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Old 4th January 2004, 04:48 AM   #1
soneil is offline soneil  United States
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Default Passive Preamp (H.H. design) problem

I've built of version of a passive preamp described here by HarryHaller. It works fine, except for the fact that when I turn the volume up it starts to distort quite a bit on the louder passages in the music.

Instead of the 2 series 22k resistors which he used for each, I've used a single 10k one, and I've also got a double-gang 100k pot instead of 2 single gang ones, which I got from Radio Shack. I'm using pretty cheap copper wire, and nearly six inches of it, so I'm sure that isnt helping. If I jumper over the 10k resistors the problem is not nearly as bad, though still present.

If anyone has any ideas on what might be going on, I'd like to hear it. This is only my second project, the first being a CMOY headphone amp. I've actually got one of these sitting inside the box with the passive preamp, along with a few inputs and outputs it makes for a pretty neat package, provided it would work correctly....
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Old 4th January 2004, 01:09 PM   #2
sreten is offline sreten  United Kingdom
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diagram taken from planet10's post.

Can't see why it would distort at high volume settings,
unless you are driving a very low input impedance load,
as output impedance is only good with this arrangement
at low volume settings.

There are better arrangements of passive pre-amps for
driving lowish impedance loads, the original circuit :

http://sound.westhost.com/project01.htm

Ouput impedance is lower at near full volume settings.

sreten.
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Old 4th January 2004, 01:22 PM   #3
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Soneil

What makes you think the passive is responsible for the distortion? Such a simple device, provided it is correctly wired and does not load excessively the source, simply cannot distort. Unless your source is a high Rp valve stage 10k should be a reasonable load. Something seems very wrong. Have you tried wiring the pot in the normal way?
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Old 6th January 2004, 06:42 PM   #4
soneil is offline soneil  United States
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Ok, so upon looking at the circuit I built further, I realized that I had the wipers of the pot connected to the ground, and the legs connected to the outputs, instead of the other way around as is shown in the original design. That cleaned the sound up quite a bit, I'm not sure why really. Shouldn't the resistance be as, um, "clean" across the pot no matter which way you are running it?

Anyway, its still not as good as I would like it, and I plan on putting in the type of control shown on the link sreten gave. Thanks for that by the way.

As for input impedances and loads and stuff, I don't really know much about that. My input is my dvd player's audio out, and my output is going to my sterio reciever. I've looked around for reading material to learn about the sort of difficulties I might run into, to no avail. Any suggestions on books, or some more helpful links?

Thanks again!
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