[comabereni]

I just bought a Millett tube amp from Voodoochile (thanks Mark) and have been kicking around some ideas for converting it to battery power. I intend to use the circuit board from a Go-Vibe as a pre-amp per Wodgy's comments on how using his META42 as a pre-amp really elevated the Millett's performance. Here's what I thought:

- The Millett amp runs on 24-30VDC. Instead of a wallwart, I would like to use two 12V 7-amp hour lead-acid batteries wired in series and located inside the case, probably under the PCB to keep the footprint as small as possible

- I would install a switching circuit that alternates between "on" and "charge", so that the 'Battery Tender' wall charger would be disconnected from the amp when the amp is operational, and charging the batteries when the amp is off

- The Go-Vibe is designed to operate at 9-volts. What must I do to make it run happily run at 12v, so I can run it off the same batteries/circuit, or should I/can I step down the voltage to 9v on a parallel circuit. Or should I use a different battery entirely? I really don't want to be manually swapping 9v batteries when I put this all together and would like the whole thing to charge off one wall charger when in the "off" position.

- Perhaps four (4) 9v batteries could be utilized instead--three in series (for 27v) to drive the Millett amp and one to drive the Go-Vibe-- a charge/on switch would send the appropriate parallel current to the four batteries during the charge cycle (this might be the best solution yet, but I don't know the availability of 9v lead-acid batteries and chargers...)

Thoughts? Recommendations?

Thanks.

 

[Nisbeth]

First off, the SLA's are great for the purpose but the 9V's are useless. They have far too little power to run the Millet hybrid. (EDIT: Oh, I thought you meant 9V NiMH's

Nevermind this comment

)

As for the "preamp" idea: You could either ask Norm if the amp will run at 27V without problems (which it might), or you can build a simple supply with an 7809 or LM317 voltage regulator. You basically need the same components as you would for an AC psu but you should of course omit the rectifier. The filter caps can either be reduced in size or omitted. If a 7809 is enough you basically only need the regulator with a 1uF film cap before and a 330nF film cap after (required for stability). Add a 470uF electrolytic before the 1uF for better dynamic performance. I would connect the regulator board across both batteries to make sure they are loaded evenly but this might not matter.

You can google for a suitable SLA-charger (a simple one with an LM317 as constant-current source will be fine) and simply use a SPDT switch to switch between listening and charging

/U.

 

[comabereni]

Okay Nisbeth-- good idea to begin by contacting Norm to find out if the Go-Vibe can operate at (much) higher voltage. If it'll run at 27v, or 12v, it solves the problem with having either two voltages or two battery types inside the same amp.

The other idea about a voltage regulator driving a separate pre-amp circuit (I assume that's what you are describing) is also good.

.

I guess the actual starting point is to wait until the Millett amp gets here and see if the Go-Vibe as preamp makes a significant difference to my ears. If not, then the whole thing is a cinch.

Anyway, this gives me some options--thanks.
Quote:

Originally Posted by Nisbeth First off, the SLA's are great for the purpose but the 9V's are useless. They have far too little power to run the Millet hybrid. (EDIT: Oh, I thought you meant 9V NiMH's Nevermind this comment ) As for the "preamp" idea: You could either ask Norm if the amp will run at 27V without problems (which it might), or you can build a simple supply with an 7809 or LM317 voltage regulator. You basically need the same components as you would for an AC psu but you should of course omit the rectifier. The filter caps can either be reduced in size or omitted. If a 7809 is enough you basically only need the regulator with a 1uF film cap before and a 330nF film cap after (required for stability). Add a 470uF electrolytic before the 1uF for better dynamic performance. I would connect the regulator board across both batteries to make sure they are loaded evenly but this might not matter. You can google for a suitable SLA-charger (a simple one with an LM317 as constant-current source will be fine) and simply use a SPDT switch to switch between listening and charging /U.

 

[Wodgy]

Hmmm, I don't recall making comments about using the Millett Hybrid with a META42 as a pre-amp. That must have been long ago. I'm not sure I'd still recommend it (if that's what I was recommending; I'm confused). The Millett Hybrid is good enough on its own.

As for running it on batteries, you won't have much luck. The necessary heater current for each tube is 150mA, for a total of 300mA. You'd be hard pressed to get more than 45 minutes of use off any reasonable size battery configuration, unless you're talking car batteries.

 

[comabereni]

Quote:

Originally Posted by Wodgy Hmmm, I don't recall making comments about using the Millett Hybrid with a META42 as a pre-amp. That must have been long ago. I'm not sure I'd still recommend it (if that's what I was recommending; I'm confused). The Millett Hybrid is good enough on its own. As for running it on batteries, you won't have much luck. The necessary heater current for each tube is 150mA, for a total of 300mA. You'd be hard pressed to get more than 45 minutes of use off any reasonable size battery configuration, unless you're talking car batteries.

