wondering about hissing

[tayloralf]

Hi guys!

So having stumbled across an old thread about hissing I started wondering about my own AER Compact 60, which I absolutely love. When I turn the unit on, it's really quiet. If I press my ear up against it I can hear a very faint "sssssss". If I turn both the gain and volume knob to pass the midpoint (12 O'clock), then I can hear the "sssssss" a step away if the room is dead quiet.

Does that sound normal? Sorry if this sound so silly, but all this talk about hissing amps have gotten me a little worried.

Thanks everyone in advance!!

[donh]

Every electronic unit has a noise floor.

Your key to understanding this is to compare the noise to the signal.

Cranking the noise with no signal to compare is a fool's errand.

If the noise is nothing compared to the actual signal the unit is created to support then you have no problem.

[Bob Womack]

Yep. It's called "self-noise." It is the sound of electrons randomly moving in the input stage of the amplifier. The better the system the less you hear it. However, if I put my ear up to the speaker of the $9,000 recording studio monitor system where I work, I still hear it. It is faint there, but it is there. Every guitar amplifier I've ever worked with had audible self-noise when you cranked it past 12 o'clock.

Bob

[Bobby1note]

Originally Posted by tayloralf

Hi guys!

So having stumbled across an old thread about hissing I started wondering about my own AER Compact 60, which I absolutely love. When I turn the unit on, it's really quiet. If I press my ear up against it I can hear a very faint "sssssss". If I turn both the gain and volume knob to pass the midpoint (12 O'clock), then I can hear the "sssssss" a step away if the room is dead quiet.

Does that sound normal? Sorry if this sound so silly, but all this talk about hissing amps have gotten me a little worried.

Thanks everyone in advance!!

Yes, as others have said, it's entirely normal,,, to varying degrees. I'm sure you've come across the term "signal-to-noise ratio". The AER Compact 60 is a particularly "clean" amp. Quite remarkable IMO.

I've often noodled around with mine, then turn around and do something on the computer, and forget the amp is still on. Even though it's only a few feet away, it's just dead quiet. The only way I know it's "on", is when I shut off the lights in the computer-room, only to see the illuminated green LED on the amp.

[JanVigne]

As stated, every gain stage exhibits random thermal noise. When I was selling audio I would often get a call from a client who had, for the first time, placed their ear within an inch of the tweeter in their speakers and detected noise from the amp. Their question was always, is this normal? The simple question I asked in response was, is that a normal listening position for you?

There are things we should be concerned about and there are things we should not be concerned about.

Different gain stage devices have differing degrees of noise. First, the more gain you ask from a single device, the more noise you are likely to have as a result. Pre amps tend to have less noise than do power amps due to the amount of gain required from each component. So 12AX7's in the pre amp and 6L6's in the power amp. When you separate the two components, pre amp and power amp, the pre amp's noise will typically be lowered since it is not in the radiated field of EMI found in the larger power amp's power supply. However, pre amps deal with signals which are much more delicate than do power amps so noise is still a constant issue with a pre amp. Remove the power supply from the chassis of the pre amp and you will lower the noise of that component. You can do so by either using an outboard ps or by moving to a battery power supply. Each alternative brings with it different trade offs in the final product.

Tubes are generally noisier than transistors and integrated circuits are typically less noisey than transistors. However, much of the noise in a circuit is the result of other components within the amplifier as a whole. Transformers hum and they induce some noise into the circuits of the pre amp and power amplifier. The incoming AC Voltage is today filled with quite a bit of RFI and EMI which needs to be filtered out of the line before it can enter the amplifier. A high quality power conditioner will go a long way toward reducing these noises before they can contaminate your amplifier's circuits as will a dedicated real earth ground to those locations where you use an amplifier. Since modern communities are tied together on an electrical grid it's not uncommon for your neighbor's (and even your neighbor's neighbor's neighbor's and so on) HVAC system to be injecting noise into the AC line running to your house. Therefore, it's not at all uncommon for someone to actually detect less noise from their components at, say, 2AM than at 2PM.

The process of rectifying AC to the DC Voltage which courses through the amp can leave AC components such as AC ripple on the power supply rails. A good deal of this noise can be filtered out of the amplifier but it requires more expensive devices and greater care in circuit design. Heavily potting critical circuits will reduce much of the induced noise and high quality shielding can minimize RF and EM pick up. All of this costs money and isn't critical in many applications such as a guitar amp. Again, your priorities will dictate just how much you think you should address the issue of noise.

The efficiency of the gain device will play a large part in how quiet the circuit will be. As I said, tubes will typically display more quiescent noise than transistors and transistors will be a bit noisier than ic's. In the output device itself much of the noise is created by electrons which are on the loose and looking for a place to land. Minimize the path the electron must travel and you minimize noise. Thermal noise, essentially the majority of what you are hearing with your ear against the tweeter, is signal energy lost to heat production. Minimize the heat and you'll minimize the noise plus, hopefully, have a cleaner signal which is more responsive to the incoming voltage changes. For this function of reducing thermal noise, low voltage ic's are a close to ideal component. Trade offs exist with the use of ic's in gain stages but here, for noise alone, they are superior in most ways.

In high end audio we talk about "blacker blacks"; stealling a term from video production but referring to the perceived silence between notes and between the placement of performers on a virtual soundstage. Rather than a soundstage which is filled with random noise components, "blacker blacks" refers to a distinct absence of noise from the components. To date, the best output devices for this sort of black background noise level are the T-amps I mentioned in an earlier thread. These devices exhibit an efficiency of 80-90% which would be typical of a class D or class T circuit using a switching power supply. A switching power supply pushes the noise of the circuit into the 100kHz range as opposed to the more conventional 60Hz (with all of its harmonics) from a rectified AC ps found in traditional class A or AB circuits. Running on very low DC voltages many of the class D and T amps can be operated from a battery power supply. The battery ps eliminates the need for a step down transformer along with the rectification and capacitive filter and storage portions of a common amplifier's power supply. The battery stands in for the storage caps as the only source of Voltage and current (Amps). Once these front end portions of the traditional amp's power supply have been eliminated along with the incoming AC Voltage, the result is extremely black backgrounds from a very inexpensive circuit which rivals the consumer audio amps selling for tens of thousands of dollars.

Many if not most of the battery powered "practice" amps on the market use these same class D or T circuits based on chip output devices. Much of the noise you would hear from such an amplifier would be from its use with an AC power source. If you have an amp with the option to power it from either batteries or a conventional AC source, try using batteries and you should notice a very distinct lowering of the noise floor.