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Swampwalker, yes, whoever wrote that is mistaken. Reducing the bias may actually get you a couple more watts, rather than less, as the B+ voltage may rise with the reduced quiescent current. But you would also have more distortion...
Csontos, It sounds like what you are looking for is more than just the correct bias setting. Bias is only one of literally a thousand different variables that can affect your stereo. Fortunately we don't have to deal with most of those variables :) |
Thank you, Ralph. 'Sweet spot' is the operative here but the one I was hoping you would elaborate on is that elusive one when nothing is wanting but never stays very long. The one that keeps Geoff in business, and all the other exotic wire and cable and 'you name it' tweak manufacturers. To be specific, perfect channel balance. Finding a way to maintain it is really the only tweak we need. |
Thx, Ralph. Again, really frightening that the Bias Measurement and Adjustment Instructions include the following statement (and I quote) "The voltage is 0.20V. Low voltage settings cause output power down by well for the tubes life". I don't mind the translation issue but from what I've read above, it sounds like the information is incorrect. Or perhaps it's technically true but not relevant due to it having only a negligible impact within the "typical" range of underbiasing??? |
No worries- if the bias is set correctly the amp *should* have the lowest distortion for the class of operation it is designed for.
For example if an amplifier designed for class A is overbiased or underbiased, the distortion will be higher. There is a sweet spot. If overbiased, that amp may also be in danger of overheating.
Class AB amps are usually set up along similar lines, but with more emphasis on heat. For example in a transistor amp you want the output devices to conduct a little, so there are no switching artifacts (crossover/notch distortions). But OTOH you don't want them running particularly warm at idle either- else you will need a very large heatsink. At low volumes the driver transistors may actually be doing most of the work.
In a tube amp, the actual operating point might be a little less critical although you have to make sure that the dissipation in the output tubes is not exceeded as in a class AB amplifier the B+ voltages tend to run higher. So the bias has to be such that with the higher B+ voltages the dissipation of the tubes has a healthy margin. In addition, you have to have some concern about the balance of the tubes, as any mismatch in DC current for the power tubes through the output transformer can cause saturation distortion in the output transformer. So you have to be able to balance the tubes.
Not all tube amps have a provision for that- if not they are expecting that the power tubes be matched.
With respect to gain- bias will not affect that in the amp until you reach some sort of extreme such as cutoff in the tube or transistor. IOW, the output device has a certain amount of gain in the circuit that is dependent on its internal structure and the load impedance (also affected by any emitter/cathode degeneration that may be present). That is to say that with a certain voltage swing at the input you can reasonably expect a certain amount of output from the circuit, regardless of bias.
Now as the bias approaches some extreme like cutoff, you will begin to experience distortion. If this becomes significant then you may experience something that seems like a change in gain but it is by no means the same thing. |
I would love to hear Atmasphere's technical and practical explanation on how bias affects an amp sonically, ie: when both channels are the same vs. not. Forgive me for taking you to task Ralph, but I gotta take the chance when I see it. How else am I going to learn:) |
Atmasphere- well, then it's pretty scary that the manufacturer or distributor put a comment in the biasing instructions that said it would. I guess you can't always believe everything you read!!! Another illusion shattered. |
Thanks for the comments Swampwalker. FWIW, adjusting the bias not only does not affect gain, it doesn't affect the output power of the amp either. |
Why not just get yourself a Lightspeed with separate volume controls? |
Someone also recommended Scott Endler's attenuators when I posted a question about a DIY balance control in a thread on Audio Circle. Just need one on the channel that is to loud for proper balance. Waiting to hear back on how small the dB reduction steps can be on the Endler attenuators. |
Don c55- I had not thought of that. I do have 10 dB Rothwell attenuators. I'll try that but I believe the difference is not that great. |
You can buy attenuating plugs that are put in between the interconnect and preamp to change the volume of one channel.