Hey Wodgy,

I went looking for the META42 pre-amp comment and couldn't dig it up again via my seearches, so maybe I read it somewhere else, or mis-read it. In any case, your comments here and now are what matters.

The 12v 7-amp hour batteries I'm talking about are rather large (computer UPS' use them), and I'm talking about using at least two of them, maybe up to four--series or parallel, or both. Whatever works. Here are some pics of the battery type I'm discussing:

Dimensions are: L: 6" W: 1.6" H: 3.9"

Using a 12AH capacity/size is also feasible. One of these things will run all the electronic and electrical components on my telescope for several nights (DC drive motor, three 3.5" computer fans, anti-dew heater, object database and locator computer, etc.), so I would hope you'd get more than just a few minutes listening from a couple of these things, but my hope may indeed be in vain. The combined size of two 12v 12ah (or three 7ah) sealed lead acid batteries is something like 1/2 the size of a standard car battery, and very heavy. I suppose it all comes down to current draw. I'll try to work up the numbers now that I have the heater current.

 

[Wodgy]

Those will work! (I was under the impression you were thinking about two or three 9-volt batteries.) Just connect two of the 12-volt batteries you describe in series and plug the result in to your Millett amp. No problem.

 

[comabereni]

Quote:

Originally Posted by Wodgy Those will work! (I was under the impression you were thinking about two or three 9-volt batteries.) Just connect two of the 12-volt batteries you describe in series and plug the result in to your Millett amp. No problem.

**COOL**

Also, eliminating the preamp detail makes this thing just that much easier. Looks like I'll be running the Millett amp on clean DC power with a charge/on switch in short order!

Keep hunting down my threads Wodgy--you seem to have all my answers.

Thanks.

 

[Nisbeth]

If you go for a series/parallel battery configuration, remember that charging batteries in parallel is not recommended. You need to find some way of either connecting them in series for charging or build a separate charger for each set. Try using one set of SLA's at first and if battery life is a problem, then experiment with other configurations if necessary.


/U.

PS: Don't forget to put fuses everywhere as the SLA's will put out several hundred amps if they're shorted (making them a potential fire hazard)

EDIT: If Wodgy's numbers are correct about the current draw, the battery life should be around 15 Hrs on a single pair of 12V/7Ah. If you set up the charger to trickle charge the batteries when you're not using the amp I don't really think you need anymore than that.

 

[PeterR]

Max supply voltage for the OPA2227 is 36V, the max spec for the AD8620 is 26V, which two fully charged 12V batteries in series will exceed.

 

[comabereni]

Quote:

Originally Posted by PeterR Max supply voltage for the OPA2227 is 36V, the max spec for the AD8620 is 26V, which two fully charged 12V batteries in series will exceed.

Should I decide to preamp, the Go-Vibes I have utilize 2227's. Is the opamp the only part in the Go-Vibe I need to worry about blowing if connected to the serial configured 12v batteries?

Quote:

Originally Posted by Nisbeth If you go for a series/parallel battery configuration, remember that charging batteries in parallel is not recommended. You need to find some way of either connecting them in series for charging or build a separate charger for each set. Try using one set of SLA's at first and if battery life is a problem, then experiment with other configurations if necessary. PS: Don't forget to put fuses everywhere as the SLA's will put out several hundred amps if they're shorted (making them a potential fire hazard) EDIT: If Wodgy's numbers are correct about the current draw, the battery life should be around 15 Hrs on a single pair of 12V/7Ah. If you set up the charger to trickle charge the batteries when you're not using the amp I don't really think you need anymore than that.

Okay, no parallel charging--I'll have to look at that. When I go about an electronics problem, unlike some of you guys, I don't immediately start thinking about how I can *build* two 12v trickle chargers and mount them internally to manage each battery

. I just looked up 24v chargers and they are relatively expensive (relative to the cost of the amp anyway: here's one for $399.95; here's another one for $159.95), I need to think more about this two 12v battery thing as I assumed an external charger like the one I have (Battery Tender, Jr.) would happily charge 12v batteries wired in parallel. There are 24v tool battery quick-chargers like those for DeWalt tools for around $75, but I doubt the circuit would work with lead-acid batteries.

As per your edit regarding the 15-hour battery life on two 7ah 12v batteries--that's just plain cool--almost an entire waking day. Perhaps I'll consider even smaller batteries if the total footprint or case size is excessive using the 7ah's.

Finally, if I don't utilize a Go-Vibe as preamp, going to battery power may not be paramount since I was primarily looking at that option as a way to keep the GV's signal as clean as possible since it wasn't designed (that I'm aware of) to filter AC line noise. I suppose I still think going DC has worthwhile advantages no matter what I do regarding a preamp.

