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Old 24th October 2005, 01:40 PM   #1
beppe61 is offline beppe61  Italy
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Default Transformer substitution on a audio power amp.

Dear Friends,


I would like to get some opinions from you about a my idea.
I have a power amp of which I quite like the overall sound.
Unfortunately is not able to source the needed current to drive adequately my woofers.
The bass is too weak and soft, also out of control.
I think that the main culprit is an underdesigned the transformer.
My intention is to replace it with a nice and high current model.
I am thinking of something like a Talema 600-800 VA, a new very big diodes bridge and maybe new caps.
Is the modification worthwhile?
It would cost me just about 100 USD in the worst case.

Advice would be very much appreciated.

Kind regards,

beppe61
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Old 25th October 2005, 04:36 PM   #2
Tweeker is offline Tweeker  United States
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Be sure your problem isnt something like an undersized DC blocking input capacitor, or both.
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Old 25th October 2005, 09:21 PM   #3
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by Tweeker
Be sure your problem isnt something like an undersized DC blocking input capacitor, or both.
Dear Mr. Tweeker,
thank you so much for your kind and valuable reply.
Actually looking at the schema I can see a 4,7 uF/50V electrolytic just before a 12K resistor to ground (a C-R high pass filter).
Is 4,7 uF too small a value ?

Moreover if you had then to upgrade the power supply would you start from the transformer or from the filter capacitors?

I look forward hearing from you.

Kind regards,

beppe61
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Old 25th October 2005, 09:54 PM   #4
Electrons are yellow and more is better!
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The corner frequency of the filter is 2.8 Hz ( f=1/(2*pi*R*C) ) so this is enough when it comes to bass capacity.

What exactly are you not satisfied with? Max level of the bass? Do you drive the amp into clipping?
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Old 26th October 2005, 07:01 AM   #5
beppe61 is offline beppe61  Italy
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Dear Mr. Peranders,

thank you so much for your extremely kind and very valuable reply.
Please let me explain better between the lines.

>
Quote:
Originally posted by peranders
The corner frequency of the filter is 2.8 Hz ( f=1/(2*pi*R*C) ) so this is enough when it comes to bass capacity.

Thank you very much for your explanation.
Nothing to replace then.

What exactly are you not satisfied with? Max level of the bass? Do you drive the amp into clipping?
I will try to explain better.
I have connected two 5 ohm /100W power resistors at the output of the amp to run it in.
The heatsink of the amp got quite warm while the resistors remained cold.
The amp did not go into protection so it was working.
With the speakers connected the amp fails to reproduce with a good impact the lower end of the spectrum (let's say <100Hz).
After all I think that much depends on a underdesignaed power supply.
The first suspect is the transformer, that I have thought to replace with some high-current 600 VA from Nuvotem-Talema.
Maybe a nice pair of big caps should be a nice addition.
The power amp sports two pairs of nice Toshiba per channel.
I am sorry because all things considered I like the sound (bass response apart, of course).

Any your suggestion would be vey welcome and much appreciated.

Kind regards,

beppe61
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Old 26th October 2005, 07:13 AM   #6
Electrons are yellow and more is better!
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Quote:
Originally posted by beppe61
I have connected two 5 ohm /100W power resistors at the output of the amp to run it in.
The heatsink of the amp got quite warm while the resistors remained cold.
The amp did not go into protection so it was working.
100 watts into a resistor gets quite soon, certainly if you use a sinus signal as input. Check first that you really get any output power/voltage AND also that you have 5 ohms!

How many VA has the transformer now and how much smoothing capacitance do you have?
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Old 26th October 2005, 07:18 AM   #7
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Default Re: Transformer substitution on a audio power amp.

Quote:
Originally posted by beppe61
The bass is too weak and soft, also out of control.
Before you dive too deeply into the amp, why don't you test your woofer with an another amp, just to see the difference? Maybe nothing is wrong really? It may be the woofer itself
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Old 26th October 2005, 07:20 AM   #8
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by peranders

100 watts into a resistor gets quite soon, certainly if you use a sinus signal as input. Check first that you really get any output power/voltage AND also that you have 5 ohms!
Dear Mr. Peranders,

I have 4 power resistors of 10ohm/100W.
I have paralleled them two by two and got 2 power resistors of 5ohm/100W.
The light of the 1W was blinking but was not on steady.
The amp has a thermal protection but the correspondig light remained off.
As the price of the amp is quite low (220 euro) I think that some savings have been done on components, transformer above all.

Thank you very much indeed.

Kind regards,

beppe61
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Old 26th October 2005, 07:32 AM   #9
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If you won't get more than 1 watts, why don't you increase the output? Turn up the volume "Full spätta"*) as we say in Sweden

220 EUR won't get you much but one thing to estimate the power is to measure the supply voltage. Theoreticly the max output power will be

Pmax = (U*U)/(R*2) where U is supply voltage and R is the load.

*) Full throttle but spätta is a flat fish
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Old 26th October 2005, 07:34 AM   #10
beppe61 is offline beppe61  Italy
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Default Re: Re: Transformer substitution on a audio power amp.

Quote:
Originally posted by peranders

Before you dive too deeply into the amp, why don't you test your woofer with an another amp, just to see the difference? Maybe nothing is wrong really? It may be the woofer itself

Dear Mr. Peranders,
the woofers are fine.
They worked nicely with two Albarry mono amps.
These monos however have a much heftier power supply.
I understand that they are known to be difficult to drive adequately.
They are current hungry.
The model is Dynaudio 24W75.
Maybe I should take a look also to the crossover.
I don't expect night and day changements.
I need just more bass, more impact on drums, pipe organ etc.

Thank you so much for your kind and valuable support.

Kind regards,

beppe61
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