Would like to try and build a Tube pre/Hypex UcD 400 power unit for Light weight bass - diyAudio
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Old 30th January 2009, 10:04 PM   #1
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Default Would like to try and build a Tube pre/Hypex UcD 400 power unit for Light weight bass

Hi All,

Names Robbie from RIM Custom Basses in the UK, Im a custom Bass guitar builder. Ive been looking at the amps coming out for bass guitars and at the moment everyone seems to be going into Class D stuff. I would like to try and build a 400/700w bass amp using a 6SN7 Tube preamp/Hypex Ucd 400/700 Power module with SMPS from Hypex also to keep it light. I would like to maybe have a Volume and tone control on the front end to keep it simple.

Any info or help on this would be great, as im a newbee to building amps. It looks doable and could be fun


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Old 31st January 2009, 04:35 AM   #2
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Location: Seattle USA
You're on the right track.
If you go to Hypex.nl and pick the UCD 400 there will be a manual.
Read the other thread in this forum about availability of SMPS for it.

You will end up with a high power, reliable, lightweight, super low distortion amp.

What's the tube for?
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Old 31st January 2009, 02:02 PM   #3
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I was thinking the valve front end would give it some extra warmth, this company makes there own tube preamp


If not valve could you help me with any other types of preamps with maybe just Volume and tone controls.


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Old 31st January 2009, 03:35 PM   #4
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Location: Seattle USA
I'm sure that since you hand build custom basses they already have whatever you're looking for in "warmth"

There's an excellent book here:

The author can often be found at:

For bass, I would suggest 6.42
Change the cap values for bass.
You could be unique and use a strat type switch.

Best of luck,
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Old 1st February 2009, 04:39 PM   #5
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Im thinking this London Power bass preamp kit could work well,



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Old 23rd February 2009, 11:39 PM   #6
stroy05 is offline stroy05  United States
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Location: Houston
Hey Robbie,

I have been working on the same thing for a couple of years. I have got something working for the past few years now and it actually great. I just posted a pic over in the Epifani club on talkbass.
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Old 24th February 2009, 12:07 PM   #7
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Hi Robbie,

You are absolutely on the right track. For some reason there are no better interfaces for guitar and bass than tubes. The input of the old Vox AC-50 with 12AU7 is one my favourites for bass. But maybe some more Eq-controls are needed.

I build guitar-amps(check my profile) but have had the thoughts of this concept for bass.

I would then take it on step further with a tube driving the Hypex via an input transformer instead of the OP-amp buffer.
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Old 24th February 2009, 07:15 PM   #8
Koifarm is offline Koifarm  Netherlands
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Lars, how would you design or buy this transformer for the UCD because the + input has 8k2 impedance and the - input has 1k8 impedance? This is a big difference to overcome.
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Old 24th February 2009, 08:25 PM   #9
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Hi Koifarm,

You have to search(google) the basics of differential amps. I am no pro but this is elementary.

Even if it sounds strange the UcD has a Zin of 2*1,8k=3,6k when connected differential.

Differential connection is also the best as you should get a better CMRR and no risk of ground loops.

So a transformer with lets say 4,5:1 gets a reflected input impedance of over 70kohm at the primaryin the midband. But a 90 H transformer(as in the example below) has an impedance of just above 10kohm at 20Hz

There are plenty of transformer out there but I can highly recommend Lundahl.

One advantage is that you also get galvanic isolation between Hypex and driver/preamp with a transformer.

Below is a theoretical example of a combination that would work with low distortion without tendencies of clipping before a Hypex UcD400 does. The secondary L1 should read 4.4 when connected 4,5:1, though.

One could also use a parafeed or balanced configuration.
Attached Images
File Type: png sehypex.png (60.1 KB, 442 views)
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Old 25th February 2009, 10:34 AM   #10
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Kevins O´Connors preamp should be good company to the Hypex.

Still bypassing the input opamp and adding a tubedriver would be nice. Mail me if you want some help with the design.

With a SMPS supply you could travel really light ! The Redrocks supply looks promising.

I also use a DC/DC 12V/(up to)210V SMPS for the tube pre. 12V for this and the heaters is available from the Redrocks PSU
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