EDIT: I just found an article written by Rod Elliott on building DC charger circuits specifically for preamps here. Rod starts off by saying "At last, remembering to turn the charger back on is no longer a problem!", which makes me wonder why he never considered wiring a switch that turns on the charger when powering off the preamp? I admit I haven't thought of the details of this on/charge switch yet either, but don't see why it should pose a problem (???)

EDIT2: More research on charging--this site shows numerous charging configurations using various battery voltages, chargers, and parallel/serial connections. One configuration shown is two 12v batteries being charged in parallel.

Not sure what limitations exist that I'm not aware of, but this is what I had hoped to do. Only I had envisioned running two wires from the + lead on the charger to the + terminals on the two batteries and the same for the - . I guess that would be going about it all wrong...

Anyway, if parallel charging is an option, then since I already have a 12v charger, all I need to do is configure a switch to change the batteries from parallel to serial while turning on/off the amp and on/off the charger at the same time. Maybe that isn't possible using only one switch (which would be best since I wouldn't need to remember to turn on the charger), but it should certainly be accomplishable by separate switches.

Thanks.

 

[Nisbeth]

As you can see from the label on the side of the battery, the charging voltage has to be a bit higher than 12V, so a 24V charger isn't going to work.

IMO, building a charger should actually be an manageble task for most people - you included

If you're nice to PeterR he might post the schematic for the one he build a while ago

I haven't heard the Millet-amp, but I think you'll find battery power to be a worthwhile upgrade. If you're not sure, then wait until you have had the amp for a while and have gotten yourself acquainted with it. Then you should be able to test if the battery power provides any sonic benefit. If it doesn't, you can just put a pair of bricks a separate enclosure. You'll then get the "instant coolness" of having an amp that seems to run on batteries but it's much easier to make since bricks don't require chargers...

/U.

 

[comabereni]

Quote:

Originally Posted by Nisbeth As you can see from the label on the side of the battery, the charging voltage has to be a bit higher than 12V, so a 24V charger isn't going to work. IMO, building a charger should actually be an manageble task for most people - you included If you're nice to PeterR he might post the schematic for the one he build a while ago I haven't heard the Millet-amp, but I think you'll find battery power to be a worthwhile upgrade. If you're not sure, then wait until you have had the amp for a while and have gotten yourself acquainted with it. Then you should be able to test if the battery power provides any sonic benefit. If it doesn't, you can just put a pair of bricks a separate enclosure. You'll then get the "instant coolness" of having an amp that seems to run on batteries but it's much easier to make since bricks don't require chargers...

Thanks Nisbeth. Note that I was editing and adding to my post above yours at the same time you were responding. Perhaps there are some things in there to respond to.

Good thing a 24v charger won't really work as the cheapest charger cost more than the amp. I guess some people use $2,000 cables with their iPods, but I'm not that kind of person. I found numerous charger schematics at this site:

Discover Circuits

I like the two bricks in a separate enclosure idea... well, not really

. I really want to go battery powered if possible. I think the pieces are coming together. Also, while the final result certainly won't clip to your belt and go on a bike ride, a battery amp could be picked up and moved from office to bedroom if desired without dragging a power cable along.

EDIT: Actually, the two bricks idea has more merit than at first blush (if I forego battery power)--I was thinking of some cool case tricks that would benefit from a few LEDs, etc. and I think running those things on separate power would be of benefit. I know when I install LEDs on a computer case fan circuit under 12v power, I no longer get near the max RPM's out of the fan--not sure what effect it would have on the current the amp would require to run optimally.

 

[Nisbeth]

Another charger option here. I've tested and built the first circuit and it works fine. It might be a little rougher on the batteries than is necessary, but for such a simple circuit it's just fine. You can build it on piece of veroboard/stripboard quite easily. Set the charging current to something like 700 mA for a 7Ah battery and remember to put a small heatsink on the regulator.

/U.

 

[JHouser]

To be honest I have no idea if this would help any but these are the batteries we use for motion picture cameras. Very small and compact.

http://www.mdsbattery.com/shop/produ...ctGroupID=1053

Hope this helps.

 

[comabereni]

Quote:

Originally Posted by JHouser To be honest I have no idea if this would help any but these are the batteries we use for motion picture cameras. Very small and compact. http://www.mdsbattery.com/shop/produ...ctGroupID=1053 Hope this helps.

Yes, thanks. These are the exact same type batteries being discussed, i.e. sealed lead acid. These appear to have unique tabs/terminals for use in camcorders, etc. but they are essentially the same. It looks like you get to choose from a lot of size and capacity options at the link you provided. Prices are also comparable.