Constant Volume Amplifier

@averagejoe Welcome, to date there is no such circuit released. I assume this would be in a solid state setup?

Yes, what I currently have in mind is for headphone/hearing aid use. High impedance input(Cap coupling) so it can be driven by a computer headphone  output, High end Audio Op AMP with neg feedback to set the constant volume with either an FET or Bipolar driver for high impedance output to drive a headphone or blue tooth input. It could be a single or multiple Op Amp setup. I’ll also have to have a + and – power supply for the Op Amp(s). At least that’s my thought at this time.

@averagejoe I would say you are on the right track, I personally don’t have experience in that department. Next thing to do is start building in sections and experiment, figure out what works and put it all together. Sounds like a fun project

Hmm, I have a proposed circuit using a High quality Audio Op Am(LT1115CN8) and a FET I think will work(2N3819). I figured the easiest way to kick this off would be told be to start by using circuit simulation. I’ve chosen Circuitlab. I’ve never used it before. They don’t have the Op amp listed so I chose a generic one(LM741) and a generic FET(J310). I’ll have to generate my own device models using the data sheets. All in I’ve spent about 30 min working on the simulation. I have the schematic identified and proposed elements defined. I made one run and the circuit(Not My design) seems to work. It’s a simple circuit using negative feedback into the Op Amp to compress the input volume swings. I tried a 1V input and then a 5V input and the output was certainly “compressed”. I’m sure I’m going to have to do some tuning. Possibly changing the circuit and adding another Op Amp and maybe use a bipolar transistor to drive the load rather than a FET. I don’t have a circuit for the Bluetooth unit I’m using so I may have to add capacitive coupling to the output. I may have to also add a resistive load to the output. It’s a start. I’m going to breadboard the circuit up top see what I get. I can then compare the simulation with the real output to see how close they are. That will take sometime since I’ve only got 30 minutes of flight time on the simulator. I am very surprised this isn’t available commercially. At least I haven’t been able to locate one for a reasonable price. The only thing I’ve found are studio level Dynamic Range Compression units that are expensive.

Well after closer inspection of the simulated output levels I’m not sure if the circuit is working the way I want. Or, maybe the simulation isn’t working they the way I think it should. The end goal of the circuit is to increase the low level amplitude and decrease the high level amplitude waveforms to provide a relatively constant volume output. That way I can hear both the dialog and action scenes in a movie without having to constantly adjust the volume level. I’m going to have to breadboard this up and see what I get. More later.

@averagejoe I have never messed with simulation but sounds useful to play with for getting basic design and theory figured out. Also may be worth researching AGC circuits. As in tube radios, using strong signals to create negative voltage to lower gain of prior stages. You need the feedback inversion to regulate signals.

I started breadboarding the circuit up. But I found I was missing  trim pot. I’ve ordered one. In the meantime I’m going to use a resistor I think will work. The more I delve into it the more I think buying a used dynamic range compression unit. After simulating various amplitude inputs I think I’ll need to add additional circuitry such as AGC to the design(see comment above). My guess is by the time I buy the components, design and have printed circuit boards fabricated, enclosure and input/output connectors I’ll probably have as much in it as a used commercial unit. However, given I’ve got most of the parts I’m going to try the original circuit to see how it performs. I’ll keep you posted on the results. 

@averagejoe All part of the fun is build it and seeing what happens! Keep up the good work

Just a simple question. Isn’t there noticeable delay in the audio when doing this? Something like video game lag where actions can be delayed displaying on some televisions?

@gbfreeman123 The answer to that is yes. There is a latency for all electronic devices. I don’t know what the latency will be for this Amp.

Hopefully it’s negligible. In a studio environment making say a DVD they would have variable delay networks built into their devices so you could sync to video with the audio. Since I’m adding this to my computer or TV I’m adding delay to the audio. Hopefully that delay will be negligible. I’ll try to remember to post the results. First I need to build it to determine if it’s going to work at all. I’m sure some tweaking will be required. I’ve already thought of one thing that will need to be added. That’s a volume control(ganged Pot? or single pots/trim resistors) to set the volume for left and right channels. The initial build is one channel. Once I get one the way I want if I can I’ll build a second for stereo.

Well, I completed the initial build. This was just a non printed circuit breadboard build. I’m also in a very electrically noisy environment. What I’ve learned so far is that I’m going to have to build it using a printed circuit board. The inputs will need to shielded an possibly the Op Amp. I used what I had for the capacitors which were 1uf ceramic. I need to buy either orange drop or polyester film caps instead. There’s too much noise being injected into the input coupling capacitor. The first thing I’m going to have to do is look into making my own printed circuit boards. Double sided would be best but I may only be able to do single sided. I was able to get a first pass look at the latency. It appears to be about 700us. With 300mv input at about 500hz I get 10V output with a 3M input resistor. That’s a gain of about 30 at that freq. It appears I have about 7V of high freq noise on the output sine wave. As much fun as I’m having the decision to buy a used commercial unit will depend on how much it will cost to build one. More to come. Thanks for your interest. 

Well, I’ve done a cost analysis and I’ve determined it’s going to be far cheaper to buy a used studio Dynamic Range Compression unit than it will be to buy my own. It wasn’t cost effective to have PCBs(Protos) made up and I didn’t want caustic chemicals in my shop for one or two sets of PCBs. So, I’ve decided to purchase a studio unit and move on. I have several other electronics projects that include restoring several old amplifiers as well as putting new surrounds on some 15″ speakers. So, I’m going to move onto those.