A40 offset problem?

[stickleda]

I have one channel of my a40 that has 27mV of dc offset on the output without an input. If I add a shorting pin to the input or conect it to a source (without dc offset) it jumps to 96mV offset. Should I be concerned. What is causing this.

stickleda

[Netlist]

It is possible that any DC coming from your source is amplified and shows up at the output. You could try a DC blocking cap at the input. 96mV is not really harmful but a bit high.

/Hugo

[Original Burnedfingers]

Try matching up another set of diff input transistors and see if it makes a difference.

[Nelson Pass]

Originally posted by stickleda
I have one channel of my a40 that has 27mV of dc offset on the output without an input. If I add a shorting pin to the input or conect it to a source (without dc offset) it jumps to 96mV offset.

Rather than matching, it may be inadequate current gain on the
input device(s). Try using a high-beta type transistor, or select
for a higher one in the extras you have, and if possible, measure
the beta of the ones you have in there.

[stickleda]

Thank you all for your responses. The differential pair is 2N5551's. I thought a close replacement for the MPSL01's. How can I test for beta. I have access to a couple of DVM's a 20meg scope and a 2meg function generator.

Thanks

[Netlist]

Here's a simple test circuit that might help you.

/Hugo

[stickleda]

Transistor 10K base resistor 1K emitter resistor Beta/Hfe
0.797 8.89 111.3179474
0.784 8.9 113.291074
0.838 8.85 105.3952412
0.819 8.87 108.0840148
0.782 8.91 113.7084399
0.877 8.81 100.2531587
0.812 8.86 108.8928696
0.791 8.89 112.1623313
0.789 8.9 112.5731331
0.769 8.92 115.7604655
0.861 8.83 102.3479863
0.784 8.9 113.291074
0.802 8.89 110.6239452
0.769 8.92 115.7604655
0.784 8.9 113.291074
0.777 8.91 114.4401544
0.777 8.91 114.4401544
0.778 8.91 114.2930591
0.779 8.91 114.1463415
0.784 8.9 113.291074
0.784 8.91 113.4183673
0.788 8.9 112.7159924
0.8 8.89 110.9005051
0.804 8.88 110.2246344
0.805 8.88 110.0877094
0.838 8.85 105.3952412
0.883 8.8 99.45891531
0.867 8.82 101.5245884
0.891 8.79 98.45389926

Conditions are 9.88Kohm base resistor_____990ohm emitter resistor
10.37volts to base resistor
15.31v to 15.25v to emitter resistor


This is per the last postings circuit. Sorry about the way it posts, i you can't understand let me know.

I will pull out the transistors that are in there and check everything again including removed's beta numbers

Don

[stickleda]

What should I be looking for as a minimum beta/Hfe for these devices in this circuit? Also at what test voltages/currents, and is the test circuit I have used adequate.

[stickleda]

Update! Beta measurements are off. Hfe is about (70) for Fairchild 2N5551, removed units were slightly less. Sorry for this, faulty test procedures that I corrected today. What is the ideal Beta for this circuit? Can I substitute Zetex ZTX653 and ZTX953 for the mpsl01/2N5551 and mpsl51/2n5401, as these look applicable with much higher Hfe at the currents that are called for?

I was incorrect before about only one channel increasing offset when shorted. Both do this! However it is much less in the channel with higher Hfe (90) and Motorola brand 2n5551.

I also finally removed the resistor substitutes for Q11 (2N5248) and was able to see and hear the reduction of the noise floor of the amp. I had been reluctant to do this before because the only replacement I could readily find was NTE312 (and this site has been pretty hard on using NTE product). No smoke and the voltages referenced in the build are pretty close after installing. Both channels are little higher than the specs.

Mr. Pass, if could answer the above questions I would greatly appreciate it.

Thanks, Don