1

MU524 Vari-Mu Tube Compressor / Re: MU524 is noisy

« on: April 21, 2026, 06:59:11 PM »

Check if any component is overheating, U2 in particular.

How does one know if U2 is overheating? Is this estimated by touch, or measured in some manner?

OttoWei's discription, and troubleshooting is the exact same issue as mine...I had to double-check it wasn't me who wrote the initial post! 

Ed

2

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: July 04, 2025, 04:02:25 PM »

If the issue is caused by a defective component, no charge, you only pay for the shipping.
In the other cases, our rate is 59 € per hour (+VAT if applicable).

I have already replaced many of the electrolytic capacitors...would you prefer I reinstall the components that came with the kit when sending to you, i.e. back to factory components?

3

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: June 24, 2025, 01:58:54 PM »

No. Regardless of the devices placement, either next to the other MU524, or by itself in a separate chassis, the noise begins after the device is warmed up for a while.

Again, I swapped the DC/DC converters...and many other components.

What do you charge for troubleshooting the MU524?

Thanks!

Ed

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MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: June 18, 2025, 02:42:17 PM »

FYI - Still having issues.

I have two MU524. The "Left" one is the one with intermittent snaps and pops and noise - it comes and goes. It happens whether in "IN" or "Linked" modes.

The "Right" unit is my control source. It's silent...no noise. It did have a problem with the P1 pot going bad - replaced.

What I did on both was to tape the entire underside (toward the components) of the cover plate leaving only the screw holes and their conductive pads exposed. That worked for the "Right" unit.

But, for the "Left", it's been a long-running process of swapping components between the two devices to try to find the component responsible for the intermittent noise.

Along the process, I did swap the voltage converters, but was not the cause.

Other attempts:

Lightly sanded the board contacts (to the 500 box), lightly added solder to the header pins connecting to the front plate (thinking poor connectivity), tried three different 500 chassis, tried moving positions in Midas L6 (and removing other devices around it), Swap tubes, swap C4 with C7, replace all electrolytic capacitors, replace C17, remove and resolder T1 & T2, swapped U3 & U1 between units, swap tube mount and socket between units, replace C10, C11 & C12, reflow ALL solder joints.

I will still swap U2 & U4, one at a time, to continue the process.

I'm probably forgetting other maneuvers as it's been a year-long process...could it be a bad trace on the PCB? That's where my head is going. It only starts when the device is well warmed...but a reflow of ALL solder hasn't helped...neither has a very tight mount of the support posts to the chassis (thinking ground issues).

I'm about ready to hit JP up for his repair service...I'm so damn frustrated at this point!

5

MU524 Vari-Mu Tube Compressor / Re: Hissing Noise on Output only - Voltage Regulator?

« on: June 18, 2025, 02:40:03 PM »

FYI - Still having issues myself.

I have two MU524. The "Left" one is the one with intermittent snaps and pops and noise - it comes and goes. It happens whether in "IN" or "Linked" modes.

The "Right" unit is my control source. It's silent...no noise. It did have a problem with the P1 pot going bad - replaced.

What I did on both was to tape the entire underside (toward the components) of the cover plate leaving only the screw holes and their conductive pads exposed. That worked for the "Right" unit.

But, for the "Left", it's been a long-running process of swapping components between the two devices to try to find the component responsible for the intermittent noise.

Along the process, I did swap the voltage converters, but was not the cause.

Other attempts:

Lightly sanded the board contacts (to the 500 box), lightly added solder to the header pins connecting to the front plate (thinking poor connectivity), tried three different 500 chassis, tried moving positions in Midas L6 (and removing other devices around it), Swap tubes, swap C4 with C7, replace all electrolytic capacitors, replace C17, remove and resolder T1 & T2, swapped U3 & U1 between units, swap tube mount and socket between units, replace C10, C11 & C12, reflow ALL solder joints.

I will still swap U2 & U4, one at a time, to continue the process.

I'm probably forgetting other maneuvers as it's been a year-long process...could it be a bad trace on the PCB? That's where my head is going. It only starts when the device is well warmed...but a reflow of ALL solder hasn't helped...neither has a very tight mount of the support posts to the chassis (thinking ground issues).

I'm about ready to hit JP up for his repair service...I'm so damn frustrated at this point!

6

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: October 16, 2024, 04:00:37 AM »

I only have the Midas L6 and two Empirical Labs EL500s - one is worse than the other...it may be them? Hmmmm...

But, strong line levels may be the issue: recording soft vocals and finger-picked acoustic guitar leaves a lot of space for noise to be apparent.

