i luvmusic 2
New member
Thanks for the info,Another question do i need to ground the Choke chassis?Thanks!
Choke specs for the Crack
- Thread starter JamieMcC
- Start date
Paul Joppa
Moderator
Yes, all the iron should be grounded to the chassis plate, for safety reasons. It's good practice to insulate the iron from the plate so it can be grounded to a single point, preferably near to where the power is grounded.
The reason is to minimize the leakage currents in the chassis plate, and keep them well away from the audio circuitry.This reduces a potential source of hum.
The reason is to minimize the leakage currents in the chassis plate, and keep them well away from the audio circuitry.This reduces a potential source of hum.
i luvmusic 2
New member
Paul Joppa said:
THANK YOU!
Wow, this thread is loaded with great information! Thank you Paul J. and Paul B. for being so open in the forums, providing to-the-point, no-nonsense, clear information and advice to all of us. I've become a fan of both of your scientific and kind responses, and this makes me even more proud to be an owner of the Crack. 
Paul J., I posted a question to you in the Crack thread at Head-Fi, but it's more sensible to reach out to you here. You've said, "I would never use less power supply capacitance (last cap) than in the output cap, and I'd prefer at least twice that - if it were my amp." Currently, I'm using three 68uF Audyn Q4 caps (2 outputs, 1 last PS), could you please explain what the difference instead would be if I used two 47uF outputs and a 100uf PS cap? Which would fare better in terms of bass roll off, the three higher capacitance 68uF caps, or two 47uF outputs and the doubled 100uF PS cap? My cans are 600ohm AKG K240 Sextetts, and I do have the C-7X choke installed. Thanks so much and take care!
Paul J., I posted a question to you in the Crack thread at Head-Fi, but it's more sensible to reach out to you here. You've said, "I would never use less power supply capacitance (last cap) than in the output cap, and I'd prefer at least twice that - if it were my amp." Currently, I'm using three 68uF Audyn Q4 caps (2 outputs, 1 last PS), could you please explain what the difference instead would be if I used two 47uF outputs and a 100uf PS cap? Which would fare better in terms of bass roll off, the three higher capacitance 68uF caps, or two 47uF outputs and the doubled 100uF PS cap? My cans are 600ohm AKG K240 Sextetts, and I do have the C-7X choke installed. Thanks so much and take care!
Paul Joppa
Moderator
I don't follow the HeadFi forum - PB is the headphone guy. Hopefully he will post based on his much greater experience. I'll just give the theoretical argument:
The issue is that the PSU cap and output cap are effectively in series in the current loop; if they are both 100uF then the effective output capacitance is 50uF. Not an issue with 600-ohm phones. At 160 ohms, there would be a bass rolloff at 20Hz - lower than that and you would likely hear a loss in the deep bass.
The issue is that the PSU cap and output cap are effectively in series in the current loop; if they are both 100uF then the effective output capacitance is 50uF. Not an issue with 600-ohm phones. At 160 ohms, there would be a bass rolloff at 20Hz - lower than that and you would likely hear a loss in the deep bass.
Is there an equation to calculate this? So, right now my effective output capacitance is 34uF from my 68uF caps, with bass roll off at 8hz using my K240. What if I increased the PS cap to 100uF but use 47uF caps for outputs? How much will the effective capacitance decrease in this case? Or does it stay the same as the output when the PS cap doubles the output?
And in that case, if using a PS cap that triples the outputs' capacitance, does the effective capacitance double the outputs (220uF last PS cap + 47uF outputs = around 100uF effective capacitance)?
Thanks so much for your response and your time. Take care!
And in that case, if using a PS cap that triples the outputs' capacitance, does the effective capacitance double the outputs (220uF last PS cap + 47uF outputs = around 100uF effective capacitance)?
Thanks so much for your response and your time. Take care!
Please correct if I have this wrong. Still trying to figure this stuff out.
The -3dB roll off frequency is calculated by 1/(2*pi*R*C).
In this case, R is the impedance of the headphones in ohms and C is the effective capacitance of the last PS cap and the output cap in farads (not micro farads).
The effective capacitance of capacitors in series is calculated by 1/(1/C1 + 1/C2 +…).
The -3dB roll off frequency is calculated by 1/(2*pi*R*C).
In this case, R is the impedance of the headphones in ohms and C is the effective capacitance of the last PS cap and the output cap in farads (not micro farads).
The effective capacitance of capacitors in series is calculated by 1/(1/C1 + 1/C2 +…).
Which K240 headphones do you have? They have come in several flavors with different impedances.
Another issue that's worth considering is that when you start shrinking the last PSU cap in the Crack, you will let more power supply noise through,which is very undesirable.
Another issue that's worth considering is that when you start shrinking the last PSU cap in the Crack, you will let more power supply noise through,which is very undesirable.
Hi Paul B.
I have the K240 Sextetts, 600 ohms.
Also, in the PSU, along with the last 68uF film cap and C-7X choke, I'm using 470uF electrolytics for PS caps 1 and 2. I'm happy to say, the background is dead silent even with the volume maxed out.
I'm just trying to decide if I should raise the last PS cap to 100uF and use two 47uF output caps. Considering my choke, 470uF PS caps, 68uF last PS cap, 68uF output caps, and 600-ohm headphones -- bass doesn't roll off till 8hz -- is there any other reason to have a last PS cap that doubles the outputs besides the bass roll off? What would you do if it were your amp and using 600-ohm cans?
Thanks so much!
I have the K240 Sextetts, 600 ohms.
Also, in the PSU, along with the last 68uF film cap and C-7X choke, I'm using 470uF electrolytics for PS caps 1 and 2. I'm happy to say, the background is dead silent even with the volume maxed out.
I'm just trying to decide if I should raise the last PS cap to 100uF and use two 47uF output caps. Considering my choke, 470uF PS caps, 68uF last PS cap, 68uF output caps, and 600-ohm headphones -- bass doesn't roll off till 8hz -- is there any other reason to have a last PS cap that doubles the outputs besides the bass roll off? What would you do if it were your amp and using 600-ohm cans?
Thanks so much!
I'd use the 470uF->choke->470uF in parallel with whatever film caps you have, then use 47uF or 68uF caps on the outputs. The extra R-C power supply node shouldn't be necessary with the rest of the changes.
The extra RC power supply node is the 270 Ohm resistor and whatever cap follows it. With the choke and the increased capacitance, the extra filtering shouldn't be necessary.larcenasb said:
Thank you, Paul for helping me understand.Caucasian Blackplate said:
But what about the relationship between the last PS cap and the outputs for the "effective capacitance"? If I don't use a last PS cap, just the two 470uF caps and the choke, how will that affect my effective capacitance? Will the 68uF outputs indeed effectively be 68uF or much less than that? Thanks!
As PJ mentioned, they are effectively in series. The larger the last PS cap is, the closer your coupling cap will be to giving its corner frequency as though it was the only cap that mattered. With 470uF as the last cap in your power supply, your 68uF cap will perform better in this regard than if the last cap in the power supply was 220uF.
More chokes will further reduce the presence of noise from the power supply.
You would need to redesign the power supply and use three chokes that are different than the two you already have.
It would be tough to do choke loading of the 12AU7 in the Crack without abandoning the direct coupling or having a separate power supply for the driver stage with much lower voltage.
