1

LA502T Tube Optical Compressor / Re: Release time, and a couple other oddities

« on: October 13, 2026, 06:02:33 PM »

2. Do you measure the gain reduction from the LEDs or the actual gain reduction on an external meter?
It is true that the LED meter is not accurate when the decay time is increased. This is because it is driven by the side chain, not by the signal itself.

4.  We haven't encountered this problem yet. Is the inductor whine annoying? If it is the case, we can exchange the module.

2

LA502T Tube Optical Compressor / Re: test procedure - having trouble

« on: October 12, 2026, 05:09:12 PM »

What DC voltage do you get on TP8 (against0V) 
- for Vout = 15VAC (no gain reduction),
- for Vout = 10.6V (-3dB),
- for Vout = 1.5V (-20dB).

What DC voltage do you get on U4 pin13 (against0V) when you turn Trimmer TR1 from 0 to the max. Should go from 0.08V to 0V approx.

What DC voltage do you get on U3 pin2 (against0V) when you turn Trimmer TR2 from 0 to the max. Should go from 0.75V to 0.2V approx.

3

LA502T Tube Optical Compressor / Re: test procedure - having trouble

« on: October 08, 2026, 05:43:25 PM »

Step 5- I only get about 15-16V max when it should be 20V
How many AC Volts max (against 0V) do you get on TP4 and TP5? (should be 11-12VAC)

Then in step 6- I get 3 gain reduction LEDS on with all 3 trimmers all the way down (CCW) and cannot adjust to only 1 LED (-3)
With PEAK at 0 What DC voltage do you get on TP8? (should be 0V).

4

LA502T Tube Optical Compressor / Re: Release time, and a couple other oddities

« on: October 06, 2026, 03:39:50 PM »

1. The decay is set by the cell itself, it is not reflected in the LEDs which are driven by the side chain. It is not very fast, about 200-300ms.
2. Not normal. It should only change the gain by 0.5dB.
Check the solder joints and components value of R44, R46, C21, C22.
3. Normal. There is a minimal attenuation caused by the input transformer and the attenuation network.
4. Not normal. Can you identify the source of the noise by pressing a pen or something against the components, most probably one coil. 


Check if any component is getting abnormally hot.

5

LA502T Tube Optical Compressor / Re: Some wrong parts in the kit.

« on: September 16, 2026, 05:20:51 PM »

I am sorry for the mistake (still a new product). We have sent the correct switches.

6

CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« on: September 12, 2026, 09:34:46 AM »

Does the LED lights up?
Check Q7 and Q9 with the DMM in diode mode. It is possible to test Q7 while in circuit and test half of Q9, the BC junction.
Test transistors
Generally DOA's are very difficult to troubleshoot because of the density and all the feedback paths. I would recommend to build a new one.

7

EQ573 Equaliser / Re: EQ573-II ran in my lab

« on: September 12, 2026, 09:11:34 AM »

Actually, this is the normal frequency response of the original 1073. About -1dB at 20kHz.
I guess that the modern reissues are modified.
You can make the frequency response flat up to 20kHz by changing the value of 2 caps:
C47 at 100pF (instead of 470pF),
C60 at 47pF (instead of 150pF).

8

CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« on: September 09, 2026, 04:07:17 PM »

at ~300 mVAC input it begins developing a large negative DC shift and its AC output collapses instead of continuing to increase.
It looks like the positive part of the output is not working. Check the solder joints of Q9, R12.
Can you explain your measurements:
R9 bottom: +0.12 VDC low -> -11.4 VDC high

9

MP573 Microphone preamplifier / Re: MP573 Build - No Output with XLR Connector

« on: September 04, 2026, 04:43:45 PM »

Too bad !

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MP573 Microphone preamplifier / Re: MP573 Build - No Output with XLR Connector

« on: September 02, 2026, 05:35:48 PM »

The burnt relay is very probably what causes the DI/Mic issue.
You need to replace it.
Mouser/Digikey: TQ2-24V
TME : AZ850-24

Sorry for the missing cap. If you have an other one, you could also replace C2. C2 and C3 must be similar for the best CMRR.

11

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 25, 2026, 06:53:28 PM »

12

MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« on: August 25, 2026, 06:52:18 PM »

Your Q2 may be damaged, try replacing it.

13

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 22, 2026, 04:52:17 PM »

Sorry !!! I mean't R35, not R39.
Double check R35. 
Check you didn't break the via which connect the bottom and top pads together. In particular on the bottom pin of R35 (near R17).

14

MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« on: August 22, 2026, 04:44:04 PM »

I can measure an AC Voltage on all TP's except TP2 and 3.
This is because there is a large DC offset which fools your meter. It is normal.

15

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 21, 2026, 10:10:37 AM »

- Remove all the socketed IC's and and check it it is still the same.
- Check the value of R39 with the ohmmeter (1 ohm). It should almost look like a short.
- Did you need to remove a component during the build?

16

LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« on: August 21, 2026, 10:05:55 AM »

17

MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« on: August 21, 2026, 10:04:29 AM »

Try tapping on components and connectors with a pen to try to locate the defective connection.
Resolder all solder joints that look suspicious.
Monitor a constant sine signal with your AC voltmeter at points TP1, TP2, TP3, TP4, TP5, TP6 to try locate the stage where the problem occurs.

18

MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« on: August 21, 2026, 09:57:31 AM »

Check you didn't swap Q1 & Q2.
Check the voltage on R46 top pin (7.4V).
Check the solder joints on R45, R46, R47.
Check the direction of C23.

19

MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« on: August 18, 2026, 11:04:36 AM »

Check that Q2 (2SK170) is in the correct position A.
Can you measure the DC voltage (against 0V) on both sides of R51: near connector (16V) and near C24 (15V) ?
Can you measure the DC voltage (against 0V) on both sides of R48: near C23 (8V) and near R41 (1V) ?

20

LA502 Optical Compressor / Re: Release time feels bit fast

« on: August 11, 2026, 09:27:39 AM »

Do you also find the release time too fast with the 10mS attack?
Check the value and solder joints of R32, C17, C18.
If still too fast try increasing R32 to 4.7M. You could also add an additional cap in parallel with R32. It will also slow down the attack time but you can compensate by decreasing R29.

21

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 06, 2026, 04:47:16 PM »

Check that R46-L is connected to the 0V. Check on the resistor leads, not the PCB pad.
Reflow the solder joints of R45, R46, R47, R48.

22

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: August 03, 2026, 05:43:17 PM »

No, that's very unlikely. The microphone is isolated from the preamp circuitry by the input transformer, so the preamp itself cannot damage it. The only parts directly connected to the microphone are the 48 V phantom power (through R1 and R2), the pad relay, and the input transformer. Under normal conditions, none of these should damage the microphone.

23

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 03, 2026, 05:34:53 PM »

You have a broken connection.
Check that R47-Right is connected to
- R46-R
- R45-L
- U4-pin2
Check that R47-Left is connected to
- D11-L
- R48-R
- U4-pin6
If one link is missing, repair it with some wire.

24

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 01, 2026, 07:48:11 PM »

Check the value of R47.
The value is 1Meg (brown, black, black, yellow, brown) but when measured in circuit, it should give you about 12k.

25

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: August 01, 2026, 07:37:53 PM »

and than the signal fades away quite rapidly.
Try to follow the signal on TP1, TP4, TP5, TP6.
Check what happens between the initial working state and the final faded away state.
Also cjheck if something happens with DC.