My Pultec MB-1 clone

[drjet_de]

Nice build! May I ask which power transformer you used?

Thank you for sharing. Might want to give this one a go.

Point-to-point seems a sensible place to start building (after a bunch of
restorations)...having all the parts laid out like that makes it look deceptively easy

Hi Jeremy,
This is the PT I used:

https://www.don-audio.com/Toroidal-Transformer-Mila-250vct-63vct-48V

Looks like it was made for NJD's Mila-1 preamp. It's nice to have a 48 Volt taps.

It really was fun  to do.  As I said before I will add a hi-Z Input and a EQ sometimes. I think for the EQ I will stay in the Pultec-Family

Best wishes from Germany
Jojo

 

[drjet]

hi,
after reading Ian´s document about grounding I changed the grounding of  my mb-1.
Close to the mains connector I have a bolt to the chassis to which til now only safety earth was connected. I had another stargroundpoint in the center of the chassis. The 0V from powersupply, preampboard and phantom-power and XLR-screens were routed to this point. My chassis is coated, so i sanded it on stargroundpoint and safety earth point to have good contact.

Now I routed the 0V form the powersupply to my safety earth bolt(common point). I soldered phantompower 0V, the preamp groundbus and XLR out pin 1 to XLR in pin 1, from where I have a wire to the common point.
Next I did my testing again: I have about 10db lower noise, at maximum 5db

But the noise is still rising on the last few degrees of pot rotation to maximum gain, but not as loud as before.  When the noise is starting I can measure about 100ohm across the pot. So maybe I will solder a 100ohm resistor in series with the pot.

Here are my measurements, pot halfway up:

 

[drjet]

and pot to maximum

 

[drjet]

I also tried to mount the torroid to the side of the chassis, but it just didn´t fit in, it was a millimeter too large.

So: lesson learnt, thank you so much for your advice

Jojo

 

[ruffrecords]

It is difficult to get a true feel for the noise from a spectrum analysis. Does it have the ability to calculate the rms values over the 20KHz bandwidth?


Looks like the grounding changes have made a good improvement but you may have reached the limit of what the design can do.

Cheers

Ian

 

[Ilya]

Another option is to use external PSU and bring rectified and (or) regulated DC to the preamp.

This should eliminate your 50Hz hum which looks like it's induced from PT or HV AC wiring.

Make sure that you use properly rated connectors for HT supply if you choose to move the PSU in the external enclosure.

 

[drjet]

Hi,
I added a Hi-Z input to the MB-1 as planned. It works fine and sounds awesome with bassguitar. The output is a little to hot, the inputs of the audiointerface are clipping when the strings are hit too hard, even at lowest preamp gain setting. So I think I need some switchable post OT attenuation.
And I have to do something with the gain control pot, as it is not very sensitive on higher gain levels. There´s almost nothing happening on the first 3/4 of the way, but then there are massive gain changes with every further degree. It´s a 10 k log pot now, maybe a 10k log reverse pot would do the job. But I think I go for a stepped gainselector.