Bias adjustment at solid state Hi-Fi amplifier.

Just a few words of caution: In the net you can find various opinions regarding the effect that bias has on the sound of a solid state Hi-Fi amp. Some people suggest increasing it, some others say that they decrease it as much as they can (!!!). But everybody says that there are a lot of cases of blown up amps from people that they do not know what they are doing or thought that since a little bias increase sounds good, a lot of bias increase will be better. Please, if you do not know what you are doing do yourself a favor and avoid electrocuting yourself or blowing up your faithful amp. If you feel that you know what you are doing, you will need the service manual of your amp (you can get it from hifiengine.com), a good multimeter which includes mV range and the appropriate low resistance multimeter cables that will guarantee you an accurate and a safe measurement. Have the amp turned off when you connect and disconnect the multimeter's cables at the amp's bias measurement points. During bias measurement there should be no signal present at the input (so disconnect the interconnection cables or shut the signal completely with the input attenuators (if your amp has them) and there should be no load at the output (so disconnect your speakers or turn them off if there is a speaker on / off switch at your amp). Unless stated otherwise at the service manual, the amp has to be left turned on for some time in order to warm up and stabilize, so do not rush the procedure! Also, prefer to use a plastic screwdriver, this way if you touch something or drop by accident your tool in the amp, no drama will occur. Last but not least, DC offset is a different thing and has nothing to do with the bias – I have seen some confusion regarding this.

I (thought) that I pretty much knew some things about bias, but as always I did not had the complete picture. Instead of overanalyzing everything, let me share with you my story.

In the past I had searched and found the service manual of my Luxman power amps. I also had a positive opinion regarding a slight overbias, so I had adjusted my amps accordingly.

A John Curl post was about to change everything. My friend Panos who has in his collection of power amps a Prarasound A21, had found this: “If you want to change to A21 for the best, learn to adjust the output bias (pots) current and push it up until the heatsinks get too hot to touch, OR the overtemp protection kicks in. Of course, keep the heatsinks off the rug, and allow free air circulation. I have actually used 'Whisper' fans on occasion to minimize overheating. The smallest fan (hopefully quiet) can make a big difference. Still air is not a good heat conductor.” (source: http://www.diyaudio.com/forums/solid-state/175484-modding-parasound-halo-a21-2.html)

So the other day Panos was thinking to play a little with the bias of his amp. Now, even though I have no reason to disagree with what Mr. J.C. states above, mortals like us have also to consider the well being of our equipment. I mean, we can’t afford to have an F1 kind of equipment, that is a top performance machine that you use for one hour and then you have to service it for two hours. So, even though we are sure that Mr. J.C. is right, we pushed the bias to a point where the amp becomes comfortably warm and that in our case was at 27.5mV (it was set at 20mV from the factory).

Up to that point, I believed that bias has just a slight effect at the sound character and that you should increase it just a little, so I was skeptical regarding our actions on the poor A21. After all, the A21 sounded wonderful as it was. However, I proved to be wrong. Increasing the bias made the low end of A21 to sound better controlled.

Even though I felt that I had reasonably increased the bias at my beloved Luxman M-03B amps, I thought about experimenting a little more. After all, the amps remained so cold even if left turned on for hours.

The service manual suggests adjusting bias at 3.5mV, +/- 0.5mV. However, the service manual of a newer model (M-117) which had exactly the same main amp boards, suggests adjusting the bias at 4mV, +/-0.5mV.

When I had adjusted the bias in the past, my voltage selector switch on the amps was set to 220V and the bias was set around 4.5mV. But when I moved the voltage switch at the 240V position, the bias dropped a little, you can read more about this at this post. As you understand, before adjusting your bias make sure you have settled regarding the 220V / 240V voltage selector switch, if your equipment has one.

Luxman M-03 is designed to be also operated under bridged mode, something that I do not do since I prefer biamping. Also, it has 1.5 kg per channel heatsinks. These two facts mean that there is safe room for bias increase and that the manufacturer bias values are somehow conservative, in order not to stress the amp when used at the bridged mode.

Adjusting the bias at a Luxman M-03B power amp.

However, since I did not want to stress my amp I proceeded slowly, checking that my amps do not get hot.

First critical listening was done with the bias set at 5mV. Things became awkward. I always had a very solid, deep, well controlled base. The base now became even stronger. I always had a lot of detail and a big soundstage. Everything just boosted. Too much. Also, I always wished my sound was a little softer especially regarding the brass instruments. Now I wished that I was deaf, what sounded a little harsh before now sounded too much harsh. I invited Panos for a listening test, we both agreed on the facts above.

For a couple of days I was thinking what to do, go forward or backwards? Lucky for me, our much appreciated tech guru encouraged me with his wise words: Be careful about the temperature but until temperature becomes an issue, go forward. Go as much (safely) forward as you can, before going back.

Gradually and slowly increasing the bias to 8.5mV.

So I did. To make a long story short, I settled at 8.5mV bias. I could go further, but I prefer to stay here in order to keep my amps just slightly warm and not hot at all. After all, I was very happy; everything that was already good in the sound had become slightly better. The annoying harshness minimized, almost disappeared, without loosing details.

After double checking the acoustic results with Panos, I happily putted everything in it's place and started enjoying listening.

UPDATE: After living with this bias adjustment for a while, I have absolutely no doubt that the Luxman M-03 power amps have literally transformed, sounding better than ever. The harsh in their sound minimized, almost disappeared.

Chris