of signal levels.

Would be trivial to build from scratch.

[martin manning]

I would just replace the line out jack with a 12A switching jack and connect the internal speaker to the switch contact. That way the level control adjusts the speaker volume unless a cable is plugged into that jack, which would disconnect the internal speaker.

[CAS]

Thank you both.

[Helmholtz]

Would my 3" 8ohm .5 watt speaker prefer line level? Should I use a different value for R2, or will that suffice? Thank you

No, line level will be too low.

When I need to hear the amp during a repair, I just wire a 270R/10W resistor in series with the speaker and connect across the dummy load.
This uses the speaker impedance as shunt part of the voltage divider and limits the power to the speaker to below 0.5W.
You could use your 0.5W speaker, but it probably has poor bass response and might not reproduce 60Hz hum.
What is the max. amp power?

[CAS]

Thanks for that suggestion Helmholtz.

I will be working on my AC30 (35Watts max power, I believe). I might eventually begin working on amps up to 100watts.

Would you be able to provide a rough diagram of what you are suggesting, please?

[Helmholtz]

1) Connect your dummy load to the amp just as usual.
2) Wire the 270R resistor in series with your speaker by connecting one end of the resistor to one of the speaker lugs.
3) Connect the other end of the resistor to the hot side of the dummy load and connect the other/unused speaker lug to the grounded side of the dummy load.