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Denon pro amplifer balance problem...??? How to balance?
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666666
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#1
15th April 2007
Old 15th April 2007
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Denon pro amplifer balance problem...??? How to balance?

I have a new Denon PMA-2000IVR audiophile amplifier.

In sum, I sense that the left side channel is hotter than the right. I ran a 1kHz test tone through the amp and measured voltage at the speaker outputs... the left side is indeed hotter.

I'm not a tech so please bare with me.....

I popped the lid on the unit and have found what appears to be some sort of adjuster pot on each of the unit's two main circuit boards ("L-ch" and "R-ch").

Might these be the amp's output trim pots? Should I try adjusting one to see if I can get the unit in balance? Or might this be something that I should NOT touch?

I sure as heck do not want to ship this thing back to Denon and have to wait 3 months for it to be fixed if I am able to fix it here in a matter of minutes. Though I am not highly experienced with electronics, I'm competent enough to not electrocute myself or make a big mess.

The pics show the location of the two adjusters, and also focus in on the left side adjuster (which is identical to the right side adjuster). To see the pics, please click below link:

How to tell if studio amplifier stereo channels are balanced?

Thanks for the help!
#2
15th April 2007
Old 15th April 2007
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MARTIN SOUND is offline
THESE PRE-SET POTS MAY VERY WELL BE TO ADJUST THE DC OFFSET OR TRIM THE QUIESCENT BIAS CURRENT SO BE VERY CAREFULL . CONSULT THE CIRCUIT DIAGRAM OR FOLLOW THE PCB TRACKS . IF THE POT IS TO ADJUST DC OFFSET IT WILL BE ASSOCIATED WITH THE FRONT END DIFF AMP CONFIGURATION , IF IT IS TO ADJUST BIAS CURRENT IT WILL BE ASSOCIATED WITH THE VAS AROUND THE BASES OF THE DRIVE TRANSISTORS . IT IS VERY UNLIKLY TO BE AN INPUT LEVEL TRIM . GOOD LUCK .
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16th April 2007
Old 16th April 2007
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Thanks! At this point I do not plan on touching the unit because I am simply not qualified. I was originally hoping there may have been a quick fix, but I can see now that there isn't.

But, for the sake of understanding this problem a little better, here's a little more info:

Note: this is an integrated "audiophile" type amp. It has one big volume knob on the front that controls both stereo channels. It also has a section with tone and balance controls... BUT, this tone / balance section can be switched OUT of the path for a pure direct amp signal... and this is how I like to use it... sounds best this way.

I just ran the 1kHz test tone into the amp again and measured the voltage at the speaker outputs at various settings on the main volume knob of the amp (tone and balance controls bypassed).

With the volume knob set at anywhere from about 5% to 35% of its total travel up from "zero", the difference in voltage between the left and right channels is close to 20%, the left side always being hotter. For example, with the volume knob at about 25% up from "zero", the left channel is putting out 0.980 volts and the right, only 0.800 volts.

As the volume knob gets closer to the 50% mark... between about 35% and 45%, the difference in voltage between the two channels gets smaller, down to around 5% difference.... left side still always the hotter side.

When the volume knob is at exactly 50% of its travel and above, the voltage difference gets even smaller, in some cases the right channel actually gets a tad hotter than the left, and the differences in voltage between the two channels remains pretty small, in terms of percentage, all the way up to full blast where each channel is putting out almost 50 volts with less than a 1 volt difference between them.

When I use the amp, whether using monitor speakers (Dynaudio BM15s) or headphones (assorted pro headphones) , the volume knob is always between about 10% and 40% of its full travel, never higher. Any higher would be "too loud" and would damage the speakers. So, I am always using the amp in the range where the imbalance is greatest. (The amp is rated at 80W + 80W @ 8ohm, and 160W + 160W @ 4ohm).

I can assure you that the imbalance is very audible, at least to the audio professional... I certainly cannot tolerate this on a brand new expensive amp.

So... based on this info... any more theories as to what the problem may be? Any other thoughts, comments?

