1

EQ573 Equaliser / Re: Hi Frequiency "Hiss" when it is at pre mode

« Last post by Alex Harmer on Today at 12:31:18 AM »

I visited the forum today to see if anyone else had had this issue.

The EQ is gorgeous. In XLR mode it is totally clean. I have operated these units successfully since early 2025 with no issues. 

Today I noticed in Pre mode that decreasing a frequency caused hiss. This happened across the highs, the mids, and the lows, but it was most noticeable with the high frequency control. 

It happened in the first 30dB of gain on the MP573. On 40dB and above, the hiss was not as bad.

I'm totally happy to just use the EQ in XLR mode, but I thought it would be worth alerting JPK.

Are you able to go in to a bit more detail about the benefits of Pre vs. XLR mode JPK?

Thanks,
Alex Harmer

2

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

« Last post by steveholm on September 08, 2026, 06:48:40 PM »

I'm tuning a CP5176 and am unable to reach the required 13.5 VAC output. The problem appears to be in the SK25 DOA.

With a 1 kHz signal and JMP1 on pins 2–3:

At 100 mVAC input, TP1 = 65 mVAC, TP2 = 1.336 VAC, and L1/SK25 output = ~0.97 VAC.
TP1 and TP2 appear to behave normally as input is increased. At maximum input I get TP1 = 1.194 VAC and TP2 = 7.54 VAC.
However, at about 300 mVAC input, the SK25 output starts behaving abnormally. L1 stops increasing and begins decreasing as input is raised.
At maximum input, L1 falls to about 0.4 VAC and develops approximately -10.5 VDC.
With the signal removed, L1 returns to 0 VDC over roughly 25 seconds.
TP1 and TP2 remain essentially 0 VDC during the fault, so the large DC shift appears to originate in/around the SK25.
TP6 remains stable at -5.5 VDC at both low and high signal levels.
SK25 V+ remains stable at ~+16.0 VDC under load.

Some internal SK25 measurements also follow the negative shift. For example:

R9 bottom: +0.12 VDC low -> -11.4 VDC high
D5 anode: -0.13 VDC -> -10.7 VDC
D5 cathode: +0.15 VDC -> -11.1 VDC
D6 anode: +0.23 VDC -> -11.4 VDC
D6 cathode: ~0 VDC -> -12.1 VDC

I've checked Q2–Q10 and D5–D9 with the meter's diode test and haven't found an obvious failed/shorted semiconductor. I also checked the assembly-guide warning about R1 contacting the D5 solder joint; R1 is definitely clear. D5/D6 are thermally coupled, and the heatsink-board solder connections look good.

The key symptom seems to be: the SK25 works at low signal, but at ~300 mVAC input it begins developing a large negative DC shift and its AC output collapses instead of continuing to increase.

Any suggestions for where to look next on the SK25 would be greatly appreciated.


Steve

3

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

« Last post by JPK on September 04, 2026, 04:43:45 PM »

Too bad !

4

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

« Last post by morast on September 04, 2026, 10:17:23 AM »

Hi,

i swapped the relay which is working now, however, i fucked up the input trafo in the process .

I'll send you an email / pm because now i need spare parts, haha.

Cheers, 
Herbert

5

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

« Last post by OttoWei on September 03, 2026, 11:00:25 AM »

Thanks for the reply. I just found some time to continue troubleshooting the problem.

• U2 is not hot at all.
• U1 and U4 are warm, but not excessively so — around 40–45 °C, estimated by touch.
• I can't find any other hot components.

Interesting to see that egelmett has a similar issue. Have you made any progress with your unit?
To be honest, I think I'm out of my depth here. If you don't have any other suggestions, I probably need to send this to your service department.

6

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

« Last post by morast on September 03, 2026, 10:23:54 AM »

Hi,

i figured it might be the relais, i'll try to (gently) replace them. 

I will replace C2 as well to be of the same type as C3.

Thanks & kind regards,
Herbert

7

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

« Last post by JPK 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.

8

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

« Last post by morast on August 31, 2026, 01:35:06 PM »

Hi,

i finished my 2nd MP573 build and I'm having troubles getting it to work & would need some advice / guidance.


When the DI board is inserted, JMP3 removed, everything works fine. However, with no DI board, JMP3 on + Mic/Signal via XLR, there is no output.

Tests / measurements I did so far:

DC Values A-K all have the correct voltage.

AC Values with multimeter on 50hz Sine wave via XLR Input:
* In+ to In- : 0,6V
* C2 to C3: 0,6V
* TP2 to T1 Pin 7: 0V

Hints:
* The kit was missing C3, which i then substitutet with a ceramic cap "n47" 100V. (see attached pic).


* RLY1 edge got burnt (see attached pic2). I don't remember if it was from soldering iron (unlikely) or due to faulty connection...




Thanks for you support & cheers from Austria

9

LA502 Optical Compressor / Re: V- Rail returns low

« Last post by JPK on August 25, 2026, 06:53:28 PM »

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

« Last post by JPK on August 25, 2026, 06:52:18 PM »

Your Q2 may be damaged, try replacing it.