turning my mb-1 clone into a channelstrip - finished

[drjet]

Hello,
last year I build my Pultec MB-1 clone, have a look at this post:

http://groupdiy.com/index.php?topic=59296.0

It really turned out very nice and I use it a lot.
I´ve been thinking for a long time to add an EQ to turn it into a channelstrip. There´s more than enough space in the chassis  and enough power from the power supply.
I want to add a passive Pultec style EQ, followed by a make up gain stage. I thought about something like the GPultec or the poorman´s Pultec.

Is it possible to hook up the EQ unbalanced right behind the output coupling cap of the preamp, as shown in the attached schematic? Or do I need a interstage transformer?

Best wishes from Germany
Jojo

 

[CJ]

what happened to to 15K:600 output?  you could run the filter from the 600 ohm winding, eh?

 

[ruffrecords]

The schematic you have drawn is fine. Placing the passive EQ after the mic pre is exactly the right place for it. You only need  to decide what to use for the gain make up.

Cheers

Ian

 

[drjet]

Thank you for your advice, I´ll try it both ways. 
I´´ll go for the GPultec, cause I´ve most of the parts including inductors laying around.

 

[drjet]

Hi there,

it took a bit of time but now it´s finally done. I added a pultec-style eq to my mb-1 clone - I call it the P-Style Channelstrip. The EQ is a modified G-Pultec. I omitted the the G-Pultec bypass-circuit and replaced it with a direct bypass via voltagedivider in the way Ian does it with his poorman´s Pultec. I replaced the 2k2 pot with 2k5, which was easier to find. In the SRPP the 470k resistor was replaced by a 500k log pot to have an output control. I build the filterboard on stripboard. All rotary switches are mbb switches, so I have no switching noises. The whole thing worked as I first switched it on, but I had an audible 50 Hz hum. I checked my grounding over and over again. Finally I sanded off the coating of the chassis on every spot the panels, pots and switches are touching eachother,connected input and output pin 1 directly to chassis, Phantompower 0V to XLR input Pin1  and signal 0V and PS 0V to the common groundpoint near the power connector, just as recommended in Ian´s " Mixer Grounding 101" (thank you, Ian, for this invaluable instruction). This solved the problem.
Here are a few impressions

 

[drjet]

and this

 

[drjet]

... and this

 

[drjet]

the filterboard, lots of wires!!

 

[drjet]

Mic preamp schematic

 

[drjet]

filterboard schematic

 

[drjet]

..... and the makeup gain amp schematic

 

[drjet]

and the stripboard layout of the filterboard

 

[jensenmann]

Well done, congrats! It´s always good to see how ideas turn into reality. That´s real DIY spirit.

 

[G-Sun]

Congratulations!
Looking stellar

 

[ruffrecords]

Which make/model of inductors did you use?

Cheers

Ian

 

[drjet]

Hello Ian,
I used Bourns RLB1014 series induktors, which I bought at musikding.de

best wishes
Jojo

 

[ruffrecords]

Hello Ian,
I used Bourns RLB1014 series induktors, which I bought at musikding.de

best wishes
Jojo

Thanks for the info Jojo. The 100mH inductor in this series has quite a high series resistance of 300 ohms. Did you think of using the Bourns RL181S series which has a lower series resistance?

Cheers

ian

 

[drjet]

There were only this series of inductors  available at musikding.  Maybe I'll look for a different source. The lower the dcr the  sharper the Q, is that right?

 

[ruffrecords]

There were only this series of inductors  available at musikding.  Maybe I'll look for a different source. The lower the dcr the  sharper the Q, is that right?

OK, I understand. The Q depends on the total circuit resistance, part of which is the inductor resistance. So a lower inductor resistance means the potential Q can be sharper. Generally you would design to try to make the inductor resistance small compared to the circuit resistance so you can better control Q. The Q of the inductor itself is often very large; something like 30 up to as much as 90, whereas you want perhaps one tenth of this value for an EQ.

Cheers

Ian

 

[rouni]

Hi! Refreshing this good very old thread... while i was searching for some infos about a hum in my last built, i found this post.... i ve just finished this one, it s an eqp1/peq1 at first stage and the makeup is an mb-1.... working as hell! The 12ax7 at first stage was a little bit too much, so i have a 5751 tube and it s a perfect match. The inductor is a soviet one with looots of taps (from zero to 800mh) so the 11 pos highboost goes from 16k to 630 htz.... the low atten has his own freq selector like the lang.
Happy new year! Manu.