1

MP573 Microphone preamplifier / Re: MP573 unstable, volume drop

« on: May 06, 2024, 11:48:54 AM »

I just the following:
I also have a second MP573/EQ573 combo in another rack, that workes without problems. I switched them out, so the combo, that was in the IGS rack before is now in the API rack and vice versa, so I can rule out, if the power supply might cause the problem.
I repeated the test and now bothe units work perfectly. I have no explanation for this.
Do you have any clue, what could have caused this?

2

MP573 Microphone preamplifier / MP573 unstable, volume drop

« on: May 06, 2024, 09:27:28 AM »

Hi JP,
My MP573 started to develop some strange behavior. After working normally for a couple of minutes, the gain starts to become unstable, going down and up again a few dB. Then after ca. 46min the gain drops considerably. The gain then very slowly goes up again.
When I use the gain switch (48min), the preamp starts working normally again, but the problem accurs again after a couple of minutes.
This problem alway occurs, no matter, what gain I set and no matter, if the EQ is in or out.
The file from the screenshot was recorded with a digital 100Hz sine.
It would be great, if you could help me with this.

3

EQP501 Passive Equalizer / Re: EQP501 Gain again

« on: July 16, 2021, 03:41:00 PM »

So I replaced R12 (1,62kOhm) with a 1k resistor and a 1k trimmer. Unity gain was very easy to dial in. Doesn't look pretty, but it seems to work.
I might check, how closely the EQ curves are matched mit Room EQ Wizard, only out of interest. Auraly, I have nothing to complain.
Thank you for this great EQ! I really like it.

4

EQP501 Passive Equalizer / Re: EQP501 Gain again

« on: July 15, 2021, 03:22:08 PM »

Thanks for the quick reply.

I measured the voltages again and there only minor deviations.
TP3 gets very close to 0,17V, if I use a square wave as an input signal, but that also changes the other voltages, of course.

The parts have the right values, but there seem to be some tolerances. I measured with all boards attached.
On the quieter unit, I measure P1 11,59kOhm and P4 11,44kOhm.
On the louder unit, I measure P1 9,78kOhm and P4 9,16kOhm.
The other values seem to be pretty accurate.

I'll try to adjust the R12/R13 value.

5

EQP501 Passive Equalizer / EQP501 Gain again

« on: July 14, 2021, 03:53:04 PM »

Hello JP,

I also have a small problem with EQP501 gain, but decided to open a new thread. If you think, they belong together, feel free to merge them.

I followed your advice from the other thread and did some measurements:

	Unit 1	Unit 2	Reference
Gain	-1,3dB	+0,5dB
TP1	1,95V	1,92V	2V
TP2	1,28V	1,63V	1,7V
TP3	0,09V	0,11V	0,17V
TP1-TP3	11,54kOhm	9,73kOhm	10kOhm
TP1-0V	12,37kOhm	10,68kOhm	11kOhm
TP3-0V	817Ohm	901Ohm	1kOhm

I can live with this problem, but if it is easy to fix, I'd be happy to do so.

all the best
Constantin