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Old 7th January 2008, 08:40 PM   #1
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Default why does a relay attenuator sounds much worst than an alps black ?

I have just tested a relay attenuator board, and I'm quite diasppointed at the moment. Something I do wrong probably, but I do not know what. If you have any experience with R-2R relay attenuator network, please let me know. If you could point anything wrong on my proto, please let me know.

This is the schematic :
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Old 7th January 2008, 08:43 PM   #2
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a picture of the top :
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Old 7th January 2008, 09:00 PM   #3
kevinkr is offline kevinkr  United States
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Contact quality at very low signal levels is extremely important.. You did use relays designed to reliably switch very low voltages, right?

Choosing a conventional power relay can result in very high distortion due to contact oxidization. The oxidation actually forms diodes. Some (many) relays will not maintain a reliable connection with less than a few mA flowing through the contacts.

Long ago I used some very nice looking relays in a pre-amplifier to select sources, invert phase and mute. None of them worked until I treated all of the contacts with a cleaner and then cramolin. They were brand new and passed dc at a few mA without issues - no high impedance low current audio got through until I treated the contacts. They were the wrong type of relay for the application, contacts being rated at a couple of amps at 24Vdc. Suitable relays will indicate that they are designed to handle very low signal levels and often have a relatively low maximum current and voltage rating due to the contact treatments used. Most reed relays are suitable if selected for the application, although form C types are less reliable - use a form A and form B to make spdt switches if you go this route.

The other potential suspect would be the resistors used. Avoid metal oxide, carbon film for linearity issues due to their construction. Carbon types in general may be too noisy for best performance. Really good resistors like Vishay and the older Holco H series metal films are warranted in this application.
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Old 7th January 2008, 09:02 PM   #4
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and this is about the bottom :

the resistors are proven 1W Beyschalgs, the relays are Takamisawa NA5W-K. The traces on the PCB are mostly the resistor legs, instead of copper trace.

The board is inserted before a line amplifier which is followed by a 2A3 PSE and AN speakers. The line amp starts with a 100k alps black, turned to full volume. The wires to and from the board are very short, 20-30mm only. The relays are run from 3 alkaline battery BCD encoders. 4.6V measured. It works perfectly on the bench, gives the logaritmic curve very accurately.

The sonic difference is interesting. It looks like to higher the high frequencies a bit. The LP noise is more significant also. Several different disk is tested and on some tracks I did not know which one is better. For sure it was different, much different. Sometime it had a bit more trasnsparent space, but after listeinig it longer I did found it to be true, rather it had too much high frequencies. Brasses decided it, very significantly. The network sounds good, but the alps has much more life, the instrument has more weight.

I was thinking about, maybe I have used wrong relays. I would be very very interested in what relay should I try.

Thanks a lot,

JG
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Old 7th January 2008, 09:10 PM   #5
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Hi kevinkr, I have very good experience with these Beyschlag resistors, this should not be a problem.
The relays are miniature signal relays, max. 50mOhm is specified contact resistance. It is sealed.

I'm now thinking about to make a "burn in" circuit. The relays are brand new, during the measurements I switched them no more than 30 time each.

Thanks,

JG
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Old 7th January 2008, 09:16 PM   #6
kevinkr is offline kevinkr  United States
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I looked up the NA5W-K and despite the bifurcated gold over silver contacts I see that the relay is designed to switch higher currents its high reliability rating not withstanding. I'd change that to a relay designed expressly to switch low voltages at very low currents. Usually the maximum contact voltage rating will be <30V and the current 50mA or less. Alternately you might find reeds better. There are relays in your existing form factor that will pop right in and have the required specs.

I'm no fan of beyschlag resistors, nothing special imo, and not what I would use in this application.

It may well also be that the attenuator is telling you an "inconvenient truth" (love that line ) about something else in your system.

Another thought is that there are a lot of contacts in series with the signal in that binary attenuator.
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Old 7th January 2008, 09:44 PM   #7
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it is interesting. I have choosen this against Omron G6K ( http://www.omroncomponents.eu/home/p...ys/upto1A/G6K/ ) because of the low contact resistance, 50mOhm against 100mOhm (and because I can't source through hole G6K around, RS, Farnell, ELFA no one had on stock).

Against the contact resistance, what you say is also logical for me. Which relay you know as the best for audio signal ?

Thanks,

JG
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Old 7th January 2008, 10:28 PM   #8
poobah is offline poobah  United States
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Look for relays that say "telecom" or "dry circuit" in the description. Then look at the specs carefully... when you are looking at the right style of relay... the data sheet should list the MINIMUM current the relay is capable of switching.

The best relay would probably be a mercury wetted reed. I don't know if they are still available though... with mercury and the envoronment issues to consider.

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Old 8th January 2008, 04:53 AM   #9
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Hi Giordano,

The relays look good to me. I would be happy to recommend them for general audio use. There are auguably better relays but I believe your problem lies elsewhere...

I think the problem may be due to the higher resistance of the relay attenuator interacting with the input capacitance of your amplifier forming a low pass filter that is audible.

You mention the connection to your amp is very short, but coax cable has considerable capacitance even for short lengths.

You could try measuring the input capacitance and lowering the resistor values. As little as 50pF of capacitance could be an audible problem with your high resistance values.

Cheers, Ralph
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Old 8th January 2008, 07:41 AM   #10
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Thanks for the replies, I have an idea, at the setting which is confortable I will solder the closed legs together on the PCB. I think I can do it today. I will let you know the results.

About reed relays, I do not know the effect of the coil around the reed tube.

Thanks,

JG
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