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Old 19th February 2004, 05:12 PM   #1
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Question Amp power supply question

I have a plate amp that is rated at 110w RMS @8ohm and 156w RMS @4ohm. The power supply section of this amp looked a bit chinsy to me so I decided to upgrade with some old parts of another amp I had laying around. It allready had two 6800mfd 63v caps and I added two more 10,000mfd caps,a 500VA transformer and and a 50 amp rectifier. Overkill? I hope so.......lol. MY question regards the voltage of new transformer and how much wattage is actually gained. The original transformer supplied +-48v and the new one supplies +-63v. It doesn't run much hotter and seems to be stable but I would just like to know someones best guess on the gain or way to figure out an estimate without having to test the amp. Thanks in advance for any replies .
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Old 19th February 2004, 07:41 PM   #2
sreten is offline sreten  United Kingdom
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a simple estimate is circa 190W into 8 ohms
and a lot more into 4 ohms.

However if the sub amplifier has well designed SOA (safe
operating area) circuitry and was designed to run near its
limits you will get barely more power.
Its possible the extra rail voltages would kick in the SOA
circuitry earlier so you'd get less power !

Or it could have no SOA circuitry, so the first estimate applies,
but your circuit board is more likely to perfunctorally expire.

As a general guideline upgrade current and smoothing
capability of a power supply, changing the output voltage
of an amplifier power supply is generally not a good idea.

sreten.
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Old 19th February 2004, 08:01 PM   #3
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By my calcualtions:

Going from +/-48 VDC rails to +/- 63 vdc rails (ignoring all other factors)
will result in an amplifier that is 2.2 dB louder. Not much. That is a best case calculation.

Will this greater voltage put your amplifier's components at risk? Hard to say. I speculate that if your expertise is at the level that you had to ask your initial question, you probably cannot answer this question on your own.

HOWEVER......

If you wanted to improve the sound of you amplifier and make it "sound louder" consider adding voltage regulation to your amplifier's power supply. Regulate the power supply down to about the original rail voltage.

Do not be surprised when it really sings.

Good luck,

Aud_Mot
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Old 19th February 2004, 08:06 PM   #4
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Thank you for your reply sreten. This is not a well built amp, lol. So I doubt there is any type of limiting circuitry. It has been running in this new config for awhile now and it hasn't burnt itself up yet. I did the"upgrades" because I was bored and had the pieces and didn't really care if I broke it or not.
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Old 19th February 2004, 08:28 PM   #5
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Aud_Mot
By my calcualtions:

Going from +/-48 VDC rails to +/- 63 vdc rails (ignoring all other factors)
will result in an amplifier that is 2.2 dB louder. Not much. That is a best case calculation.

Will this greater voltage put your amplifier's components at risk? Hard to say. I speculate that if your expertise is at the level that you had to ask your initial question, you probably cannot answer this question on your own.

HOWEVER......

If you wanted to improve the sound of you amplifier and make it "sound louder" consider adding voltage regulation to your amplifier's power supply. Regulate the power supply down to about the original rail voltage.

Do not be surprised when it really sings.

Good luck,

Aud_Mot

Whilst I agree about the posibililties of regulation or capacitance
multipliers, in this particular case such additions are arguable,
due to the overspecced transformer.

sreten.
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Old 19th February 2004, 08:43 PM   #6
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My post was assuming that he already installed the overkill transformer. So what to do now? Wait for the mod to blow up his amp? My suggestion would do two things 1) return his amp to using applied voltages that should be safe 2) give him the sonic advantages of a regulated power supply.

I do not understand the point of the over specced transformer. Since I recommended a lowering of rail voltage, the only penalties of a "too big" transformer are physical size (taken care of) and expense.

Aud_Mot
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Old 19th February 2004, 09:09 PM   #7
sreten is offline sreten  United Kingdom
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Sorry,

we are talking about 2 different things, at crossed purposes.

If the transformer is overspecced, VA wise then adding
regulation or a multiplier is poor engineering, due to it
being overspecced VA wise it will have good regulation.

You are perfectly correct that adding regulation to reduce
rail voltage to the original value is the sensible option,
and IMO the best option for similar VA transformers.

Boils down to why you are adding the regulation.

If the amplifier works at the higher rail voltages then
everything thing is fine, adding regulation will lose power.

If it doesn't, then regulation is required to drop rail voltages
and the overspecced VA rating of the transformer doesn't
matter.

sreten.
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Old 19th February 2004, 10:39 PM   #8
Eva is offline Eva  Spain
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This increase in supply rails may be reliable as long as loads below 8 ohm are no longer used

To be fully reliable it would also require all transistors in the circuit to have enough blocking and thermal capabilities and biasing schemes good enougn to not come into runaway with higher supply rails

From the point of view of dissipation and SOA of the output devices, running them with +-63V and 8 ohms load isn't much worse than running them with +-35V and 4 ohms [except if they are old 100V Vce devices like TIP35/36C]
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Old 19th February 2004, 10:59 PM   #9
SimontY is offline SimontY  United Kingdom
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Hi Anonymous,

This is a long-shot, but is your plate amp by any chance the RCM Acustik Detonator DT-110 (or something like that!)?

If so, then it's the same as mine. I added 40,000uf to the PSU with no apparent problems. It sounds more gutsy, but still isn't going to use my Tempest driver to the full...
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Old 19th February 2004, 11:12 PM   #10
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No, mine is the no name "Part Number 300-792" that they sell at parts express. I really appreciate every bodies input here, but the first answer probably suites my question the best. Like I said before, I'm not having any problems with the amp after the modifications I did. Its been running a 12" 4ohm sub for quite some time now with no ill effects. I was really just interested in what the average gains of such a change would be. Oh, and I dont know how much it helps the amp but I used heat sink epoxy to glue little heat sinks on every last transistor on the pcb.
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