Loudspeaker Relays

[Bob Cordell]

Use a "double mute" arrangement whereby the input to the power amp is muted as well. This second muting relay operates after the speaker is connected so that there is no possibility of the L/S relay contacts operating when there is a large signal present. Similarly on switch off this second relay removes the signal to the power amp first. I think there is more of a problem if the relay has to work for real, the inductance of the speaker will draw a large arc, many relays would fail under these conditions. I have thought about designing a protection circuit that does away with the usual time constant of traditional D.C. offset protection, along the lines of a comparator that would track the input and output's of the power amp and react if there were any difference. If it could be made 100% foolproof a triac as used in the old Quad 405 may be another posibility to include along with the relays. Maybe we could devise a "test network", lowish wattage resistors or perhaps a small filament bulb in order to test protection circuits. It's to late when a few thousand pounds worth of speaker goes up in smoke.
Regards Karl

If we have an amplifier that can withstand (e.g., not self-destruct) an output short circuit, a Triac that shorts the output of the amplifier to ground under a d.c. voltage event is certainly attractive. I have not tried this, but I think I have seen it in some professional amplifiers.

One question in this regard would be whether or not the presence of a shunting open Triac across the speaker line would cause any distortion or other sonic degradation.

Thoughts?

Cheers,
Bob

 

[Bob Cordell]

For many years now I have been using a specific Amplimo relay that has two parallel contact. At closure, a 100A (!) tungsten contact closes first, followed by a gold plated shunt across the main contact. At opening, the reverse takes place.

It's not a very well known relay but has been specially designed for audio power amps. I've used dozens of them , never had any problem.

I did some THD measurements with/without the relay in the chain and couldn't detect any differences.

I anyone is interested I can get the details; last time I looked it cost about 4 euro's from Amplimo, the tube transformer people.

Jan Didden

Hi Jan,

Thanks. Yes, I would like more information on this relay. Please post or email me.

Cheers,
Bob

 

[nemestra]

Yes, I would like more information on this relay.

Go to www.amplimo.nl

Click on Amplifier modules and scroll all the way to the bottom.

James

 

[Bob Cordell]

Avoid relays if you can is my advice.

_-_-bear

Hi Bear,

This is good advice, but it must be balanced against alternative means to protect the loudspeaker from an amplifier fault that could place enough d.c. on the output to destroy the loudspeaker. This is especially important in the rare but possible case of the output transistors failing shorted and the rail fuses (if there are any) not both blowing.

What is your favorite approach to protecting the loudspeaker?

Thanks,
Bob

 

[myhrrhleine]

Hi,
If you wire the relay so as to be NO and short the output on failure mode, then there is no need to worry about contact quality.
Similar to the TRIAC crowbar.
(include appropriate value current limit device as desired)

 

[Mooly]

Hello Bob, The Triac is certainly the "final" solution. One false trip could spell disaster for many an amp, but in fairness to Quad I think it was to provide failsafe protection for their electrostatic speakers. Nevertheless it is a very valid approach, I have the original details somewhere if your interested. Personally it is the small time constant introduced by all the protection schemes (that I have seen anyway) that bothers me. An output device going dead short and feeding say 60volts across the speaker is going to do some damage with the typical delay of most integrating networks. We all concentrate on the performance of the amp, the protection network is like a neccessary evil. Hence my thought on the comparator, no time constants, and the ability to detect a fault if there were more than say a 3% variation between input and output.
Regards Karl

 

[jan.didden]

Go to www.amplimo.nl

Click on Amplifier modules and scroll all the way to the bottom.

James

Yes that's the one; the ones I have are from a previous batch and are blue; however I doubt that that changes the sound

See pic of the relay on my ec amp protection board for size. I use one relay per channel and parallel the contacts.

My buddy Ward at Pilgham Audio is stocking these relays; I'm not sure what they would cost in singles from either Amplimo or Pilgham though.

BTW I am in no way affiliated with Amplimo, but it really is a great relay.

Jan Didden

 

[hoka]

nice protection pcb, where did that come from?

 

[jan.didden]

nice protection pcb, where did that come from?

Buy the May issue of Elektuur / Elektor and all will be revealed

Jan Didden

 

[peranders]

Hi Jan,

Thanks. Yes, I would like more information on this relay. Please post or email me.

Cheers,
Bob

Those relays Janneman mentions has good inrush current properties but this is hardly necessary because the speaker relay normally breaks up with little or no current. The relay has to perform only in emergencies. Since the relay will break a high current only a couple of times you can actually have a bit under rated relay. I would have used 2 x 8 A hardsilver in parallel. This combination can break 25-30 A many times and can carry continious current of 20 A at least.

Becuase of RHoS AgCdO has been replaced by AgNi (hardsilver) which is a bit better for low currents. AgCdO was not good at mA current

I have sold (not anymore) relays (Schrack, Elesta, Relpol) since the 80's so I know some about real world applications.

Bob, if I should recommend something I would have taken Elesta SGR282 types becuase Elesta is one of the better brands in that segment, unfortunately they are a bit hard to get hold on and also a bit expensive.

Elesta has also a wolfram/silver relay but I don't remember the type SGR something.

