Designing a circuit (transistors etc) ...

[zcacogp]

Chaps,

A practical one this ...

I have a HiFi at home, which comprises of a CD player, playing into an NAD pre-amp, and a couple of studio amps. (Studio amps are Crown D45's, as I posted about here.)

The studio amps are not very sightly, so I want to install them somewhere where they won't be seen. The HiFi is in the lounge, but the speaker cables are routed through our basement to the speakers, so I could just put the amps in the basement, run a signal cable from the pre-amp to the amps, and speaker cables back up to the speakers.

However, if I do this, I need to be able to turn the amps on remotely. They have big chunky controls on the front of them, and no remote-controller, so this will need a bit of thought.



Having dismantled the amps, the on-off dials are just simple switches, with two terminals on the back. If I bridge these terminals, the amps turn on. This could be done with an external circuit, but it would need to be able to handle a bit of power - the amps are rated at 80w each, but the turn-on circuit would probably need to be around twice or three times that. So, about 200w.

I have also dismantled the pre-amp, and discovered that it has a power circuit in there that goes up to a (very very steady) 5.00v when the unit is turned on. It goes back to 0.00v when it is turned off.

My thinking is that I can use this to turn the power amps on. BUT clearly I will need some circuitry to enable this to happen. I am thinking about using the +5.00v to switch a transistor on, which can in turn switch a relay on, which turns the amps on. So, a circuit something like this:



I am assuming that the current draw from the +5.00v input can only be minimal, so a matter of a milliamp or so. (If the current draw is too large, the pre-amp will refuse to turn on.)

The transistor will need another power supply to drive it, but I have enough spare mobile 'phone chargers and the like to provide this. The figures marked on the picture (10V, 200mA) happen to match one that I found in my cupboard.

A couple of questions ...

1. Will this approach work? If not, why not ...
2. (The biggie!) What components should I be choosing for the transistor and the relays? I can shop at Maplin or RS (I have an RS account), but simply don't know what to choose. Does anyone have any suggestions?

Thanks!


Oli.

[GreenV8S]

Relays (suitable Voltage and current rating)?

[dtmpower]

Are you sure the power switches are the full rated power ? Are they not some sort of relay or transistor switch telling the unit to power up the amps ?

[zcacogp]

GreenV8S said:

Relays (suitable Voltage and current rating)?

Yes, as suggested in the piccie above (did I use the wrong symbol for a relay?)

The question is, what rating, and which one? And (more importantly) what transistor should I choose?


Oli.

[zcacogp]

dtmpower said:

Are you sure the power switches are the full rated power ? Are they not some sort of relay or transistor switch telling the unit to power up the amps ?

I think they are the full rated power, given the thickness of the wire going into them and the general industrial build of the amps. And the fact that they pop noticably when they turn on. (They are studio amps, not home hi-fi ones, so are made rather less elegantly than the sort of thing I'd choose to put in my lounge. The upside is that they sound delicious, and were free!)


Oli.

[jeff m]

I could dig you out a number for a "switcher" and rec a couple of mercuries, but why mess with the innerds of your amps.
Two very very cheap X10 appliance modules will handle these amps very easily,
one X10 trans/rec, plug into the room of operation, one small remote.
(signal will go through your house wiring)

[Mars]

jeff m said:

I could dig you out a number for a "switcher" and rec a couple of mercuries, but why mess with the innerds of your amps.
Two very very cheap X10 appliance modules will handle these amps very easily,
one X10 trans/rec, plug into the room of operation, one small remote.
(signal will go through your house wiring)

+1

In fact B&Q sells some remote plug-in switches for pennies.

[zcacogp]

Interesting idea ... I like the seamless nature of the amps turning on with the pre-amp. It's one less button to press when you turn on, and more integrated and discreet.

And I like the project.

What do you mean by a "Switcher, and rec a couple of mercuries"?


Oli.

[jeff m]

Switcher = sutible transister
Mecuries = relays able to handle back emf with no arcing

But really rec the X10 module route, just make sure you get appliance modules and not light modules. Set them both to same code then one signal will turn them both on.

[Globulator]

You need to limit the current to the base, or you will immediately burn out the transistor (Base to collector is just a small diode). Between 1k and 10k should be fine.

I'd also go for a darlington transistor to reduce the base loading onto the 5v power line - they have far more current gain in this application.

In addition you need a diode across the relay coils to snub the 'kickback' spike which will otherwise take out your transistor.

[zcacogp]

jeff m said:

Switcher = sutible transister
Mecuries = relays able to handle back emf with no arcing

Yes, but what sort of transistor would be suitable? rswww.com comes up with about 3800 of the things! here

And, come to mention it, what sort of relay? And why does it need to be able to handle back EMF?

jeff m said:

But really rec the X10 module route, just make sure you get appliance modules and not light modules. Set them both to same code then one signal will turn them both on.

OK, thanks. I prefer the one-switch method, but I'll look into this if I can't find out what I need to ...


Oli.

[zcacogp]

Globulator said:

You need to limit the current to the base, or you will immediately burn out the transistor (Base to collector is just a small diode). Between 1k and 10k should be fine.

