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Old 16th June 2004, 01:04 PM   #1
ABO is offline ABO  Netherlands
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Default input cap or output relay...

That's the question.

I did some listening with different types of input coupling caps. They all seem to degrade the sound. I tried:

- Intertechnik KPSn (tin foil) cap: good on trebles, but not in midrange, expensive

- Intertechnik Audyn cap (Polyprop): muddy! don't use these!

- Intertechnik MKP+ (low inductance polyprop): very good, perfect midrange, but slightly dull top-end and very expensive.

Then I think: why not use a output relay? If all, even the best, caps degrade the sound?

I currently have a DC coupled signal path straight from the DAC to the speakers. With very expensive drivers I think this is asking for trouble. If anything fails, my woofers will join the failing part.

So, what is the verdict on output relays? To quote Charles Hansen: 'Everything makes a difference.' Understood, but some things make more of a difference than others. So, caps or relay?

I know that there are kitsets available, but none seem to be of 'high-end' quality. I'm looking for a 'stand-alone'speaker protection kit that uses very good quality relays.

Thanks
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Old 16th June 2004, 03:09 PM   #2
ekaerin is offline ekaerin  Sweden
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ABO,
If you don't want to do it from scratch you may want to get a cheap kit
with a nice topology (I assume it's the relay making up the cost) and
change the relay.
However, finding a relay that can handle both brute force and still deliver details needs some thinking.
Myself I'm not that "worried" about input caps, maybe I should be....

Anyhows, adding an input cap does not protect your speakers from an
amp failure. Guess which one is more likely to fail, your DAC or amp ?

/ Mattias
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Old 16th June 2004, 03:26 PM   #3
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ABO, before you do anything, why don't you test these caps together with an another person? I think you are influenced by opinions. Why do I say this?

I have designed an amp with BUF634 as output buffer. The pcb has an option for "medium speed" and "high speed" of this buffer. Some people here claim that they really can hear a significant difference. Myself and two more persons have tested this, jumper on...jumper off...jumper on... jumper off. I _wanted_ to hear a difference but I couldn't. Neither could the other persons. With this I wanted to say that despite my still rather good hearing the difference is VERY small but other people do really claim that they _can_ hear the difference. The thing is that those people never told us _how_ they could detect this difference.

With this I want to say, get second opinion before you are digging yourself into a too deep hole.
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Old 16th June 2004, 03:35 PM   #4
sam9 is offline sam9  United States
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The value of the input cap rather than the brand is probably more important. The type has some importance as well. I came to this conclusion accidently in the course of some simulations followed by actually constructing an amp. If you look just at the roll of a 4.7uF metal film cap looks good enough for most circumstances. If your speakers can deliver really deep bass (20hz?) then 10uF metal film appears to do the job.

However, on further investigation, 10uF can be a source of noise and distortion (as comparred to DC coupling) well up t in to the spectrum - nearly to 1kHz - even though the frequency response is unaffected. To get noise and distortion down to the level of DC-coupled, it looked like you need a metal film cap of about 20uF. These are really big! For audio purposes the application is usually in speaker crossovers where the large size is less of a problem. On a PCB with limited space you will sometimes find a pair of oppossed (neg-to-neg) 47uF/25V tantalum caps, but these are expensive. I had very limited PCB space to wook with and found an even more atypical (and heretical) solution - a pair of opossed 100uF/25V aluminum electrolytics. They may not be as aptimal as a very large metal film cap, but they are highly effective and very inexpensive. If this bothers one's "audiophile concience", one could use Black Gates which will dent your wallet enough to make you notice.

The above is based only on my recent experience and should not be taken as authoritative - just an option to consider.


PS- there are also bi-polar electrolytics, both ordinary and BGs.
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Old 16th June 2004, 05:20 PM   #5
ABO is offline ABO  Netherlands
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P-A,

Of course my opinions are influenced, just like everybody else's. Just like you didn't hear a difference because you didn't expected it to exist.

OTOH, I realise this. I try my best to be as objective as possible. The differences between the caps is VERY clear. You'd have to be deaf not to hear the difference between DC coupling and a standard Audyn cap (or perhaps your system is not up to the job). With the other caps the difference is far more subtle and I wouldn't want to do a blind test. But A/B-ing a few times makes the difference clear: voices sound less palpable with KPsN and cymbals are less sharp with MKP+. The differences are there.

What I was really looking for are opinions about output relays. I have established sound degradation from input caps (to the best of ability- that's good enough for me). But I don't know about relays.

Ekaering is right. That's why relays would be the best solution: I would not need any coupling capacitor at all and not be afraid that any failure may wipe out my beloved woofer.

So let's have some opinions on relays!

.. and of course some concrete recommendations for kitsets or other easy to build solutions.
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Old 16th June 2004, 06:09 PM   #6
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Quote:
Originally posted by ABO
P-A, Of course my opinions are influenced, just like everybody else's. Just like you didn't hear a difference because you didn't expected it to exist.
You are wrong here. I did actually _want_ to hear a difference.

If your problem is input caps, I think you will get even more problems with the relay contacts. Since I sell relays at work in know quite a deal about those but the knowledge is more "industrial" if you know what I mean?

The main problem is low voltage over the contacts and small currents...and powerful relays.

One contact material you should avoid is SilverCadmiumOxide AgCdO.

Avoid gold flashed contacts. They are only for protection and will wear out very fast.

Hardsilver is OK.

Goldplated (with thick gold) contacts are OK but _very_ rare.

A good relay, one of the best in it's category, is Elesta SGR282 but there is one problem. The delivery times

We sell Elesta at work but we are changing brand because of too high prices (not the biggest problem) and rediculoius long delivery times. That's too sad because those Swiss made relays are really good. They even sound good, the click, like a Mercedes door!

But the state of the art comes from NAIS. They make really, really god relays.
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Old 16th June 2004, 06:34 PM   #7
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There are relays with double contacts, one contact of some
heavy duty material that can withstand quite strong arcs and
one with some good contact material, gold, beryllium or
whatever it is. It is designed such that the heavy duty
contact always makes before and breaks after the other
contact and thus protects it. Just don't ask
me where to get these relays, but I remember it has been
discussed before on the forum.
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Old 16th June 2004, 06:53 PM   #8
ABO is offline ABO  Netherlands
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I remember some sort of DC protection where a triac is used to shorten the powerlines. This would be a good solution. Nothing in the signal path and only a blown fuse in case of a mishap.

That's what I need!


BTW,

P-A,

Surely you know that such a test (opa 627/buf) doesn't mean anything. Everything is tied to something else and everything influences everything else.

For instance: what load did the BUF drive? Was it a 47 kohm amp input? Or did you use PMA's termination resistor? The latter would mean load resistance of 100 ohm, quite a difference with the former case. And such a difference may very well be audible.

I hate it when people draw generalized conclusions (nothing personal here). Usually this means misinforming other members.

ABo
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Old 16th June 2004, 07:06 PM   #9
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Quote:
Originally posted by ABO
I remember some sort of DC protection where a triac is used to shorten the powerlines. This would be a good solution. Nothing in the signal path and only a blown fuse in case of a mishap.

That's what I need!

Yes, it is called crowbar circuit, in case you want to search
for it. Slone has a schematic of one such circuit in his book,
for instance.
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Old 16th June 2004, 07:06 PM   #10
sam9 is offline sam9  United States
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"between DC coupling and a standard Audyn "

What was the value of the cap?
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