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Old 6th May 2007, 02:36 PM   #21
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Quote:
For two channels try to find a 4 secondary toroid. One pair of secondaries for each amplifier.
Did I miss something. Are you running this as a single rail?
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Old 5th July 2007, 12:50 AM   #22
Arx is offline Arx  Canada
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Default Re: minimum gain

Quote:
Originally posted by melbsp
Really Rf is a pot so the gain can go as low as 0. The noninverting side is grounded. Anyway, with the Gain at 6.5 and no speaker/load, it no longer gets extrememly hot. only a tiny bit warm.

If I do set the gain to be -1 (Rf=Rin= 14.4k), then w/o the speaker the op-amp does get really hot. Why would they design an op-amp to limit the minimal gain for your design? I'd like to be able to turn the volume all the way down to low some times. I could do this using the IPOD, but it would be nice to have one knob do everything.

power for the op-amps(instead of the two dc power suplpies I have now 12V & 1.3A), will the op-amp (741 & the LM3886 only draw enough current as what they need?).

Volume controls usually don't actually change the gain of the amplifier.

Make Rf a fixed resistor set for a reasonable gain (maybe 20) and just set up a pot as a voltage divider on the input (often called a "passive preamp") Ipod's signal to one end of the pot, ground on the other, and input of the opamp from the wiper.

You can turn it all the way to 0, no problem. Full volume will be whatever level the ipod's control is set to.

As for opamps, the 741 is old and slow and probably making your amp sound much worse than it actually is.

I would probably just leave it off, since the Ipod should have enough current capability to properly drive the 3886 directly. Even if it is a bit weak, it'll probably distort less than a 741.

-Nick
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Old 5th July 2007, 01:01 AM   #23
Arx is offline Arx  Canada
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Default Re: Re: thanks Greg

Quote:
Originally posted by AndrewT

Hi,
transformer + rectifier + smoothing = PSU for a chipamp.
For reliable maximum into an 8ohm speaker use a 25Vac to 28Vac transformer. For two channels try to find a 4 secondary toroid. One pair of secondaries for each amplifier.
100VA to 150VA is adequate for one channel.
Wow, I think 28Vac is pushing it a tad. The maximum supply differential the chip is rated for is 84V.

With the transformer I'm using at home rated 600VA w/ 2x25VAC secondaries I get 40VDC

Based on that It seems to me that a 28V would put me over the 84V rating.

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Old 5th July 2007, 02:37 AM   #24
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Default Re: Re: Re: thanks Greg

Quote:
Originally posted by Arx


Based on that It seems to me that a 28V would put me over the 84V rating.

-Nick

30 vac = 42 vdc.
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Old 5th July 2007, 07:08 AM   #25
AndrewT is offline AndrewT  Scotland
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Quote:
600VA w/ 2x25VAC secondaries I get 40VDC
Hi,
there's something odd here.
40Vdc implies about 15% regulation, about three times worse than one would expect from a 600VA transformer.

I would expect 28Vac to give 40.8Vdc on open circuit with one rectifier and about 40Vdc with twin rectifiers.
Once the amp is coupled and drawing quiescent current, both these voltages will drop a bit.
When the mains is above nominal supply voltage these voltages will rise, but hopefully not too much and not for too long.

When you measure the transformer AC voltage and DC voltages what is the corresponding mains voltage?
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Old 5th July 2007, 07:42 AM   #26
Nordic is offline Nordic  South Africa
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Do not forget there is a forward voltage drop over the chip itself...

You will need big heatsinks though.
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Old 5th July 2007, 07:25 PM   #27
Arx is offline Arx  Canada
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Default Re: Re: Re: Re: thanks Greg

Quote:
Originally posted by MJL21193
30 vac = 42 vdc.
Yeah, I think my transformers are a bit higher voltage than spec. Still, that puts it right at the maximum voltage rating. If the line is overvoltage at all it'll be beyond the spec of the chip.

I would probably suggest ~20V secondaries assuming you're not aiming for maximum power. I ran my old amp from 18V into some 8ohm speakers and it had plenty of power.

At 20V it'll be a little happier with difficult speakers.


If you're aiming for max power, go for it, but remember that you're stuck with 8 ohm loads.

-Nick
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Old 10th April 2011, 02:15 PM   #28
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Default input attenuator

If you want to use only the knob on the unit to control loudness, you could simply gain the amp for the maximum and use a potentiometer or step attenuator for the input signal and leave the gain alone.
This way you could achieve the total range of loudness with the smaller heatsink.
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Old 11th April 2011, 02:14 AM   #29
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My stereo 3886 does not oscillate but required 300 square inches without a fan to cool running on 32 volts under heavy drive.
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Old 30th July 2012, 01:13 AM   #30
weiyan is offline weiyan  Hong Kong
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Stumbled on this thread. I saw some pre-build diy use chassis as heat sink. Is this sufficient?
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