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Old 5th August 2011, 07:01 AM   #11
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Quote:
Originally Posted by richard.C. View Post
Hi Mike,
yeah i was gonna short the signal between the preamp output and amp input. Or i was just thinking of shorting the power supply on the DC side maybe with the same method. Short + to ground and - to ground using resistors to drain the charge in the caps. I'll try that one too.

If there is an easy way to use one tranny to power both i would like to do that. Mooly suggested a zener/resistor circuit, but i wouldn't know how to go about that.

rick
Hi Richard, I'm sure Mike will say the same as me on shorting PSU's to discharge them. It's a definite NO

The PSU I posted a few minutes ago... it really is that easy.

The ground is the main ground in your chip amp. The - and + inputs are from the main reservoir caps... so it just connects across your -36 - 0 + 36 rails.

The resistor is chosen to allow a few millimaps to flow through the zener... if you tell us how many opamps it's feeding in the preamp we can work the value out. It's very non critical though.
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Old 5th August 2011, 07:03 AM   #12
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Do you mean you wouldn't know how to connect the above PSU to your preamp ?

That's easy...
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Old 5th August 2011, 07:11 AM   #13
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I've just looked at your link in the first post.

The PCB has on board regulators. So we can still retain those. It says they are 78/7912 type.

The only difference to my circuit is that we would choose a higher zener voltage, say 18volts and feed the original connections on your board. You can feed DC into the bridge rectifier with no problem.
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Old 5th August 2011, 07:13 AM   #14
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Originally Posted by richard.C. View Post
Now i do!!
So that diagram, thats not from the 240VAC mains is it??? No it's from the secendary AC side of the tranny?? or the DC side of the rectifier?
That post slipped in... I didn't see it

Yes its fed from the DC side of the rectifier, the -/+36 volts
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Old 5th August 2011, 07:29 AM   #15
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Originally Posted by Mooly View Post
The resistor is chosen to allow a few millimaps to flow through the zener... if you tell us how many opamps it's feeding in the preamp we can work the value out. It's very non critical though.
How many opamps?? there's the pre-amp feeding the two mono chip amps boards, one for each channel.

And shorting the caps is a no no, haha i just tried it with two 33ohm resistors, 33ohm + to ground, 33ohm - to ground. What would happen if i was to continue going about it in this manner?

Anyway i just finished the circuit and all seems to be a success. The start up "pop" has turned to a slight cone movement(i'll tweek a little), and the nasty sounding power down "pop" is completely gone. Allthough i'm shorting the caps at the moment to solve the last "pop" issue.
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Old 5th August 2011, 07:38 AM   #16
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Quote:
Originally Posted by richard.C. View Post
How many opamps?? there's the pre-amp feeding the two mono chip amps boards, one for each channel.

And shorting the caps is a no no, haha i just tried it with two 33ohm resistors, 33ohm + to ground, 33ohm - to ground. What would happen if i was to continue going about it in this manner?

Anyway i just finished the circuit and all seems to be a success. The start up "pop" has turned to a slight cone movement(i'll tweek a little), and the nasty sounding power down "pop" is completely gone. Allthough i'm shorting the caps at the moment to solve the last "pop" issue.
I hadn't looked at the photo of the preamp when I asked that... a single 5532 draws around 10milliamps give or take.

Shorting the rails is a no no... what caps are you shorting ?
It's not clear what you are doing
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Old 5th August 2011, 07:55 AM   #17
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The caps i'm shorting are those 4x 50V 10,000uf on the PSU board i previously pinned up on my other posts. I'm using 33ohm 5W resistors, i tried it manualy with no spark on contact and the resistors remain cool. What would i be damaging in doing this, i've previously seen caps explode with over volt stress. I would have thought it would have been Ok to do this to quickly drain the remaining charge in the caps.
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Old 5th August 2011, 07:58 AM   #18
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And what wattage should the zeners be, 5W at least? and what resistance should be added do you think?
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Old 5th August 2011, 01:15 PM   #19
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Your not damaging anything with 33 ohms 5 watt resistors across the 10000 uf caps... although personally IMO it's not a good way of overcoming the problem.

I'm still not 100% sure whether you mean you have the resistors connected permanently or just that they connect via a relay when the power goes off.

The zeners... it works like this. 36 volts to begin with. The 7812/7912 regulators need around 15 to 25 volts input. Each reg draws around what ? 3 milliamps at a guess. The 5532 draws around 10 to 15 milliamps. So thats around 18 milliamps per rail. Call it 25ma for a good margin.

So for the resistor we have 36 volts less the zener volts (lets settle on an 18v zener) which gives 18 volts. Thats the voltage the resistor has to drop. We allow a zener current of around 10 milliamps.

So the resistor needs to handle 18 volts at 35 milliamp maximum. That is R=V/I which is 16/0.035 giving 514 ohms. Call it 470. Wattage is (v*v)/r which is (18*18)/470 giving 0.69 watts. So a 1 watt carbon or metal is ideal.

The zener wattage is the same. 18 volts at 35ma.
That is worst case calculation for the zener meaning no current drawn by the preamp (not connected). When the preamp is connected then the preamp current is subtracted from the zener current. The zener current falls (still always with 18 volts across it) and so the zener wattage drops as the load current increases.

The above values are very conservative... and worst case. Typically a zener current of 5 milliamps (rather than 10) would be more than adequate and the preamp load current will be less than the 25ma I allowed.

So that means you can increase the value of the resistor to reduce current and heat.
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Old 5th August 2011, 09:05 PM   #20
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Hi Mooly,
When the power goes off. I've got a 4PDT relay, so i'm able to switch from common(PSU side)to either the resistor circuit on power down, or complete the DC power circuit to the amps on power up.

And for the zener your saying a 470ohm 1 watt resistor will do the trick. I just had a look at your paint brush diagram, and the resistors are in series on the rails, with the zener and cap shunting, is the cap a polarised type, running same polarity as the zener?

rick
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