From business, Elesta had the lowest failure rates, in fact too good (and too expensive) in many applications.

http://www.elesta-gmbh.de

Not the same company (they are now divided)
http://www.elestarelays.com/de/index.php

 

[peranders]

Go to www.amplimo.nl

Click on Amplifier modules and scroll all the way to the bottom.

James

I wonder who has made them. It looks like a chinese brand but it could be something else. Amplimo has not made them

 

[jan.didden]

I don't know who makes them. Amplimo has them since the early 90's so I doubt they are from Chinese origin.

Jan

 

[Juergen Knoop]

high inrush current relays with advancing tungsten contact were at least made by Schrack and MS Mikrosysteme (obv. a company from the deceased GDR).
Regards

 

[peranders]

I don't know who makes them. Amplimo has them since the early 90's so I doubt they are from Chinese origin.

Jan

If that is the case it ought to be some europeen manufacturer.

 

[jacco vermeulen]

The Amplimo is on the market longer than that, i bought them after i saw them advertised in RB/Elektor in 1987.

the best compromise is a Matsushita (Nais, Panasonic, SDS ...) S Relay.

S4, 2 relays in parallel makes 40A, 8000VA AC or 800W DC from 8 contacts.
Factory box NOS SDS S4's were posted for 1 Euro/pc on this forum.

Or else:
these

 

[fotios]

I would like to learn what experiences people here have had with relays in the output path of a power amplifier, and what recommendations anyone might have for a very good-performing PC-mountable loudspeaker relay.

When I built my listening comparison AB switch box, I spent quite a bit of time making distortion measurements on relays as candidate for use in that application, and found substantial differences among different relays. Most of those relays were large, high-current relays that were not PC-mountable. Some were automotive relays. PC mountability and size were not an issue in that application.

Cheers,
Bob

Hi again Bob (if you permit me to call you with your little name - in such way we say in Greece the first name)

I forgot a significant remark. Peavey, as usually, has some currious practices. Instead the usually used relays, in all of its models use those of 18Vdc coil! During repairing a CS 1200X, i observed a strange thing in the voltage supply of coil . When the relay it is armed (a normal condition) the supply voltage meassured accross the coil terminals it is 21Vdc! From curiosity i searched the data of relay. In these reffered that the maximum allowable Vdc for the coil it is 21,6 Volts. This placed me in wonder. But after little thought i supposed that this practice maybe it is usefull. I have some JTN open type relays, and i observed that by increasing the coil voltage from its nominal value, the contacts pulled between them with more force. I don't know with certainity if my thought it is correct, but i suppose that the more voltage from the nominal (and a little bit lower from the maximum allowed) to supply the coil, the more force accordingly from the armature to join the contacts between them. Maybe this reduces a little bit the resistance between the joined contacts. For certainity, i measured a second amplifier and the results was the same.
I ask from any of you if he has objections about this practice, to address these directly in Peavey and not to me. I am simply an observer of this incident and maybe my assumption it is wrong.

Fotios

 

[hoka]

Buy the May issue of Elektuur / Elektor and all will be revealed

Jan Didden

Mei issue is nog niet eens uit???

 

[Cal Weldon]

hoka,

If you wish to write in another language, please include the English translation as well.

Thank you.

 

[peranders]

Mei issue is nog niet eens uit???

I think I'll understand this without checking a dictionary

The may issue isn't out yet?

 

[Bob Cordell]

Hello Bob, The Triac is certainly the "final" solution. One false trip could spell disaster for many an amp, but in fairness to Quad I think it was to provide failsafe protection for their electrostatic speakers. Nevertheless it is a very valid approach, I have the original details somewhere if your interested. Personally it is the small time constant introduced by all the protection schemes (that I have seen anyway) that bothers me. An output device going dead short and feeding say 60volts across the speaker is going to do some damage with the typical delay of most integrating networks. We all concentrate on the performance of the amp, the protection network is like a neccessary evil. Hence my thought on the comparator, no time constants, and the ability to detect a fault if there were more than say a 3% variation between input and output.
Regards Karl

Hi Karl,

Yes, the Triac (or shorting relay) approach should only be used on an amplifier that will definitely non-destructively protect itself from an output short. In one appraoch I like to use, my amplifier shorts the MOSFET output gates to the output rail with a latching action if the current through any output device ever exceeds, say, 10 Amps, for more than, say, 10 ms.

You are right about the worst-case failure scenario, where the amplifier suddenly has one or more of its output transistors short, instantly applying full rail voltage to the loudspeaker for a certain amount of time until either a rail fuse blows or the dc offset detector has time to respond. Such a scenario could happen as a result of a random circuit failure or due to an output transistor SOA failure. The large glich might last 10-100ms, and might very well damage a loudspeaker.

Any fast-acting approach to avoiding this possibility would seem to be fraught with the danger of unnecessary triggering, such as when the amplifier clips. I can't think of any easy answers here.

In any of these approaches we always want to be careful of falsely triggering on a large 20 Hz sinewave, for example.

I'm curious what others may think of any possible audible effects of hanging a Triac across the speaker output for speaker protection. I would think a Triac would be cheaper, smaller and more reliable than a shorting relay (if output shorting is the approach to be taken for speaker protection).

Cheers,
Bob