That's helpful, thanks. Hadn't thought of that. Should the resistor go before or after the transistor in the circuit (or doesn't it matter - I imagine it doesn't.)

Globulator said:

I'd also go for a darlington transistor to reduce the base loading onto the 5v power line - they have far more current gain in this application.

When you say 'more current gain', you mean that the transistor will draw less current from the +5v supply than other types? RS comes up with a couple of hundred of them - here
Part of my problem is that I don't know what the various specifications refer to - what is Ic, what is Vceo, what is the difference between an NPN and a PNP, what is Pd (power dissipation?) and what are the different packages?

Globulator said:

In addition you need a diode across the relay coils to snub the 'kickback' spike which will otherwise take out your transistor.

OK, is this what was referred to as back EMF by JeffM? Presumably this goes across the coil of the relay in the opposite direction to the current being supplied to turn it on?

Thanks for your help.


Oli.

[Pigeon]

Your circuit is a bit upside down, and that transistor will be always on.

Try this:



A TIP110 should do for the transistor and any of the 1N400x diodes for the back EMF catcher. For the relays, look through the book and pick something with a coil voltage to suit your power supply and a contact current rating to suit your amp... well I'd go for 10A or more thinking of switchon surges... does the existing switch have a rating printed/embossed on itt anywhere?

[spikeyhead]

Pigeon said:

Your circuit is a bit upside down, and that transistor will be always on.

Try this:



A TIP110 should do for the transistor and any of the 1N400x diodes for the back EMF catcher. For the relays, look through the book and pick something with a coil voltage to suit your power supply and a contact current rating to suit your amp... well I'd go for 10A or more thinking of switchon surges... does the existing switch have a rating printed/embossed on itt anywhere?

That's close but will draw about 10mA from your 5V supply, which may be too much. Increasing the base resistor to 4k7 will reduce the base current to 1mA ish but you'll then need to darlington the transistor to get enough current gain to drive the relays.


ETA, I've now read the TIP110 datasheet and it is a darlington device, so there'll be enough current gain once the base resistor is increased.

Edited by spikeyhead on Tuesday 26th February 18:58

[Pigeon]

10mA will be fine from a regulated logic-level power line. It's not going to be that feeble.

The TIP110 is an integrated Darlington... but Darlingtons, especially with power devices, never "live up to expectations" so I've given it a good whack of base drive. You probably could get away with less, but it's not exactly a micropower design, it's a make jolly sure it works when zcacogp tries it design

[Parrot of Doom]

Part of me is trying to understand why you don't just switch the mains into the power amps on/off together with the mains into the preamp, with a wall-mounted switch.

Unless you're trying to eliminate the 'pop' from the power amps as the preamp turns on?

[klootzak]

Why don't you just stick 'em in the basement and leave them powered up all the time? Undriven, their power consumption should be negligible and being pro-amps they're designed to be left on for long periods anyway.

k

[zcacogp]

Pigeon,

Fantastic! Thanks very much - that's exactly what I wanted!

So a component like [url]this|http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4859648[/url} should do me nicely? Only snag is that they come in a box of 5 minimum. Heigh ho - it'll let me blow a few up in the making, eh?

Resistors - thanks. I'll have a hunt around in the box of bits and see what I can find. Ditto the diode.

As far as the input current from the pre-amp goes, I'll try the higher value of resistor (4k7) and see where that gets me. If not, I'll put a lower one in. I found out (by accident) that if you draw too much current from that part of the pre-amp, the pre-amp simply refuses to turn on, so I won't be doing any damage there. Thanks for your input there spikeyhead.

PoD, one mains switch would be difficult as the amps will be a distance away from the pre-amp, in the basement. I could have simply run a power cable down to the basement and switched them on and off from there, but then the power cable would be running alongside the signal cable (from pre-amp to amp) and the risk of mains hum would then be a concern.

And (more importantly), it wouldn't have been as much fun!

Klootzak, I could have done that as well. But again - I like the project! And the power use would have been small, but slight. And they do get hot. And I guess I don't like leaving things on when they aren't being used.


Oli.

ETA: Pigeon, no, the switch on the inside of the amps has no indication of current rating. It's a pretty simple thing - twist, two detents, couple of terminals which line up. 10A is probably a wise choice, I'll see what sort of relays are out there. Thanks again.

Edited by zcacogp on Tuesday 26th February 22:27

[Pigeon]

zcacogp said:

So a component like this should do me nicely? Only snag is that they come in a box of 5 minimum. Heigh ho - it'll let me blow a few up in the making, eh?

Yep, that's fine...

Try http://www.grandata.co.uk/ for small quantities of semiconductors - you can buy singles and postage is only a quid. Phone to check their stock first though, because the website won't say and they'll send you a refund in stamps.

[zcacogp]

OK, a year later and I finally got 'round to doing this ...

... and it worked!

Many thanks for your inputs chaps. Much appreciated. The circuit is still on my workbench - hasn't been wired up to the amps yet - but the relays are pulled in when the pre-amp is turned on and open up again when it is turned off.

I now need to run a bit of wire from the lounge to the basement and move the amps downstairs ... but this is not hard at all.

Thanks again. This is the first time I have ever built anything like this and it's most pleasing!


Oli.