diy LA2A gain calibration problem

[ckbeck]

Hi.
I make a diy la2a.
I ran into a problem while gain calibration.

This is the gain calibration process.

1. R3 : fully counterclockwise.
R37 : fully clockwise
COMP/LIM switch : COMP
2. input : 1Khz 4.5Vpp(+6dBu) signal generator
output : oscilloscope
gain/peak reduction knob : fully on
--> output clip( about square waveform )
wait a minute
3. Slowly turn R3 clockwise until output decreases to about 8Vpp (+11.5dBu) on the oscilloscope.
R3 fix

But, my la2a cannot set 8Vpp at output. My la2a minimum Vpp is 28Vpp at R3 minimum.

How do I solve this problem?

This is the my voltage at each position.

 

[beatnik]

Didn't know about this calibration process, where did you find it ?

I believe R3 has to end up around the fully cw position for maximum signal going to the EL panel.

R3 adjustment is definitely required when stereo coupling to a second La2a where the T4b units and associated circuitry might have some imbalance, that results in one channel compressing more than the other.

From what you are describing seems like possibly not enough signal is going to the EL panel so there is not much compression happening.

What reading are you getting on the gain reduction meter ?

 

[ckbeck]

Didn't know about this calibration process, where did you find it ?

I believe R3 has to end up around the fully cw position for maximum signal going to the EL panel.

R3 adjustment is definitely required when stereo coupling to a second La2a where the T4b units and associated circuitry might have some imbalance, that results in one channel compressing more than the other.

From what you are describing seems like possibly not enough signal is going to the EL panel so there is not much compression happening.

What reading are you getting on the gain reduction meter ?

Thank you for replay.

This is the link.

https://help.uaudio.com/hc/en-us/articles/215479643-Replacing-Tubes-in-Your-UA-Analog-Hardware

 

[ckbeck]

I found my mistake. I input the 4.5V between 2 and 3 pin of XLR. It make that the input signal is the half of 4.5Vpp. I reconnect and test it. ( I connect the balance input +- 4.5V signal to XLR)
The output voltage drop to 18Vpp at now. I cannot make the 8Vpp util now.