# 2-way speaker switcher: feedback and advice on schematics needed

Discussion in 'The Projects Forum' started by micheled, Jun 22, 2015.

1. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
Hi all, thanks in advance for the time spent on helping me out on this project. I greatly appreciate!

Goal
To build a dual-channel, 2-way speaker switch to control the output from the hifi amplifier to 4 speakers, speaker set 1 and 2, that are located in two different rooms.
The idea is to be able to activate speakers set 1, 2 or 1+2.

The numbers
- Amplifier output: 70W + 70W (4 ohms at 1 kHz) DIN Power output. (As stated on the service manual)

I know this is fuzzy figure. The amp is an old Sony TA FE-370. For the sake of start designing, I have assumed this figure to be the constant output power from the amp, leaving headroom for possible spikes in power. As soon as I will receive a new multimeter, I will attempt a more detailed measurement of the output, choosing the final components accordingly.

- From those numbers, using Ohm's law, I derived volts and amperes:

P = 70 watts
R = 4 ohms
Output voltage = √P * R = √70 * 4 = 16.7 volts
Output current = √P / R = √70 / 4 = 4.18 amperes

- Total resistance of the parallel circuit:

Number of resistors = 2
Resistance = 8 ohms
Total resistance = resistance / number of resistors = 8 / 2 = 4 ohms

Design

Approach

The schematics above shows the design for a mono-channel switcher, which will be duplicated for the actual use (example is wired to Left channel).
Following the idea on this Wikipedia page, https://en.wikipedia.org/wiki/3-way_lamp, the DP3T switch controls the positions necessary to activate each set of speakers at one time, or both at the same time (1, 2 or 1+2).

A dummy load replaces one of the speakers when just the set of speakers 1 or 2 is working. This is in order to maintain the overall resistance constant.

Technical data for components

DP3T switch:
Contact Voltage AC Nom: 250V; Contact Voltage DC Nom: 28V; Contact Current Max: 5A;

Resistance: 8ohm; Power Rating: 50W; Resistance Tolerance: ± 1%;

Questions

1. I'd appreciate a general feedback on the design. Is it well designed? Is it gonna work? Any drawbacks you can think of, or possible improvements?

2. Given the numbers for current and voltage, would the circuit be safe to use as it is, once wired?

Thanks a lot, please feel free to ask for more details and/or slap the total newbie!
Michele

2. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
Drawing of the circuit updated.

3. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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771
Why do you need a dummy load ?
Amplifier will run fine without the dummy you know

4. ### micheled Thread Starter New Member

Jun 21, 2015
10
1

But don't I need to keep the resistance constant when one set of speakers are not powered? I might be a bit slow here...

Thanks a lot!

5. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
Nope.
Without a dummy you will be fine.
You showed a mono block, so I assume if you use stereo you would use two identical connections, if so you will be using both channels. No need for dummy.
If you use a mono one you will always be using one speaker so no need for dummy here too.
Dummy is used to test amps without destroying your hearing.

You will do fine with a relay of 5A, 30VDC contact rating. The coil voltage will be of your PSU choosing.. Mostly 12V~24V coil rating are used.

6. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
Okay, Okay, got you. For some reasons I wanted to utilise the maximum power from the amp at 4 ohms no-matter-what, but in fact half of it would be totally wasted anyway on the dummy load. Thanks for pointing out a very basic thing...

7. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
Dummy wastes power on load and amp. You will load the amp uselessly.

8. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
OK, will do further reading on relays and hopefully buy the parts today. Thanks for your help

9. ### #12 Expert

Nov 30, 2010
16,705
7,355
Only vacuum tube amplifiers become stressed about the load. Modern transistor amplifiers have no such problem.

10. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
Hi #12, thanks for adding to the conversation.

Did some reading today about fuses, relays and circuit breakers - especially informative were the articles of the textbook here on AAC.

I realised that adding a relay as suggested above by R!f@@ could actually be counter-productive, if not planned properly. Spikes of power from the amp could trip the circuit, when actually these spikes are part of the normal operation of the amp as I think I read elsewhere. I redesigned the circuit (below) without any additional load, or safety measures. The last idea is part of the assumption that the amp has already got built in some form of protection from over-current, therefore making the addition of another relay possibly superfluous. Am I wrong here? Is this bad practice? Thanks, I know I've got a lot of questions, possibly banal ones, but I'm new to this

11. ### #12 Expert

Nov 30, 2010
16,705
7,355
You really threw a wrench in that drawing. The switch doesn't switch anything. Please clarify.

12. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
- Switch in top position powers speaker 2

- Switch in middle position powers speaker 1

- Switch in bottom position powers speaker 1 and 2 in parallel

- This is a switch for one channel, actual switch will have two, Left and Right channels.

Edit:
I may see what you mean, just having a second, harder think about it.

Last edited: Jun 23, 2015
13. ### #12 Expert

Nov 30, 2010
16,705
7,355
I think you need a 2 pole, 3 position switch to do an A, B, A+B activity.
Works better (in my mind) with the wipers connected to the speakers.

14. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
Like so? Cheers

15. ### #12 Expert

Nov 30, 2010
16,705
7,355
Top jumper is wrong. It goes from 1 to 3, not 2 to 3

16. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
Yeah, right. Sorry, attention span shortening a bit now (past midnight here in the UK). Thanks, you're a patience man

17. ### #12 Expert

Nov 30, 2010
16,705
7,355
Not what I said to do, but that is one of several configurations that will work. It reads out, A, A+B, B
ps, I only appear to be patient because you are not assuming I am secretly feeling hostile. Right now, I'm just quickly spewing out facts without emoticons because I have too many chores on my plate.

18. ### micheled Thread Starter New Member

Jun 21, 2015
10
1
That's because I prefer to be straightforward with people and tend to expect the same. Life is hard enough as beginners, have a good evening and a big thank for your help anyway

#12 likes this.
19. ### R!f@@ AAC Fanatic!

Apr 2, 2009
8,792
771
PS. Adding relays is not a problem.
As speaker is always connected and disconnected via relays with in the AMP