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 Post subject: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 12:40 am 
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Two vintage stereo receivers in my shop recently I've noticed no speaker protection relays or delayed turn-on of speakers. Direct coupled output stages using fuses as protection. I happened to have a voltmeter connected across the speaker terminals when this Harmon Kardon is turned on, DC voltage swings widely about for a few seconds until the various supplies and circuits settle down. A bit unnerving! No wonder they make a thump and you can see the woofer cone move in and out an inch or so when you turn the things on. Kind of makes me want to design a cheap delay circuit to make the units more "modern"!


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 1:40 am 
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And yet, many still exist today along with their attached speakers.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 3:25 am 
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More common than you might think, even today. Only this month I had two high end (very expensive) audio amplifiers on my bench and none of them had any DC protection (relay) circuit. Looks like some engineers are overconfident and relays in the audio path are baaaad ! (according to audiophiles). Both amplifiers were DC coupled with output transistors shorted, putting over 50VDC on the speaker. Of course the fuse had blown, like the speakers connected to the amp. (burned and stuck voice coil).
And what about the highly valuable and collectible multi-hundreds watts vintage U.S power amps rightly renamed "Flame Linear" (I let you guess why)


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 2:10 pm 
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They were not the only ones; early Crown, Ashly, and other amps were made the same way. My old Magnavox Stereo 1000 also has no DC protection on the output.

At one time, there was a kit available to add that circuit to existing amps, but I haven't seen it offered for years; I plan to build my own for the old Mag.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 2:59 pm 
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There are several versions of relay based time delay/protection boards currently available on eBay such as:

https://www.ebay.com/itm/393475686076?h ... SwClZeorEh

Some styles are small enough that they might fit internally to the amp/receiver. Most require a non-grounded AC power source, but this can sometimes be obtained from the transformer winding powering incandescent lamps in the receiver/amplifier.

-- Jim


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 5:42 pm 
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SteveT wrote:
Two vintage stereo receivers in my shop recently I've noticed no speaker protection relays or delayed turn-on of speakers. Direct coupled output stages using fuses as protection. I happened to have a voltmeter connected across the speaker terminals when this Harmon Kardon is turned on, DC voltage swings widely about for a few seconds until the various supplies and circuits settle down. A bit unnerving! No wonder they make a thump and you can see the woofer cone move in and out an inch or so when you turn the things on. Kind of makes me want to design a cheap delay circuit to make the units more "modern"!


Standard operation, without extremely careful parts matching to make sure both voltages come up and track each other, nearly unavoidable. Transient DC within reason is not terribly damaging. The direct coupling is a very good thing that you want, the last thing you want in an output circuit is a transformer or (typically enormous) series capacitor - this is why transistor and hybrid amps are far superior to tube amps. It is generally safe enough for permanent damage - the fuse just has to blow before the woofer voice coil burns out, which have a huge margin.

*Many* modern, new-manufacture power amps also have no relays or turn-on protection, they have components matched well enough or rise-rate controls on the regulators to prevent significant transient DC/low frequency thumps. It suddenly appearing is a sure sign something is going over the hill - which may well be the case in these vintage amplifiers, parts aging can easily cause that. Preamps, on the other hand, almost always have something like this, if nothing else, if you turn stuff on in the wrong order, you greatly amplify it even if the power amp is perfect. That's why you always want to turn on the preamp first, let it get done doing the same thing, then, the power amp.

Brett


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 8:01 pm 
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Thanks Jim for the source of those protection circuits!


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Tue 24, 2021 8:38 pm 
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Tubologic wrote:
And what about the highly valuable and collectible multi-hundreds watts vintage U.S power amps rightly renamed "Flame Linear" (I let you guess why)

Funny you should mention that…. Got a Flame Linear D-500 welder on the bench right now. The power transformer is the size of a washing machine motor, and weighs about as much.,…

But it’s got 3 relays. Munch on THAT, audiophiles. How a relay can affect sound……. Beyond me

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Wed 25, 2021 2:31 pm 
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Yes, thanks for the link; I never thought to check EBay. There's no reasoning with audiophiles. There is nothing wrong with feeding audio through a relay!

In my days as a theatre-sound tech, I always cautioned the operators to turn the processor and preamps on first, and off last. Fortunately, the later amps did have protection relays.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Wed 25, 2021 7:47 pm 
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A system designed to last maybe 10 years is still working 40 or 50 years later and you're questioning the reliability of the design? As the Beatles once said. "Let It Be". You're overthinking it.

Larry

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Wed 25, 2021 7:50 pm 
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Speaker protection relays are fine until their contacts get dirty or corroded. Lately I've had a run of intermittent speaker protection relays -- one channel or the other drops out, give the relay a tap and it comes back. Pop the cover and clean the contacts with alcohol and they seem to respond, but I replace the relays if they are standard parts. Still, I much prefer that they be there in the design even if they need some attention or replacement later in life.

