Bias troubles.

Nixxuz

Super Member
TO EDIT: Apparently I'm not using the correct terminology. It's a balance and gain issue.

How is biamping in regards to imaging and bias? I'm asking because I got a monster of a pro audio amp that's EXTREMELY loud. I'm still messing with all the different configs because I have 4 different volume levels to deal with already. The configuration is as follows below with a pair of Jamo C-109 towers for my L/R.

  1. Source- a tablet running Android with PowerAmp. This has a handy touchscreen volume wheel and EQ that I can adjust from my listening position. It's also where I store all my digital music and it runs out to a DAC X-6. I tried a W10 laptop, but for some reason every program wouldn't let me adjust the volume.

  2. Little Dot Mk2 as a preamp. This is wanted, but almost needed, because the power amp is pro audio and I kind of have to boost the signal anyway, due to the higher output of most pro sources.

  3. QSC ISA280 power amp. This guy has TONS of volume to offer, and can easily take 2-8 ohm loads in stereo. My inclination was to crank both channels all the way up and handle everything from the Little Dot and the tablet...but the sheer output is EXTREMELY touchy. Like a tiny increment on the volume from either device can cause it to go from 65db to 90db VERY FAST. So I have to set the volume lower than max. This poses a bit of a problem because I don't think the pots on this unit are quite level. One channel almost always seems slightly louder than the other, which becomes a huge pain when you are looking for that spot on imaging that the Jamos are capable of. Instead, it ends up sounding like 2 speakers projecting sound instead of 1 large stage with individual players. I've tested this over multiple songs that I am very familiar with and can't quite peg it to where it's transparent.
Then, on top of everything else, I'm running dual subs that are affected by each having it's own volume and crossover. This isn't my primary concern as the 109s are nearly capable down to the same rang as the subs themselves. The SUB-1500s start to roll off around 28hz, and I feel the Jamos are currently rolling off around maybe 40hz. But it's hard to tell in that lower range. The subs signal is from the Little Dot, but fairly independent of the 109s as they are hooked to the QSC for wattage.

So one of my thoughts is to run my previous power amp, a Dayton APA-150, as a semi isolated unit at full bore to the 109s HF posts. That way I at least KNOW that the volume will be consistent across the HF range for imaging. The QSC might have a db or 2 of variation across the LF, but seeing as I don't know the crossover range for the 109s, I'm not really sure how it will turn out.

Last option would be to just get another APA-150, (I sold the 2nd one I had when I got the QSC) and monoblock the 109s. Then I'm pretty much rid of any biasing issues that I'm getting now with the QSC. I have no real idea what to do with the QSC though...I suppose I could monoblock it for a DIY Ultimax build or something.

Any thoughts?
 
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I'm not sure I make the title connection to the question and I'm not sure I understand the question
 
I'm having problems adjusting the various volume controls on my system so as to not have my left and right speakers outputting as the exact same db level. It has to do with volume bias. It's not about biasing tubes.
 
"Bias" typically refers to a DC voltage applied to the base of a transistor or control grid of a tube to ensure that its operating point with no signal is within its linear operating range. Setting the bias correctly minimises distortion.

I too am not clear what's being asked in the original post.

The issues with the QSC (point #3) don't sound like biasing problems, they sound like gain problems. A line attenuator might resolve the problem.
 
FWIW, the input sensitivity of the QSC amp is not anything unusual, only 1.15V required for full output. Probably why it seems to get loud fast, relatively speaking.

As far as checking/setting channel balance, I would disconnect the speakers then use a Y cable from one channel of the source to input a 1kHz test tone to both channels of the amp. The Y cable ensures both channels receive the same signal. Then, measure the voltage output at the channel 1 and channel 2 speaker terminals with a multimeter or an oscilloscope.

If you don't have a Y cable you can test just one channel at a time, but do not change any settings between tests and use the same one channel from the source to check both channels of the amp. Using the same channel from the source precludes an imbalance in the source causing the appearance of imbalance in the amp.

For best signal to noise ratio its usually better to output a hotter signal sooner in the chain with less gain downstream.

I would set the source to 100%, put the little Little Dot volume about 1/2 to 2/3, then adjust the gain/levels on the QSC to whatever setting makes it about as loud as you'd ever go. From then on just control the volume with the Little Dot.

Understand the convenience of using the on-screen volume adjustment and that's fine as long as you don't hear any degredation doing it that way. Adjusting volume digitally usually causes a loss of dynamic range, worse it gets the lower you set the volume that way.
 
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FWIW, the input sensitivity of the QSC amp is not anything unusual, only 1.15V required for full output.

As far as checking/setting channel balance, I would disconnect the speakers then use a Y cable to input a 1kHz test tone to both channels of the amp. The Y cable ensures both channels receive the same signal. Then, measure the voltage output at the speaker terminals with a multimeter or an oscilloscope.
I think I've got it mostly sorted at this point. I'm still pretty interested in running my APA-150 for the HF anyway. I've never bi-amped before so it's something fun to try. And the APA-150 has a line in and a line out so I can easily run source to preamp to Dayton to QSC with little hassle.
 
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