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Old 23rd October 2006, 08:00 PM   #1
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Default anyone using a battery powered pre?

I was putzing around the ESP site and found a neat circuit for a float chatger and another for a sensor circuit to disengage the charger when it sees "turn on" of an amp or pre. I've got a couple of 100Ah SLAs but when I looked around last night I could not find a pre that uses a 12-0-12 supply. My current pre's, Kenwood L-07 and B&K Model 1, are pretty good but it would be nice to DIY if the results would be worth it. All I'd need would be line level.
TIA, John..
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Old 24th October 2006, 11:35 PM   #2
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Thumbs up Battery Preamp

Hi John,

I do. I've built several preamps with great success, but none as clean and flawlessly noise-free as my current unit powered by two 12V, 1.2Ah Gel Cell batteries.

The preamp design is a very basic op-amp design with much attention paid to quality and precision. I use a Burr-Brown OPA2604 (my next design is almost done and uses an LM4562) op-amp, switched attenuator, very clean grounding, and decent Teflon wire. All components are hand-matched and are decent quality parts, but nothing fancy with big (expensive) names. I simply used what works and measures well.

The two batteries form a +/- 12VDC supply that is heavily filtered due to the nature of Gel Cell's impedance to rise with frequency. I use an off-board charger that I plug in after every 20 or so hours of use. The battery-supply provides an absolutely noise free voltage source that is ultra stable and has extremely low impedance accross the full audio band. I have never measured a regulator circuit, regardless of design or complexity, that is as "quiet" as this battery circuit.

You mentioned 100Ah batteries - um, slight over-kill, but will still give you +/- 12V. Most preamp designs will run just fin off +/- 12V, but will limit your output voltage swing to about 4VRMS. Still, that's more than adequate for most any commercial amplifiers to hit full output. I highly recommend trying it - you may never go back to regulators or mains-powered preamps again.

Cheers
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Old 25th October 2006, 01:58 AM   #3
paulb is offline paulb  Canada
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Mercator and I are in the middle of building one. We're also using 2x12V batteries. We're using different gain blocks. I can send more details if you like.

Paulb
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Old 25th October 2006, 02:08 AM   #4
Leolabs is offline Leolabs  Malaysia
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Wondering these 12V battery able to run a power amp or not???
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Old 25th October 2006, 02:37 AM   #5
hacknet is offline hacknet  Singapore
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if you stack enough of them together.. i know someone who had a battery powered gainclone..
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Old 25th October 2006, 03:32 AM   #6
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Quote:
Originally posted by paulb
Mercator and I are in the middle of building one. We're also using 2x12V batteries. We're using different gain blocks. I can send more details if you like.

Paulb
Yes please. I already have the somewhat large 100Ah batteries so I guess I could use Class A if I wanted to. Heck, I have a dozen of them. I just want to try something different.
John......
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Old 25th October 2006, 07:04 AM   #7
scottw is offline scottw  United States
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Quote:
DCPreamp wrote:

...a +/- 12VDC supply that is heavily filtered due to the nature of Gel Cell's impedance to rise with frequency.

Just curious, do you use any inductance in your filter? I've heard of using chokes in series with a battery supply but never any specifics on the size of the chokes.


Thanks,

Scott
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Old 25th October 2006, 04:43 PM   #8
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Default Battery preamp

Hi scottw,

No inductance used. My goal was to have very low impedance across the entire audio band so the op-amp does not need to rely on PSRR. With an inductor in series, the problem would be compounded at higher frequency. So, I use 16mF (yes, milli) electrolytics plus 4 x 100nF mono-ceramics parallel with each battery. I also use additional mono-ceramics, small electrolytics, and tantalums from rail-to-rail. Take a glance at my avatar Ė thatís the actual PCB. Itís overkill, but at a DIY level, the cost is not an issue and the performance is outstanding. With my scope at maximum resolution, signal ground, +Vcc, -Vcc, Vin, and Vout are indistinguishable with effectively zero noise. Zero noise at Vout is, of course, with 0dB attenuation. Every mains-powered preamp, regardless of design, and every regulator circuit, regardless, Iíve tested clearly shows measurable noise under the same test conditions.

Iíve seen inductors used in car audio, 12VDC stuff, but thatís more intended to reduce alternator ripple and charging noise. In car applications, higher impedance at higher frequency is desirable.

What else can I say? My preamp is as ďnoiselessĒ as I can measure and hear, and is as close to a passive preamp in clarity, but with gain and a low Zout, as I can get. 2Hz to 200Khz, 13dB of gain, 24-step attenuator, 90dB separation, and non-existent coloration. I donít have equipment measure distortion, but Iím pretty sure it way, way down there. I love listening to music through it. It is truly a pleasure to use plus that DIY comfort knowing it could probably go head-to-head with $1K ďaudiophileĒ units and do very well.

Best regards
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Old 26th October 2006, 02:10 PM   #9
paulb is offline paulb  Canada
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The two preamps we are building will use different gain stages. Mercator's using an ESP P88, while mine will have a Valkyrie board from an article in AudioXPress (actually probably Audio Amateur) many years ago. Dave oops I mean Mercator's P88 uses OPA2134 op amps. Both will have differential input and outputs, based on another ESP design, in addition to the single-ended ones. I may end up using mine mostly as a headphone amp, with the differential output for audio distribution through my house.
We're using a capacitance multiplier on the audio power supply rails, and a charging circuit, based on 2 TI UC3906s, that charges when the preamp is not being used. Input switching is done using reed relays, volume control uses a motorized pot. I will use an RF 4-button remote, while Mercator will control his from a PC that itself has an RF remote. The control / charging power is completely electrically separate from the battery/audio power.
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Old 26th October 2006, 07:03 PM   #10
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Quote:
Originally posted by paulb
...The control / charging power is completely electrically separate from the battery/audio power.
Wow, that sounds really hi-tech! I sure would like to hear one of those some day
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