MyRefC build guide

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Now that would be handy.... nah, dual mono all the way! ;)

I thought you needed 160VA per channel?

I asked about this back on page 9 and this is the reply that AndrewT gave me

a 100W amplifier can operate quite well with a transformer rated from 100VA to 200VA. any bigger becomes uneconomic.

A 60W Chipamp can use a 60VA to 120VA transformer.

Unfortunately a very low VA transformer has very high regulation.
I recommend that the transformer should be >=160VA.

A two channel amplifier of 60+60W would require 120VA to 240VA and add in that "avoid high regulation guidance" and you get 160VA to 240VA.

I don't know where 300VA came from. It is icing on the cake. An uneconomic way to improve an amplifier. But some builders do hear a sound quality benefit and consider the bigger transformer as offering good value for their budget.

I would consider a 300VA as good for a 100W + 100W two channel amplifier.

Unless I've understood this in completely the wrong way, our 2 channel amps (being around 60W per channel), could be run from a single 225VA transformer with 24-25V dual secondaries.

AndrewT, could you confirm that I've understood you correctly.

Thanks.
 
A two channel amplifier of 60+60W would require 120VA to 240VA and add in that "avoid high regulation guidance" and you get 160VA to 240VA.
I asked about this back on page 9 and this is the reply
Unless I've understood this in completely the wrong way, our 2 channel amps (being around 60W per channel), could be run from a single 225VA transformer with 24-25V dual secondaries.
I thought I made it clear.
60+60W can be run economically on a transformer rated from 120VA to 240VA.
The condition of avoiding a high regulation transformer raises the lower limit from 120VA to 160VA.
The 225VA proposed is right in between the suggested lower and upper limits, i.e. 160VA < 225VA < 240VA.

Did I confuse some Members?
 
Now that would be handy.... nah, dual mono all the way! ;)

I thought you needed 160VA per channel?
I think you will find that a two channel amp will run quite well from a transformer where the VA = total maximum output power.
However better performance is usually achieved by using a higher VA transformer. All it costs is money and space and weight. some go as far as saying that 300VA is best for a chipamp. I don't disagree with that opinion.
But there is a downside to using the minimum size of transformer.
Low VA transformers have a much higher regulation.
The effect of this is that off load they produce a high DC voltage. This potentially damaging voltage must not overload the chipamp.
As the load increases the available DC voltage falls (sags). All transformer rectifier smoothing cap PSUs suffer from this characteristic.

It has been found that avoiding the smallest transformers helps improve performance by reducing the sag in PSU voltage with increasing load.
I have suggested that the lower limit be set at 160VA. That has absolutely nothing to do with whether the chipamp is capable of working or not working with a smaller transformer. It is an economic decision that gives better performance than saving a few $/£ on a 80VA or 120VA transformer. It comes down to good value.
 
I think you will find that a two channel amp will run quite well from a transformer where the VA = total maximum output power.

Certainly its possible, but please be aware that such a transformer will be seriously overloaded when delivering the maximum output power and substantially overloaded well prior to that point. That its possible to get away with this without burning out the transformer is because of the peak to average value of music. Classical music aficionados will get away with it more readily than head-bangers.

In order to see how overloaded a transformer will be at maximum output, take into account both the efficiency of an average class-B amplifier at full power (of the order of 70%) and the non-linear load presented to the transformer by the rectifiers and capacitors.

Transformers' VA ratings are specified with a resistive load, this draws current throughout the mains cycle. However, rectifying and smoothing the voltage from a transformer results in current only being drawn at the peak of the mains cycle. Typically under load, the transformer will be supplying current for 20% of the time, the remaining 80% of the time the amp is supplied by the reservoir caps.

A rough estimate then is that the current pulses from the transformer will be five times greater than the peak current into a resistive load. This results in five times greater heating (heating is proportional to the square of the current) than with a resistive load.

