Would there be any adjustment to resistor values ...

[Laudanum]

Obviously the value of the bleeder resistors at the headphone jack were determined based on the design and components of the stock Crack.   I have mentioned before that I and a couple others have measured a higher (than 9 vdc mentioned in the manual) DC voltage spike at start-up AFTER upgrades with Speedball and film caps.  I know that I and another member measured about 20 volts and a third measured around 14 volts after Speedball.  Im not sure if we all had replaced the stock 100uf electrolytics with film caps.  I had.  

I want to make clear that I havent had any problems at all and obviously neither have the vast majority.  And just for extra precaution, I also dont have my phones plugged in when I turn the amp on.  So, Im asking just out of curiosity here ...  would the value for the bleeder resistors at the headphone jack have be calculated differently if Speedball and 100uf Film output caps were part of the original design?  Or do the resistor values have nothing to do with voltage of over 9 vdc appearing at the jack AFTER Speedball and Film cap upgrades?   Just trying to get a better understanding of how this works, I dont know how the resistor values are calculated.  I think that I read that they determine the rate at which the voltage is bled off, but beyond that, I know jack

Thanks

 

[Grainger49]

The bleeder is in parallel with the headphones.  As long as the value of that resistor is high it doesn't hog the current or change the impedance "seen" by the amplifier.  If you lower it too much it will affect the response and power delivered to the headsets.  

Changing the resistor to a 1k would bleed the DC off faster, it would not limit the leakage through the output capacitors.  It is just a guess but the film cap might be what allows more voltage through.  And I might be wrong.

 

[jrebman]

My understanding is that the resistors are there to charge the cap, not discharge it, and the calculation is based on the size of the cap and inrush spike wanted more than anything else -- again, as best I can remember and without having the circuit description in front of me.  Basically I believe that if you are going to a larger value film cap to help with lower end response on lower impedance cans, you want to adjust the resistor to keep roughly the same time constant for the circuit.

Hopefully one of the Pauls will chime in and educate us further on all this.

-- Jim

 

[Grainger49]

Jim,

The 2.49k resistor does create the RC network and time constant to charge the output capacitor.  (And I see now that I misread the value on the schematic in my above post.  I had thought it as 249k.  I will change the value in my above post.)  

But doesn't it also give the leakage DC a path to ground.  Otherwise it would stay there till the headsets were plugged in and put the captive DC on the headset.   I had an amp once that could amplify DC.  A quick power outage ruined a pair of Dynaudio woofers when my tube preamp started up leaking DC.

 

[Laudanum]

Well, what Im gathering here is that the resistor value, atleast in part, is chosen based on the 100uf value of the cap.  So Im assuming that whatever calculations are made would be the same with 100uf film caps or any type of 100uF cap.  If thats the case, then it's probably the property of the film caps versus that of the electrolytic that causes the higher measured DC voltage peak after turn on.  So would it be safe to say that different brands of 100uF film caps (or different electrolytics for that matter) could/would result in different peak DC voltage at turn on based on the internal characteristics of the partiuclar capacitors?

 

[jrebman]

Desmond,

Yes, I'm sure that plays a role here -- the equivalent series resistance and inductance of film caps is much lower than electrolytics, and among film caps different films also have different typical ESR/L figures.  Also different types and quality electrolytics will also vary too, but the best of them will still be some orders of magnitude higher than film caps.

HTH,

Jim

 

[Laudanum]

Thanks gents.

 

[Paul Birkeland]

Hello Desmund,

Yes, different values and types of capacitors at the output will change the charging characteristics of the amplifier.

If you find that your modifications cause that charging voltage to be a little too high, you can decrease the value of that resistor.  Decreasing this resistance will also decrease the overall load presented to the cathode follower, and this can eventually impact the sonic qualities of the amp.  All of this, of course, entirely depends on the impedance of the headphones you are using. 

-PB

 

[Laudanum]

Thanks Paul.