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Hi,
Does the uBitx have AGC? It was discussed in some of the previous threads, but the website only has this comment: "The audio amplifier uses one section of the TDA2822 – a quieter device compared to the LM386. It has two audio channels, the second channel is kept free for use as an AGC audio amplifier."? I am not sure if that means an AGC can be added later, or is it already present?
Thanks!
Jason W4JAL
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If so could it be adjusted, with a panel mounted pot.
Phil - G7BZD
On Sun, Dec 10, 2017 at 8:01 PM, <jettoblack@...> wrote: Hi,
Does the uBitx have AGC? It was discussed in some of the previous threads, but the website only has this comment: "The audio amplifier uses one section of the TDA2822 – a quieter device compared to the LM386. It has two audio channels, the second channel is kept free for use as an AGC audio amplifier."? I am not sure if that means an AGC can be added later, or is it already present?
Thanks!
Jason W4JAL
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No AGC on the production uBitx. The original schematic for the uBitx here: ? shows the second half of the TDA2822 used as an AGC amp, attenuating the audio through the 2n7002 nfet. Apparently Farhan found that he preferred using the rig without that particular AGC scheme. As Henning/DK5LV has shown in the distant past, that npn audio pre-amp stage for rx is a dynamic range bottleneck. if AGC is applied after it around the volume control as the above AGC scheme does. I think something like the original uBitx AGC level detect could work, but we'd be better off if the attenuation occurred? somewhere prior to the npn audio pre-amp: ? ? /g/BITX20/topic/5913954Jerry, KE7ER
On Sun, Dec 10, 2017 at 12:01 pm, <jettoblack@...> wrote:
Does the uBitx have AGC? It was discussed in some of the previous threads, but the website only has this comment: "The audio amplifier uses one section of the TDA2822 – a quieter device compared to the LM386. It has two audio channels, the second channel is kept free for use as an AGC audio amplifier."? I am not sure if that means an AGC can be added later, or is it already present?
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Search for AGC in this forum and you will see lots and lots of schemes to implement it.
The thread I pointed to is just part of a discussion, I think somebody renamed the thread and?
we continued the conversation from there.
At some point, somebody pointed out that the multiple pin diodes form a pi network attenuator
that presents a more or less constant characteristic load impedance, which can be of critical importance.
Jerry
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On Sun, Dec 10, 2017 at 12:33 pm, Jerry Gaffke wrote:
I think something like the original uBitx AGC level detect could work, but we'd be better off if the attenuation occurred?
somewhere prior to the npn audio pre-amp: ? ?/g/BITX20/topic/5913954
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This is a good question, but I think that the answer is No at the moment, in the schematic description on hfsignals site i can read? some tipa :"AGC as audio" in second channel of tda2822 and an IF AGC as Improvements of a better designed IF section. The homebuilder will have some occasions to work on It.?
Best 73, IK8YFW, Giuseppe
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Jerry,
I am in the process to do the following on my "original" BITX40 (yes I got one from Farhan):
use an opamp with a gain of 10 (20 dB) and an audio AGC system using a SSM2166.
Fortunately I purchased a pcb from ZL1CVD with the DIL chip via ebay years ago... Unfortunatley he does not sell this pcb any more...
The SSM2166 has a dynamic range of 60 dB, the opamp in front of it instead of the first discrete amp aftger dthe demod will amplifiy the audio into a range the SSM2166 can handle.
Also the SSM2166 has a provision of outputtting a AGC voltage -- may be used for driving an S-meter.
Let?s see what comes out.
Henning Weddig
DK5LV
Am 10.12.2017 um 21:33 schrieb Jerry Gaffke via Groups.Io:
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Show quoted text
No AGC on the production uBitx.
The original schematic for the uBitx here:
shows the second half of the TDA2822 used as an AGC amp, attenuating the audio through the 2n7002 nfet.
Apparently Farhan found that he preferred using the rig without that particular AGC scheme.
As Henning/DK5LV has shown in the distant past, that npn audio pre-amp stage for rx is a dynamic range bottleneck.
if AGC is applied after it around the volume control as the above AGC scheme does.
I think something like the original uBitx AGC level detect could work, but we'd be better off if the attenuation occurred
somewhere prior to the npn audio pre-amp: /g/BITX20/topic/5913954
Jerry, KE7ER
On Sun, Dec 10, 2017 at 12:01 pm, <jettoblack@...> wrote:
Does the uBitx have AGC? It was discussed in some of the previous
threads, but the website only has this comment: "The audio amplifier
uses one section of the TDA2822 – a quieter device compared to the
LM386. It has two audio channels, the second channel is kept free
for use as an AGC audio amplifier."? I am not sure if that means an
AGC can be added later, or is it already present?
