Are Green or Stock Muff layouts less noisy than the Triangle Muff?

[powerplayj]

....built the Triangle but rather than cranking the gain I have better experiences running it lower with low gain and using an LBP, MOSFET or OD to boost the front end. I was just curious if the Green and stock versions were lower gain and possibly have less noise?

[petemoore]

  Mostly the same as noise goes, actually different from one to the next.
  The main noisy things are the transistors, HG/LN types like 2n5089 or MPSA18?
  they all use 4 transistors.
  The voicing is different, the topology is [mostly] the same, compare schematics and correct this statement if necessary.

[R.G.]

Noise is a slippery subject.

The 2N5089 and MPSA18 are very high gain - but also very low noise. They are specifically designed for low noise audio input stages.

General purpose amplifier transistors like the 2N4400 series are not as quiet. Noise on things like the 2N3900 series is not all that well specified.

Resistors and active devices are the only things that generate noise. Capacitors and inductors can't.

Poor solder joints and intermittent, dirty contacts are big noise generators.

For resistors, the inherent noise is all thermal. It's proportional to the resistance and the absolute temperature, degrees kelvin. For low noise, use small resistors and dunk them in liquid nitrogen - not always the most practical solutions, since sometimes you simply have to have high resistance and don't have a tank of N2 available.

So you pick resistors that have the lowest practical noise. That means, in decreasing noise order: carbon film, metal film, wirewound.

Different resistive materials also have excess noise, above the thermal noise. Carbon composition is the classical example. A carbon comp will have excess noise that is simply not there in a film resistor. This is made worse by high currents. So on sensitive inputs,
- don't use carbon comp
- if you must use high value resistors (and you usually do) arrange them so that the currents through them are low. This last is the basis of "noiseless biasing".

Damage to transistors is a big contributor to noise. A really high class low noise transistor like the 2N5088/5089/MPSA18 can be permanently made noisy by reverse breaking over the base emitter junction ... even one time. Some circuits have capacitors internal to them that are charged up during operation to near the power supply that can hold a base at near 0V at turn on or mid-power supply at turn off and reverse break the base-emitter when power is cycled. This noise damage accumulates with time. You stop this by putting a reverse biased diode in parallel with the base-emitter so that the diode prevents the base-emitter from ever seeing more than a diode drop reverse and never breaks over.

Noise is also an input phenomena. The noise of the input stage is amplified by the entire rest of the circuit. If your input stage has a gain of 10, you can use ten-times-worse noisy circuits after the input stage before you contribute as much noise as the input stage. The input stage is critical.

Also, if you get a lot of gain at the input, you need less gain after the input, so the noise made by the input stage is amplified less. So get a lot of gain in the first active device your signal touches for lowest noise.

How does this apply to the Big Muff?
- use metal film resistors and a low noise transistor for the input stage.
- Convert the input stage to noiseless biasing
- Use reverse base-emitter diodes on all the transistors but especially the first one
- and it goes without saying - use clean contacts and good solder joints.