They're a lot better than just after the build. With your advice, like I wrote previously, it helped to go over solder joints and make sure C12 wasn't impacted.

Now, I'm dealing with P1 having failed on one of the MU524 - asking you about the part number or if I can buy a couple from you, in another post.

7

MU524 Vari-Mu Tube Compressor / MU524 - P1 Pot Part Number (10k LogB)

« on: October 15, 2024, 04:18:59 AM »

JP,

Could you please share the exact part number for P1? I think I have one going bad...

When audio is input, the levels go to max (12dB) on the meter, and no adjustment of the In pot makes a difference.

I've removed it from the circuit it seemed to test fine (8.5k). It seems that other than Pins 1 & 2 being joined on the PCB, when in-circuit and the control panel disconnected, it should test fairly closely. However, when placed back on the pcb (control panel still disconnected), I'm getting values higher than 10k.

I've checked for shorts elsewhere in the circuit, i.e. R1 and R2 and continuity to pins 7 & 10 on T1, there doesn't seem anything strange but the P1 pot itself.

If I could get the pot part number, or if you are willing to sell a couple, or even have other ideas to check, I'd appreciate it.

Thanks!

Ed

8

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: September 09, 2024, 04:45:10 AM »

After taking your advice (reflow all film and ceramic capacitors), the odd noise related to the cover is gone. I did this to both units.

I did remove the transformer to examine C12 for any perforation. There were none. As a safety measure, I sanded down the closest transformer pins for clearance, and added electrical tape around C12...just in case.

Still, the EL500 case is too noisy to use the MU524 in it. The transformer picks up hum from the power supply, even when the MU524 is farthest away from the power supply.

And, analog being analog...there is still some inherent noise. On loud sources, not a big deal. For me, this is my opinion, too noisy for delicate sources like voice and acoustic guitar.

I see you have a solid state opto compressor kit! 

9

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: September 03, 2024, 06:09:19 PM »

JP,

It's been a while since my last post on this. Even when used for Line In/Line Out (i.e. no preamp input), there is audible hiss and I'm not using them.

Along the lines of moving slots, I tried one in an Empircal Labs EL500...and the noise was even worse in either slot. I cannot use them with the EL500 in any way. The MPU566 I built are not noisy in the EL500.

I will go over the capacitors again, as recommended...but this is happening on two MU524s...surely, I can't be that inept at incorrectly soldering two units?

Any ideas?

10

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: February 27, 2024, 02:26:34 AM »

Yes, the hiss would clear after removing the cover, and only reappear when securing the screws to the posts I mentioned.

11

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« on: February 24, 2024, 07:45:02 PM »

JPK,

Thanks for the suggestions. As far as I can read with my meter, it was around 1mVAC.

I tried different slots, and a different 500 chassis with no changes. I was also sure to check, and then check again, the DC converters were different considering stereo usage.

However, in the process of testing one of the units today, while taking off the panel, I noticed a decease in the hiss I am concerned about. Through some more testing, I had to isolate two of the four screws that secures the cover plate from making contact with the plate. I isolated the points on the plate from the posts and screws. Those were the screws near U2 and the one near TP5. Both devices exhibited this noise, one way worse than the other.

It was odd though. Only the screw near TP5 did this at first, and once I isolated that screw, the screw near U2 started doing the same thing, so I isolated it as well. Luckily, there were no further surprises.

I did look under the main PCB to see if perhaps some unwanted contact was being made when tightening the screw. None found. I even tested this by taping off the whole underside of the cover plate thinking perhaps some intermittent contact was taking place along the plate. I couldn't find an issue there either.

In the end, I applied isolating electrical tape on the cover holes (both sides) near TP5 and U2 in order to isolate the screw from the plate, on both devices.

Any ideas why this might be?

Connecting this back into my system, I found upping the output of the MP556 and lowering the output of the MU524, reduced audibly, any remaining noise -  aka better gain staging.

12

MU524 Vari-Mu Tube Compressor / Light Hiss/Noise

« on: February 11, 2024, 09:30:57 PM »

My intention for the pair of MU524s was as a subtle buss compressor for mixes.

I noticed that there's a light, but audible, hiss/noise coming from these devices.

I understand this may be inherent to analog, especially tube/valve, devices, but is there anything to look out for that may cause this noise?

Thank You,

Ed

13

MP566 Tube microphone preamplifier / P1/P2 100K LogB Pots

« on: February 11, 2024, 08:51:07 PM »

Is it possible to purchase a couple of these (P1/P2 100K LogB) pots from Sound Skulptor?

The "Out" pot on one of my MPU566 has gone intermittent where I need to tilt the post (i.e. wedge a guitar pick under the knob) in a particular direction for it to function. 

