A twisted tale about a logarithmic relay attenuator

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#343
joshua tree attenuator "pop" at step 64

my recently assembled JT attenuator is making a "pop" at step 64.

as has been suggested on this thread, i checked for DC offset at the input and output. i found that i have a 2-3 mV dc offset coming from my marantz processor to the output of the JT.

i have two questions:

1. why is the "pop" always at step 64?

2. given that i cannot modify the marantz to remove the DC offset, is there anything further i can do to eliminate the pop?

i would like to avoid using a capacitor to remove the DC component of the signal as i was told that this can negatively impact the sonics (is this true?). however, if a capacitor is my only option, what is a appropriate capacitor choice and where would be the best location in the circuit for it (across the poles of the output rca connector ?)?

thanks.

fyi the pic is programmed with V1.2
 
#344
DC blocker

Hi,
if you have to resort to a capacitor block it has to go in line with the live input lead (i.e. not the ground return).

You should not cancel the DC offset at the output of the next amplifying stage. That would upset the bias conditions set up in the amplifier.

The other option is to apply DC correction (servo) to the source component, but that will require a design level understanding of the output circuits and the knowledge of how to integrate it into the circuits.

The only sensible solutions are to put up with it or add a DC blocker.
 
#345
thanks andrew,

if you have to resort to a capacitor block it has to go in line with the live input lead (i.e. not the ground return)
Click to expand...

gotcha.

1. do you have any capacitor recommendations (values and type)?

if you have to resort
Click to expand...

2. does this mean that you don't really like this solution?


You should not cancel the DC offset at the output of the next amplifying stage. That would upset the bias conditions set up in the amplifier.
Click to expand...

3. in other words, i should not place the capacitor at the output of my preamp (which will have a buffer at its output)?

that will require a design level understanding of the output circuits and the knowledge of how to integrate it into the circuits
Click to expand...

since i just learned what bias was 🙂 that might be a little beyond my abilities at this time.

The only sensible solutions are to put up with it or add a DC blocker.
Click to expand...

putting up with it, is something i have a hard time doing after investing so much work.

4. Is the DC blocker really the only solution? it seems strange to me that the pop always occurs at the same step. Is this a conincidance or is there a reason? since the volume steps are so fine, i don't think i would miss step 64 if there was a way to skip it.
 
#346
okapi said:
4. Is the DC blocker really the only solution? it seems strange to me that the pop always occurs at the same step. Is this a conincidance or is there a reason? since the volume steps are so fine, i don't think i would miss step 64 if there was a way to skip it. [/B]
Click to expand...

Yes, there is a reason. At that step the most number of relays is switching, and the most significant change in current flow occurs. 🙂

I would try a 10-22uf Solen or similar film cap at the input of the JT if the pop bugs you. You probably will not notice any change at all, but the pop will be gone.


Cheers!
Russ
 
#347
thanks so much,

having an understanding of the reason for the problem, even if i fail to solve it, helps a lot.

i will definitely try using a capacitor.

theoretically, if i had the ability and equipment to reprogram the pic, and could make it skip step 64, would that make the pop go away or would it simply move to another position?

last but not least, the fine volume control that 128 steps provides is fantastic . you can always find an appropriate listening level without having to "bracket" your way to the position. THANKS.
 
#349
Typo, MUTE Relay and visual feedback LED?

Hi Russ and Brian,
On your website you have written:

"Read more about the "Twisted X" lineup herehere."

Typo shows up in several places.

I was wondering what software you use to create those great 3D images of the pc board and components in your http://www.twistedpearaudio.com/docs/TX/TX.pdf

Also, what would it take to get a HEX code for the Joshua Tree that has one of the un-used PIC output lines go HIGH each time the volume position is changed, then output a LO when the change has completed.

In otherwords, I would like to operate a MUTE relay during changes in the volume position as I mentioned in an earlier post on the forum.

Also, it would be desirable to have another unused PIC output pin blink an LED whenever the volume position is changed. I am thinking of using a motorized potentiometer for the volume control so I can implement IR remote control easily. Having an LED blink on the front panel during changes in volume gives a great indication that the IR signal is being received by the JT. See what I am after?

Your thoughts?

Thanks!
Russ :>)
 
#350
"Read more about the "Twisted X" lineup herehere."
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Fixed. Thanks.

As for the mute relay and LED, you could easily do that from the remote control circuit.

Alternatively, you could mod the JT code to actually handlt the remote control functionality using the space pin outs on the controller board.

I did make a seperate remote control volume using a motorized pot a while back, I think it's in the thread somewhere. It was cheap, and powering a relay and LED from that would be simple.

We also have a master controller in the works that will control the JT and Darwin relay boards with remote and optional LCD, but still a little ways off.
 
#351
lin attenuator, log pot

Hi, I have not read through the whole thread so sorry if this has been suggested already.

How about if the relay attenuator board is made with linear steps, and the control pot is logarithmic? Would that work ok (would it provide log attenuation for the audio signal)?

The point of doing this is so a motorized pot can be used. Linear motorized pots are very difficult to get hold of compared to log. Alps RK16 motorized looks good with a reasonable price.
 
#352
Re: lin attenuator, log pot

mr.duck said:
Hi, I have not read through the whole thread so sorry if this has been suggested already.

How about if the relay attenuator board is made with linear steps, and the control pot is logarithmic? Would that work ok (would it provide log attenuation for the audio signal)?

The point of doing this is so a motorized pot can be used. Linear motorized pots are very difficult to get hold of compared to log. Alps RK16 motorized looks good with a reasonable price.
Click to expand...


Yes this is indeed possible, but it would not hurt just to use the log pot with the curve as it is. It would simply be a sharper log curve.
 
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