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Passive preamp resistive design? [message #7700] Wed, 16 June 2004 09:08 Go to next message
akhilesh is currently offline  akhilesh
Messages: 1275
Registered: May 2009
Illuminati (3rd Degree)
HI everyone,
Quick question. If i am using a pot with a 1 Kohm resistor in parallel to it, my amp sounds alot smoother than if I increase the parallel resistor to say 2 Kohms...it then sounds brighter.
My source output imp is 600 ohms, the pwer amp input is 150 Kphms. My pot is 1 Kohm linear, with a series resistor of say 10 Kohms.
My question:
Am i killing some portion of the frequency spectrum with a low parallel resistor to the pot?
-akhilesh

Re: Passive preamp resistive design? [message #7701 is a reply to message #7700] Wed, 16 June 2004 09:52 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)
Hi Akhilesh,

You could be loading the preamp output so that frequency response is changed. But it also could be that you are simply limiting the drive signal to the main amp, possibly preventing it from being overdriven.

If you want, we can put the scope and generator on it next week and find out which is the case.

Wayne

Re: Passive preamp resistive design? [message #7702 is a reply to message #7701] Wed, 16 June 2004 10:30 Go to previous messageGo to next message
akhilesh is currently offline  akhilesh
Messages: 1275
Registered: May 2009
Illuminati (3rd Degree)
HI Wayne,
Thanx for your repsonse. THe impedance of the passive preamp is going to be less than 1 kohm...say around 700-800 ohms. I wonder if reducing the impedance to around that of the CD source (or maybe even a little less) causes anything. It actually sounds better though (more bass)! Clarity is there, without any harshness. If i increase the impedance, it sounds a little harsh...(more digital i guess)...hmm maybe i just like rolled off...
putting the scope &generator on would be excellent...really appreciat eyour getting it if you can
-akhilesh

Re: Passive preamp resistive design? [message #7703 is a reply to message #7702] Wed, 16 June 2004 11:46 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18670
Registered: January 2001
Illuminati (33rd Degree)
If the load impedance is less than output impedance, it does generally cause problems. It can cause distortion or frequency response aberrations. The most common symptoms are distortion and loss of bass. But the output impedance of most devices like this is 600 ohms or less, so you're probably just fine.

You can match impedance between the output stage of one amp to the input stage of the next amp to obtain maximum power transfer between stages. Or you can connect a low output impedance of an amp to a high input impedance of the next stage to obtain maximum voltage transfer. Both are acceptable methods, and each will work just fine depending on what you are trying to do. The one thing you don't want to do is to connect an amp with high source impedance to a low impedance load.

If the output device characteristics aren't known, it can be checked with a test load resistor using a scope to see how much voltage drop results. Using a test signal at bandwidth extremes, one can also find out where is rolls off and if this frequency changes with different load resistances. A CD player, tape player or turntable require test disk, tape or record to provide the signal. But for a preamp or main amp, a signal generator can be used instead.

Re: Passive preamp resistive design? [message #7704 is a reply to message #7703] Wed, 16 June 2004 14:07 Go to previous message
akhilesh is currently offline  akhilesh
Messages: 1275
Registered: May 2009
Illuminati (3rd Degree)
Thanx Wayne. Very helpful indeed. I guess seeing the input and output signals visibly isone way to see if ther is distortion. I must admit the 1 k ohm in paralle (meaning close to 600 ohm in total) sounds good to me...but it's a loss in treble i think.
-akhileshb

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