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Title: Need: a simple voltage-controlled audio amplfier Post by: AB2EZ on January 07, 2007, 11:12:23 AM Hi!
I am looking for a readily available, voltage-controlled, dc-coupled, audio bandwidth (0-10 kHz or better) audio amplifier to build an AGC stage into an off-air monitor. Ideally, an integrated circuit in a DIP package would be fine. Alternatively, some kind of voltage-controlled variable resistor to use as the feedback resistor for a plain vanilla op-amp would be ok. Solid-state only. I don't need super high fidelity or the ability to drive a speaker... just an analog voltage-controlled gain that is variable over 20 dB of range. Thanks Stu Title: Re: Need: a simple voltage-controlled audio amplfier Post by: w3jn on January 07, 2007, 12:15:21 PM Use a FET in the FB loop of the op amp. The gate of the FET would be the control voltage.
Title: Re: Need: a simple voltage-controlled audio amplfier Post by: steve_qix on January 07, 2007, 12:49:27 PM Hi! I am looking for a readily available, voltage-controlled, dc-coupled, audio bandwidth (0-10 kHz or better) audio amplifier to build an AGC stage into an off-air monitor. Ideally, an integrated circuit in a DIP package would be fine. Alternatively, some kind of voltage-controlled variable resistor to use as the feedback resistor for a plain vanilla op-amp would be ok. Solid-state only. I don't need super high fidelity or the ability to drive a speaker... just an analog voltage-controlled gain that is variable over 20 dB of range. Thanks Stu Hi Stu, These devices are available. Check the company THAT corporation. They have a number of VCAs and other things. I'm using a THAT corp VCA in my new audio processor design. I'm also fooling around with some analog optoisolators in the feedback of an op-amp for another application. This might work out better than the VCA for my particular application, where fast attack times are not needed. The optos are not super fast. The VCAs are ! I'll share the circuit when I have it working, for what it's worth. I'm using a FET in the feedback loop of an op-amp currently for my multi-band audio processor. This works well, although the THAT corp VCA is probably better. The distortion figures for these VCAs are absolutely amazing ! Talk later and Regards, Steve Title: Re: Need: a simple voltage-controlled audio amplfier Post by: AB2EZ on January 07, 2007, 03:37:04 PM John
Steve Thanks! I've checked out the FET / THAT approach, and also the opto-resistor approach. For this application, I'm going to use a Perkin Elmer VTL5C# "Vactrol" (LED + opto-resistor), which I was able to purchase easily on line using eBay ( 2 for $6.99 + $2.65 shipping). I'm going to post my initial circuit design on my web site. The application is an AGC amplifier to place between the off-air sensor and the main unit of my REA modulation monitor. Best regards Stu Title: Re: Need: a simple voltage-controlled audio amplfier Post by: Steve - WB3HUZ on January 07, 2007, 04:17:19 PM The application data sheet for the Vactrols has a circuit that appears to be close to what you need Stu.
Title: Re: Need: a simple voltage-controlled audio amplfier Post by: AB2EZ on January 07, 2007, 05:42:23 PM Steve
Thanks! Here is my initial design. I'll update it after I build it... probably sometime in the next week. http://mysite.verizon.net/sdp2/id18.html Best regards Stu Title: Re: Need: a simple voltage-controlled audio amplfier Post by: WA1GFZ on January 07, 2007, 08:45:42 PM Stu, AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
I've seen the FET in the input side of op amps so they can be referenced to ground with the control also referenced to ground. I have the FET part of the gain setting or an up front attenuator. Motorola had a good AGC circuit in their BASE units to AGC audio from dispatchers. They put the fet in front of the op amp so you don't have to float the control or isolate it. |