LYNX amp problems...Help

[JojoD818]

Hi all,

Just finished building Jan Dupont's Lynx amp found at http://www.audio-circuit.dk. i used a +/-35V test power supply and measured 3V at the output. Idle starts at 75mA and cannot be lowered anymore. If I connect the Zobel network, the DC at the output becomes 5-6V and the current goes 300mA+.

I've checked everything, even the resistors. They all passed. All transistors are ok, OPS have no short to heatsink.

Am I missing something? Please help me.

Regards,
JojoD

[sajti]

It looks as some oscillation. Did You check the output with scope?

Sajti

[Jan Dupont]

I've been checking the copy of the PCB layout you emailed me.
Can you tell which make the MPSA42 and 92 are...
Collector and Emitter swaps places in different brands!
Your layout fitts Philips brands, but if you are using transistors
from ON Semi, then you have to turn them 180 degrees....

[JojoD818]

Still here...

sajti,

I don't have a scope, so I cannot verify if the amps oscillates.

Jan,

I'm using the one from On Semi , can you be more specific with the pins for the mpsa42/92 transistors?

Thanks,

JojoD

[Jan Dupont]

If you are using ON's, then your layout should be OK on the MPSA's

[Jan Dupont]

Have you tried disconnecting the output transistors from the PCB ???

[JojoD818]

Jan,

I've tried disconecting the power transistors already, the dc at the output only decreases to 1.4-1.8V dc.

When the power transistors are connected, I cannot connect the zobel network or even a speaker because the dc only gets higher!

Surprisingly, at +/-12V i can connect a speaker and hear a hum when I touch the input which is normal right.

I'm out of ideas and any suggestion would be helpful...

JojoD

[Jan Dupont]

Some of the transistors must get pretty hot.
Please inform us which ones............

[JojoD818]

From the schematic, the transistors that gets hot are Q103, Q104, Q110, and Q111. Resistors that are hot are, R118, R119, R111, and R112. Output transistors are not even warm. Opamp used is an OPA134 which I tested (again) on a mic preamp I use.

JojoD

[Jan Dupont]

Please examine your pin connections to Bias transistor Q105.
Try placing the decoupling cap C104 directly on transistor pins instead of on the PCB.... If those transistors/resistors (q103/104 and R111/112) get really hot, it can be caused by that Q105 almost acts like an "short circuit"... What are the Voltage readings across R111 and R112??

R118/119 will normally not be very hot, however if some reason the OpAmp oscillating they will get hot...............
Try putting a 100-500 pF cap from pin 2 of opamp to GND.