[ozshadow]

While my amp is turned off and charging via the trickle charger, I get a reading on my headphone output of -20 to -40 mV and ground to ground reading of about 1.5 mV

Is this ok ?

 

[Garbz]

Define "ok". No it's not ok the amp is off there should be no current on the output of the circuit. Yes it is ok your headphones will not fry.

 

[threepointone]

what amp is this?

 

[ozshadow]

It's my PPAS which uses a separate circuit for a Pint-like trickle charger. The ground of it goes directly to batt - and the wallpower -. I am not quite certain what is going on with this.

 

[ozshadow]

Ok, so now I have a little more time to post exactly what is going on.

I have a Tread-setup that outputs a solid 24.6 volts. My PPAS has a trickle charger circuit inside to power the amp and charge the batteries, which are two 9v = 18 volts.

Here is the trickle charger, which now has a 47 ohm resistor in it.
http://www6.head-fi.org/forums/showp...3&postcount=15



When the amp is on, with or without wallpower, everything is as it should be. 0.6 mV and 1.4 mV DC offset. Zero across the grounds.


When the amp is off, no source connected, but the wallpower charging, I was reading NEGATIVE 20-40 mV on the output, like you were measuring DC offset normally. Across the grounds it read 1.6 or so mV.

I have the amp ground, batteries ground and wallpower grounds all connected to each other. All my jacks are isolated.

Any ideas ?

 

[ozshadow]

Maybe an isolation issue ? The tread-type regulator is homemade and in a plastic case. Its powered by a 30v DC Ratshack wallwart.

 

[tangent]

Quote:

Originally Posted by ozshadow While my amp is turned off and charging via the trickle charger, I get a reading on my headphone output of -20 to -40 mV and ground to ground reading of about 1.5 mV

I question whether your amp is truly "off", then. Can you draw up a block-level schematic showing the battery, the charger, the power switch, and the amplifier itself? It's important to show how these blocks connect together, particularly the power and ground connections. I'm not so much interested in the details of what is inside each block.

 

[ozshadow]

The amp is a PPAS. Let me know if more details are needed.

Thank you.

Oh, trickle charger:

 

[FallenAngel]

Add a diode on the V+ leading to the regulator. I needed to do that to my PINT + TREAD, otherwise the battery would discharge into the tread.

 

[ozshadow]

Oh, and now that the batteries are fully charged, with the amp off I am reading positive 1.2 to 1.5 mV dc offset while its plugged in. Without a meter, you would never know it.

No wallpower, amp off readings are zero.

I am not really worried about discharging into the tread because if it is plugged up, I have the PS on.

 

[tangent]

I also wanted to see the power switch. I suspect you have it in the wrong place.

If you redraw the schematic, can you rotate it 90 degrees to the left, so V- is along the bottom, and V+ along the top? Also, it would help if you just drew each major piece as a box with just the V+, V- and virtual ground connections (if any). This isn't just pedantry: if you draw it following existing conventions, that makes it easier for people familiar with those conventions to understand it, so you increase your chances of getting a fast, correct answer.

 

[ATAT]

Well the power switch is a RK97 wired on the PCB.. so I doubt you can have it in the wrong place..

Show me the bottom side of your charger.

Of course.. what could be happening is some weird case-grounding issue. are all your jacks and pots isolated?

 

[ozshadow]

Yes, the RK097 is PCB mounted.

As for the charger, the pic is there.

The 2nd pin goes through the resistor and ties to the bottom red wire which is Batt +

The 3rd pin is a straight wire that also goes down to the bottom red wire.

The middle red wire is Amp+

The top red wire is Wallpower+

That entire circuit is heatshrinked.

The jacks are all isolated.

The pot's not isolated - simply mounted on the PCB and bolted in.

Is it possible the rechargeable batteries themselves are carry some sort of charge on their casings ?

Quite frankly I am stumped. The PCB has electrostatic packaging material on the underside. I got no conductivity through that with a meter.

 

[ATAT]

Check for any loops between your power supply V+ and any components. Im guessing that the case is acting as a path..

 

[amb]

If you're sure that the battery charging current is blocked from powering the amp in some way, then the amp is off. In such an event, its output transistors (inside the output buffer chip) are not turned on, and hence your outputs are basically floating. It's possible to measure strange small voltages with your DMM in such a scenrio but it's no cause for concern.