[gtp]

Hello,

I am putting together a Cmoy and need to decide on a battery configuration. I am planning on using rechargeable 9V or 9.6V. Opamps will be the usual suspects: 2227, 8620, 2132, and perhaps 8397 if I can work out all of the stabilization schemes from Tangent's website.

I am planning on using a Serpac H65, so I figure I can jam a small Cmoy and 2x9 volts in there. So the question is: Is there audible improvement with 18V? Or is there merely an improvement in the life before recharges? Are the differences significant or is it a waste to go 18V?

Thanks in advance to all responders!
GTP

 

[sbelyo]

I noticed a huge difference. A CMOY circuit will run on 9V. That would give you -/+ 4.5V for the opamp. Some of the opamps you listed have a -/+ 5V minimum. So right off the bat you are down 1 volt.

 

[Wyloch]

Go 18V, without a doubt.

 

[kramer5150]

x3... I run my gmoy at 18V. 4 NIMH's in series / parallel. Recharge them with a rat shack 18V charger, cat # 23-349.

Its my transportable gaming amp, so I wanted the extra capacity and extra voltage potential for use with my K240s or DT770

Garrett

 

[prof.anderson]

Quote:

Originally Posted by gtp I am planning on using rechargeable 9V or 9.6V.

First, I hope that you have found some 8 cell batteries that will really give you 8.4 volts or more. Commercial "9 volt" rechargables usually drop down to 7.2 volts pretty quick. Running those op-amps on a 7.2 battery could be sketchy.
Second, you want to use NIMH batteries with the lowest internal impedence you can find.

I've never bothered to try to do both at once -- I just use dual 7.2 volt NIMHs. Sounds better than a single 9V alkaline, but that could have something to do with the high impedence of alkalines.

 

[Rix2357]

Whether you need the two 9V batteries depends on the gain you use. I know that the 2227 and the 2132 work fine on one 9V battery. In my build, the only reason why I used two was for the extra run time. One battery yielded me about 9 hours run time. Two yielded 22 hours. Mine is set at around 6 gain.

 

[mono]

With the aforementioned opamps and low-Z cans you should be ok with one *9V" rechargeable cell. I disagree with one post's suggestion to look for lowest impedance. You do not need particularly low impedance for a CMOY, it's but a few mA. Further, lowest impedance cells are a different internal design that compromises runtime for this impedance drop, at least when the specs are accurate, some battery specs are questionable on off-brands.

With 2 x 9V, you get almost no additional runtime at all in the series (for ~ 18V, actually more like 15-17V) configuration. There is a very minor benefit as the cells get almost depleted, staying above the floor for a particular opamp, but realistically not enough to make it wortwhile for that purpose rather than the issue of whether you needed higher gain.

H65 is relatively large for a CMOY. I would wonder whether it's possible to use 10 AAA cells, if you coudl optimize the CMOY to be low-height then except for the POT at the front and the jacks, the entire (remaining) length of the case should fit at least 8 AAA cells. That would only be to address any unusually long usage patterns, a CMOY is going to run pretty long off of a *9V* battery too, but with only one 9V, a Serpac H65 seems like a big case to use.

 

[tangent]

Quote:

Originally Posted by mono I disagree with one post's suggestion to look for lowest impedance. You do not need particularly low impedance for a CMOY, it's but a few mA.

If you think low supply impedance doesn't matter for a CMoy, try leaving out the power supply capacitors. If you don't think that's a fair test due to the CMoy's virtual ground weakness, go ahead and replace the divider with a TLE2426, but leave the PS caps out, and use alkalines. See how it sounds.

Quote:

With 2 x 9V, you get almost no additional runtime at all in the series

I see a bunch of short statements in this thread like this one that can be true, but may well not be. I wrote a fairly detailed article on this topic because the issues are complex. Anything less detailed is going to miss important factors, and so can lead to bad results.

 

[gtp]

Quote:

Originally Posted by mono but with only one 9V, a Serpac H65 seems like a big case to use.

The reason for the H65 is to better buddy-up with my Ipod. I want to use velcro or rubber bands to semi-perminently combine the two. I found that smaller amps are too small for this.

 

[gtp]

Quote:

Originally Posted by tangent If you think low supply impedance doesn't matter for a CMoy, try leaving out the power supply capacitors. If you don't think that's a fair test due to the CMoy's virtual ground weakness, go ahead and replace the divider with a TLE2426, but leave the PS caps out, and use alkalines. See how it sounds. I see a bunch of short statements in this thread like this one that can be true, but may well not be. I wrote a fairly detailed article on this topic because the issues are complex. Anything less detailed is going to miss important factors, and so can lead to bad results.

Tangent, I read your article (multiple times, and BTW I feel the need to buy you a beer or beverage of choice if I'm ever in NM!).

I'm in one of those engineering-tradeoff spaces. I have a Cmoy board bought on Ebay that uses a TLE2426. It has room for 10mm electrolytic caps. Running at 18V restricts me to smaller cap values to fit 10mm, say 470-680uF. Running at 9V allows me to use larger caps, up to ~1200-1500uF. I don't have the experience to know which trade-off would cost more sound-wise, i.e., 9V+big caps, or 18V+smaller caps.

Thanks,
GTP

PS: I will be using NiMH batteries at either 9V or 9.6V (probably not 8.4V unless I decide to go with two of them and 16V caps (haha, maybe there is my compromise????)).

 

[mono]

Quote:

Originally Posted by tangent If you think low supply impedance doesn't matter for a CMoy, try leaving out the power supply capacitors. If you don't think that's a fair test due to the CMoy's virtual ground weakness, go ahead and replace the divider with a TLE2426, but leave the PS caps out, and use alkalines. See how it sounds.

I don't claim that low impedance power does not matter, not that the caps can be removed, but rather, the caps are why the battery need not be lowest impedance.

 

[prof.anderson]

My understanding is that for a given capacitor used to decouple the power supply, the higher the impedance of the power supply the more likely power oscillation will occur. Of course whether that ever happens depends on the load, and that means the gain etc.

But higher impedance limits the other design options.

 

[mrjat]

Quote:

Originally Posted by gtp I'm in one of those engineering-tradeoff spaces. I have a Cmoy board bought on Ebay that uses a TLE2426. It has room for 10mm electrolytic caps. Running at 18V restricts me to smaller cap values to fit 10mm, say 470-680uF. Running at 9V allows me to use larger caps, up to ~1200-1500uF. I don't have the experience to know which trade-off would cost more sound-wise, i.e., 9V+big caps, or 18V+smaller caps.

my take would be the 470uF-680uF with 18V. going higher than 470uF would give minor (if there's any) improvement compared to doubling the voltage.

 

[Tomo]

Hi,

220uF-1000uF caps can be quite small if you reduce their voltage tolerance. Just so you know caps with big voltage tolerance can be quite large even if capacitance is low.

Can't you use really high quality electrolytics with high current capability like OSCONs? You can probably use smaller capacitance.

If you have headphones with less than 100 ohms impedance, you probably don't need to have the opamp swing a lot. I used to have single 9V CMOY running my Ety4S. You need like 10-20mW. Wouldn't it amount to a bit less than 1-2Vrms? If so, it is worth less to go 18V. ...

Lastly, I think Fixup had some really nice amp running on two AAA's. You can also use some digital CMOS invertors driving your cans; PSU voltage MUST be less than 5V. (Irony

)

You can have alotta fun. Tomo