ADA Preamps > Original MP-1

Getting rid of noise in my MP-1

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hansgraudal:
Roughly ten years ago i acquired an MP-1 sporting a rev.C PCB and a v1.37 eeprom, which turned out to be very noisy.
Back then i followed the paint by numbers noise mod procedure, which sadly didn't cure the issues.

I've decided to give it another spin on the workbench, and see what happens :) Having acquired a degree in electronics engineering since then has hopefully raised the odds to my favor...

For reference - this is ground zero:

MarshallJMP:
How do you "measure" the noise level?

hansgraudal:
Step 1: Make sure the supply rails are clean and healthy!

Unfortunately i didn't save scope dumps for the first couple of steps, so you'll have to take my word for this:
Examining the supply rails i had almost 200mV ripple over the filter caps, but oddly there was still around 100mV 50Hz ripple coming out of all four DC regulators.
The +/-15V rails were closer to +/-14.5V and the 5V rails were both down to 4.7V
Also the 47n bypass caps around the regulators are way to small compared to the 330n caps specified in recent datasheets for the 78xx/79xx series.

As they must have had a hard life, I decided to replace the old 78M05, 78M15 and 79M15 regulators with new 7805, 7815 and 7915 regulators as they can handle currents in excess of 1.5A where the 7xMxx regulators are only rated for 0.5A.
I replaced the large filter caps with some 3300µF 50V low ESR caps (I'll have to do something about those. It's not too pretty as is...  ;D), and the 33µF 25V electrolytics (C121, C124, C126, C128) were upped to 220µF 50V also low ESR types.
The 47nF ceramics were replaced with 470nF MKP types.

The result of this was 100mV 50Hz ripple over the filter caps and just 6mV ripple on all four regulated supply rails. The frequency of the ripple on the regulated rails has shifted to something in the MHz range. I guess I'll have to look at the decoupling caps in the digital part...

All in all a great improvement. The rails are spot on their intended DC values and ripple is greatly reduced. All in a good hours work:

hansgraudal:

--- Quote from: MarshallJMP on June 15, 2018, 01:16:22 PM ---How do you "measure" the noise level?

--- End quote ---

I have an abundance of oscilloscopes as well as a R&S UPV audio analyzer at my disposal.  :thumb-up:

hansgraudal:
I have installed sockets for all op-amps in the circuit, and put in recent production TL07x for a start.
With shorted input terminals, OD1@10, OD2@0, master@10 and output volume at max, noise measured at the output jacks was 40mVpp (-35dBu).
With the opamps replaced throughout the circuit the measurement is down to 2mVpp (-61dBu).
Raising OD2 also raises the noise floor somewhat. The next I'll be doing is a recap of the tube board, and I'll try replacing the carbon film resistors on the tube board with metal film resistors as well.
Let's see what happens...

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