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Old 24th September 2004, 09:58 AM   #1
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Question Power transformer and Capacitor

I have a 2x100watt at 8Ohms(with the same output at 4Ohms) amp (McIntosh) with 2x22000uF capacitors and I would like to change the transformer and capacitors with bigger ones.I am thinking of a 500Va Plitron LoNo toroidal transformer(not cheap) and at least 44000uF per channel.I also consider the solution of 2x250Va transformer(dual mono) but I do not know if there is going to be any significant improvement.I am not lookin for more watts but for better performens at difficult loads (2.8-3.0 Ohms).Any opinions and suggestions?
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Old 24th September 2004, 10:46 AM   #2
johnnyx is offline johnnyx  United Kingdom
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It's unusual for a ss amp to give the same power in 8 & 4 ohms. I looked on their website, and some of their ss amps use output transformers, which is unusual, but explains the constant power level with different load impedence. They look like good (expensive) amps. The limiting factor may be the amp circuit rather than just the power supply. I would get some more information before spending on a new power supply. Is it for a ribbon speaker, or just an awkward one?
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Old 24th September 2004, 10:58 AM   #3
Jennice is offline Jennice  Denmark
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I'm slightly concerned that a bigger PSU may not be the (complete?) answer to your needs.

What you're describing is a nasty load on any amp (except a few car amps which are designed to work with 2 ohm loads).
Maybe it's your output stage that is having a hard time handling the current. If so, a better PSU may not need to be the answer.

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Old 24th September 2004, 11:02 AM   #4
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It is for my B&W 805 Nautilus speakers.They are 2way speakers.They go down to 4.2 Ohm(factory results)and this propably means 3.4-3.8 Ohms at real life.I have not checked the VA in my power supply yet.Yes they are expensive amps.
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Old 24th September 2004, 11:22 AM   #5
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Hi! For me their values are very high, for an amplifier with 100W off course.
And my opinion for the use of two trafos, never, never! But this is my mouth speaking...
With one trafo, tension doesn't float a lot and regulates the stages of voltage gains.

I already heard an amplifier of the same mark and they don't need none alterations, they play very well.

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Old 30th September 2004, 12:52 AM   #6
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Default Re: Power transformer and Capacitor

Quote:
Originally posted by Drioannis
I have a 2x100watt at 8Ohms(with the same output at 4Ohms) amp (McIntosh) with 2x22000uF capacitors and I would like to change the transformer and capacitors with bigger ones.I am thinking of a 500Va Plitron LoNo toroidal transformer(not cheap) and at least 44000uF per channel.I also consider the solution of 2x250Va transformer(dual mono) but I do not know if there is going to be any significant improvement.I am not lookin for more watts but for better performens at difficult loads (2.8-3.0 Ohms).Any opinions and suggestions?
If the amp is transformer coupled to the speakers, then there is no need to upgrade the power supply. There should be no changes made to the circuit. The solution lies in the output transformers.

Here is the problem with driving such low impeadence. The windings on the secondary of the output transformer are wound for 4 or 8Ohms. Any impeadence applied to the secondary of a transformer is reflected to the primary side, and thus the output drivers. Applying a lower impeadence will create problems just like in any other typical SS amp design(class H excluded). This makes the drivers have to put more current into the primary winding. Not only is the outputs driven into saturation and thus producing lots of symetrical distortion, the current can saturate the output transformer core and cause all kinds of nasty noise.

Hope is not compleatly lost though, 2-3Ohms can be driven by this amp, but rewinding the secondary to match the lower impeadence is needed. (less turns, thicker wire)
Don't know how difficult it would be but I wouldn't do it unless you are sure of what you are doing. This sounds like a nice amp and would be a shame to destroy it by messing with it. If it is not broken, don't fix it. However, if it is to be done, keep track of how many turns = 4Ohms, then use half as many turns for 2Ohms, but go to a larger gauge magnet wire. Consult a wire table for info on this.
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Old 30th September 2004, 04:58 PM   #7
djk is offline djk
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My McIntosh MC2120 is 120W into 4 or 8 ohms, autoformer coupled.

The power transformer is 600VA, I see no point in making it larger.

The filter caps are adequate at two 20,000F.

With respect to low impedance loads, I can drive a 2 ohm load on the 8 ohm tap without a problem. If I turn off the PowerGuard and drive it hard into clipping the foldback current limiting will activate and make a nasty sound.

"If the amp is transformer coupled to the speakers, then there is no need to upgrade the power supply. There should be no changes made to the circuit. The solution lies in the output transformers. "

I am tired of you, a lot of what you say is wrong. It takes too much time to prove someone like yourself wrong, and even then you(they) don't seem to get it. You are way off base here. The output transformers McIntosh use have less phase shift at high frequencies that the transistors used as the output devices. If you had bothered to read the spec sheet you would know this.

"Here is the problem with driving such low impeadence. The windings on the secondary of the output transformer are wound for 4 or 8Ohms. Any impeadence applied to the secondary of a transformer is reflected to the primary side, and thus the output drivers. Applying a lower impeadence will create problems just like in any other typical SS amp design(class H excluded). This makes the drivers have to put more current into the primary winding. Not only is the outputs driven into saturation and thus producing lots of symetrical distortion, the current can saturate the output transformer core and cause all kinds of nasty noise. "

Why can a MC2300 drive a 0.25 ohm load in mono? 20hz~20Khz plus or minus a fraction of a dB? At a fraction of a per cent distortion? In a thirty five year old design?

"Applying a lower impeadence will create problems just like in any other typical SS amp design(class H excluded). "

Why are you excluding class H? They really blow up big time on low impedance loads. Now maybe if they were designed properly (like a McIntosh).

"Any opinions and suggestions?"

Call McIntosh with the model and serial number and get the service manual for your amplifier.

Add power supply bypass caps where needed, upgrade the coupling caps in the signal path with film types. Cut off all the high current push-on connectors and solder (speaker relay, bridge rectifier, etc). Solder a 0.1F film cap across the speaker relay contacts.

After soldering, clean the relay contacts with Caig R5 or D5. Clean all the low level push-on connectors, switches, board edge connectors, input jacks, etc, with Caig.
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