I don't disagree with Andrews analysis, this is merely to point out the potential hazards of going to a transformer whose VA rating is equal to the rated output of the amp. It would be a no-no commercially as CE marking requires certain sustained power output tests without overheating.
 
this is merely to point out the potential hazards of going to a transformer whose VA rating is equal to the rated output of the amp. It would be a no-no commercially as CE marking requires certain sustained power output tests without overheating.
this is precisely what commercially is done.
Putting in a smaller transformer that can only just do the job of giving out a music signal without catching fire or failing in the first year of operation to save a couple of £/$

I normally recommend a 20dB of overhead between maximum output power and average listening power.
Some may go as low as 10dB. That would require 6W of average output power in a 60W amplifier.
Look at the chipamp datasheet. How much heat/power does it dissipate when delivering 6W, most will be around 10W. That's a total 16W. Two channels of headbanging music would consume ~32W. That is just 27% of the smallest transformer (120VA for 60+60W) that any sensible builder would use.
If 160VA were used for 2channels then the consumption reduces to 20%.

I don't see the problem.

If the builder were to do some continuous maximum power testing, I believe that same builder would be very aware that temperature is critical to reliability. He would do the testing in an appropriate manner to avoid damaging any of his equipment.
 
Hi All,

Thanks for the detailed responses. I intended to over-spec my trafos, but not by that much! :rolleyes: I must have picked up the larger recommendation (160VA per channel) from somewhere or the other. No worries here, though. As I have four of these, and 8 channels of MyRef (four Ultimate BOM), I can try all configurations to see what suits me and my system.

The irony is that with the two trafos in my chassis, I cannot use the 2 big 'sinks I have planned (one for each amp), but could configure 2 'sinks and 1 trafo, or 2 trafos and 1 sink in each chassis. I bi-amp here.

Cheers

Jon
 
this is precisely what commercially is done.

How would they pass CE then? You're saying that the manufacturers are fraudulently self-certifying? Or evading EU directives?

Look at the chipamp datasheet. How much heat/power does it dissipate when delivering 6W, most will be around 10W.

That would imply the amp was around 38% efficient at 10% output. Its considerably worse than that.

Have a look here for an efficiency table for a class B amp.

A figure of 20% efficient is more like it and that figure is optimistic as it takes no account of PSU regulation.

Two channels of headbanging music would consume ~32W. That is just 27% of the smallest transformer (120VA for 60+60W) that any sensible builder would use.

I note you've ignored the power factor issue I explained above. Do you think I'm wrong in that? If you do I'm interested to learn why.

If the builder were to do some continuous maximum power testing, I believe that same builder would be very aware that temperature is critical to reliability. He would do the testing in an appropriate manner to avoid damaging any of his equipment.

As far as I recall, CE testing does not require any continuous maximum power testing. So yes, I agree with you, its prudent to do such testing very carefully.
 
Offboard Rectification and connecting this to the circuit

If I have my power supply in a seperate enclosure then I would be wanting to rectify this at the transformer and only send DC across the power umbilical.

How would I connect that to the board as the board assumes the rectification is done by a bridge onboard?
 
If I have my power supply in a seperate enclosure then I would be wanting to rectify this at the transformer and only send DC across the power umbilical.

How would I connect that to the board as the board assumes the rectification is done by a bridge onboard?
Nothing happens by delivering AC current through an umbilical cord, but you could rectificate before if you want to. Don't mount the rectifiers on the board, assemble the bridge on a piece of veroboard in the transformer enclosure. Then connect the umbilical cord + and - to the holes of + and - tracks. And the center tapped out of the trafo to ground.
 
I have built a ground (loop) breaker Like the one Regi suggested from the ISP site Earthing (Grounding) Your Hi-Fi - Tricks and Techniques .

1) I don't understand how this device works or how I can test that it is working correctly?

2) I read that I should have used crimped connectors rather than solder, I understand the thinking here, but is it really necessary?

3) I should use one for each channel?

Can someone kindly point me towards a method to test this device and perhaps a little reading which might help me understand the principle of how this works? I have two decent DMM's, a dc bench supply to 32 volts, a variac, (and isolation transformer), and a bulb tester.

