Meters On Pass X Series Amps


2002-01-31 5:48 pm
Quote from

"The meter on the face plate of the X1000 measures the current drawn by the output stage. At idle, it sits slightly less than half full scale, reflecting the standing bias current of Class A operation. As the amplifier exceeds its class "A" rating the needle on the amplifier will register the increased bias being drawn by the output stage."

High-end meter

If you want high-end metering without too much effect on sound, consider the following:

1. Choose the optimal or most convenient place to measure. This could be at output, one MOSFET source or drain, input current etc. etc. etc.

2. Choose sensing element. I would suggest current transformer on input or output stage as this is non-intrusive and measures true drive + has the bonus of providing a balanced output signal at whatever DC level you require. Other options are typically resistive. High-end sensing means an accurate .01 Ohm or so resistor with kelvin terminals. Low end sensing means standard resistor of low value.

If you need to measure current drawn from supply you basically have to measure at the power supply.

3. Choose amplification (if required which it typically would be). May I suggest cheapest op-amp available? Remember you will need some cheap power as well.

4. Choose low pass filter topology and frequency of same. May I suggest simple RC filter at 1-5Hz?

My personal favourite would be current transformer sensing of primary. You can check up on current transformers at or wind your own. Here is a photo of one of my favourites up to 10 amps non-contacting, and probably even better .007 Ohms current type at .1V per Amp ...

Off topic


The inductors made by Coilcraft are typically of too low value to be very effective for things other than RF rejection. If you want to build a coil loaded power supply you should consider looking elsewhere. Where? Well, I am not sure where to send you. For low current applications there is always Hammond, and I am making my own inductors these days.

If you want the ultimate in high current inductors, consider getting into some distributed airgap toroids from and wind your own. It will take you some time but you can do some serious things. I hand-wired 150mH common mode chokes that were the size of a dime ... on a "w" material core from the same company as one un-related example.

If we are talking LC filters here, loading chokes are made by virtually all major transformer mfgs. Lundahl in Sweden Sowther in UK , among others...A few extra windings on this would probably give you the few microamps necesseary to drive a meter...

For measuring DC load current, an op-amp sensing the voltage drop across one of the emitter ballast resistors will probably do the trick....

Otherwise, - theres a lot of more or less complicated circuits one can think of..:)
Mark A. Gulbrandsen said:
What exactly do these meters monitor???? And how do they work to display what ever they are monitoring?
Mark Gulbrandsen

the meter is monitoring you monitoring the meter :D

do you think that is really necessary?

It's daaaam KEWL :cold:, that's what it's for i think.

Some other amps give you some kinda scale, but I guess nobody cares to read it out.
Hi all

If one does it the way Nelson Pass suggested then there is no need for rectification at all. You just have to care you don't mess up the circuit's properties.

I once did it on an ordinary class AB amp (abouit 20 years ago). I simply connected the series connection of a POT and a cheap meter across the emitter resistors of the output stage. No rectification, no filtering (the latter might be beneficial, specially in in my case). What the meter then showed whas the current load on the amp. On a highly biased amp like the X-series amps it is an indicator showing the actual "working area" of the amp. And as already remarked it looks quite cool :cool: