Threshold T200 biasing??

[Elan67]

Hi all,
in few days i will get a Threshold T200 in my lab for biasing (I think).
Owner says that the left heatsink is colder than the right one. After some time it sounds distorted too.
I was not able to find any T200 biasing procedure on the net...
Do you have any hints?

Thanks so much in advance.

[Zen Mod]

ask in Pass subforum

just in case that some of regulars aren't around

[Elan67]

I didn't not post it on Pass subforum since AFAIK T200 was not designed/developed by Nelson Pass, isn't it?

[Nelson Pass]

Yes, it was not.

That doesn't mean that you can't bias it. All you have to do is locate the
bias pot and measure the voltage drop across the output transistor power
resistors of the "good channel" and duplicate that on the other.

Adjust in small steps only and wait an hour to be assured of thermal stability.

The heat sinks can safely operate to about 50 deg C (10 second hand hold).

[Elan67]

Well I did it.
It was easy, just remove the back cover,de-solder XLRs and place back the input pcb. You will have a comfortable access to 4 trim pots (2 for each channel).
So you can easy adjust both dc offset and the bias current.
Following "His Majesty Nelson Pass" advices I have found that 50 deg C is equivalent to a bias current of 2A. i.e. you should measure around 20mV at the bias probe resistance terminals (0.01Ohms on the power supply pcb) for both channels.
More than 25mv will lead to heat protection in few hours.
I had to change a 1n5236b zener diode (on left channel power pcb) since it was out from specifications. Both channels needed bias and dc offset re-adjustments. Initial starting measurements were 1.2A for the "good" channel and .7A for the "bad" one...the amplifier was cold as a class D one!




Thanks so much!

[Nelson Pass]

Hooray!

[nigelwright7557]

Just because a heatsink is cold doesnt mean the bias is wrong.
Some amps will run cold with correct bias.
Peavey especially run with low bias on their amps.

I set my bias on my designs using sig gen and scope and just have enough bias to get rid of crossover distortion on the scope. Any more is a waste of power and generates unnecesary heat..

[Elan67]

Hi nigelwright7557,
In general I set the bias trying to minimize zero cross distortion too for class AB (and similar) amplifiers where it makes sense. Especially if I do not have any service manual.

T200 is claimed to be a 100w class A amplifier, if it sounds a bit distorted and it is too cold there is something wrong with bias.
At the beginning bias current was 1A and .6A for L and R channels ( -+50 voltage supply), for a class A amplifier there was no hope to get 100w power but a fractional.
Regards.