Guitar headphone amp with aux input questions.

[johnr66]

I'd have to assume it is clipping. Without the input pot, it is like running it wide open. Guitars can make a pretty strong voltage into high impedance loads. Using a 50k or 25k pot will calm it down and allow for level adjusting.

[ddietz]

John, Thanks for the response. I have a 1M pot in line with the guitar input and that made all the difference. It sounds fine now. I tried other values also but this sounded the best and there is plenty of headroom for the guitar. I suspect 500K is correct but I did not have that. I'm using a 10uf el and 10K alps pot in series for an adjustable gain. On the Aux input, I'm using a different 10K alps pot and with most source a gain of 20 is enough.

I only have one issue now. I've built your layout and I believe I've double checked everything for errors. The issue is, when the pot is off there is a huge hiss that disappears when the pot is on, even just barely on. I did this to keep the CD channel off when not in use to conserve battery. I guess that won't work though. Does that cause some sort of loop with the guitar 386 to the cd 386 when the cd chip is powered off. Its no biggie if the other is on when the unit is on, I just thought I was being smart

The noise/static issue was actually caused by the cheap a$* RadioShack 1/4" mono jack I used. Switched to a different jack, virtually no hiss.

Thanks all!

[digits]

Originally Posted by johnr66

I'd have to assume it is clipping. Without the input pot, it is like running it wide open. Guitars can make a pretty strong voltage into high impedance loads. Using a 50k or 25k pot will calm it down and allow for level adjusting.

As a kid I blew a set of AR's I think, torn clean out of the spider, plugging an electric guitar in through my dad's amp.

[johnr66]

Originally Posted by ddietz

John, Thanks for the response. I have a 1M pot in line with the guitar input and that made all the difference. It sounds fine now. I tried other values also but this sounded the best and there is plenty of headroom for the guitar. I suspect 500K is correct but I did not have that. I'm using a 10uf el and 10K alps pot in series for an adjustable gain. On the Aux input, I'm using a different 10K alps pot and with most source a gain of 20 is enough.

I only have one issue now. I've built your layout and I believe I've double checked everything for errors. The issue is, when the pot is off there is a huge hiss that disappears when the pot is on, even just barely on. I did this to keep the CD channel off when not in use to conserve battery. I guess that won't work though. Does that cause some sort of loop with the guitar 386 to the cd 386 when the cd chip is powered off. Its no biggie if the other is on when the unit is on, I just thought I was being smart

The noise/static issue was actually caused by the cheap a$* RadioShack 1/4" mono jack I used. Switched to a different jack, virtually no hiss.

Thanks all!

The chips will always draw some bias current even with the pots turned all the way down. If you get a strong hiss with the pot down and not when turned up a little, something has to be wired wrong as something causes instability.

[ddietz]

The hiss was only there when the power was removed from the aux chip. I had a switch in the Vcc line to pin 6 of that chip. It works perfectly when power is applied, regardless of the input volume level.

Thanks again.

[mrbell321]

Hi guys,
I'm new the the forum and I'm bringing back this old thread because I'm just getting into this. I've read through all of the explanations, but there is still something I don't quite get:
Given that a CD/MP3-input is likely already designed to drive headphones, why do we need the amplifier circuits on that side?
wakibaki, you said "Everybody wants to mix after the 386, but nobody would ever think of doing it that way if the output were 5 Watts." and you are right in the context. However, we are not pushing 5W. We are driving tiny headphones with a supply that was designed to drive tiny headphones.
Were we driving something larger and wanted to mix in 5W from the guitar w/ a 5W output from a CD-player, what would your solution be?
I'm hoping you can straighten me out on this.

Thanks,
Tyler