I don't understand how the amp works either but I can test that with my ears...this is a safety issue! I want to be sure my grounding is secure.

Thanks
Bill
1- If you measure impedance with your DMM, you should get 10r. The only way it will conduct without an applied voltage is through the 10r resistor. If you apply more than aprox 1,5v or 2v, it will conduct current without almost any resistance. The paralleled capacitor will allow current to flow in high frecuencies (to allow the chassis acting as a more effective shield)

2- Not necessary to crimp, but it will be safer in case of HIGH currents flow. No worries because you have a high current bridge and fuses (or you should).

3- If you use one for both channels, you will have to join grounds from each amp board. Less channel separation. I am going to use 2, so they will join at the Safety Earth ground, where all my equipment (not only audio) is meeting.

Regards,
Regi
 
I forgot to quote the most relevant points from the article:
-Don't earth the internal electronics, or use a simple 'loop breaker' circuit to allow the case to act as a shield for radio frequency interference, but no solid connection is made (this is a common approach). This provides protection should there be a failure from the incoming mains to chassis

-The loop breaker works by adding a resistance in the earth return circuit. This reduces circulating loop currents to a very small value, and thus breaks the loop. The capacitor in parallel ensures that the electronics are connected to the chassis for radio frequency signals, and helps to prevent radio frequency interference. Finally, the diode bridge provides the path for fault currents. The use of a large chassis mounting (35A) type is suggested, since this will be able to handle the possibly very high fault currents that may occur without becoming open circuit. Note the way the bridge is wired, with the two AC terminals shorted, and the two DC terminals shorted. Other connection possibilities are dangerous, and must be avoided.
 
I think I would rectify in the amp enclosure.
1: The boards rectify for you if you put in the MUR820's included in the kit
2: You will need heavier cable to carry the higher current to the amps
3: If the AC picks up some noise on the way to the amps case you can destroy most of it with the rectifiers. With DC what will you do to destroy the noise besides push it through the caps?
Uriah
 
Thanks all - I'd read elsewhere about doing rectification at the traffo side of things and then having only DC on the umbilical.

I guess I could use IEC sockets to transfer the AC - Neutral and Live conductors could be for AC1 and AC2 and the earth socket for PG. I'm planning on running a seperate safety earth between the boxes that will be bolted to the casing at either end, so that would be all bases covered.

Cable could be mains 13A stuff - unlikely to need such a high rating, but it adds to the safety, plus I can get 10A or 16A rated IEC male and female plugs/sockets.
 
Thanks all - I'd read elsewhere about doing rectification at the traffo side of things and then having only DC on the umbilical.

I guess I could use IEC sockets to transfer the AC - Neutral and Live conductors could be for AC1 and AC2 and the earth socket for PG. I'm planning on running a seperate safety earth between the boxes that will be bolted to the casing at either end, so that would be all bases covered.

Cable could be mains 13A stuff - unlikely to need such a high rating, but it adds to the safety, plus I can get 10A or 16A rated IEC male and female plugs/sockets.

Not suggested to us mains type connectors for secondary voltages. To much chance of a wrong cable being plugged in.

I "suggest" you use the Neutrik power connectors or mic jacks...
 
C4 and C5 came supplied with 10 mm lead spacing and the circuit board has 7.5 mm spacing. How are you dealing with that?

TIA,

rick

I am guessing everyone is in the same boat with C4 and C5. Are you just bending the leads to fit the 7.5mm spacing? If so, do you bend both leads 90 degrees against the bottom of the cap and another 90 degrees to point down again? Just leave one straight and bend the other? What is best?
 
All else being equal, AC transport is much more immune to noise pickup than DC because of the extra components before the final use at the amp circuits.

With that said, usually all things are not equal and doing a CRC circuit is what is typically used.


This is the first time I've heard such a thing

Regards,
Regi

A LOOOOONG time ago Mr. Erath and Mr. Gerald taught me that just because I hadn't seen or heard of it doesn't mean they hadn't done it before I was even born. :D I am SURE that applies around the world.
 
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