Battery-powered supplies

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SLA powered "passive preamp"

Greetings,

I recently built a buffered "passive preamp" that is powered by SLA batteries. The sonic results are stunning.

I used Shallco stepped attenuators with Resista resistors and OPA627 opamps as buffers. The batteries are rated at 7AH.

<center><img src="http://207.115.74.87/vcm/WH/images/DCP_2316.JPG"></center>
 
<center><img src="http://207.115.74.87/vcm/WH/images/DCP_2305.JPG"></center>

What did you use for charger? I'm thinking about similar preamp.

I used two Velleman SLA charger kits. Initially I was going to build a charger from scratch or buy two of the circuits that Chris Brady uses for the Teres. The Velleman works very nicely since it automatically switches from deep to trickle mode and can remain connected to the batteries 24/7. In addition, I have a switch on the front panel of the preamp to take the charger out of the circuit while listening.

Is that two stepped attenuators that you have used instead of one stereo?

Yes. I struggled with this for some time. I really wanted a single control but didn't want to add an additional balance pot to the signal path. The Shallco switched can be ganged together but they already require mucho effort to rotate.

And before anyone asks, where did you get those knobs?

I had a local machinist spin them for me. They are 2.25" in diameter and 1" high. The cost was very reasonable.
 
Yes. I struggled with this for some time. I really wanted a single control but didn't want to add an additional balance pot to the signal path.

Hi Mac, I have never had a problem with channel balance when using a stepped attenuator! You must have very fine steps on your attenuators, or quite a big imbalance! Do you know what is causing the imbalance?

The Shallco switched can be ganged together but they already require mucho effort to rotate.

Yes, this is nearly always a problem with lightweight equipment like hi-fi. I once built a 'two-switch' stepped attenuator using two 12 position rotary switches and ganged them together but the feel was just not right.

I like the feel of Bloore Edwards switches that I use now, but only after replacing the springs with something 'lighter'.

I had a local machinist spin them for me. They are 2.25" in diameter and 1" high. The cost was very reasonable.

I'm surprised that 'decent' knobs aren't more widely available! It's very difficult to find any good looking knobs 'off-the-shelf'.
 
Nuuk said:
I'm surprised that 'decent' knobs aren't more widely available! It's very difficult to find any good looking knobs 'off-the-shelf'.

I think <a href="http://www.kilointernational.com/">Kilo's</a> ML series knobs are quite nice for an off-the-shelf:

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<img src="http://www.kilointernational.com/new/Knobs/ML_Series/mlknobs.jpg">
</center>

The two rings are like rubber O-rings. Come in diameters from .5 to .95 inches, for .125 inch, .25 inch and 6mm shafts and available in black and clear with gloss or matte options.

se
 
I have never had a problem with channel balance when using a stepped attenuator! You must have very fine steps on your attenuators, or quite a big imbalance! Do you know what is causing the imbalance?

Sorry, I wasn't clear. What I struggled with was the decision to either use a single volume control with an added balance control or just go with dual volume controls. I don't have a balance problem; I just wanted the capability to change it.

The audio taper posts have 45 positions and provide 1db resolution through most of their upper range.

I was advised by Shallco to not adjust the spring tension and took their recommendation at face value. The silver contacts and wipers are rather robust and I assumed that lightening up the tension might effect their make-before-break action.

The largest Kilo knobs that Steve mentioned were just too small to provide adequate torque for the Shallo controls.
 
I think Kilo's ML series knobs are quite nice for an off-the-shelf:

Once again we can only envy you chaps over there Steve. I had a look for a similar UK supplier but haven't found one yet. Instrumentation knobs yes, but sexy ones for hi-fi?

One day, I will get around to moulding my own in casting resin but it's a lot of outlay to make the moulds etc for just a few knobs!
 
diyAudio Senior Member
Joined 2002
Subdued He Answered...

Hi,

The Shallcos are hard to come by in Europe...which leaves us with the Blore Edwards which at first glance seem to offer similar quality.

Re: the torque issue, I find the Elmas great in that respect... for my needs the 23 useable steps are more than ample, I use 3dB steps at the less used positions and 2dB steps at the more popular positions.

To the human ear a 3dB step is clearly audible, anything beneath that is much harder to detect.

Another advantage is that the Elmas take any kind of standard knob.
RS Components in the U.K. carry a range of knobs, nothing all that fancy but nice quality nonetheless.

Cheers, ;)
 
I posted this at the just anothe gainclone thread but it belongs here. The batt ps is the one at the bottom the gc is in the middle and the batt ps detail at bottom pic.
 

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