A twisted tale about a logarithmic relay attenuator

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#281
Stevox said:
Hi,

I was going out for a series resistor, so okay that was wrong.
I measure before resistance at the output and yes it was constant ~750R.
Yet I connected one channel with mine cd player and on the other end mine amp.
And yes its works only there is one step when I degrease the volume its goes harder sometimes a plop and then it goes lower.
The other thing is that it doesn’t start for from zero.
And yes there was a little hum. But okay it was only for test.
So now I want it to build behind the Bozos preamp from kari so I can play balanced with it.
Thanks Guys I let you know how it goes.

THX
Peter.
Click to expand...

It sounds like you may have the JT 1.1 firmware which has a "roll over" feature toward the end of the pot travel. 🙂

Talk to Brian about getting the latest firmware.
 
#286
Yes, I am afraid so, but your attenuator should still work fine for now, it will just simply roll back over to -63.5 db at about 75-80% pot travel. I am sorry if there was some problem with the chip. I did not burn it myself, so I am not actually sure there is any problem. I am simply giving you my best guess. I have no "1.2" chip from Brian to test. I am only saying this is a possibility because Marc happened to have a chip marked "1.2" which had 1.1 code on it.
 
#287
Hi Russ,

Very thanks, for support.

I have yet Build it in, so that I can control the volume balanced on the output of the Bozos-pre (Kari).
I must say it sound very very good , no hum
Only hear sometimes a plop
One question what is the value of attenuator 10K or 5K ???

Thanks
Peter
 
#288
Stevox said:
Hi Russ,

One question what is the value of attenuator 10K or 5K ???

Thanks
Peter
Click to expand...

Neither. 🙂

It does not work like a pot or a serial attenuator. The input impedance ranges from 2K or so to around 10k depending on the volume. So you should size your output cap from source (if you have them) for the lowest input impedance which is about 2K.

The output impedance is constant (at 750R in the stock version) which is one goal of the JT design. The other primary goal being a true logarithmic curve, which it has.

You should be able to place the JT in an most applications where you would use a 5K pot.

Cheers!
Russ
 
#289
joshua tree 1.1 software looping relays at some settings ?

Hi Russ

Finally got JT in balansed configuration to the point of testing it a bit, there is still a lot to do for whole Twisted Bosoz and Darwin.

There seem to be two positions of volume pot, where JT 1.1 software will change the state of relays all the time (at least many many times, for tens of seconds at least), one fast loop somewhere at two thirds of maximum setting and a longer change loop at about 90 % of maximum setting.

This effect does not happen with JT 1.0 software , so I will use it for now. I know that JT 1.0 also has some problems, but my PS voltage should be at the safe side (about 10 volts).

Should I do more debugging of the system (quickly measured input impedances with JT 1.0, seem to change quite OK), or can that loop problem be caused by the JT1.1 code overflow problem ? I dont have a PIC programming tool at least for now, so I cannot check JT 1.2 software.
 
#296
Hi Russ and Brian,

I just installed the JT in my BOSOZ. I'm happy with the sound and how easily it went together.

At the mid-point of the volume I'm experiencing a POP during the relay transition. Also when I urn the volume pot slowly past the mid range, the relays are all toggling on and off in an unsteady state until I move the volume pot again. This is repeatable almost everytime. The PIC is labeled 1.2.

The shaft for the volume pot is too short for my chassis. I'm plannig on buying a POT with a longer shaft. Is the supplied POT 5K linear or audio taper? I seem to remember it was liner.

Thank you for your help.

-David
 
#297
The pot is linear.

I wonder if your JT relay board is getting enough power. My JT experienced similar "motor boat" behavior with a 6V transformer. Remember the 78xx regulator really needs an input voltage 2 volts above the regulated output voltage.
 
#298
Hi, you need 2V margin for regulator drop when the voltage input is at it's worst. In addition the low input voltage is measured at the bottom of the trough caused by the PSU ripple. The actual DMM measured voltage exceeds the minimum trough voltage by half the ripple peak to peak voltage.

Adding these two effects together demands that the pre reg voltage needs to be at least 4V and preferably more above reg out voltage when mains incoming is at nominal supply voltage.

At maximum mains supply voltage the regulator could be dissipating ten times the power it has to cope with at mains minimum. Check temperatures at the extremes and use an appropriate heatsink for worst case.
 
#299
orthoefer said:
The pot is linear.

I wonder if your JT relay board is getting enough power. My JT experienced similar "motor boat" behavior with a 6V transformer. Remember the 78xx regulator really needs an input voltage 2 volts above the regulated output voltage.
Click to expand...

Yep,
6v didn't do it for me either. I had the same 'motorboating'. Go higher and I bet it clears up.
 
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