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A DIY 1/2 Rack Bass Preamp

#41
Passinwind said:
Coming next: a matching 1/2 rack power amp.
Click to expand...

After an epic marathon dweeb out, up and running:

PW700_guts.jpg


First impressions are very positive, I'm sure there'll be a great crop of new miniamps using this module. My build came in just under 7.5 pounds, all-in. There's a lot of aluminium in this thing.
 
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#49
MarkA said:
Looks good! Are the two holes marked "in" for switches to take the HPF and EQ in and out of the circuit?
Click to expand...

The right one is for the PEQ. I think I may decide to leave the HPF in all the time, so the left switch may be for low and high impedance input switching -- most of my builds are low or medium impedance only since I don't own anything but active basses. The second switch could be used as a mid shift for the fixed mid control instead though. There's also one switch on the back panel that'll probably be used for EFX/DI routing, and gain and master controls for the DI/EFX send, which is pre-HPF and can drive a power amp with no problem.
 
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#50
During test fitting of the new front panel a few new rabbit holes appeared due to slightly different control placements. Metalworking is really not my forte, but I got it done OK and all the filing and grinding is invisible once everything is assembled. Need different trim washers for the switches too, but that's nothing too tragic. Edit: now sorted.

PW7B2_newpanel.jpg



So I've had the preamp/power amp rig out at two open mikes so far, with very good results on both EUB and fretless slab. It'll be interesting to see how the preamp works with passive basses now that I've installed a 330K ohm input option.
 
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#51
I like the shade of blue. If the washers are stainless you can mount them in a drill and spin them with a file held on the edge to grind them down to the desired diameter.

What input impedance did you have before the 330k option? 1M?
 
#52
beans-on-toast said:
I like the shade of blue. If the washers are stainless you can mount them in a drill and spin them with a file held on the edge to grind them down to the desired diameter.

What input impedance did you have before the 330k option? 1M?
Click to expand...

Nope, no accommodation for passive basses at all. This one now switches from 18K to 330K. It's just a courtesy thing since my gear gets used at jams a lot.

Good call on the washers, I have plenty of extras and they are SS.
 
#53
I was recently looking at a preamp design from the 1940's. They had two inputs, one each for crystal and magnetic pickups. It featured a variable input impedance. Something that I've never seen before. Before standards were adopted I suppose.

With two fixed resistors like you used it would be quieter.



image.jpg
 
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#54
beans-on-toast said:
I was recently looking at a preamp design from the 1940's. They had two inputs, one each for crystal and magnetic pickups. It featured a variable input impedance. Something that I've never seen before. Before standards were adopted I suppose.

With two fixed resistors like you used it would be quieter.
Click to expand...

Cool, I love going through old tube design texts.
 
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#56
And on to some cosmetic detailing. New front panel for power amp module:

Halfrackstack3.jpg



I've played out with this mini stack quite a few times over the last few months and everything seems to work just as expected. I found a decent hard case for the preamp on eBay and a great laptop bag at the Salvation Army that holds the power amp perfectly, plus cables, tuner, etc. in other compartments. Maybe a slightly less convenient schlep in some ways than a rack case, but I'm still at a very easy two trip load in or out, as good as it's ever likely to get for me really.
 
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#57
Passinwind said:
And on to some cosmetic detailing. New front panel for power amp module:

Halfrackstack2.jpg



I've played out with this mini stack quite a few times over the last few months and everything seems to work just as expected. I found a decent hard case for the preamp on eBay and a great laptop bag at the Salvation Army that holds the power amp perfectly, plus cables, tuner, etc. in other compartments. Maybe a slightly less convenient schlep in some ways than a rack case, but I'm still at a very easy two trip load in or out, as good as it's ever likely to get for me really.
Click to expand...

very nice rig!
 
#59
beans-on-toast said:
Silly question Charlie, how do you get power into it?
Click to expand...

No free lunch today David, just an IEC connector for the preamp and a Neutrik Powercon for the amp. The unusual placement of the Speakon connector is a function of using this case style, as it would be a major PITA to machine the thick extrusions if placing the connectors along the long side, which seems to be what the ICEpower folks had in mind.
 
#60
Passinwind said:
No free lunch today David, just an IEC connector for the preamp and a Neutrik Powercon for the amp. The unusual placement of the Speakon connector is a function of using this case style, as it would be a major PITA to machine the thick extrusions if placing the connectors along the long side, which seems to be what the ICEpower folks had in mind.
Click to expand...

I should have known! I love those powercons. I've never used one in an amp. I've seen them in lighting applications.
 

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