Thanks.
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16th April 2007
Old 16th April 2007
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1. Are distortions the same on a low level?
2. Are they equally hot when idle?
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16th April 2007
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Quote:
Originally Posted by Wavebourn View Post
1. Are distortions the same on a low level?
2. Are they equally hot when idle?
I do not know how to measure distortion, but per ear, both channels sound fine (no audible distortion or problems) other than the balance issue.

Equally hot when idle? I have not checked. I assume you mean is the voltage output the same at say 25% volume when no music or tones are being played through the amp. Is this correct? I will check it.
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16th April 2007
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Quote:
Originally Posted by 666666 View Post
I do not know how to measure distortion, but per ear, both channels sound fine (no audible distortion or problems) other than the balance issue.

Equally hot when idle? I have not checked. I assume you mean is the voltage output the same at say 25% volume when no music or tones are being played through the amp. Is this correct? I will check it.
You said, the left side is indeed hotter. Is it hotter with a signal only, or without it as well?

Also, can you connect your scope to volume regulator pot and see if voltages are the same on it's halves?
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16th April 2007
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Hi
Presuming that your definition of 'hotter' means louder (from your measurements as well) then try it with the level pot at max (reduce the input suitably).
I suspect the output voltages will be close to identical. I think it is simply that you gave a lousy volume pot fitted. Log pots are notorious for not 'tracking' at less than say 40% scale and a tolerance of 3 or 4dB is common for 'cheap' units in this area of rotation. An expensive amp should have a decent pot fitted so it may well be out of tolerance so should be referred to the manufacturer.
You say the levels are more matched at higher physical settings which is where the pot would 'like' to be used. Fitting an attenuator to allow 'normal' listening at about 60 - 70 % rotation is usual and sensible.
I would expect the amp itself to be fine. As mentioned above, the pots would be unlikely to affect GAIN so leave them alone.
Matt S
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16th April 2007
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Quote:
Originally Posted by Matt Syson View Post
...Fitting an attenuator to allow 'normal' listening at about 60 - 70 % rotation is usual and sensible...Matt S
Thanks Matt.

I was thinking this too... that if for some reason the manufacturer is unwilling to replace the volume pot with one that tracks better, maybe I could just add a fixed attentuator somewhere in the path so that the volume pot would be running in its 50% - 80% zone (where it acts more balanced).

If I did want to add an attentuator, where would be a good place in the path to add it? Since this is an integrated amp with many individual switchable inputs, it would not be practical to add attenuators at the inputs themselves. However, it would be easy to add attentuators at the speaker outputs... if this makes any sense. (Again, please bare with me, I am not a tech.) Could I simply add some resistors between the speaker outputs and speakers to achieve the goal of having the volume pot run at 50% or greater without any other problems or downsides? Would this degrade audio quality? Or?

I'd like to explore the attentuator option a bit more just in case.

Thanks
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17th April 2007
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When you said "hotter" I started thinking of a temperature so of a quiescent current or oscillations.

I'd select a better potentiometer because an attenuator in one channel equalizing gain in one point don't guarantee it will be equal at the rest of the scale. At the maximal gain it will be definitely out of scale if both channels are good and properly tuned. I connect them to a 12V laboratory power supply, put a pocket size multimeter between wipers, rotate and select which one shows less during rotation.

However, if you have to use the control in the beginning of rotation (counterclockwise) it indeed means too high input signal that has to be attenuated. You will need or stereo attenuator, or less sensitive input, or less gain in the signal source. Attenuating speaker output is not a good idea: modern speakers are designed to be driven by very small impedance, otherwise they will sound boomy.
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17th April 2007
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Hi
Basically it is 'unfit' for your purpose and as it is an expensive amp the manufacturer should sort it out.
If you want to try an attenuator it should be after the input switching and before the power amp part (an insert point in 'mixer' language). Many amps have pairs of phono sockets with a link, which could be removed and an attenuator inserted. Use something like 10K in place of the link and say 3K0 from the 'hot' of the power amp input socket to ground. 1% resistors will be fine as you are dealing in the imprecise world of potentiometers so better tolerance is completely wasted.
Talk to the shop where you got the thing and get them to loan another or replace yours.
NO attenuator in the speaker cabling.
Matt S
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