[cgrums]

Hey all,

After spending the last month changing jobs, moving and generally being out of the DIY scene, I've finally been able to sit down and finish up my Dynalo.

After powering the board up with two treads and no opamps installed I took initial measurements of DC offset and found 47 mv in one channel and 32 in the other. I powered down, installed the opamps and powered the board back up. I then observed the offset drop to ~ 5 mv in each channel. Perfect.

I plugged in the SR60s and heard beautiful music...for about two minutes. After about two minutes the offset shot up to 5 volts in one channel and 3 in the other

and made terrible sounds in the 'phones. I'm very happy I wasn't using the SR225 at the moment.

Any thoughts? Comments? Any help would be much appreciated.

 

[Pars]

You need to watch the offset for several minutes (or perhaps a couple of hours) without the servos installed. Initial offset should be adjusted when warmed up (20 minutes).

However, offsets going into the volts range does seem strange... hope your SR60s aren't toast, but I would expect them to be at that kind of DC voltage.

Have you looked at the construction manual on Dan Gardner's site? He also had a nice diagram of DC operating points throughout the circuit which might help pinpoint where the problem is.

What does the power look like (V+ / V-) when the offset jumps this much?

 

[cgrums]

Quote:

Originally Posted by Pars /img/forum/go_quote.gif You need to watch the offset for several minutes (or perhaps a couple of hours) without the servos installed. Initial offset should be adjusted when warmed up (20 minutes). However, offsets going into the volts range does seem strange... hope your SR60s aren't toast, but I would expect them to be at that kind of DC voltage. Have you looked at the construction manual on Dan Gardner's site? He also had a nice diagram of DC operating points throughout the circuit which might help pinpoint where the problem is. What does the power look like (V+ / V-) when the offset jumps this much?

I found out a few things:

1. My leds, while matched, aren't the spec'd 1.6 V but slightly higher causing a 1.2 V drop across the resistors at the head of the CCS. I know that this will carry through each subsequent stage of the amp but would it cause the offset to go all wonky?

2. The regulators on both of my treads get very very hot. Almost too hot to touch. I'm assuming this is because the over spec'd leds are drawing too much current but I could be wrong. My heat sinks for the treads haven't come yet so I'll wait to do any more testing until those come in.

Any thoughts? Anything else I should be looking at?

 

[nikongod]

thermal runaway?

im not sure if that would cause the DC issues you speak of, but it could.

the fact that LED's are not the EXACT voltage spec'd should not be a terrible issue unless it causes current issues. the REAL key with the led's is that they are the same voltage drop.

edited: you should also let the amp run for a bit of time with no opamps on initial startup. the amp behaves VERY differently when cool and warm. thermal issues and all.

 

[cgrums]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif thermal runaway? im not sure if that would cause the DC issues you speak of, but it could. the fact that LED's are not the EXACT voltage spec'd should not be a terrible issue unless it causes current issues. the REAL key with the led's is that they are the same voltage drop. edited: you should also let the amp run for a bit of time with no opamps on initial startup. the amp behaves VERY differently when cool and warm. thermal issues and all.

Interesting points. I'm going to let it run for a good long time as soon as I get some sinks for the treads. Thanks for the input.

 

[amb]

Keep an eye on the output transistors' temperature. If they get hot enough to be uncomfortable to touch, then either the LED voltage is too high, or you've run into thermal runaway problems. Measure the DC voltage across each of the 25 ohm output resistors. What do you read?

 

[cgrums]

Quote:

Originally Posted by amb /img/forum/go_quote.gif Keep an eye on the output transistors' temperature. If they get hot enough to be uncomfortable to touch, then either the LED voltage is too high, or you've run into thermal runaway problems. Measure the DC voltage across each of the 25 ohm output resistors. What do you read?

I'll check this as soon as I get sinks in for the treads. Thanks amb!