• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Adjustable Feedback on a Switch

Has anyone got a schematic of the Icon Audio Stereo 40 (mk I, II, III, or IV)?

This amp is fairly conventional, except that it has a three position switch which can be used to select "H", "L", or "0" feedback. It makes for a very interesting listening experience to switch between the three.

I would like to see how they implemented this feature.

SO, schematic anyone????
Use a 3 position 2pole switch for ease of stereo control. They make nice rotary switches like that. One end of three resistors tied together for the feedback input and the other end of the resistors to the switch. Output of the switch to the feedback path and you’re done! Don’t switch it while the amp is on unless you want a massive damaging spike!
Thanks for your replies. Yes, I was pretty sure that was how Icon implemented it. You can actually see the wiring to this switch on the attached photo.

I would still like a schematic, however. Anyone go one?


  • Icon Audio b.png
    Icon Audio b.png
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As was stated in Post # 5 above, the gain versus the "H", "L", and "0" settings will be different; must make the gain from the CD player through the amplifier channels, to the speakers be equal.
Here is how.

You have a pair of identical speakers.
Get a Potentiometer volume control to be able to set and reduce the signal level into channel 1.
Set channel 1 negative feedback to "L". Re-wire channel 2 switch for "H" negative feedback.
Put the Left channel signal from a CD player into Both channel 2 input, and to the external potentiometer that feeds channel 1.
Set the speakers facing 1 inch away from each other.
Connect channel 1 speaker cord in reverse, cord + to speaker -, cord - to speaker +.
Turn the channel 1 potentiometer until the sound level reduces to a minimum (now, the channel gains are identical, but the out of phase speaker connection provides a sound level null when the gains are equal.

Re-connect channel cord to the speaker back to normal. Leave the speakers close together, but face both of them forward.
Then get a switch that allows output from the CD player left channel to alternately connect to channel 1 external potentiometer, or channel 2 input.
When you connect the CD player Left channel output to only channel 1 external potentiometer, and then connect the CD left channel output to only channel 2 input, you can listen for any differences in sound.

Then, start the whole process over to compare "L" to "0" negative feedback; then one more setup to compare "H" to "0" negative feedback.

If the above is too much trouble, you will not be able to do any A - B testing, without relying on your 'memory' of the sound differences,
or without purchasing a second identical amplifier, and purchasing 2 more speakers.

Happy Listening.
... Don’t switch it while the amp is on unless you want a massive damaging spike!...
Unless one of the FB settings is unstable causing oscillation there is no such spike.
(The really disastrous case would be a triode-ul-pentode switch.)
I fitted a simple on-off-on switch to different amplifiers and it can be thrown either way live and on the fly without even the slightest thump.
The only thing which can be heard is the change in loudness which makes it problematic to assess which setting "sounds better".
For the "interesting listening experience" the FB switch can be ganged with a properly calibrated attenuator.
Mmmmm. I beg to differ. I had a switch on a hacker amp I built for experimenting with different tubes and accidently switching the feedback while the amp was on wasn't pretty. I got a pretty massive thump and saturated the output transformer to the point that I had to wait several hours to ake measurements again.
Different amplifier circuits and different amplifier topologies respond differently to changing the amount of negative feedback on the fly.
Better to be safe than to be sorry.

That is why having 2 stereo amplifiers and 4 speakers is the best way to test on the fly.
Just switch the CD or other signal source between one stereo amplifier and another stereo amplifier.
Plus, if the amplifiers each have volume controls, you need to reverse the phase of the 2 speakers on one amplifier (with the power off, than power-up), and run all 4 channels and all 4 speakers at once. Then adjust the volume of the one that has less negative feedback (higher gain), so the sound nulls out (now the gains are equal).
Then turn the amplifier off that has the speakers out of phase, and reconnect the speaker connections in-phase.
Turn the amplifier on again, and now you are ready to switch the signal source between one amplifier, and the other amplifier.

Sorry, but this really fair test is only available to those who have 2 stereo amplifiers.
Otherwise, you have to use a single stereo amplifier, and use a single signal that is paralleled to both channels.
You have to reverse one speaker connection, and then adjust the individual channel gains for a null. Now, re-connect the reversed speaker connection, and switch the signal source to one channel at a time.

(if you are not able to hear any differences between the 2 gain adjusted channels with just the Left or just the Right channel signal source,
Then it is doubtful that you will hear differences between the 2 stereo amplifier 4 speaker setup.
Differences are easier to detect when there is less complex signal to listen too (like stereo versus Left only or Right only).

I have done the 2 channel 2 speaker listening myself, so I know what I have experienced.

1. Do not switch negative feedback with the amplifier on.
2. Do not reverse the speaker connection with the amplifier on.
2. Do not un-reverse the speaker connection with the amplifier on.

Have fun!