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Messages - duclon

#16
Amplifier Discussion / help with issue in my amp
June 27, 2007, 08:32:25 AM
I've built a small 12v DC practice amp. Here is a simple issue, for someone other than me. i searched but did not find an answer. When i have nothing plugged into it, there is a loud hum, or buzz. when i plug in an instrument it goes down, not away. when I touch the instrument jack or the metal that all the pots and jacks are mounted to it pretty much goes away. So I know this has to be simple but this is my first amp and i am missing it.
Any help?
Mike
#17
thanks for the reply guys, i think I'm too worried about the impedance. As stated, it might just be unnecessary to go that far. It might be best to just use the best instrument available, my ears. if i hook up my instrument to my amp and like the sound then go with it.  :)
Mike
#18
Sorry if this has been discussed before but I searched and could not find a definitive answer.
Is there a way to accurately measure the input impedance of the amp I'm building.
I need 1.8 M ohm or a little more for the instrument I'm designing the amp for.
anyone know how using some basic test equipment, multi-meter, oscilloscope?

Mike
#19
Preamps and Effects / Re: Stereo Preamp Question
April 19, 2007, 12:25:27 PM
Thanks this is helpful. I think I can get it to work now.


Quote from: teemuk on April 19, 2007, 11:04:01 AM
You can do it with so many ways... For example, see how the outputs of each module are summed in this:
http://www.schematicheaven.com/voxamps/sspa50.pdf

I assume you have nearly identical scenario (two "channels" feeding their individual tonestacks - output signals from the stacks are summed together). In the example only simple "mixing resistors" were used but for better result I would separate each tone stack from the "mixing" circuit by using a buffer in between.

The simple configurations always have some problems like the one described in http://www.best-circuits.com/booa.htm (See the section about "inverting adder").

How you want to do this is entirely dependent on how complex you want to make the circuit.
#20
Preamps and Effects / Re: Stereo Preamp Question
April 19, 2007, 09:51:15 AM
Thanks, would I go from my inputs to my 2 tone stacks then to v1 ad v2?

Quote from: teemuk on April 19, 2007, 09:42:35 AM
You can, for example, try something like this as input or mixer:
http://www.art-sci.udel.edu/ghw/phys245/05S/classes/images/opamp-adder.gif
And configure it for gain of 1.
#21
Preamps and Effects / Re: Stereo Preamp Question
April 19, 2007, 09:46:56 AM
Well I just need the stereo to have the separate tone for each side of the instrument. One side is bass, one is melody, after I get by the tone I can use mono or it can be mixed. It will end up going through one speaker. And I guess one amp?


Quote from: syndromet on April 19, 2007, 09:39:06 AM
In order to have true stereo, you need two sepparate tonestacks and amps. If you just want to blend both signals before it hits the preamp, you need a buffer for the signals before you blend it. Sorry for the short answer, but I have don't have a lot of time right now. Let me know if you nedd a "fuller" answer.
#22
Preamps and Effects / Stereo Preamp Question
April 19, 2007, 09:10:12 AM
I've built a small amp based on the Ruby. I added a tone stack between the input and amp and it works well. what I need to do is have stereo inputs as I have a stereo instrument. Both need the tone stack. I tried just adding another input with a tone stack and it lost a lot of volume and didn't sound very good.  It hadd some weird issues when I adjusted one the other would drop in volume. that may have been a wireing error.
So can anyone advise me how to place the tonestacks.  and combine the 2 signals.
Do I need some boost on each signal.
Do I need a resister inline of each input signal beore the tone stacks.
The pickups I have a passive.
Mike