I think I'll change my name to "Hiss Man"...

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3/29/2000 4:46 PM

Dave James

I think I'll change my name to "Hiss Man"... Troops,     Ok, here's the deal...EVERY OP-AMP BASED GAIN STAGE I'VE EVER MADE HISSES!!!!     From the range of exotic circuits to bare-bones, I cannot end up with one that doesn't hiss. Obviously, I don't know what the heck I'm doing.     The latest effort was a split-supply, direct coupled, inverting stage. Metal film resistors were used. The input resistor was 56K, the feedback resistor was 100K and a 56K was used from the (+)side to ground (Yes, it should be a parallel value of the input and feedback resistors). The op-amp was a (good) Signetics NE5532 dual speced at a nominal 5nV/root Hz.     It HISSES.     Can somebody, anybody enlighten this poor man?     Confused in Los Gatos,     DJ

 

3/29/2000 5:36 PM

R.G.

Well, does it help to know that every opamp based gain stage anyone has ever made hisses?     Just on the face of it, your resistances are a bit high for true low noise operation. I try to keep resistors under 10K for low(er) hiss in my designs. However, it doesn't look all that bad. What are you doing to amplify the signal after it leaves the opamp?     All resistors and active devices hiss, the only difference is the degree, not the presence of hiss. The only way to get no hiss is to have hiss that you can't hear because it's below some masking threshold - that is, hidden.     What changes is that some devices have lesser intrinsic hiss than their brothers, as in metal film versus carbon comp (which is the resistor high-hiss champion), or NE5532's versus 741's. Within a given device, the operating conditions can change the amount of hiss - low resistances hiss less than high resistances, resistors with high voltages across them hiss more than with low voltages, hot resistors hiss more than cold ones.     The other variable is what you do after a gain stage. If you make any opamp circuit and then pipe it into a super high-gain guitar amp, it will likely have hiss that you can tell is greater than the existing hiss from the amp itself.     There's a great book on getting low noise, I think it was "Low Noise Electronic Design"; I'll dig out the title and author if you need it.

 

3/30/2000 12:30 AM
Cole	RG -     Don't inverting op-amp stages like 1k-10k resistors for a low DC path to ground for lowest noise? Maybe a non-inverting op-amp stage would be better in this case?     Come to think of it, maybe a simple single transistor stage would be better?

 

3/30/2000 7:38 AM
paul perry	The best practical stuff I fver read on guitar preamps is in Boscorelli's "Stompbox Cookbook"   which you can buy from eg Lutherie Supply, justr   search in www.google.com for 'boscorelli' & you will find sites selling it.

 

3/30/2000 4:41 PM

Dave James

All, thanks for the feedback.     R.G. - please dig up the reference you mentioned.     Paul - thanks for your reference; I'll check it out.     I'll lower the resistances and re-check the "subjective" hiss level.     I listen to the hiss of my Crate VC-15R with a shorted input and then swap in the gain stage with its input shorted and do an aural comparison (not done at high level settings on the amp).     I cannot differentiate the noise out of the gain stage from the noise floor of my scope visually (5mV/div is the lowest).     Again, thank you everybody.     DJ

 

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