L-pad attenuator on preamp output - diyAudio
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Old 30th July 2014, 05:31 AM   #1
Hikari1 is offline Hikari1  United States
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Default L-pad attenuator on preamp output

I am having some balance issues related to my unusually shaped listening room. Speakers are about 8 feet apart, listening position is 7 feet from speakers. The "center" image (voices) are about 3 feet off center to the left. The room is unusual as the ceiling slopes from about 25 feet high on the left to 10 feet high on the right. I believe this is causing the imbalance.

I've checked my preamp (Pass BA-3) with a 1kz test tone. AC volts are even on both channels. I've checked the amp (Pass f5), even both channels. I've swapped speakers...same thing. Pretty much ruled out equipment as the issue.

So I hook up an L-pad to the preamp out on the channel that is louder...used a 3.1K resistor in series and a 10K resistor to ground. It worked. The image is centered...I believe it is giving me about a 6db pad.

But now I am worried about output impedance. The Pass f5 has a 100K input impedance...I believe my L-pad arrangement is giving me about a 2K out impedance on the preamp. Not good I think. Also, I notice that the bass isn't exactly centered with the vocals. This could still be a room-relaed issue.

Anyways, this is driving me crazy. Anyone suggest a different or better solution? Would lower value resistors in the L-pad be a better choice? This would give me a lower output impedance. Is it better to reduce db with a pad after the volume pot but before the preamp? I need a 6-8db reduction I think.

Any ideas are welcome. And thanks.
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Old 30th July 2014, 09:02 AM   #2
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To check for imbalance i use a mono signal feeding both channels . This should give a well defined central image at all frequencies , it also shows up phase reversal ie ...if one pair of speaker wires has been reversed .

Last edited by epicyclic; 30th July 2014 at 09:07 AM.
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Old 30th July 2014, 10:12 AM   #3
DF96 is offline DF96  England
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Resistors in the K or 10's of K region should be fine. Go too low and you place too much of a load on the preamp. The attenuator should preferably be at the power amp input end of the cable, as that means it doesn't have to drive any cable capacitance. If your interconnects are quite short then don't worry; just put it where convenient.
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Old 30th July 2014, 03:19 PM   #4
Hikari1 is offline Hikari1  United States
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Quote:
Originally Posted by DF96 View Post
Resistors in the K or 10's of K region should be fine. Go too low and you place too much of a load on the preamp. The attenuator should preferably be at the power amp input end of the cable, as that means it doesn't have to drive any cable capacitance. If your interconnects are quite short then don't worry; just put it where convenient.
Pad at the amp input is a good idea. Thanks. Interconnects are short...about .5 meter.

How about a pad after the volume pot but before the preamp? Would that be better or worse? Then it would not effect the preamp output impedance, right? The pot is a 25k stepped attenuator.
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Old 30th July 2014, 03:46 PM   #5
rayma is online now rayma  United States
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Quote:
Originally Posted by Hikari1 View Post
The "center" image (voices) are about 3 feet off center to the left. The room is unusual as the ceiling slopes from about 25 feet high on the left to 10 feet high on the right. I believe this is causing the imbalance.
This would be unusual. I agree that you should feed a mono signal into the system for testing. Use a signal that is CERTAIN to be identical in both channels, like a Y connector off of one channel of a CD, tape, or FM tuner.

Then, using this mono input, see if the balance changes with position of the volume control, it could be defective. If not, swap the preamp outputs to see if the balance shifts.

If not, then swap the speaker wires at the back of the amplifier. If the balance does not change, swap speaker wires at the back of the speakers. If not, swap speakers (you said you already did). If you cannot shift the balance after all this, it MUST be acoustical. Are you sitting near a wall?

Last edited by rayma; 30th July 2014 at 03:57 PM. Reason: change
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Old 30th July 2014, 04:12 PM   #6
Hikari1 is offline Hikari1  United States
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Originally Posted by rayma View Post
This would be unusual. I agree that you should feed a mono signal into the system for testing. Use a signal that is CERTAIN to be identical in both channels, like a Y connector off of one channel of a CD, tape, or FM tuner.

Then, using this mono input, see if the balance changes with position of the volume control, it could be defective. If not, swap the preamp outputs to see if the balance shifts.

If not, then swap the speaker wires at the back of the amplifier. If the balance does not change, swap speaker wires at the back of the speakers. If not, swap speakers (you said you already did). If you cannot shift the balance after all this, it MUST be acoustical. Are you sitting near a wall?
I have swapped cables and speakers... even fed a 1kz mono test tone and the imbalance is constant, always same side. Also, i have measured the voltage on the outs of both the amp and preamp....it is the same on both channels when fed a test tone. It has to be the room. It is a strange shape...ceiling height is different from right to left and one side is open.
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Old 30th July 2014, 04:25 PM   #7
rayma is online now rayma  United States
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Quote:
Originally Posted by Hikari1 View Post
It has to be the room. It is a strange shape...ceiling height is different from right to left and one side is open.
Sorry that you are having such an unusual problem. Try this: have someone hold a small wool area rug or similar absorbent material behind your head while in the listening position. If there's still an imbalance, it's not a reflection from behind or nearby.
I've seen this happen before in the bass, but not in the higher frequencies.
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Old 30th July 2014, 04:35 PM   #8
DF96 is offline DF96  England
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A pad attached to the output of a volume pot will modify the law of the pot so you will get a variable stereo balance as you change volume. The pad should go at a low impedance point, so preamp output is a good place (assuming a competent preamp design).
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Old 30th July 2014, 05:16 PM   #9
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Behind the partition on the left of photo is that perhaps a reverberant room ie with little furnishing .How far is it from the right hand speaker to a wall on the right .
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Old 30th July 2014, 05:58 PM   #10
Hikari1 is offline Hikari1  United States
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Yes, behind that partition is the kitchen. The right speaker is about 2m from the side wall, the left is a little farther...maybe 3m.

It's the left side (when looking at the speakers from listening position) that is louder. I believe the space behind the half wall on the left and the corner of the room to the left is creating this. I just experimented placing a small mattress around the room. It was interesting.

When the mattress was placed in the left corner the balance got better. When placed on the right side it got worse. When behind I did not notice much effect. Even with the mattress in the left corner the sound was still not perfectly balanced though.

Sorry about the messy room, I have small children. Maybe I can make them stand in the corner as room treatment.

My wife is not going to like a mattress in the living room. And my 1 year old will have no where to sleep. I better look at some sound panels.

Thanks for the help guys.
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