Simple Pre-amp

[danrogers]

Simple Pre-amp / volume control

Hi all

I have just purchased some studio monitors. These are powered (built in amps) and will be connected to my PC sound card. I need a really simple pre-amp, just a volume knob (pref a single knob for L&R channels) to sit in between the PC sound card and the input too the monitors and adjust the line level.

I'm not sure how it could be done, with audio quality being a very important factor? Any help or suggestions very welcome

 

[Dragon Tamer]

I'm assuming that since the monitors were built to work with a computer that they should have a built in preamp. What you want is some sort of volume control to avoid rupturing your ear drums first thing in the morning (I try to wait an hour or so before rupturing mine). But for what you want using a simple dual audio taper pot would be the quickest way to go with little interference of the signal. If it's a decent quality pot (not one of those cheap chinese ones) there should be no audible distortion other than the decibel level.

 

[danrogers]

Hi Dragon Tamer, Yes thinking about it a pre-amp does amplifiy the signal I think? And as you said, I only need to adjust the line level voltage (I think!)

Could you recommend where to look for these dual audio taper pots please? Do they come in different resistance values?

 

[Dragon Tamer]

Behold, the power of internet shopping: https://www.mouser.com/Search/Refine.aspx?Keyword=dual+audio+pot just pick the one you want to use. I'm personaly attracted to the ones with volume control motors, bt that may not be what you are looking for.

I'm not 100% on this so you may want to have this fact double checked, I think a liniar pot has an expodential effect in amplifiers (double the resistance, quarter the signal from the previous setting). The way around this is a log pot but they have to be installed a certain way.

 

[danrogers]

Thanks

what kind of resistance do you think I should go for?

 

[Nigel Goodwin]

Thanks

what kind of resistance do you think I should go for?

What is the input impedance of the monitors?, and the output impedance of the sound card?.

Have you considered (or even tried?) just connecting it directly, and using the PC volume control.

 

[danrogers]

Hi Nigel. I don't have the monitors yet, only just ordered them. They are Alesis M1 mk2's. I'm sure I could connect them directly but I wan't to have a 'hardware' volume knob instead of having to adjust it via the PC

How could I measure the impedance of the soundcard output? It's not a top spec one and pretty old so I'm not sure that I could find a spec?

Thanks

 

[crutschow]

Likely the output impedance of the sound card is reasonably low. Do you know the input impedance of the speaker amps? You could probably go with a 10k dual audio taper pot.

 

[danrogers]

Hi crutschow. This is from the user manual I have just downloaded.. Input Impedance: 20kΩ balanced 10kΩ unbalanced

I think I will be running them unbalanced, so 10k as you say I guess. Why are they different? If I wanted to run them balanced, I guess I would need to get 20k?

Thanks for the help

 

[Dragon Tamer]

I think unbalanced is just a fancy way of saying mono input, while balanced is a fancy way of saying stereo input.

 

[gabeNC]

I think unbalanced is just a fancy way of saying mono input, while balanced is a fancy way of saying stereo input.

Not really. Balanced connections are used to reject noise especially on longer cables. This is important in studios and driving microphones. These will be mono signals, if you pushed a stereo signal through (and kept it stereo) the line would then be unbalanced.

Thought this was a good link that explains it.

http://www.nullmodem.com/Audio.htm

 

[Johnwoo]

I think a 10k - 50k pot will work.
but make sure you got the right type, I mean these 'volumn purpose' pot.
it changes resistance exponentially which matche human's ear.

 

[danrogers]

I really don't know what I'm looking at here to be honest.

How about this https://uk.rs-online.com/web/p/trimmers-potentiometer-rheostat/7293694/

the following one states that it is a 'log' pot but its a lot more money? https://uk.farnell.com/vishay-sfernice/p9a2r100fisx1103ml/pot-log-dual-10k/dp/8791384

 

[danrogers]

just found these two, they have two versions. One is log scale and one is standard?

https://www.rapidonline.com/Electronic-Components/10k-ECO-16mm-Logarithmic-Dual-Control-65-1460
https://www.rapidonline.com/Electronic-Components/10k-ECO-16mm-Linear-Dual-Control-65-1442

 

[Dragon Tamer]

The difference between the parts are that log and liniar scale the signal voltge differently. A log pot is better than a liniar pot for audio aplications. Log pots are more dificult to make than liniar pots, this may be a contributing factor to why the pots in your first 2 links are so different in price. The price difference may also be a factor of the company that makes the part. A more expensive part implies a higher quality part.

After looking at the data sheet, the one shown in the Farnell link (the one made by Vishay) has a longer mechanical and electrical life than the one shown in the RS link. It's mechanical life is 25,000 cycles minimum vs. 15,000 cycles for the linear pot. IMO you are getting more for your money with the log pot.

 

[audioguru]

The inexpensive volume control from RS (it is a log audio volume control) is not bolted to the chassis like the expensive one from Farnell so it might easily break.
It will be fine if you can secure it from moving back and forth.

 

[danrogers]

Hi Guru, hope you are keeping well!

Thanks for the help everyone, I'm going to order the bits later. I'm not thinking of adding a LM317 and a small LED bar graph. Is this likely to interfere or reduce the quality of the signal at all?

Thanks again

 

[audioguru]

I'm not thinking of adding a LM317 and a small LED bar graph. Is this likely to interfere or reduce the quality of the signal at all?

An LM317 DC voltage regulator was not talked about in this thread. What would it power?
Two LM3915 LED bar graph drivers will not have any effect on the audio.

 

[danrogers]

Sorry, you know me better than I do, I meant LM3915's

Is there anything I would need to do or set to make them work with a line signal voltage? Also from what I can see ref adjust is the way you fine tune the display?

Thanks for the help as always

 

[Dragon Tamer]

Ref adjust is used to by the internal comparators to get a refference on what voltage they should trip at. You will need a peak detector for your circuit though otherwise you will just have a bunch of LEDs flashing too fast to see.