TNT-200, adjusting the bias and DC offset

Read more on the Acoustat TNT-200 in our home audio section

Lou2004-02-27 17:20

I have a TNP and a TNT 200 . I would very much appreciate any information you can give me about adjusting the bias and DC offset on these amps. I would think the DC offset would be measured at the output, though I have also heard something about adjusting it for --18 volts at some point? Thanks! Best Regards, Lou.


Andy Szabo2004-02-27 17:20

Setting the bias and DC offset for the TNT-200 is relatively straightforward. Each channel\'s PC board has two potentiometers, labeled, if memory serves correctly, ”DC Offset”, and ”Bias”. If they are not labeled, you can identify them by value: the DC offset pot if 100K-ohms, and the bias pot is 200-ohms.

For both adjustments, disconnect both inputs and outputs on the amplifier. Adjust the bias first. Remove one (either one) of the rail fuses (5-amp) for the channel to be adjusted, and replace it with a DC ammeter capable of measuring at least 300-mA. Turn on the amplifier, and allow it to stabilize for a few minutes. If you have the means to adjust to the mains voltage, adjust it to 120-volts AC (or whatever the amp is wired for). Adjust the Bias pot for a reading of 300-mA. Repeat this procedure for the other channel.

For the offset adjustment, connect a DC voltmeter across the output of the amplifier, using a meter scale capable of reading down to a few millivolts. Turn on the amplifier, and allow it to stabilize for a few minutes. Adjust the DC Offset pot for a reading of zero millivolts. Repeat this procedure for the other channel.

I am not familiar with any other methods of setting the DC offset. The method described above was the standard procedure used by Acoustat. For the smaller TNT-120, these procedures are the same, except that the bias should be set to only 200-mA.

If you are not able to obtain either of the desired values, then there is something wrong with the amplifier.

Tom Schmidt2004-02-27 17:22

Tom Schmidt provided the following answer (September 2003):

I would add the following to Andy Szabo\'s excellent instructions for adjusting the Acoustat TNT-200:

From the Twin-200 owner\'s manual:

\"Acoustat does not recommend increasing the bias above the factory setting of 300ma. This will cause the amplifier to run hotter than normal, with no sonic improvement.\"

Tom Schmidt

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