barthman.de
Well-known member
I have been struggling with a La2a replica for several weeks now, which I built using P2P wiring. Unfortunately, the device does not work properly yet and despite extensive attempts and tests, I am not able to achieve my goal. One thing first: the tubes are not the problem, I've already replaced all tubes once.
The problem is:
When I look at the plug-in emulations of the La2a, the signal level at the input should also be at the output if the gain control is in the “30” range (without compression). The La2a should have a lot of gain to compensate the peak reduction. Unfortunately, this is not the case at my unit: When I feed a signal (sine 1 kHz; + 4dbU = 1.228V = 0VU) into the input, a corresponding voltage is only applied to the output in the "80" position and has a THD of approx. 20%. So something can't be right.
I've looked through the circuit more than once and can't find any fault with the wiring.
So in order:
I used this circuit:
http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_1968.jpghttp://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_layout.jpg
With the following components:
UTC A10 input transformer (XLR hot/cold wired to pin 1 an 6 – no C.T. used)
UTC A24 output transformer (XLR hot/cold wired to pin 1 an 6 – no C.T. used)
R34 10K
R29 4k7
R38 and C13 installed
I only write this because there are different La2a revisions.
If I understand the circuit correctly, the tube amplifier part of the circuit can operate independently of the compressor and VU meter part. So I'm currently focusing exclusively on the amplifier when troubleshooting.
In order to test the voltages of the amplifier in the first step, I proceed as follows:
R3 fully CW
R16 fully CW
Peak Reduction fully CCW
Gain fully CCW
The following voltage is applied to B +: 276V
NE2 (lamp) 64.5V
I measure the following DC voltages at the pins of the 12AX7A tube:
1 - 122.8V
3 - 1.03V
6 - 124V
8 - 1.16V
I measure the following DC voltages at the PINs of the 12BH7:
1 - 99.7V
3 - 3.97V
6 - 227V
7 - 74.1V
8 - 104.8V
The voltages look pretty good, only the voltage between R11 and R22 is significantly lower than noted on the circuit diagram and is around 18V instead of 27V. R11 and R22 have the correct resistor values.
I also measured the signal voltage (AC):
I go with a sinus voltage of 1 KHz; + 4dbU = 1.228V (0 VU) from my Focusrite 4i4 into the La2a.
Then I measure:
A10 Pin1 and Pin 6 (input): 0.668V / -1.30dBm
A10 at pin 7 to ground (output): 5.51V / 17.04 dBm
12AX7A / pin 2 or middle pin of R1
Gain at 30: 79mV / -19.78 dBm
Gain at around 80: 1.228V / + 4dbm
A24 Pin7 to ground (input)
Gain at 30: 483.9mV / -4.1dbm
Gain at 80: 6.85V / + 18.9dBm
A24 Pin 1 and Pin 6 (output)
Gain at 30: 88mV / -18.9dBm
Gain at 80: 1.228V / + 4 dBm
Here is the frequency chart (Gain at 80):
So I’m stumped, any ideas?
The problem is:
When I look at the plug-in emulations of the La2a, the signal level at the input should also be at the output if the gain control is in the “30” range (without compression). The La2a should have a lot of gain to compensate the peak reduction. Unfortunately, this is not the case at my unit: When I feed a signal (sine 1 kHz; + 4dbU = 1.228V = 0VU) into the input, a corresponding voltage is only applied to the output in the "80" position and has a THD of approx. 20%. So something can't be right.
I've looked through the circuit more than once and can't find any fault with the wiring.
So in order:
I used this circuit:
http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_1968.jpghttp://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_layout.jpg
With the following components:
UTC A10 input transformer (XLR hot/cold wired to pin 1 an 6 – no C.T. used)
UTC A24 output transformer (XLR hot/cold wired to pin 1 an 6 – no C.T. used)
R34 10K
R29 4k7
R38 and C13 installed
I only write this because there are different La2a revisions.
If I understand the circuit correctly, the tube amplifier part of the circuit can operate independently of the compressor and VU meter part. So I'm currently focusing exclusively on the amplifier when troubleshooting.
In order to test the voltages of the amplifier in the first step, I proceed as follows:
R3 fully CW
R16 fully CW
Peak Reduction fully CCW
Gain fully CCW
The following voltage is applied to B +: 276V
NE2 (lamp) 64.5V
I measure the following DC voltages at the pins of the 12AX7A tube:
1 - 122.8V
3 - 1.03V
6 - 124V
8 - 1.16V
I measure the following DC voltages at the PINs of the 12BH7:
1 - 99.7V
3 - 3.97V
6 - 227V
7 - 74.1V
8 - 104.8V
The voltages look pretty good, only the voltage between R11 and R22 is significantly lower than noted on the circuit diagram and is around 18V instead of 27V. R11 and R22 have the correct resistor values.
I also measured the signal voltage (AC):
I go with a sinus voltage of 1 KHz; + 4dbU = 1.228V (0 VU) from my Focusrite 4i4 into the La2a.
Then I measure:
A10 Pin1 and Pin 6 (input): 0.668V / -1.30dBm
A10 at pin 7 to ground (output): 5.51V / 17.04 dBm
12AX7A / pin 2 or middle pin of R1
Gain at 30: 79mV / -19.78 dBm
Gain at around 80: 1.228V / + 4dbm
A24 Pin7 to ground (input)
Gain at 30: 483.9mV / -4.1dbm
Gain at 80: 6.85V / + 18.9dBm
A24 Pin 1 and Pin 6 (output)
Gain at 30: 88mV / -18.9dBm
Gain at 80: 1.228V / + 4 dBm
Here is the frequency chart (Gain at 80):
So I’m stumped, any ideas?
A classic - R12 has to be 470k - I had installed 470R
Now I have enough gain and can optimize the cable routing a little.It appears that the connection between R17 and 12BH7A is particularly sensitive to interference.Thank you for helping me isolate the problem.
