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attenuator hook up?

mr mojo

Member
2005-06-06 3:58 pm
Iowa
Howdy folks,

My DIY is almost done-but I've got a question about installing my attenuator. I'm using a ganged 500k, make before break, 30 step with detent stops audio log attenuator from Shalco.

There are two rows of three soldering tabs, each set has "In, C and out." I've no problem showing my ignorance, so is this "In" for audio in, "C" for common or ground and "Out" for audio going to the grid of my first gain stage? It's my understanding that at each step of the attenuator the resistance decreases, shunting less and less of the audio signal to ground-am I right on this?

I've also got a question about gain. The amp needs .475v to drive it to a full 40 watts. I'll be using this with a CD player, which is anywhere between 1 and 4v output.

How can I measure the output of my CD player since I can't find any specs? I'm guessing it's anywhere between 1 and 2 volts which will overdrive the amp from the get-go.

Any and all help would be greatly appreciated.

Best,
mr. mojo
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> is this "In" for audio in, "C" for common or ground and "Out" for audio going to the grid

Yes.

> my understanding that at each step of the attenuator the resistance decreases, shunting less and less of the audio signal to ground-am I right on this?

Usually a voltage-divider, not a simple suck-down. Shalco? Then it could be constant-impedance T-pad, which is pointless for going into a grid. Why did you get 500K? What is the real output impedance?

> How can I measure the output of my CD player

"All" home hi-fi CD decks are 2V RMS output. 2.8V peak is a nice fit for the +/-5V that they run the DAC on. Oh, with low-pass filter it may be 1.8V or 2.2V, but who cares?

> will overdrive the amp from the get-go.

That's why you have the attenuator in front. You won't turn it up any higher than you need for the volume you want.

> The amp needs .475v ... output of my CD player.... 2 volts

It is "2V" only if the mastering engineer cranked the level to digital maximum. That's bad form. True, most pop and quite a lot of classics ARE recorded up to the last bit. But many are not. I've seen tracks, even whole works, that didn't reach -6dBfs, or 1V.

You are fine. I'm a little worried about frequency response with a "500K" pad feeding more than a few inches of wire. If it is truly a 500K or even 125K output impedance, a foot of wire can pull down your highs.
 

mr mojo

Member
2005-06-06 3:58 pm
Iowa
PRR,

Many thanks for the help, I surely appreciate it!

As far as the 500K attenuator, the circuit I'm building is from an old article in "Electronics World" from @ 1961. Stereo, single chassis, dual 5AR4s, 7591s in PP AB1, 0A3s for screen regulation and 6AN8s for gain/phase splitting. The article states since the amp only needs .475 volts for full output it may be more convenient to use a ganged 500K pot rather than the two 480K dropping resistors listed.

The 6AN8s are *similar* in terms of gain to a 6GH8, which is used in my Scott 299C. 6AN8 pentode=7800 micromho transconductance, 6GH8 pentode=7500 micromho transconductance. However 6AN8 triode mu=21 and 6GH8 triode mu=46, so the triode portion of the 6GH8 will have twice the gain.

The Scott schematic shows a ganged 500K pot with a 150k isolating resistor going to the grid of the 6GH8 pentode. The circuit I'm building shows a 15k isolating resistor to the grid of the 6AN8 pentode. I figured the 500k attenuator *should* work since the 299C uses it and the article suggests it-but like I said, I've no illusions about my ignorance.

Your comment on the attenuator feeding a few inches of wire brings up a question-the attenuator is located on the amp between the RCA inputs and the 6AN8s so I have about 2.5" of wire going from it to the grids-does this negate the issue you mentioned?

Lastly, a question about the attenuator itself. How do I know if it's a constant-impedance T-pad? Maybe more importantly, what IS a constant-impedance T-pad :D and why would that present a problem?

When I ordered the attenuator I asked for a ganged 500K audio log taper-I was thinking I was getting basically a pot with 30 separate resistors on each half, shunting less and less of the signal to ground with each step. Is that what I have or do I have a constant-impedance T-pad? How can I tell the difference?

Hope you don't mind being bombarded with all these questions-I'm a graphic artist, not an engineer! I could paint you a beautiful picture of an amp, but actually BUILDING one represents a learning curve I'm still struggling to scale.

Thanks again,
mr. mojo
 

jlsem

Member
2003-06-17 1:12 am
Dallas,TX
When I ordered the attenuator I asked for a ganged 500K audio log taper-I was thinking I was getting basically a pot with 30 separate resistors on each half, shunting less and less of the signal to ground with each step. Is that what I have or do I have a constant-impedance T-pad? How can I tell the difference?

There is no reason to think that the Shallco is a T-pad. Most Shallco audio attenuators are ladder-type pots and should work great in your application.

Shalco? Then it could be constant-impedance T-pad, which is pointless for going into a grid.

I don't know where this comes from. I've never seen a T-pad with 30 steps.

John
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> I don't know where this comes from.

Daven, Shallcross, Shalco; the trademark and ownership changes but the parts stay the same.

http://www.shallco.com/frameset.html Audio Attenuators and Networks

[IMGDEAD]http://www.shallco.com/images/unmountd.jpg[/IMGDEAD]

"T" and Bridged-T Circuits
".... Increments from 0.1 dB to 2 dB are standard with ranges of 10, 20, 30 and 45 steps in most impedances. ... we can provide custom designs for special applications. ... Standard impedances are 600 ohms but others are readily available..."

> I have about 2.5" of wire going from it to the grids

Fine.