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Old 8th October 2012, 04:16 AM   #2841
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What is the voltage necessary to turn on Q1 and Q2? I looked at the charts and I think that it's 10V, am I correct? What is the symbol for voltage at which a MOSFET turns on and off?

So far I'm thinking that the voltage to switch off Q1 and/or Q2 is lower than the voltage needed to turn it on, and that it might be the cause for why I can force turn on the amp to keep it running.

The issue that I have described in my earlier post occurs when the batteries are at about 8.30V.

Last edited by HurlockVanguard; 8th October 2012 at 04:32 AM.
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Old 8th October 2012, 05:13 AM   #2842
agdr is offline agdr  United States
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Quote:
Originally Posted by HurlockVanguard View Post
What is the voltage necessary to turn on Q1 and Q2? I looked at the charts and I think that it's 10V, am I correct? What is the symbol for voltage at which a MOSFET turns on and off?
Vgs(th), the gate-to-source threshold voltage. 2Vdc to 4Vdc for the N-channel device and -2Vdc to -4Vdc for the P-channel device. The higher (lower, p-ch) the voltage goes from there the more "on" the device goes, up to the gate to source maximum (Vgss) of 30Vdc (-30Vdc p-ch), although things level off pretty quickly. At 8.3Vdc x 2 = 16.6Vdc (rail to rail voltage across the gate to source) both devices will be fully "on" from graphs 1 and 2 in the respective data sheets:

http://www.fairchildsemi.com/ds/FQ/FQPF10N20C.pdf

http://www.fairchildsemi.com/ds/FQ/FQU11P06.pdf

The problem isn't the mosfets. RocketScientist included a hysteresis circuit around the comparator (R25 and C1) that works by making the "trip on" voltage a tad higher than the "trip off" voltage. That is a common thing to do in comparator circuits to prevent oscillations around the trip point. The "8.4Vdc" NiMH batteries actually run closer to 8.5Vdc - 8.7Vdc through their useful charge range, from here:

http://support.radioshack.com/suppor...h-9v-lodis.gif

NiMH 9-Volt Battery Engineering Data Sheet

By the time one of the batteries is down to 8.3Vdc is is pretty well shot and in need of a recharge. So the circuit is working as it should - it is just saying "recharge me", essentially. Would have been handy if RS had included a LED to indicate when the power management circuit had tripped, for diagnostic purposes if nothing else.

One suggestion is try a different brand of batteries. It is common for batteries to "self-recharge" a few fractions of a volt once the load is removed, another reason RocketScientist put in that hysteresis circuit, and the reason he had to change the values in that circuit once when the recharge voltages turned out to be a bit more than he expected. It sounds like your particular batteries are not recharging at all when the load is removed. Given the battery recharge phenomenon, the low battery behaviour most people seem to get is off/on osciallation as the battery load is cut, then recharges past that hysteresis trip point, turning the amp on again, then dragging the voltage back down below the "off" trip point. Since your batteries don't seem to be doing the load-off-recharge thing, your amp just goes off and stays off with the battery voltage drops below the hysteresis point. A different brand of battery may get you the low battery oscillation behavoir, which at least is more indicative of "recharge me" time.

Last edited by agdr; 8th October 2012 at 05:29 AM.
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Old 8th October 2012, 05:24 AM   #2843
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Thank you very much, it makes sense to me now.
Sad thing is I can get about 1 hour of additional runtime when I let it run uninterrupted.
Perhaps that's what I get for buying cheap batteries.
Now it's time for me to add a bassboost option.

Best regards.

Last edited by HurlockVanguard; 8th October 2012 at 05:28 AM. Reason: typo
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Old 8th October 2012, 06:55 PM   #2844
agdr is offline agdr  United States
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Hey one correction to what I posted. Working through the math on RocketScientist's power management circuit the per-battery "off" voltage is about 7.07Vdc per battery and the "turn on" voltage is around 8.33Vdc. So it probably isn't the case that your batteries are not doing the "recharge voltage rise" thing once the O2 is off, but rather they are not quite recharging enough, 8.30 vs. around 8.33Vdc. But a solution is still trying different batteries. Another thing to try would be going back to RocketScientist's original values of R25 = 2.7M and R9 = 40.2k. That would lower the upper trip point (power management circuit "on") to 7.0Vdc and the lower trip point (PM circuit "off") to 6.3Vdc. He made the change to prevent the power management circuit from oscillating when the battery got low, but it wound up doing that anyway due to the battery voltage rise behavior when the load is removed. He eventually decided the oscillation was a good "recharge the batteries" signal.

Also, if you happen to be using the bass boost modification I posted in the mod thread, I just posted a warning that it won't work if the O2 gain is set to 1 (R17 and R21 clipped out in the low gain position). Been meaning to do that for awhile. The details are here: O2 amp CRC, diode, cap, and heatsink mods

Last edited by agdr; 8th October 2012 at 07:18 PM.
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Old 9th October 2012, 04:18 PM   #2845
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Thanks for the heads up. Luckily I am using "stock" settings.
I am going to switch the resistors when I'll be installing bassboost.
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Old 10th October 2012, 10:23 PM   #2846
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Hi all and sorry for being a bit late into the post... Is anyone planning any group buys for the o2 amp in the near future?

Got 2 boards from a kind member and would like to do one using the cheapest components and the other using "the best" components. I dont want to measure or analyse the output, i just want to hear if there is any difference. If there is... the wife can have the cheap one
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Old 10th October 2012, 10:31 PM   #2847
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Your bias is showing
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Old 11th October 2012, 02:08 AM   #2848
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So I successfully built my O2 when it was first released and it's been a long time since my last DIY project. I'm a bit rusty and I don't know what to do to fix the issue I'm having.

I was sitting listening to music when a random burst of DC went through the right channel of my headphones. It sounded like pulsing distortion. I measured it with the multimeter today and it measured something crazy like 1.5 volts (it might not have been 1.5 volts, but it was absurdly high)

What's weird is that it doesn't do it on startup, but it takes about a minute or two for it to kick in.

Does anyone know where I should start? My guess would be the ICs, but what do I know.

thank you.
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Old 11th October 2012, 06:07 AM   #2849
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You can't read DC with a multimeter? Maybe we should start there...
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Old 11th October 2012, 08:37 AM   #2850
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@littlerick - there are unlikely to be any more group buys for this project - the last one, run by Jokener, had a few problems, and now Stefan has set up his shop where you can buy everything you need at a reasonable price. Just look in the commercial section of the forum.
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