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DPA mma-6000 8.4V NiMH battery tests

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mountainhop:
not sure if this belongs in battery boxes/preamps section or here, its power related and contains some other useful info on other batteries so ill leave it here for now, moderators can move it over there if appropriate

I was doing some runtime test on my MMA6000 and was dismayed to see it hit red light with alkaline 9Vs after just a few hours. Documentation from DPA is slim, per usual, other than the fact that they say either alkaline or NiMHs are acceptable. I saw a little bit of discussion here on powering the MMA6000  (like here and here) but they were ancient and not really full of good info so i decided to go from scratch

The problem with alkaline batteries in this device is the red light comes on at about 7.6V. With the steep slope of a 9V alkaline discharge curve this means that it is hitting red light with over half of what we consider capacity on the table

at typical gain levels the mma6000 pulls an estimated 20-30 mA, and we see that Li-ion batteries are about the same in regard to leaving half the battery capacity on the table at red light. This is actually of little consequence because the Li-ion capacity is so high that half of its capacity is still close to 20 hours. This and the fact that keeping your Li-ion discharge to 40% capacity or above is a general rule of thumb, the Li-ions should live a nice happy life and power the pre in the right range



that leaves us with the NiMH batteries, which come in two flavors, the typical 8.4V 7-cell, and the less-common 9.6V 8-cell. These nameplate voltages are somewhat misleading as the voltage can be substantially higher than that at low current draw, particularly when fresh off a charger.

As has been posted here before, high voltages (from even alkalines) can put 406x caps at risk with the wrong circuit


--- Quote from: Len Moskowitz (Core Sound) on September 13, 2020, 07:20:39 PM ---DPA used to publish a schematic of a powering circuit for the 4060-series that showed 9 Volts DC through 10 kOhms, like the link below.

https://jwsoundgroup.net/uploads/monthly_05_2014/post-2730-0-02421800-1399370626.jpg

If you tried that on your own, you'd face a 50/50 chance of destroying your expensive 4060-series capsules. That's because a fresh 9 Volt battery can put out as much as 9.6 Volts. That small difference was enough to often destroy the 4060's FET.

We changed it to use a 13k resistor. That solved the damage problem and retained the extra headroom feature.

--- End quote ---

That said, if 9.6V can put a 406x at risk, anything over 10V should probably be treated with caution.

as we can see, at low currents, the 9.6V NiMH batteries start out higher than 11V and remain above 10V for basically their entire cycle




the 8.4V NiMH seem to be a safer bet and a better match for the MMA-6000 (or any battery box powering 406x, really). Its discharge curve is exceeding flat and right in the proper range for most equipment
 
these can read above 10V right off the charger (ive seen as high as 10.4V), but (thanks?) to the high-self-discharge of NiMH they tend to be around upper 9s after sitting around for a day. I was also successfully able to burn off the surface charge by running the MMA6000 at max gain with no mics attached for 30 minutes. This got it under 10V but in the future ill probably run it down to under 9.5V

so this test
start voltage: 10.1V, didnt bleed off any charge
MMA6000 at max gain, +37.5V
capsules were 4015 on MMP-G cables, which should draw similar power to a 406x



impressive! 22 hours to red light at max gain. this is why i would be comfortable running it down to 9V in advance to have close to 20 hours of operation on a very flat curve

i was curious how long it would run after red light, so i did a separate test after letting the unit run overnight.



it was about 45 minutes before it started to nosedive. If you like your NiMHs you shouldnt really take them below 0.9-1V per cell (6.3-7V in this case). With this info in mind it really appears as if the battery light is in fact calibrated to work perfectly with an 8.4V NiMH. While an alkaline will run the MMA-6000 for several hours past red light, witht he scant information we get from DPA its difficult to know what the design spec is for the main voltage rail in that preamp. many preamps and ADs will start to lose headroom when this voltage is crossed, so although it may sound ok at 6-7V on an alkaline its difficult to say whether this is a good range of operation, particularly for very loud sources or high-gain applications.

That said, based on Len's advice above, if you are unsure of the circuitry of your battery box, or if it was built according to DPAs original recommendations, it might be best to even burn a little charge off an alkaline to get it safely under 9.5V before using it to power a battery box, or even 9V but that leaves you less room before you hit the 7.6V red light.

I know some of you (Gutbucket!) have used the 9.6V cells to power miniature DPAs to success, but am unsure if that was with straight battery boxes, or preamps, and what those particular circuits involved. perhaps i might email chris and have him weigh in, since hes also designed and built a lot of pres for these mics.

Lacking further details on this preamp, its hard to say what the acceptable voltage range is. Without someone like like Bruce Myers to answer tech details, Longmont is pretty much void of tech people (or at least those willing to talk), and getting straight answers out of denmark is nearly impossible on current products, let alone discontinued ones. We do know that the external power input is regulated to some extent as it requires 12-30V, and the unit kicks into a different indication mode when external power is present. im personally not tearing mine down to find out how internal power works

Its also hard to say how common 9.6V NIMHs were back then. They did exist, but the manual vaguely specifies "9V NiM or NiCd", which, like alkalines, never exceed 9.5-10V in normal operation

Personally im playing it safe an staying away from 10-11V. I'm only providing the information in context of what it is. Hopefully someone with an MMA-6000 finds the runtime tests useful

aaronji:
DPA probably configures the MMA6000 to regulate the power going to the mics, as they do with the P48 adapters...

I would also, personally, take Len's comment with a grain of salt. It seems odd that DPA would publish a schematic which fried 50% of the capsules. Certainly, Len has a bit of an ulterior motive to claim his (proprietary) solution is best. Anecdotally, many 4060 users have used 9V non-DPA battery boxes with all kinds of batteries and I don't recall reading about any being destroyed (not saying it hasn't happened; just that I haven't seen it over many years as a 4060 user).

aaronji:
In other words, you have no clue despite the long-winded post? Science at it's best, as usual 'freak...

Gutbucket:
Thanks for the testing and discharge graphs.

I power DPA 4060/4061 with Church Audio preamps (2 and 4 ch CA-UGLY) using 9.6V NiMH, yet first confirmed that voltage to the mics was <10V through those preamps with the battery freshly and fully charged. That was a long time ago and I don't recall the details. 

I've not used MMA6000 in years, and am trying to remember if I ever used 9.6V NiMH in it or if I measured voltage through it.  I expect I did at some point later on, but most of the time that I was using that preamp regularly I was using alkaline 9Vs in it.

I've never used a battery box to power these mics, but if I did I would use 8.4V NiMH, or a resistor that otherwise limited voltage to <10V to be safe.

aaronji:
This is from an e-mail with Longmont's service chief in 2013 (previously posted). This contradicts what Len said, but also suggests that the DPA power sources, including the MMA6000, do constrain the voltage to the mics.


--- Quote from: DPA ---These are very good questions, the mic ideally like 8-12V to run. I have seen people use as high as 18V with no problems and as low as 5V.  If you get lower than that the mic gets really noisy. Most of our little power supplies put out around 7-8V.

--- End quote ---

In a follow-up question, I asked if a 16V power source would be OK (can't for the life of me remember to which box I was referring) and this was the reply:


--- Quote from: DPA ---16V is fine and it will not void the warranty.

--- End quote ---


I have always just used DPA power sources (MPS6030 or MMA6000), but it would be nice to get a conclusive answer to this question. Once and for all...

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