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Old 14th October 2005, 09:12 AM   #1
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Default undersized Transformer?

hey guyz

wot happens to the sound and effects of the amplifier itself if the transformer is not big enough?


thanx
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Old 14th October 2005, 12:44 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I believe the bass will be first affected since the Vrails will sag badly on sustained notes.
This will also affect the upper ranges causing the stereo image to wander as the intermodulation causes gain variation and injects distortion all over the audio spectrum.
The overall effect is probably best described as sounding "cheap and nasty" and since this is resulting from penny pinching is aptly deserved.
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Old 3rd November 2005, 08:04 AM   #3
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Hello,

My name is Raveen and I am 16 years old. I built a fully discrete stereo amplifier which is rated at 60 watts RMS into 8 ohms. Previously, I used a 200VA EI power transformer to power both the amplifiers along with 12,700uF filtering per supply rail. However, there was a slight buzzing sound in the tweeters of both channels and so i thought it was the transformer causing the problem because it does vibrate and gives out a audible buzz itself(transformer).

So, I would like to change it to a 500VA nuvotem-talema toroidal power transformer with 2x 30 volts AC output. I would like to know whether this would solve the problem and is a soft start circuitry required for the transformer? My house supply here is 220V. A.C. @50Hz and I have measured the DC level in the power line to be 25mV. I would really appreciate it if someone could help me with this. Thank you.
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Old 3rd November 2005, 08:28 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi Raveen,
many, myself included, would say that 200VA is ideal for a 2 times 60W output power amp. VA rating is about 1.5 times the total output power for a classAB power amp. Your PSU smoothing is a little light. Allow about 2mF to 3mF per amp of peak output current. In your stereo case that comes out at 16mF to 24mF per Vrail.

Your buzzing from the treble is much more likely to be from a poorly connected set of grounds.

If you go to toroid you may need to add a DC block and slow start. Your existing EI is much more resistant to both these problems. By the way 500VA would allow you to source enough current to drive 4ohm loads from the stereo amp, but only if you drivers and output stage can take it. Then you would also have to double your smoothing caps to 30mF to 48mF (stereo). And then look at your on board decoupling. Maybe simpler to stay with 8ohm capability.
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Old 3rd November 2005, 01:18 PM   #5
Eva is offline Eva  Spain
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If the said "buzzing" comes only from the tweeters and/or has only high frequency components, it would probably be a sign of triggered oscillation. Check amplifier outputs with an oscilloscope.
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Old 4th November 2005, 08:19 AM   #6
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Good Day,

Thank you for the advice, Mr. Andrew T and thank you Mr. Eva for the suggestion for using the scope to see the output of the amplifier. I have viewed the output of both the amplifiers using an oscilloscope and the noise level was around 10mV peak to peak. This should be normal because my amplifier's output stage is a complementary bipolar design using the Motorola MJ15003/MJ15004 and 2SB669/2SB649 drivers.

I have installed a 10 ohm 2 watt ground lift resistor between the signal ground at the amplifier and the transformer's centre tap. I had to do this because when I connected the signal ground directly to the centre tap of the transformer, all of my output transistors blew(both channels). So the I decided to use a low value resistor in series with the signal ground connection. Will using a ground lift resistor in any way affect the sound quality?

By the way, suppose I want to drive a 4-ohm load, I would have to use the toroidal I bought(I bought it before subscribing to this forum and the thing costs US$63, which means I better make us of it). Is it necessary for a 500VA toroidal power transformer operating at 220VAC@50 cycles a second, to have a soft start? My amplifier's output stage can handle a 4-ohm load and I am using two 35-amp 1000V PIV Bridge rectifier connected in parellel to avoid rectifier destruction due to surge currents.

Thank you all for replying to me. I really appreciate it.
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Old 4th November 2005, 12:50 PM   #7
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Quote:
Is it necessary for a 500VA toroidal power transformer operating at 220VAC@50 cycles a second, to have a soft start?
Soft-start circuitry is required not to let the power-on current blow your wall fuses. At 500VA and higher you may start worrying about the inrush current because of large capacitor banks. At more than 2000VA the transformer alone can reset the fuses (transformer core made of steel is able to hold a constant magnetic charge, and when it is being recharged, currents can exceed 16A).
With a 550VA transformer I've used a amodified version of the circuit on Per Anders' site.
For my current 2500VA toroid I use a NE555-based circuit with a 2 second delay. The ballast resistor is in fact a 189mm 1KW floodlight lamp. It's very cheap and doesn't generate very much heat.
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Old 4th November 2005, 01:45 PM   #8
Eva is offline Eva  Spain
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Quote:
all of my output transistors blew(both channels).
This once again points towards instability and oscillation (or maybe poor RF filtering in the input of the amplifier).
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