Poor preamp Singal To Noise Ratio

[steevo]

noisy preamp

Not sure what you are trying to say here, are you saying that a TL071 with a gain of 1, the most basic circuit will have a noise ouput figure of 9micro volts. which is most certainly does not, from what ive found. (try building a simple voltage follower yourself and see).

Steevo

 

[Juergen Knoop]

the most basic circuit will have a noise ouput figure of 9micro volts. which is most certainly does not, from what ive found. (try building a simple voltage follower yourself and see).

my bad english again
the TL071 in its most basic circuit would have a noise of app 3uV.
The quads input related noise will maybe 9uV. I calculated this from its input sensivity (500mV) and its signal to noise ratio (95dB)
the quad uses TL071 (LM301 in another shematic I've found) in inverting circuit with a feedback resistor of 390k.
The resistor may be responsible for the quads noise.

The scope readings give you no reliable numbers.

 

[steevo]

preamp noise

ok i will try my preamp with my giovanni stchino power amp which uses entireley discrete components througout and has a very low noise figure. If what you say is true then I should see virtually nothing on the amplifier output when I connect my unity gain premap to the input.

Steevo

 

[steevo]

noisy preamp

Incidentally I have a quad 306 which does not use an opamp the amplify the input signal, uses discrete transistors, It does however use an opamp for the DC ouput correction but this is not in the signal path. So I should see hardly any ouput noise when i connect my preamp up to this.

steevo

 

[steevo]

noisy preamp

The noise from my quad 306 with the preamp switched off is bearly audible and measures about 1.5 mv pk although its difficult to average out, with the preamp switched on the total noise measured is about 2.5 mv. For some reason with scope connected to the speaker ouput and the other channel connected to the preamp output the noise that I had seen on the quad 405-2 appeared. When I disconnedted the scope from the power amp output the noise dissappered, very strange. Dont know what on earth is going on with this at all.
The noise from the speakers with preamp connected is not much more than without the preamp connected. HArdly Notciable. I am wondering if the same problem was happening with the quad 405-2. I am going to try it.

Steevo

 

[steevo]

NOISEY PREAMP

Your correct the Quad 405-2 input stage is producing huge amouts of noise when I connect the unity gain preamp. The quad 306 by comarison is perfect.

Steevo

 

[steevo]

noisy preamp

Wwhen I switch of the mains power supply to the preamp and 2 seconds later the noise dissapers completely, when I use the quad 405-2. that is strange.

But I think the point is that the preamp work perfectly with the Quad 306.

Steevo

 

[kevinkr]

Re: preamp

I dont beleive this correct this would give you 0.707 micro volts, which is almost microscopically small and no where near the value i get. I dont think this figure relates to the internally generated background noise by the resistors, transistors thermaly generated noise etc in the opamp circuit. I think it relates to noise superimposed on signals amplifed by the opamp over and above the internal noise by the circuit.

steevo

No, actually this is correct, but it is the noise referred to the input so it would be gained up by the amplifier's input to output gain. (I haven't verified your math.)

The TL071-74 are extremely noisy for audio applications as you have already found out, and should be used only in higher level applications where the high level of input voltage noise is not a major issue.