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I am using the AGC from the original uBitx design, but found out the time constant of 1uF and 100k to be far too low which makes it a very fast AGC, not giving a nice sound to voices. I changed to 1,5 uF and 4M7 and now the voices of stronger stations are nice and clear (they 'push' the noise level down ) and the gain comes up nice and easy when the QRG is clear. I think I'll keep it this way.
73, Alex
PA1FOX
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Henning,
Interesting part, easy to use SOIC14 package, $3 in large quantities, Analog.com might sample them.
Primarily meant for speech compression on transmit, though compression can be disabled.
The part has been mentioned here before, but hadn't occurred to me it might be a good fit for receiver AGC.?
Could use two, one for the microphone and another for receiver audio.
Maybe automatically increase mike gain above 10mhz to compensate for the drop at the uBitx IRF510's.?
Though I appreciate Farhan's approach of using basic npn amps.
The SSM2166 would be an unfathomable black box for some tinkerers.
And cost 200 times as much as the MMBT3904's. ? ;-)
Jerry, KE7ER
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On Mon, Dec 11, 2017 at 03:48 am, Weddig, Henning-Christof wrote:
Jerry,
I am in the process to do the following on my "original" BITX40 (yes I got one from Farhan):
use an opamp with a gain of 10 (20 dB) and an audio AGC system using a SSM2166.
Fortunately I purchased a pcb from ZL1CVD with the DIL chip via ebay years ago... Unfortunatley he does not sell this pcb any more...
The SSM2166 has a dynamic range of 60 dB, the opamp in front of it instead of the first discrete amp aftger dthe demod will amplifiy the audio into a range the SSM2166 can handle.
Also the SSM2166 has a provision of outputtting a AGC voltage -- may be used for driving an S-meter.
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Have others tried the original uBitx audio AGC circuit? Looks like it would be simple to add on a small perf board with selectable FAST/SLOW front panel switch using different values for C70 / R71.?? AL VE3RRD 
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Hi there Alex.
Thanks for the info.
I am thinking of doing the same as AGC is needed with some QRO signals in the bands.
Based on the attached schematic that I use for reference I have a couple of questions.
C78 on the top schematic needs to be removed or not?
R253 needs to be changed from 10K to 1K as in the bottom schematic or not?
Of course C78 has to be removed and R70 replaced with a 2K2 one. This is obvious.
Thanking you in advance,
73 de Konstantinos, SV1ONW
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On Mon, Dec 11, 2017 at 03:48 am, Weddig, Henning-Christof wrote:
use an opamp with a gain of 10 (20 dB) and an audio AGC system using a SSM2166.
I am working on something similar for the microphone side with a SSM2167, and am doing a lot of thinking about how to do that on the RX side.? Someone recently used a programmable TI audio amp chip for compression and limiting.? I was thinking of using the SSM2167 in a similar way, and leaving the stock ?B audio amp in place.
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I'm thinking along the same lines.? I built a Mic amp version using the SSM2167 and thought it might work also for the Receiver. It's essentially the Sch seen on ebay but added a level pot on the output side. glenn vk3pe 
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Where did you get the "production" schematic for the ubitx? The hfsignal.com web site schematic is different. It shows no C78.?
Both the old schematic and the one of the web site shows R253 as being a 1K resistor.?
The 1K resistor and .1uf capacitor would for a low-pass filter with a 3db frequency of about 1500hz. I'm not sure why that is there but I can only assume it is needed to tame the audio circuit.
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Thanks a lot for the info. I got the schematic from a link through this site. I do agree about the filter. Anyway I will give it a try and see how it works. R70 on my board is has a 100 value.
73 de Konstantinos, SV1ONW
On Feb 8, 2018 04:51, "Tim Gorman" < tgorman2@...> wrote: Where did you get the "production" schematic for the ubitx? The web site schematic is different. It shows no C78.?
Both the old schematic and the one of the web site shows R253 as being a 1K resistor.?
The 1K resistor and .1uf capacitor would for a low-pass filter with a 3db frequency of about 1500hz. I'm not sure why that is there but I can only assume it is needed to tame the audio circuit.
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Arvo W0VRA