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Old 17th August 2004, 01:56 AM   #11
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I like both Caddock and Vishay S102. It seems like Vishay is a bit more refined, and Caddock a bit more bold, but both will sound good. Rikens are slightly more bright. You preference will depend on the rest of a system as well.
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Old 17th August 2004, 10:50 AM   #12
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Hi,

Quote:
The reason I brought up the biasing is a normal CD player will put out 2 volts.
2 Volts RMS means also 6 Volts P2P.

Since you're using a shunt with the shunt element variable, you could make the series resistor larger.
Or you could install an attenuator network straight at the CD input of the preamp.

Either way the series resistor(s) could/should be Caddock or your prefered type for both if you'd like to be consistent.

Cheers,
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Old 17th August 2004, 03:52 PM   #13
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Default Vishay source

Hi G,

If you're interested pursuing the Vishay option, I'd give Texas Components a try. They make an unencapsulated version of the Vishay S102K which, because having shed the usual rubber and plastic shell, sounds a touch more transparent than its encapsulated brother. Pricing is not out of the ballpark either .... I think their resistors sell for about US$6 each, and they fill orders of any quantity (they like to say you must order at least one resistor). Their website is here and a link to their unencapsulated "audio" resistor can be found here. Arbie is your contact person. Just note that one must be careful handling these resistors as the leads can easily break away from the ceramic substrate, which is the first step in the lead actually breaking off the bulk metal foil resistive unit. Cheers.

Tom
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Old 19th August 2004, 10:20 PM   #14
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Hi,

Gavin,

While taking a second look at the attenuator diagram I noticed you sacrificed pos. #1 for the muting of the signal input.

In practice I don't think this is very useful and by doing so you lose a useful position for attenuating, you only have 12 to begin with.

Which in turn made me notice that all other positions of the atten. are occupied by dividers.
This implies that there's no position foreseen with no attenuation at all.
You may want to change that as it may be useful with sources having a rather weak output and for the occasional fault finding.

The muting can easily be taken care of by a simple DPDT switch if you take the hot signal wire at the output and run it to the switch, shorting the output to ground when the mute is activated.

In this way the signal doesn't even see an extra switch contact in normal operation and you gain a position on the attenuator.
The output of the preamp is muted iso of the input and so will be the input of the amp.

Also keep in mind that not all tube circuits appreciate this kind of variable g' and may well show an altering frequency response depending on atten. position.

Cheers,
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Old 21st August 2004, 02:51 AM   #15
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Hi Frank,

I realize it's not perfect but with my disposable income situation being bad right now it will have to do. It will attenuate either from -1dB to -40dB or from -21dB to -60dB by adding a SPDT switch. VoltSecond says he has gotten good feedback on the design so I'm going to give it a shot. I have attached a accurate diagram of the circuit below. I took your advice about the mute. I will place it at the output of the preamp. Do you think the 75 ohm resistor looks right in that position? I have a nice scientific calculator but I have not tried to calculate decibels (on my calculator) in so long that I am just going to wing it. Thanks for the reply.
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File Type: gif series shunt attenuator (small).gif (17.0 KB, 314 views)
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Old 21st August 2004, 11:54 PM   #16
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Hi,

Quote:
Do you think the 75 ohm resistor looks right in that position?
For the mute switch you mean?
If so, you don't want any resistor in that position at all, you just short the signal straight to ground at the output of the linestage, not the output of the attenuator.

As there is no crosstalk to speak of in a DPDT switch you can simply fit it to the center of the frontpanel of the linestage and take the signal for L and R channels to the top solder lugs of the switch, the bottom lugs to the star groundpoint.
Closing the switch will effectively short the output to ground.

Cheers,
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Old 22nd August 2004, 12:15 AM   #17
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Quote:
Originally posted by fdegrove
Hi,



For the mute switch you mean?
If so, you don't want any resistor in that position at all, you just short the signal straight to ground at the output of the linestage, not the output of the attenuator.

As there is no crosstalk to speak of in a DPDT switch you can simply fit it to the center of the frontpanel of the linestage and take the signal for L and R channels to the top solder lugs of the switch, the bottom lugs to the star groundpoint.
Closing the switch will effectively short the output to ground.

Cheers,
What I did was add the 75 ohm resistor in place of the mute. I don't know what kind of step that is in dB but it does give me one more step in the attenuator. I will add the DPDT switch in the output stage. Thanks.
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Old 22nd August 2004, 12:22 AM   #18
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Hi,

Quote:
I don't know what kind of step that is in dB but it does give me one more step in the attenuator.
I see...You had me confused as it said MUTE a bit higher on the drawing.

Anyway, I'm sure I have the logarathmic conversion tables and formulas here somewhere in project a folder.
I'll dig them up and scan them for you later on next week.

Cheers,
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