Simple Universal Speaker Delay Using A Triac - Page 4 - diyAudio
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Old 9th December 2012, 07:58 PM   #31
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Try looking at the "filtered DC Supply" of the unit with a spectrum analyzer and you will be surprised at how much noise actually comes from even a small transformer current draw imbalance! It also grows with time and is reduced by power on off transients.

It just depends on how fussy you want to get. I just suggest adding a bridge.
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Old 10th December 2012, 10:39 AM   #32
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Hi Simon,

I wanted to measure this for real. With the relay driver across one winding of a 15-0-15 transformer (that also supplied a bridge and two 6800uf caps) I measured around 0.2 mv pk/pk "glitch" directly across these reservoir caps when the driver was connected. There was no load on these reservoir caps. As soon as any load is added (such as the amplifier this circuit is aimed at) then the noise and ripple increases many hundred fold which swamps the 0.2 mv glitch caused by the relay load.

The 22uf pulls around 150 to 200ma current peaks from the transformer (give or take depending on the feed resistor you select for the relay) but these don't seem to impact on the main amplifier supply.

Not sure if thats what you meant
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Old 10th December 2012, 02:51 PM   #33
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It is not the glitch. When you load a transformer with a single diode, this causes an unbalance in the current draw. This causes a slow magnetic shift in the operating point and slowly reduces the efficiency of the transformer. It is not large but can amount to a few percent if the system were to be left on for a month or so. When you switch the gear on the ac waveform is at some random voltage and this wipes away much of the residual magnetism. (But not all, it is a random process and averages out a bit.)

As to rectifier noise attached are a picture of the current draw and the spectrum analysis of a center tapped bridge.

All rectification causes noise. With a single diode you start at the line frequency and draw twice the current for 6 db more noise. The full wave bridge will start at twice the line frequency and with 6 db less amplitude.
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File Type: jpg 9 Noise from PS.JPG (74.4 KB, 163 views)
File Type: jpg 6 Power Supply Current Pulse.JPG (88.5 KB, 156 views)
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Old 10th December 2012, 04:32 PM   #34
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I'll be honest Simon, I have never heard of degradation due to that effect

When you say the efficiency is reduced what do you mean ? Do you mean that what might have been a 300va transformer is now only capable of (say) 280 va, or that the regulation figure is reduced ?
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Old 10th December 2012, 05:36 PM   #35
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I showed this in Audioxpress. This is the tabular data showing loss with time. Note the change after a power failure.

Also attached is the DC offset measurement circuit.


AC Line Out Ratio Loss % Loss
125.2 60.0 0.4792 0.0000 0.00
125.5 60.0 0.4781 0.0011 0.24
126.6 60.5 0.4779 0.0014 0.28
125.6 60.0 0.4777 0.0015 0.32
123.9 59.1 0.4770 0.0022 0.47
125.2 59.6 0.4760 0.0032 0.67
125.1 59.7 0.4772 0.0020 0.42
125.4 60.0 0.4785 0.0008 0.16
.88 VDC offset
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File Type: jpg Balanced AC.JPG (35.4 KB, 149 views)
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Old 10th December 2012, 05:57 PM   #36
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Thanks for the info.

Don't know what to say really... I've never seen it presented as a measured effect before.

Was it a small transformer I wonder... interesting though.
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Old 10th December 2012, 09:28 PM   #37
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I really don't keep around small 60 volt transformers! But I don't remember exactly which one it was.
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Old 11th December 2012, 02:27 AM   #38
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smart design! I like it.

As the circuit was meant for switching a speaker I'd also like to see the relay doubles a protection functionality on top of its initial intended muting@power-on purpose. Perhaps an NPN transistor or a low Rdson MOSFET can be paralleled onto the triac across A and K, when a faulty condition occurs (DC at output, over-current, etc), this transistor/MOSFET turns hard on for a very short moment bypassing the current in the triac and therefore turing it off. would this idea work?
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Old 11th December 2012, 03:12 AM   #39
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To answer my own question, I guess it does not work as the circuit will cycle on as soon as the transistor/MOSFET turns off. May be cutting the power to the circuit is a better way of speaker protection.
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Old 11th December 2012, 06:59 AM   #40
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Quote:
Originally Posted by nattawa View Post
smart design! I like it.

As the circuit was meant for switching a speaker I'd also like to see the relay doubles a protection functionality on top of its initial intended muting@power-on purpose. Perhaps an NPN transistor or a low Rdson MOSFET can be paralleled onto the triac across A and K, when a faulty condition occurs (DC at output, over-current, etc), this transistor/MOSFET turns hard on for a very short moment bypassing the current in the triac and therefore turing it off. would this idea work?
Thanks...

I think this would work as one option. Having a normally on device such as the NPN shown here. Pulling the base down to ground even for an instant would turn the relay off. (if the trip input trigger were a transient fault then the circuit would reset, if not then the circuit would stay tripped)

(remember that as originally intended the whole circuit doesn't need a "ground" that is referenced to the amplifiers "ground". This eneables it to be powered in a variety of ways such as "across" the transformer with no reference to the amplifiers circuits. Adding a trip input like this means that the triac circuit needs its ground line connecting to the amplifiers ground line or you could add an opto-isolator to pull the base of the NPN transistor low)
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File Type: jpg Trip Input.JPG (144.6 KB, 128 views)
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Last edited by Mooly; 11th December 2012 at 07:06 AM. Reason: More info
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