Leach amp - need suggestions for output relays

A friend of mine, much better in audio electronics than myself, finally finished a leach amp for me. :)

Even there is no mentionable DC voltage on output, just for case I have decided that speaker protection with relays on output should be implemented, to protect my expensive speakers.

The easiest way for me to buy relays is through Farnell, but almost all I have found, with higher current (30,40Amp) has contact resistance of 0.1 OHM, which is too large I suppose and can affect output impedance.

Would someone experienced propose which relays I should use to make the least effect on sound, and that could preferably could obtain through Farnell.
 

djk

R.I.P
2001-02-04 4:23 am
USA
I'm sure it is much less than 0.1 ohm.

A 10A relay should be adequate.

I use a 3P relay, the third pole is part of the inrush limiter circuit.

TYCO ELECTRONICS / POTTER & BRUMFIELD|KUMP-14D18-24|POWER RELAY, 3PDT, 24VDC, 15A | Newark.com

KUMP-14.jpg
 
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if a 30A relay had 0r1 resistance across the contacts and the lead outs, then that relay would drop 3V before you get outside.

You will be unlucky if you can measure >300mV Vdrop across the relay @ 30A, indicating ~10milliohm sum of the maximum resistances.

Imagine dropping 3V supplying a pair of 12V car headlamp filaments!!!!!
 
Ebay has a speaker protection circuit for $9.90 plus shipping or I have seen the same one that is put together for $11.00 I bought two of the kit ones and they sense DC disconnect immediately upon turn off and take 2.5 seconds to latch up. They have a 15A relays.

Since I have decided to ruin the sound with relays, I hope that if I pay more, I will get relays with better contacts that will modify the sound less..

I do not have confidence in complete kit with relays for such low amount of money...

What worries me is contact pressure, contact material and contact size (area). Why am I investing in very expensive connectors, expensive cords when I have relay with loose contact of only few square mm.... :(
 
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Since I have decided to ruin the sound with relays, I hope that if I pay more, I will get relays with better contacts that will modify the sound less..

I do not have confidence in complete kit with relays for such low amount of money...

What worries me is contact pressure, contact material and contact size (area). Why am I investing in very expensive connectors, expensive cords when I have relay with loose contact of only few square mm....

Well, a simple response of none would have been far better. If you believe that relays modify the sound then so be it. Buy the most expensive relays that you can afford by all means. I've already tested these cheap relays that you want no part of and found them to be ok and suited for MY application.
 

tryonziess

Member
2007-04-13 12:15 am
Velicko,
you can always use output capacitors and fuses.
Many more knowledgeable than me claim you will not hear the capacitors. That is a debate I wish not to get into. There also is the output transformer for protection. You could also add in a servo. All of these fixes have there drawbacks.
I do not use any protection other than a set of fuses on my Leach amps. I constructed my own speakers and would rather not loose them if at all possible, but, things do happen. The latching time of the relay is a pretty long. Is it something you trust to work effectively when called upon?

Good luck, Tad
 
I suppose that I am assuming a few things like rail fuses and relay contacts with a higher rating. Also, one could use a multislope V-I limiter in the output stage to limit the current in the event of a short or this relay action. Given the rare circumstance, it would seem a viable option in that you would be protecting hundreds of dollars or more of speakers from a failing amp module as it were. Just a thought
 

tryonziess

Member
2007-04-13 12:15 am
If you are concerned about frying your speakers you could install a set of output transformers like Mcintosh uses or a pair of large capcitors.
My experience with the Leach amp layout is that it is extremely well designed and you could if you wish spend more time selecting CD's and listening to the tunes than worrying about your speakers. I do not mean to throw caution to the wind but I have never destroyed a set of speakers with any amp I have used. Many times the magic smoked appeared in my circuit boards and never did the speakers suffer. I have heard of many a fried coil, but, I guess I have been lucky.
If you have a pair of Leach amps built by your friend with care and love then you have a truly nice audio setup. You should be proud to own one of the many BJT amp designs that is known for its quality and robust design.
Long live Leach in his work which he gave us for free.

Tad
 

Bonsai

Member
Paid Member
2003-07-25 10:44 pm
Europe
www.hifisonix.com
Stop worrying about the relay. Just use a decent quality one (parrallel then is also ok, but then use one of them as a sacrificial relay).

Most good quality relays - especially the auto ones, have very low contact resistances - damping factor is not all its cracked up to be since your cable and crossover resistances will dominate.

There can be no doubt the most detrimental things in your system are not the 10 to 30milli Ohm relay contacts.