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Old 8th February 2004, 11:17 PM   #1
John10 is offline John10  United States
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Default DSelf Precision Preamp Balance Control

I've recently completed the DSelf Precision Preamp and it sounds great except for one odd problem. The balance control is almost totally ineffectual. If I turn the balance pot all the way to the right the left channel is only slightly attenuated and it seems as if the right channel becomes slightly louder. I get the same effect in reverse by turning the pot to the left. I've double checked all of the component values and placement and the only modification I made to the original circuit was to use a rotary switch for input select instead of individual push buttons. Could it be that I'm using the wrong pot (10k lin taper) for balance? It's what the schematics specify. Has anyone else encountered this problem? I noticed that other people in this forum have built this project sucsessfully so any input would be greatly appreciated-Thanks!
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Old 9th February 2004, 08:22 PM   #2
Steven is offline Steven  Netherlands
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I suppose you're talking about the Precision preamp in the 1996 version. DS published a previous one in 1983.
Making some quick calculations it seems that the gain of the opamp section doing the balance control is as follows:
Wiper in min position: Gain = 0dB
Wiper in center: Gain = 1.13dB
Wiper in max position: Gain = 5.62dB
You're right that attenuation is hardly noticable with 1.13dB and gain is just noticable with 4.5dB.
You'd better choose youre listening position carefully. If you need more than 6dB difference between two channels, you're in a position where the stereo image is poor anyway.

Steven

PS If you need a slightly bigger range, change R34 from 1k to 3k3. Then you get the following values:
Wiper in min position: Gain = 0dB
Wiper in center: Gain = 1.63dB
Wiper in max position: Gain = 10.83dB
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Old 9th February 2004, 11:57 PM   #3
John10 is offline John10  United States
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Default Re: Precision Preamp

Thankyou Steven for your comments. I was mostly concerned that I had made some sort of construction error. Maybe I'll experiment with different values for R34 as you suggested. Honestly, I never did understand the need for such extreme balance control as you would see in most 'conventional' preamp designs other than this was the easiest way to do it. Thanks again-John
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Old 10th February 2004, 05:56 AM   #4
Steven is offline Steven  Netherlands
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Hi John,

I noticed this was your first (and second) post. So, welcome to this forum. I hope you will enjoy it, as I do.
If you have time, please post some pictures of your new preamp. Because you indicated as hobby electronics and woodworking, I am very curious to see how it looks like.

Steven
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Old 10th February 2004, 04:07 PM   #5
John10 is offline John10  United States
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Hi Steven, I'm actually still in the process of completing the casework for the preamp, but I will definitely post some pitures when I have time. Your guess is right. I did combine those two hobbies. Partly because I love working with wood and partly because finding a decent chassis is so difficult. There used to be an excellent source of chassis's in California called Spire Audio, but they went out of bussiness. Wood is a great material to use as far as making enclosures for audio projects, but it does present some challenges. I once made an enclosure for a phono stage out of solid maple wood in which one night the bottom cracked so suddenly and violently that it sounded like a gun shot! I couldn't find the cat for two days. Another problem is that wood offers no EMI shielding. Not such an issue with amplifiers but certainly one with very high gain projects such as preamps. To deal with this in my current preamp, I mounted everything inside a plain metal box and then placed that inside a walnut enclosure. The bottom and top are cabinet grade birch plywood that I veneered with walnut. This makes the box much more stable. The selector , balance and volume knobs will be walnut and the four equalization knobs Brazilian rosewood. I'll post a piture when it's completed.-John
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