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Some progress made on the hisssssssssssss...

:

11/23/1999 8:39 PM

Some progress made on the hisssssssssssss...

For those interested...

After changing out all the carbon film resistors with metal film, little or no decrease in noise was noted. So, I went after the op-amp used.

The unit uses a TL072, dual op-amp; somewhat noted for its low noise rating. I checked the data sheet on it and found its typical noise was 18nVperRootHertz (sorry, I don't have a square root symbol on this 'puter). That certainly looks low. Interestingly, the spec had no upper limit. Hmmmmmm.

I then checked the data sheet on an NE5532, dual; definitely noted for its low noise rating. I have used and seen them used in all sorts of hi-fi circuits. Its typical noise level was listed as 5nVperRootHertz with no upper limit. "A" parts (NE5532A) have an upper limit of 8nVperRootHertz.

Getting really anal, I checked the data sheet on an LTC1124, dual; not many people have heard of this one. Its typcial and upper limit were 2.4 and 4.0 nVperRootHertz respectively. I'm now attempting to get a sample of it.

Measurable Differences...

I socketed the op-amp in the Daddy-O and scoped the output with both output level and distortion controls maxed. With a shorted input, the output pedal noise was:

TL072 - 2mVP-P
NE5532 - about 1.5mVP-P

Enough to warrant tearing the box apart? Maybe. It's up to your level of tolerance.

I'll have more info when the Linear Technology part arrives.

Later,

DJ

11/23/1999 10:12 PM
Gus	Dave upto about 10k ohms bipolar device are better for noise. The spec s have to be looked at for current noise. The point as a gen rule bipoar to 10k fets above 10k for low noise

11/24/1999 8:43 PM
Dave James	Gus, I appreciate the response. But I have no idea of what you're trying to tell me. Sorry for being dense. Can you elaborate on your previous comments? Thanks, DJ

11/24/1999 9:29 PM
Gus	the resistor that defines the input z of the opamp or the resistor to 1/2 Vcc to the + input of the opamp.

11/24/1999 9:53 PM

Is this roughly the input impedance of the circuit?

In other words, if you are trying to maintain a high impedance, such as in a tube circuit, you would select FET technology because they have better S/N performance in this region than a bipolar device? Conversely, a bipolar device is basically a low impedance (wrt/ tube circuitry impedance) device, and operates with a better S/N ratio in circuits of nominal impedance 10k and below. When trying to use a bipolar device at higher impedances, it would exhibit a higher noise figure than an FET.

Do I have the general idea?

11/26/1999 4:37 AM
Gus	Yes. This is explaned in one of the Jung books. I think it opamps for audio??

11/30/1999 4:45 PM
Dave James	Thanks Doc, you cleared up my question. And it confirms what I've been told (and read) here at work. So this now raises my original question - does any body have a schematic for this beast??????? Later, DJ

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