I suspect that maintenance isn't the reason that audiophools don't like them. Probably something about oxygen levels in the contacts interfering with "air."

-David


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Wed 25, 2021 10:45 pm 
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The shop I moonlight for replaces every speaker relay in every Pioneer, Marantz, Sansui etc that comes through the door. Experience has shown that while they *may* be able to be successfully cleaned, the time and labor to do so properly far exceeds the cost of just replacing the relay. Brand new ones are plentiful, and cheap, and just a few replace the vast majority of them out there.

50, 60 years later ..... the relays have done their job and deserve to go to the great relay in the sky ;-)

But NO, they do NOT alter the sound unless they don't make proper contact, in spite of what the audipiles think.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Wed 25, 2021 10:59 pm 
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dberman51 wrote:
Speaker protection relays are fine until their contacts get dirty or corroded. Lately I've had a run of intermittent speaker protection relays -- one channel or the other drops out, give the relay a tap and it comes back. Pop the cover and clean the contacts with alcohol and they seem to respond, but I replace the relays if they are standard parts. Still, I much prefer that they be there in the design even if they need some attention or replacement later in life.

I suspect that maintenance isn't the reason that audiophools don't like them. Probably something about oxygen levels in the contacts interfering with "air."

-David


They also routinely bypass the speaker protection (and input power) fuses, or use "audiophile-quality" fuses. When I was working on hi-fi systems, I told them all up front that I was going to restore any fuses I found bypassed and add them if they had been removed - don't like it, don't leave it with me.

A properly-sized fast-blow fuse is a perfectly good way to protect the speaker from DC, and the woofer will take an amazing amount of abuse and still keep going. DC is blocked out of the tweeter by the crossover. so even though they are extremely fragile, DC and turn-on thumps are not prone to destroying them. Leaky crossover caps, high frequencies generated by clipping, and high-frequency instability are far more likely to damage the speaker, as long as you have fuses.

Brett


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Thu 26, 2021 2:40 am 
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Doing some home recording once long ago, I found that you could fry tweeters with feedback, even at frequencies beyond your perception. Dunno what I did, switched to monitoring with headphones?

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Thu 26, 2021 2:57 am 
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jimtech wrote:
There are several versions of relay based time delay/protection boards currently available on eBay such as:

https://www.ebay.com/itm/393475686076?h ... SwClZeorEh

Some styles are small enough that they might fit internally to the amp/receiver. Most require a non-grounded AC power source, but this can sometimes be obtained from the transformer winding powering incandescent lamps in the receiver/amplifier.

-- Jim

This looks like a good solution, and certainly an economical one. However, those look like very small relays.
How would one specify what values to use for given amplifier power ratings? The (approximate) maximum output voltage of an amplifier should be fairly easy to calculate, but what about output current? Thanks.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Thu 26, 2021 11:58 am 
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Since one seldom drives a power amplifier to its maximum RMS ratings, the smaller relays are more than adequate except for power levels over, say, 200 watts. I have seen one or two that showed some heat deterioration on the relay leaf springs, but VERY few. Most are just oxidized contacts from sitting on the shelf for decades.

The "welder" class of power amp uses the larger P&B relays, but those are few and far between.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Thu 26, 2021 9:14 pm 
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The guy that sold me my big Jensen model 6 speakers roasted the tweeters, and their level controls, by playing with his equalizer at high volume. I bought them cheap and repaired them, 40+ years ago.

Barry, what do you guys use as a replacement relay for the Pioneers? I should probably replace the one in my SX-1010.

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Fri 27, 2021 12:46 am 
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Replacement Relays are off the shelf at Mouser, Digikey, Parts Express etc. There are a few different physical case types used and I don't off the top of my head know what unit uses which. Most are soldered in, even though the relays are usually designed to be socketed. Very few used the sockets, and now being this old, I consider that a plus really. One less contact to worry about.

Usually about $12 or so in quantity (they buy a minimum of 25 per order). But not too much more singly. There are also a few different coil voltages used and varies by manufacturer

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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Fri 27, 2021 1:08 am 
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Also the type of terminal varies. Some are intended to be soldered to a PC board, some can be PC mounted, socketed, or direct wired.

-David


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 Post subject: Re: Disturbing findings in early transistor stereo hifi
PostPosted: Aug Fri 27, 2021 5:29 am 
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Tim Tress wrote:
The guy that sold me my big Jensen model 6 speakers roasted the tweeters, and their level controls, by playing with his equalizer at high volume. .


A variant of the old classic, "I was testing at 16 khz, but I couldn't hear it, so I turned it up"!

Brett


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