I reflowed all solder points on the pot to eliminate a cold joint. No such luck. It seems my pot is bad.

Thank You,

Ed

14

MU524 Vari-Mu Tube Compressor / Re: Is a THAT1246 a Suitable Substitute for OPA2134?

« on: December 16, 2023, 11:31:03 PM »

I received the replacement OPA2134...and minus my resistor mistake, once corrected, everything works!

15

MP566 Tube microphone preamplifier / Re: LED Goes Green, But Not Red

« on: December 16, 2023, 11:29:56 PM »

I was able to get a replacement from Digikey...all is working.

Thank you.

16

MU524 Vari-Mu Tube Compressor / Re: Tone Imbalance Between Two MU524

« on: December 16, 2023, 08:51:07 PM »

Figured it out.

I had the wrong value resistor at R18. The right value is 470k ohm.

Interestingly, my second kit came with two 470 ohm resistors per the BOM. There should only be one.

When it came time to install R18, I mistakenly used the extra 470 ohm in its place.

I did a 'stare and compare' between my first build and second, and with the BOM, was able to confirm 470k was the proper resistor for R18. Clearly, I had the wrong resistor installed.

Sound test completed: only a .5dB level difference, with a pretty much exact tonal response through both.

17

MU524 Vari-Mu Tube Compressor / Re: Tone Imbalance Between Two MU524

« on: December 16, 2023, 08:25:09 AM »

I've performed the setup again on both units - same issue. One has a higher frequency output than the other.

I've also tried new cables, different slots pairs...it seems related to one MU524.

Ideas would be appreciated.

Thank you.

18

MU524 Vari-Mu Tube Compressor / Tone Imbalance Between Two MU524

« on: December 15, 2023, 03:12:24 PM »

Running my two 524s in stereo, I notice about a 13dB difference in level around 10k. The levels stay parallel until around 700Hz, then gradually differ in the higher spectrum.

I'm thinking I need to redo the setup on both.

Is there anything in the circuit that would most affect the high frequency content?

19

MU524 Vari-Mu Tube Compressor / Re: Proper Stereo Use

« on: December 15, 2023, 03:09:41 PM »

Thank You, JPK!

20

MU524 Vari-Mu Tube Compressor / Proper Stereo Use

« on: December 15, 2023, 08:02:28 AM »

I was able to get both MU524s working...whew!

I'm using a Midas L6, which has a "comp bus" link between pairs of slots.

Should  I be using the Midas' link function along with the Link switches on the 524s?

Is only the detector slaved between the two units when linked?

Are the controls independent despite the link enabled?

Thank You,

Ed

21

First Aid / Re: MP566 - Missing DPDT Switches

« on: December 09, 2023, 08:02:10 PM »

Thank you!

22

First Aid / Re: MP566 - Missing DPDT Switches

« on: December 07, 2023, 04:37:41 PM »

JPK,

Can I get the manufacturer's part number on the DPDT switches for the MP566, please?

Thank You,

Ed

23

Discussions about the products / MP566 & MU524 - Sonic Delight!

« on: December 07, 2023, 02:52:14 AM »

I just finished one each, MP566 and MU524 (building a pair of each) and I must say, they sound frickin' AMAZING!

As a part of the audio testing, I ran a drum loop out of Cubase as an external plugin insert, through the MP566, then into the MU524 (and then back into Cubase) and was blown away with how big and round the sound was. The MP566 boosted the level some, not into saturation, and then the MU524...which, is a REALLY nice compressor for gentle averaging...it's not snappy, and it doesn't pump...but wow, does it really bring the whole signal to life.

Thank you, Sound Skulptor....great stuff!

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MP566 Tube microphone preamplifier / Re: LED Goes Green, But Not Red

« on: December 06, 2023, 10:17:46 PM »

Bad LED.

When I jumped R46, there was no change.

So, I was able to clip onto the bad LED legs (i.e. signal right off the PCB) and clip onto the LED from my second kit, and it lit RED as expected: when saturated.

Not sure if I damaged it, or bad from the factory.

How can I replace this?

Thank you again, Jean-Pierre!

25

MP566 Tube microphone preamplifier / Re: LED Goes Green, But Not Red

« on: December 06, 2023, 01:22:46 AM »

Input was 1VAC 1kHz sine (from the Signal check):

1. -14.08
2. 13.57
3. 4.82
4. 15.27
5. -0.18
6. 4.82
7. -14.08
8. -8.6
9. 4.83
10. -0.05
11. -15.33
12. 4.83
13. 3.27
14. 14.19