Google "RCA attenuating plug". |
Can't take credit, myself. Had a friend over who's a sound engineer and we were monkeying with speaker placement. Him sitting in the hot seat, he's got me moving the speakers backwards and forwards by tiny increments. "So, what exactly is this all about?" earned me the equivalent of "why, balancing the image, what the hell did you think?" If your room ain't symmetrical, why would you expect your speakers to be? The simplest solutions are often best. Cheers. |
Mezmo- Great idea. It works!!! |
Or you could use an outboard digital EQ ie an Ultracurve and use it as a balance control. I have hearing loss in my left ear, and though I still cannot achieve an absolutely perfect center image like before, it is close with this set up. I have found that the Ultracurve is 100% transparent and quiet in my system. |
Practically speaking, kiss channel balance goodbye if you want to use bias as a 'balance' control as you've described it. By rendering channels 'unequal' in regards to signal level, all you will accomplish is the destruction of the stereo image, ie: point source imaging, depth, extension, and low level resolution. You will have the exact same issue you started with, only worse, much worse. A balance control is an attenuator, not a signal reduction. |
I'll tell you what Dennis Had told me: "Move your chair", but that is easier said than done, especially if you don't have a dedicated listening room. |
I've found that the best way to do accomplish this without a balance control is to move the speakers. You can very accurately, and in whatever incriment you desire, shift the balance just by moving the desired channel either closer or further away. Image is a little shifted right, just move the right speaker 1/2 to an inch further way from you. Try it. You might be surprised how well it works. |
I agree with the Almarg....
Depending on how "desperate" you are I see four possibilities.
1) Try Rrog's ideal of changing your room. (Probably the hardest to do if a relationship is involved).
2) Add a preamp with balance control. (Probably the hardest on the finances).
3) Add a voltage divider network just before the volume control on the channel that you wish to lower. (Probably the most time consuming).
4) Have your integrated modified to have either left and right volume controls or adding a balance pot. (Definitely the most trouble to restore to original operation). |
Al- Thx, after thinking about it more, I began to believe that the approach, which would limit the power output, would not have much effect at typical listening volumes, but I do not have the technical background to be sure.
Rrog- If my integrated amp HAD a balance control, I surely would use it. Re-arranging the furniture is not a viable option, it's OUR living room and my wife has compromised enough by having one end of it full of my gear.
Atmasphere- Well, that pretty much settles it ;-) Missed seeing you in NY this past weekend. I imagine exhibiting in NY must be very $$, but the show could have used another good tube set-up. Too many uber-detailed, unmusical, megabuck ss rigs.
All- Sometimes, thinking outside the box is thinking outside reality. Case closed, bad idea. Thx for your help. |
Bias has no effect on gain until the tubes begin to approach cutoff.
Reducing the bias will likely increase distortion, assuming that the amplifier design is optimized.
So the simple answer is 'no, this will not work.' |
If you have to resort to biasing two channels or two amplifiers differently in an effort to achieve balance, where is the advantage of not having a balance control? It would seem the balance control would be much less intrusive. The other option is a preamp with a left and right volume control.
I think the bias idea is a bad idea. I would try balancing the room by rearranging furniture, window treatment, room treatment or whatever. |
Hi Michael,
I'm not 100% certain, but I don't think that approach would accomplish what you want. I believe that gain, which is what you are trying to change, will vary very slowly as bias is adjusted. If you want to unequalize the gains by say 2 or 3 db or more, I suspect that you would have to change the bias setting by much more than the maximum recommended reduction of 10%, even if you were to raise the setting in the other channel by a reasonable amount. That would most likely result in the sonic quality of the amp, as well as its maximum power capability, being considerably degraded.
Best regards,
-- Al |
All- sorry about the misspelled subject.
Zd542- I thought of that but desperate times call for desperate measures ;-) Also I was thinking about only going from 0.20v nominal to 0.19v. So far the bias seems rock steady so I might go to a more sensitive scale on my VOM and try going up 0.05v on one and down 0.05v on the other. I realize that the differential is the same but...who knows. I readily admit my technical ignorance so i thought i'd throw this out there any see if anyone has ever tried it. |
Doing that will probably make the 2 channels sound different from each other. I have no idea if you would be able to hear the difference if that's the case. But its something to consider. |
Very interesting idea. I am also trying to deal with an asymmetrical room and a preamp without balance control... |
