Recommend a Trimmer Pot

[kptseng]

Hi All,
I'm planning to upgrade the trimmer pot in my amp (OA21s).
Anyone can recommend a good brand and model?
Thanks.

[unclejed613]

bourns and piher and clarostat immediately come to mind, ...... or alps......

[Nordic]

even the vishay trimpots are quite nice these days... aslo there are more options now... like finger adjustable trimpots (have a little knob ontop) etc..

[kptseng]

Thanks Uncle Jed,
Looked at Farnell and they do have quite many variaties.
Unfortunately I'm stuck now as I'm still trying to find out how to measure the quiescent current to balance (not to increase) the bias..............
Any idea? I'm using OA21s

Hi Nordic,
I think I saw some with a knob that you can adjust with fingers.

[unclejed613]

you mean to balance the input stage? for DC offset?

a 100 ohm (or less) trimpot is usually used for this, insert it at the "T" between the input diff emitters.

[kptseng]

Hi Uncle Jed,

You mean I can just measure the current across the 2 pins of the MJ15003?

I tried measuring the Vdc across between the red and black of the speaker output. However the reading keeps "decreasing" starting from about 500+ Vdc then dropping about 4Vdc every second........

How do I measure DC offset? Is it true that I need to get as close to 0Vdc on DC offset as I can?

Many thanks.

[kptseng]

Hi Uncle Jed,

One more thing - which is more superior in quality? Cermet, Plastic Conductive or Carbon?

Thanks

[unclejed613]

offset is measured at the speaker terminals. the reansistor current is measured by measuring the voltage across the emitter resistor and dividing by the resistance. offset should only be 100mV or less (any more than that will give a turn-on thump). many amps use 0.1. 0.2, or 0.47 ohm emitter resistors. so if you are measuring 20mV across a 0.47 ohm emitter resistor, 20mV/0.5 (i always round the 0.47) ohms=40mA.
usually BJTs run anywhere from 5 to 100mA and MOSFETs run from 50 to 500mA for idle current depending on the amp manufacturer.

for an offset vontrol, go wirh plastic film or cermet. for an output bias control, any type will work, but cermet types are more reliable.
cermet is also a low noise resistance, and you definitely want to minimize noise sources in the input stage.

[Sigurd Ruschkow]

I would go for a Bulk Metal Foil trim pot. If you want the best. They are available in many sizes and values.
Texas Components sells them.

http://www.texascomponents.com/





Sigurd