Vox VT15 Hiss

[earache]

Hey folks

Have a Vox VT15 on my bench that’s gottta lotta hiss. All other functions work fine. Hiss really increases when the power level knob on the back of the amp gets turned up. Wondering if any of you kind people have had similar issues with this amp and have successfully solved them or just tossed out the amp.

Thanks in advance!

[Chuck H]

Do we know if this condition is abnormal? Hiss is usually a consequence of gain that is unavoidable in a fixed design. Johnson noise due to circuit resistance.

At less than a hundy to purchase another (used) and being a discontinued model I don't imagine it's worth too much effort unless it's important to you for reasons other than monetary. It's basically a solid state amp with a single preamp tube in it. I don't know what that tube is doing because I don't have a schematic. If someone replaced that tube then it could be a noisy one. Many new tubes are noisy right out of the box. Sometimes silicooties become hissy with age. I don't expect that amp is old enough to suspect that right off, but a blast from a can of freeze spray will reduce hiss in a noisy transistor. If you ID one (or more) you can simply replace them.

[Jazz P Bass]

I cannot seem to locate a schematic for the VT15 but if it is anything like the VT 30 you can narrow down the issue by using the headphone out.

That is before the power amplifier section.

[earache]

I should have stated that the hiss is much more than normal or acceptable.

I’ll try the freeze spray route. Nothing inside looks burnt or is loose.

[Chuck H]

Like jazz said, it's likely in the preamp and not the power transistors. Looking at pics it seems those are mostly on the inaccessible side of the board? I don't know if you can lift the board and keep the amp in operating condition to blast the preamp transistors or not. Some reading on line would indicate that these amps are notorious for hiss. I didn't look any harder for a known culprit, but that might be worth a shot.

[Jazz P Bass]

Just so you know, this is not a simple amplifier.

Input into a codec, AD conversion, CPU, DA conversion.
This is just the preamp.

That is why I suggested trying the headphone out.
It is before the power amp.

You need to narrow down the problem.

VT30 Block Diagram.pdf

http://dealers.korgusa.com/svcfiles/VT30_SManual.pdf

[DrGonz78]

I remember working on a Line 6 pod for a friend and it hissed like a bastard. Does the hiss start up right away when turning on the amp or does it take a few moments to start? Like that line 6 I traced the audio signal through the unit to see where the hiss originated. Start at the analog side of things first look at first gain stage if that is possible. What I found was the first Opamp was not hissing. Then it fed the signal into the DAC where all the effects are created, no hiss. Then I traced the analog outputs from DAC and found my left mono channel was hissing, so either the DAC or one of the many digital processing components were to blame. Strange enough my right mono output coming from DAC was fine and not hissing. I simply cut some traces and made the right channel feed into the left mono channel output. So this allowed the unit to be used in mono format and it is still working today.

Of course your amp is different than my example. I only offer this story to explain that the hiss could be something in the digital realm of the signal path. For me trying to find the source of the hiss in the digital part of circuit felt nearly impossible.

So employ a simple signal tracer to monitor the hiss. Identify the DAC chip and do a search for its data sheet. Locate the pins that input the analog signal into the DAC(AD), probe those pins with signal tracer. Any hiss there? Locate pins on the DAC(DA) where it outputs the analog signal, probe with signal tracer. Any hiss there? If signal path is hiss free before the DAC(AD) but hissing after it, then it's a digital problem. Start there to see...

Note: I was on my phone when I wrote look for the DAC chip data sheet. The schematic that Jazz provided has the pins clear labeled for input and outputs for AD and DA. Also, is that knob on the back like a master volume or is it related to the tube stage on the amp?