No Headroom on my J99/990c

[midimonster]

Hello Tim,

I completed my rack of A12's, N72's, and J99's, however there is significantly lower headroom on my J99's with 990c's running at +/- 24V.

Using a scope and a 0.5Vp-p 1KHz test signal, here are the results I am getting....

A12's saturate at 56Vp-p

N72's saturate at 49Vp-p (just as described in you cal procedure)

J99's saturate at 38Vp-p (+/- 24V) and 34Vp-p (+/- 22V).


In either case on the J99's, they saturate before before my Soundcraft Ghost console line inputs limit out. Also, the J99's saturation becomes obviously noticable when using an Audix D2 dynamic mic and singing into it. It seems that the output is always 10V less then the power rails. Is this normal for the J99's?

I have read on this forum that changing the opamp voltage from +/-24V to +/-20V will decrease the headroom from +27dBu to +25dBu, however in your N72 assembly instructions you state that the 49Vp-p output is +27dBu. If this is true, then I am clearly getting low output.

Lastly, I would like to comment that both of my J99's are showing the same results.

Please let me know what you would like me to measure...

Thanks,
Eric

[midimonster]

As stated above, there is a comment in this thread regarding the maximum output level at +/-24V verses +/-20V.

http://seventhcirclestudios.com/SCA/phpBB/viewtopic.php?t=528

However I looked at the J99 spec sheet, and it shows the Maximum Output Level @ 600ohms is 20dBu for the AD797 at +/-15V. I am only getting just a little more output with my John Hardy's. I still think something is wrong.

I have also measured the output of U1, and it does not clip until the last position of the rotary switch with a 0.5Vp-p 1kHz signal on the input. The output of U2 will start clipping with the rotary switch in the 4th position. I can back adjust R17 to lower the output until its clean again. When the switch is in the 5th position, the output will always be distorted.

Is it possible I got a bad batch of John Hardy Opamps? Normally I would expect an Opamp to saturate at its power rails, but these seem to saturate 5V below the power rails. I have my regulators set at +/-24V and the output won't swing above +/-19V.

Any advice would be appreciated...

Thanks,
Eric