First Lm3886

[Redshift187]

Originally posted by PJPro
My practical implementation will be to run a wire from the earth connector on the power inlet module and and bolt it directly to the chassis. This bolt will service no other connection other than to accommodate this wire.

Should I assume that this is the correct approach?

Yes, this is correct.

[PJPro]

OK Redshift187. Thanks. At least I can be certain that whatever I do with regard to the disconnecting network, I can be confident that the safety earth is in place.

But I am going the implement the disconnecting network and want to allow for the addition of this circuit when laying out the components in the case. Last night I did this and marked up where I need to drill my holes. Things aren't as roomy as I expected mainly due to the large heatsink. Still it does all fit.

I do need to buy some more metal, to subdivide the case internally and create a spacer for the tranformer, which I hope to do today. I'm also going to get a flat brass bar to attach the chip to the heatsink.

[PJPro]

There are a number of outstanding issues to be resolved in my own mind until I am confident that I have all the circuitry I need.

The first of these is a soft start. Do I need one of these? What's the consequence of not having one? Is it a bit of a pop when I turn the amps on or is it a constant blowing of fuses?

The other two relate to filters and were mentioned by Andrew right at the start of this thread, namely high and low pass filtering.

I'm assuming the high pass (DC) filter needs to be applied to the signal coming in from the RCA socket. I believe I placed two whacking great caps on the signal path to counter this when making my Tangent variant of the CMoy headphone amp. However, I have subsequency read a number of posts (here and elsewhere) saying that caps in the signal path are to be avoided. So what's the best way of addressing DC offset?

In terms of low pass filtering, I have gone some way to address RF by buying a power inlet module which incorporates a filter. However, as was pointed out earlier in this thread, there are other sources of RF (RCAs and speakers?). What's the recommended way of intercepting these sources of RF?

[PJPro]

The simplest soft start circuit I can find is this one by Rod Elliot, where A is the live and SA is the soft live.



It is placed in series with the supply to the transformer. More details here.

I'll have a go at calculating the resistor values (I will probably get this wrong, so do not use as a guide unless confirmed by a more knowledgable poster)....

I = VA / V (1) Where VA is the VA rating of the transformer, and V is the mains voltage used

Therefore,

I = 160 / 240 = 0.67A

At a limit of 200% of full power current, this is 1.3A AC. Using

R = V / I (2)

gives...

R = 240 / 1.3 = 180 Ohms

This suggests that I can use a single 180 Ohm resistor for the above circuit.

Earlier I bought a 5W resistor for use with my disconnecting network. This is massive. Can I use a 2W version....and how does one go about calculating this?

[Ted205]

if you are using a 160va transformer and the chipamp psu you will not require a softstart.

will you be running a preamp/buffer ? as these will cause power on 'pops'

"caps in the signal path are to be avoided" in theory is true as adding anything into the circuit will contribute it's own distortion. In practise it's just bad publicity in my opinion and comes down to the quality of the cap, but others will disagree

i've used a 3.3uf similar to these as the input dc blocking capacitors. As used in Rod Elliot's chipamp.

This capacitor forms a high pass filter with R1 (22k) so low frequencies (in this case <2.2Hz) will be attenuated

[PJPro]

Originally posted by Ted205
if you are using a 160va transformer and the chipamp psu you will not require a softstart.

That's good news. Thanks.

Originally posted by Ted205
will you be running a preamp/buffer ? as these will cause power on 'pops'

Initially, I'll be using my Beresford DAC (it doubles as a preamp). I'll probably build a pre-amp to replace this.

Originally posted by Ted205
"caps in the signal path are to be avoided" in theory is true as adding anything into the circuit will contribute it's own distortion. In practise it's just bad publicity in my opinion and comes down to the quality of the cap, but others will disagree

i've used a 3.3uf similar to these as the input dc blocking capacitors. As used in Rod Elliot's chipamp.

This capacitor forms a high pass filter with R1 (22k) so low frequencies (in this case <2.2Hz) will be attenuated [/B]

Wow! Those caps are massive.....and expensive! How do you go about mounting those puppies?

[Stuey]

PJ, I'm pretty sure that soft start diagram is just meant to be conceptual, showing the principle behind the soft start in placing resistors in the circuit to be short circuited by the relay. There's no relay delay incorporated into that circuit.

Stuey

[AndrewT]

PJ,
the input filtering does two different things.
The high pass filter determines the low frequency foll off and also blocks DC from the source, if any.
The low pass filter determines the high frequency roll off and also attenuates RF/interference coming in through the interconnect or from a source.
So the 4 components needed at the input do in total four jobs. Can't ask for better value than that.

If you know that all your sources can NEVER pass DC on to the next stage, you can delete the DC blocking capacitor from the power amp input. But you still need to determine the low frequency roll off of the amplifier.
The choice becomes yours once you have informed your self of the way your sources and your power amp interact.

[AndrewT]

Pj,
the 160VA transformer should never draw more than 667mAac of current if it is working to it's maximum rating.
A T800mA fuse is adequate for this.
Overload the transformer and eventually the T800mA fuse will blow.

Try starting the amplifier with that correct fuse on the primary side. It may blow or it may blow at the seventeenth time of switching on.

To allow the correct rated fuse to be used you may need a soft start.
Don't believe all those posters that say you only need a soft start when the toroid exceeds 300VA. It depends on how the transformer has been designed and how unlucky you are with voltage at the time of switch on.

You may find that your amplifier will operate perfectly well with T600mA or even a T400mA fuse on the primary. I recommend that you fit the smallest fuse rating that allows your amplifier to play in all the situations (loudness) that you are likely to encounter.

If it won't start reliably (nuisance blowing) on this small fuse then add a delayed relay and some power resistors.
The delay can be between 100ms and 300ms. Some have suggested that the delay can be even shorter but I have not experimented with this.
The power resistors must be able to survive the transformer start up current for the time delay you have chosen.
They will generate some self heating during the delay time but if you keep it short, <=300ms, the resistors will be OK.
For bigger transformer I use a bank of 5off 10r 5W resistors in series. For your 160VA where the current to start the toroid can be much lower try using 4off 27r or 22r 5W resistors in series.
I do not recommend putting higher value resistors in parallel. The high value resistors are wound with very thin wire and will be less reliable than the thick wire low resistance version during the high current start up phase.

BTW,
I don't like ESP's use of an aluminium shroud around that mains connected resistors. I think that is an accident waiting to happen.

Fit a cover over the mains fuse holder.

[Redshift187]

Softstart:
The image you posted is only a small part of the total circuit, and you need at least 5W resistors for it (according to Rod Elliott). Given the information on the site you linked, I had calculated the resistors would see close to 1000W (total, not each) for an instant. I had done my own research into the pulse capability of power resistors and had chosen 3X 10W resistors that have a pulse capability of something like 250W or 300W. If you think 5W resistors are huge, you should see the bigger ones with heatsinks built in!

DC blocking / high pass input filtering:
An additional potential side effect is not only low frequency roll-off, but also low frequency phase shift. That said, AndrewT has great suggestions regarding low frequencies with correct component selection for the feedback components, input filtering components and power supply components (yes it matters!). Another option to get rid of DC without using capacitors in the signal path is to use a servo setup, but that is not really possible without the PCB being designed for it.

If you do not use DC blocking capacitors at any point in your setup, use a DC detection and protection circuit on the output!