solid state amplifier hiss

[nosaj]

Is there a general course of action for determing what is causing hiss in a solid state amplifier(like the geofex tube amp troubleshooting tips.)
Thanks,
nosaj.
In this case the amplifier is a fender Accoustisonic SFX II

[Jazz P Bass]

The only thing that I would recommend is narrowing the issue down to the preamp or the power amp.

[Chuck H]

I don't have much experience with it, but...

Most of what I see here for cause, as well as my own couple of experiences have been the result of failing transistors. On instruction from the forum I used freeze spray to locate two problematic transistors and replacing them stopped the hiss. In my case the units were no longer made so I had to locate NOS for one and an "equivalent" with a different pinout for the other.

[michaelfpattison@gmail.com]

I’d like to join in here as I have a hiss problem on my B200. I have a B200 and B200H, exact same power, preamp and direct out components, speaker output varies a little with the horn bypass (the 200 has one, the 200H doesn’t). I have an audible hiss on both speaker and direct out which increases as I turn up the volume, somewhat audible when I increase the different frequencies, but more so on the lower ones. Nothing plugged in the send/return, only guitar and cable into the passive jack. Not a 60Hz hum, but a hiss like you’d hear moving between radio stations. Narrowed it to the preamp board as one has no hiss in either chassis, the other has hiss in both chassis’. I’ve pulled all the small electrolytic caps, all test at least close to listed values, but replaced a couple that were pushing the range, no change. Pulled the volume and gain pots, show good sweep from 50K and 250K respectively, deoxed everything, replaced the volume pot with a new 50K, hiss persists. Board looks pristine, no scorching, solder joints intact. What next? There are four ICs, three are 8-pin, one is a 20-pin. Would one of those be the culprit? I don’t have or know how to use an oscilloscope, equipment limited to Variac, VOM and a capacitor/transistor checker. Thanks

[Helmholtz]

Already excellent advice given here. I also like to use freeze spray.

Apart from that, hiss is a signal and can be traced and scoped.
The effect of controls' settings on the hiss can help to isolate the source.
As always, supply voltages and opamp output DC voltages should be checked.

[Mick Bailey]

I sometimes use a lead with a probe at one end, a clip at the other and a 0.022uf cap spliced in. Useful in grounding the AC at various points to narrow down the fault. Also with SS circuits an audio probe can be useful.

[g1]

If it's not a grossly obvious amount of hiss, sometimes you can save a lot of time if you can compare to another unit of exact same model. Some amps are just noisy and can be big time-wasters (chasing a non-fault).

[x-pro]

In any guitar amplifier, if there are a large number of control elements in which the main source of noise is poor contact between the slider and the resistive layer even in static mode, as well as, connectors, mechanical switches, electrolytic capacitors in AC circuits, this also includes feedback circuits, circuits that create an artificial common point. Further, the use of general purpose ICs, natural or operational aging of components, especially active components, result in the manifestation of noise.

The above amplifiers have a lot of noise from the nature of their creation.
Mediocre components, wiring and assembly. The low price point of the model.
As correctly noted, you should start by checking the wiring, the quality of the power supply, its decoupling by nodes, and the power supply to the stages. Often tracing is done for the convenience of the location of controls.
The most noisy node is determined - preamplifier, tone, effects amplifier, output.
Nodes are disconnected from each other, both signal and power. And each of them is checked with quality measuring equipment.
Of course, you can do it by ear. In this case, the check starts with the power amplifier, as it will be further required for auditory control.

It makes no sense to fight noise in a guitar amp. This is not Hi-end technology, where some people are not music lover (listening to music), but audiophiles (listening to the faults of their amplifier).
Does the amplifier provide the main task? And when there is a signal, noise is masked by a strong signal.

[michaelfpattison@gmail.com]

X-pro and g1 thanks for the response. As mentioned, I have two of these, both the same, one preamp board has excessive noise in both chassis, not just background that can be played over, louder when volume is increased, there even when no guitar is plugged in. The other preamp board has zero hiss in both chassis. I suspect a surge of some sort damaged something, had hoped I was one of the caps, but apparently not. The ICs are inexpensive. I’ll swap out those one at a time and see if I get lucky.

[Chuck H]

Well, just to state the obvious...

Hiss can be a byproduct of gain in normal function if the amp isn't carefully designed to avoid a high noise floor. This has already been hinted at previously. So some hiss may be normal. It's usually a result of too much series resistance used for buffering in the signal chain where a more careful design would avoid that. The Soldano and Peavey amps come to mind as tube products go. Which is more my wheel house so I've seen the schematics and recognized the issue. But certainly this could be the case for SS amps as well.

Sometimes modification is an easy matter when there's no consequence to matching impedances with lower series resistance values. Other times it's clear that a high series resistance was used as the easiest solution if a designer didn't want to change anything else about a circuit. Even vintage guitar amps qualify as high gain amplifiers and I'm often surprised by the use of high series resistances in sensitive circuits since "I" take great pains to avoid that in my own designs.

I haven't looked at the schematic yet and it's getting late (for me ) so maybe I will tomorrow. If I have any clever suggestions I'll post them.

[The Dude]

I'm with Jazz. Try to narrow down the section of the amp that is creating noise, first. The amp has a tuner out after the front end and an insert jack a few gain stages down hill from there. If you use those points plugged into another amp, you may be able to narrow it down to a section of the amp. Also, you may have a shorted op amp and checking voltage on output pins may tell you which one.

[Tom Phillips]

...Some people are not music lover (listening to music), but audiophiles (listening to the faults of their amplifier)...

What a great insightful statement!

[Mick Bailey]

That made me smile.

[Chuck H]

Well it's a human condition to be grumpy about something. Otherwise it's not okay to be happy about anything!?! Why not music?This is tongue in cheeck of course. Landing on an MO that makes it impossible to listen to and enjoy music is a huge mistake and a demonstrates a character flaw. At home I use really nice console speakers but at work I commonly use my phone.There's a pretty wide fidelity rift between them. But it's more about having music in my life than the actual reproduction quality.

... This posted on a forum that focuses on the sound quality of guitar amplifiers !?! Yes, we can hear the difference. But there's so much to enjoy about music that it seems a shame to moil over reproduction quality TOO much. Buy a good system, yes. But then try to enjoy what you intend to listen too. Otherwise the point is lost.

[nosaj]

On this Fender accoustisonic reflowing the input jacks and pots quieted it down. But now i have freeze spray on hand which I didn't before.
nosaj