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Hi All,
I am looking for a little advice regarding a noisey circuit.
I have designed and built a stereo volume control as prescribed in the datasheet (p14), but there is noise on the output of the amplifier stage (before the Darlington buffer). Any ideas how to reduce this noise?
thank you in advance
Steve
Hi Hooman,
The schematic is the same as on p14 of the datasheet for the device (below for your convenience).
I am hearing a white noise (hiss) through headphones following a headphone amplifier. I tested the headphone amp and the noise does not come from there. I also eliminated the buffer pair and the noise still persists
Hi Steve,
Some comments / questions:
1. Headphone Testing: What is the impedance of your headphone amplifier? I'm assuming that you are AC coupling it to the LM13700 output?
2. Setup Confirmation: Have you applied an audio source to the circuit to make sure that you can hear it on the headphone (to verify circuit operation)? Also, are you able to change gain / volume by varying the DC voltage that you apply to VC? BTW, what is the range of DC voltages you're applying to VC?
3. Expected Noise Behavior vs. Setup: Are you able to see any audible difference in noise with VC varied? Figure 15 predicts a higher noise with higher IABC (increased VC voltage). Does your setup confirm this? If your setup exhibits the same, this indicates that the noise is related to the LM13700 inherent noise but if it does not, it'd indicate a different dominant noise source!
4. Node Voltages / Gain Verification: Can you please measure the following node voltage (DVM is ok unless noted) and please report back:
a) Output pin
b) Input pin(s)
c) Diode bias pin
d) Output pin swing / waveform for a given audio source voltage, and at least one VC (DC) voltage using an oscilloscope.
5. Simulation: I've simulated your circuit and I don't see anywhere the amount of noise that could be audible. Here I see 33uV_RMS total audio noise at maximum gain amounting to 1.8pW_RMS power across a 600ohm headphone. I don't think this low a noise can be heard! However, I'm not 100% sure that the TINA-TI macromodel of the LM13700 models the device noise (as this is a fairly old device).
Here is the TINA-TI simulation file for reference:
LM13700 E2E Hooman 1_25_17.TSC
If you have the capability of measuring the output swing / noise with a lab instrument (instead of a headphone), it'd help a lot so that we can get some numerical values of the noise you're seeing. Do you have access to an audio analyzer, or at least an oscilloscope? If you are able to hear the noise, the noise must be fairly large and even a scope might be able to show it.
Please let me know.
Regards,
Hooman
Hi Steve,
Sounds like the NE5532 delivers the DC voltage to the Vc pin that sets the volume value / gain of the LM13700 and you've found out that the NE5532 output is "noisy" and the LM13700 can respond to that noise which then modulates your audio at LM13700 output current pin and you hear it as noise. If so, I recommend that you adequately low pass filter the NE5532 output pin voltage so that the Vc pin is as noise-free as possible since you're only delivering a DC voltage to this pin anyway.
If so, then the noise specs of the NE5532 (or whatever device you replace it with) will be immaterial!
Having said that, you have a multitude of devices to choose from that can swing all the way down to the bottom rail (V-) called "RR output" and can operate off of +/-15V supplies. I usually use Selguide to fine tune my OPA choices, shown below.
Here are some of the choices (Vs= +/-15V, RRO, Single, cost < $1.20, GBW < 4MHz, Voltage noise < 15nV/RtHz):
Regards,
Hooman
