Buffer adding hisssssssss

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Optical
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Post by Optical »

I built a buffer using a TL071 (standard non inverting) and a charge pump for 30V.
It does the job, but adds a lot of hiss ive noticed. The noise floor is quite high now - at band volumes you have to shout over the hiss to give an idea of how loud it is.

Before I go blindly swapping in other types of opamps and trying to lower the noise floor, is this normal? Would a jfet buffer or something like an opa134 lower the noise floor significantly?

I believe a fet would lower the noise a bit, but also some gain would be lost which I want to avoid.

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Post by bajaman »

try inserting a 100 ohm resistor in series with the output from the charge pump to the opamp pin 7, and then connect a 220uf (at least 35v) capacitor from pin 7 to ground ;-)
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Post by ppluis0 »

Hi Optical,

After implement the RC filter suggested by bajaman, inspect your layout to see if the charge pump is using the same ground tracks of the audio section.

Perhaps the noise you're hearing can be produced from the ground conductors.

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Post by Optical »

RC filter on the power supply? I'll try that, thanks
The ground paths for the audio and power circuits are separated and joined only at one location (input jack)

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Post by plush »

What kind of hiss do you experience? Can you measure it's frequency? What type of IC/charge pump you've used for 30v conversion? Is there any power line ripple (switching noise).

If it's switching noise, LC filter would work way better than RC. But in some cases it could be defective power IC (I've tested a bunch of 7660S and tc1044, a number of them operated at audible frequencies even with boost mode on, which is nuts).

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Post by The G »

If that's a standard op amp voltage follower and you're using an IC socket, you could just take out the IC, connect the input to output in the socket and see if the hiss is still there.

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Post by soulsonic »

Also a slight chance the JFETs in the opamp got zapped, which can make them turn hissy
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Post by idy »

Why a charge pump for a buffer? If you wanted lots of clean boost I would understand, but buffering means gain around 1, so unless you are buffering a signal already close to 30v(!) that is wasted headroom and parts and power and... an invitation for trouble... Maybe it's part of something bigger?

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Post by Optical »

idy wrote: 26 Jun 2020, 20:36 Why a charge pump for a buffer? If you wanted lots of clean boost I would understand, but buffering means gain around 1, so unless you are buffering a signal already close to 30v(!) that is wasted headroom and parts and power and... an invitation for trouble... Maybe it's part of something bigger?
You're onto it - part of a much bigger circuit which needs the headroom and is switched in an out.

IC is a smd sioc package so a bit of a pain to swap out

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Post by Optical »

plush wrote: 26 Jun 2020, 07:37 What kind of hiss do you experience? Can you measure it's frequency? What type of IC/charge pump you've used for 30v conversion? Is there any power line ripple (switching noise).

If it's switching noise, LC filter would work way better than RC. But in some cases it could be defective power IC (I've tested a bunch of 7660S and tc1044, a number of them operated at audible frequencies even with boost mode on, which is nuts).
It sounds the same as shot noise hiss, spectrally flat and constant. I haven't tried and measurements to characterise it though
No difference swapping between a battery or psu

Pump chip is a 1054. I tried some others but like you've pointed out many were noisy or not up to the current demand i wanted.

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Post by plush »

Optical wrote: 27 Jun 2020, 06:21
plush wrote: 26 Jun 2020, 07:37 What kind of hiss do you experience? Can you measure it's frequency? What type of IC/charge pump you've used for 30v conversion? Is there any power line ripple (switching noise).

If it's switching noise, LC filter would work way better than RC. But in some cases it could be defective power IC (I've tested a bunch of 7660S and tc1044, a number of them operated at audible frequencies even with boost mode on, which is nuts).
It sounds the same as shot noise hiss, spectrally flat and constant. I haven't tried and measurements to characterise it though
No difference swapping between a battery or psu

Pump chip is a 1054. I tried some others but like you've pointed out many were noisy or not up to the current demand i wanted.
Oh i see. Can you post your circuit diagram here?

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