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Old 25th September 2011, 09:44 AM   #1
neazoi is offline neazoi  Greece
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Default ultra low noise low distortion preamplifier needed

hello,
I need an ultra low noise very low distortion preamplifier for driving mainly headphones.
The source will be the output of the FET in the attached radio circuit. I will replace the opamp so a good headphone preamp is needed there that will be capable of handling the very low power input signals produced by the detector. Very low noise and distortion are needed.
I know it may be hard to beat the opamp, but I would like to make it using discrete components.

Any proposed schematics to look for?
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Old 25th September 2011, 09:56 AM   #2
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Do you know the S/N of the detector output? I would expect it to be much worse than the opamp. In that case, it is useless to mod the preamp for extremely low noise because you will still hear the noise in the source signal.
The switchable low-pass at the detector output will also give a hint that noise at that point may be problematic.

jan didden
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Old 25th September 2011, 10:21 AM   #3
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I really do not know that. I just need a good quality preamplifier made out of discrete components, that will be able to amplify the very low power detector signals up to the headphone level, without adding much more to the receiver audible noise and distortion.

Since I need practical circuit it would be very convenient to me if anyone could propose a such tested and well known circuit. Class of operation is not of a concern, although class-a may be better for lower distortion.
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Old 25th September 2011, 10:40 AM   #4
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Quote:
Originally Posted by neazoi View Post
I really do not know that. I just need a good quality preamplifier made out of discrete components, that will be able to amplify the very low power detector signals up to the headphone level, without adding much more to the receiver audible noise and distortion.

Since I need practical circuit it would be very convenient to me if anyone could propose a such tested and well known circuit. Class of operation is not of a concern, although class-a may be better for lower distortion.
For very low level & noise, you could look to a mic preamp. These are designed for such a purpose. There must be many here on the forum.

jan
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Old 25th September 2011, 01:07 PM   #5
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That is a slightly strange circuit, a cross between a regenerative grid leak detector and infinite impedance detector. The output will not be particularly low noise anyway, or low distortion. Any decent opamp should easily exceed the performance of the RF part of the circuit. Alternatively, two or three discrete BJT stages would do, at greater expense and complication.
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Old 25th September 2011, 02:24 PM   #6
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N1TEV states in his article that is good compared to a single conversion superhet in terms of audio quality as it uses only one mixer stage and is avoids the 3db noise that the image signal would add in such a superhet. Anyway, do you think a microphone preamplifier would be suitable, and could you suggest such a good circuit using discrete devices?
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Old 25th September 2011, 09:30 PM   #7
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Having only one mixer does not affect audio quality. This circuit does have image noise, its just that the image is 3kHz away instead of 2x455kHz away. Either you have misunderstood N1TEV or he has misunderstood what he is doing. In my opinion it doesn't need anything as sophisticated as microphone preamp. On the contrary, it would benefit from a severely restricted frequency range - say 300Hz-3kHz. My advice is to build it as shown in the circuit. You will learn quite a lot about electronics by playing with it.
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Old 26th September 2011, 08:17 AM   #8
neazoi is offline neazoi  Greece
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Thanks,
I was not aware of the image existence in such receivers. I thought that the regen amplifies only a portion of the signal and then detects it, I am still not sure how the "image" signal occurs if it occurs.
See this conversation where they claim no image signal exists in such receivers. I am trying to make another post there to claim about it.

I think I am going to try one of these auto amplified speakers first to determine the levels needed.
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Last edited by neazoi; 26th September 2011 at 08:20 AM.
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Old 26th September 2011, 10:21 AM   #9
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When receiving AM that circuit, like most circuits, will pick up both sidebands. When oscillating, to receive SSB or CW, it will receive both the desired signal and the unwanted image. It does not have anything like enough selectivity to reject the nearby image. It is possible, with much more complex circuitry, to eliminate the image either using filtering or phasing but these simple circuits can't do it.

People may be getting confused by so-called 'zero IF' receivers, also known as 'direct conversion'. Some of these do eliminate the image by using image-cancelling mixers. It does not follow that all DC receivers eliminate the normal superhet image - simple ones just have it adjacent to the wanted signal.

You need to bear in mind that radio websites and discussion boards can contain just as much nonsense as audio ones!
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Old 26th September 2011, 11:00 AM   #10
neazoi is offline neazoi  Greece
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I am sorry to turn this conversation more into a radio domain but the information you provide is very useful!

I think I can understand the close-to-frequency image signal in oscillating (CW/SSB) mode now: The detector behaves as a mixer in this mode, which mixes the amplified RF with the beat frequency produced by the oscillation. This beat frequency is not on 455KHz, like common single conversion superhets, but NEAR the actual receiving frequency (1-3khz or so). So the image signal will be close to the receiving frequency then. The audio output of the detector will contain a sum of the wanted signal and the image signal. This implies to direct conversion receivers too.

Please tell me if I have understood the above concept well?


Now, just a consideration:
The use of the regen in AM (non oscillate) mode may be different though. Here, there is no local oscillation but only a multi-amplified RF and detection. Whereas in a single conversion superhet, both signals (wanted and image that are far apart) would appear on the 455KHz, in a non-oscillating regen only the amplified desired signal (carrier and sidebands) would be detected. This is because the AM signal is quite wide for the detector to distinguish it from others. AM is just too wide and the detector can cope with it with no other far-away image signal reception. No other image signal here. I tend to think of it more like a tuned radio frequency design with RF amplifier feedback in this mode.
I may be wrong on the above...

I appreciate your answer
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Last edited by neazoi; 26th September 2011 at 11:18 AM.
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