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White noise

 

I'm considering ways of reducing the white noise of the uBitX.? Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.? You needed to use the filter between your ears and just listen to the one tone and ignore the rest.? You could do it because the rest of the audio chain was perfectly quiet.? Disconnect the antenna and virtually no noise whatsoever.? With my uBitX, the white noise is annoyingly present even without an antenna.

I think it is the LM386.? I've seen this before.? I think the LM386 will actually produce some gain all the way up to 100KHz.? I think they call it thermal noise.? What can be done to make it gone?

The LM386 data sheet talks about a bass boost using a series 10K resistor and 0.033uF capacitor between pins 1 (gain) and 5 (output).?

Using Elsie software, I wanted to see the frequency response of the series R/C with this simple circuit:



Below 500 Hz it attenuates the feedback quite a bit.



Looking at the chip's internal workings you see that there is some feedback working here to knock down the higher frequencies and leaving the listener with the sensation of accentuated Bass.? Consider the R/C between 1 and 5...

This simple R/C is clearly not very selective.? But what do you think would happen if a proper filter were designed to notch out the desired passband of 0-2.5KHz or so?? The high frequency hiss would be knocked down and the desired passband would be accentuated?

Ford-N0FP

 

I think you are on the money. Last year, during the lockdown, I built a direct conversion radio, then I added a discrete filter made from 4 transistors. The audio was so sweet!


On Sun, Mar 20, 2022, 10:45 PM Ford Peterson <ford@...> wrote:
I'm considering ways of reducing the white noise of the uBitX.? Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.? You needed to use the filter between your ears and just listen to the one tone and ignore the rest.? You could do it because the rest of the audio chain was perfectly quiet.? Disconnect the antenna and virtually no noise whatsoever.? With my uBitX, the white noise is annoyingly present even without an antenna.

I think it is the LM386.? I've seen this before.? I think the LM386 will actually produce some gain all the way up to 100KHz.? I think they call it thermal noise.? What can be done to make it gone?

The LM386 data sheet talks about a bass boost using a series 10K resistor and 0.033uF capacitor between pins 1 (gain) and 5 (output).?

Using Elsie software, I wanted to see the frequency response of the series R/C with this simple circuit:



Below 500 Hz it attenuates the feedback quite a bit.



Looking at the chip's internal workings you see that there is some feedback working here to knock down the higher frequencies and leaving the listener with the sensation of accentuated Bass.? Consider the R/C between 1 and 5...

This simple R/C is clearly not very selective.? But what do you think would happen if a proper filter were designed to notch out the desired passband of 0-2.5KHz or so?? The high frequency hiss would be knocked down and the desired passband would be accentuated?

Ford-N0FP

 

Hi Ford,

The gain capacitor (C75) is another item in the support devices around the LM386 that can impact the frequency response.? If you look at the TI datasheet for the LM386, you will see that capacitor is 10?F, not the 1?F that is in the v6 design.? You can significantly reduce the "Hiss" from the audio section by adjusting the gain capacitor and adding the bass boost.

73
Evan
AC9TU
 

Yup, remember the ARC5 well.?
Mostly fondly.
I was using an ARC5 receiver and transmitter on 40m CW while a college student in the 1970's.
Designed in the 30's, they built hundreds of thousands of them in the 1940's.

It was a superhet, not a direct conversion radio.
IF bandwidth was quite wide as you say because the IF used LC filters, not crystals.
I believe the "Frequency of CW Oscillator" in table 3-1 on pdf page 31 of the following
document tells us the BFO frequency, it varied with the receive frequency of that
particular unit since they had to balance IF bandwidth against image rejection.

A fun manual to browse through if you ever warmed your hands on any of this gear.
More references here:??

Jerry, KE7ER


On Sun, Mar 20, 2022 at 10:14 AM, Ford Peterson wrote:
Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.
 

They had the best tuning capacitors ever produced.
- f

On Mon, Mar 21, 2022 at 5:18 AM Jerry Gaffke via <jgaffke=[email protected]> wrote:
Yup, remember the ARC5 well.?
Mostly fondly.
I was using an ARC5 receiver and transmitter on 40m CW while a college student in the 1970's.
Designed in the 30's, they built hundreds of thousands of them in the 1940's.

It was a superhet, not a direct conversion radio.
IF bandwidth was quite wide as you say because the IF used LC filters, not crystals.
I believe the "Frequency of CW Oscillator" in table 3-1 on pdf page 31 of the following
document tells us the BFO frequency, it varied with the receive frequency of that
particular unit since they had to balance IF bandwidth against image rejection.

A fun manual to browse through if you ever warmed your hands on any of this gear.
More references here:??

Jerry, KE7ER


On Sun, Mar 20, 2022 at 10:14 AM, Ford Peterson wrote:
Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.
 

¿ªÔÆÌåÓý

I distinctively recall the ARC-5 series.? I tore apart one of the transmitters (TVI generators?) to build my first legal transmitter over half a century ago.

Gerry Sherman, VE4GKS

Sent by the Thunderbird

On 2022-03-21 06:29, Ashhar Farhan wrote:

They had the best tuning capacitors ever produced.
- f

On Mon, Mar 21, 2022 at 5:18 AM Jerry Gaffke via <jgaffke=[email protected]> wrote:
Yup, remember the ARC5 well.?
Mostly fondly.
I was using an ARC5 receiver and transmitter on 40m CW while a college student in the 1970's.
Designed in the 30's, they built hundreds of thousands of them in the 1940's.

It was a superhet, not a direct conversion radio.
IF bandwidth was quite wide as you say because the IF used LC filters, not crystals.
I believe the "Frequency of CW Oscillator" in table 3-1 on pdf page 31 of the following
document tells us the BFO frequency, it varied with the receive frequency of that
particular unit since they had to balance IF bandwidth against image rejection.

A fun manual to browse through if you ever warmed your hands on any of this gear.
More references here:??

Jerry, KE7ER


On Sun, Mar 20, 2022 at 10:14 AM, Ford Peterson wrote:
Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.
Jack, W8TEE
 

When I was a Novice, I bought an ARC5 and I think I paid $3 for it! It was in a garage sale!

Jack, W8TEE

On Monday, March 21, 2022, 02:29:49 AM EDT, Ashhar Farhan <farhanbox@...> wrote:


They had the best tuning capacitors ever produced.
- f

On Mon, Mar 21, 2022 at 5:18 AM Jerry Gaffke via <jgaffke=[email protected]> wrote:
Yup, remember the ARC5 well.?
Mostly fondly.
I was using an ARC5 receiver and transmitter on 40m CW while a college student in the 1970's.
Designed in the 30's, they built hundreds of thousands of them in the 1940's.

It was a superhet, not a direct conversion radio.
IF bandwidth was quite wide as you say because the IF used LC filters, not crystals.
I believe the "Frequency of CW Oscillator" in table 3-1 on pdf page 31 of the following
document tells us the BFO frequency, it varied with the receive frequency of that
particular unit since they had to balance IF bandwidth against image rejection.

A fun manual to browse through if you ever warmed your hands on any of this gear.
More references here:??

Jerry, KE7ER


On Sun, Mar 20, 2022 at 10:14 AM, Ford Peterson wrote:
Perhaps it is my nostalgic quest to reproduce the audio of an old ARC5 direct conversion tube radio.? Remember the ARC5?? 10 KHz wide you could hear every CW signal above and below the BFO.


--
Jack, W8TEE

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