Crackatwoa Upgrades

[hpjun]

Where should I start upgrading the Crackatwoa?

So far mines is stock without the step attuenator.
I was thinking of replacing the output capacitor first, with a 100uf 350v mundorf evo al-oil or maybe 47uf 600v mundorf supreme evo oil (I think the -3db point is at around 10hz might be a problem with phase below 100hz.)

5998 or 7802 output tubes to lower the output impedance, not sure what to get here.

Also any other worthwhile upgrades on the crackatwoa?

 

[Rocketman248]

I put the Mundorf EVO Aluminum/Oil caps in mine, but I may end up replacing those at some point.  They're a little too warm for my taste.

I also have the TwoQuiet attenuator upgrade on order.  I'm looking forward to that one.

 

[fullheadofnothing]

I also have the TwoQuiet attenuator upgrade on order.  I'm looking forward to that one.

Packed today. Shipping tomorrow.

 

[Rocketman248]

Excellent!

 

[Paul Birkeland]

You can save a considerable amount of money by buying 100uF or 200uF film caps at a much lower voltage.  Parts Express has several choices in the 250V-400V range. 

I believe that if you install the attenuator first, listen for a while, then install the caps later, you will learn a lot about what each modification can bring to the table. 

Beyond that, this is an amp that comes very nicely outfitted right out of the gate.

 

[hpjun]

Was thinking about getting a 7802 tube for power output, is there any trade offs going from 6080 to 5998 or 7802? I wanted to decrease the impedance and increase sound quality.

7802 is so rare, I might end up with a 5998 or 7236.

Still need to know what the trade off of getting a 5998 is, I heard it's also louder than stock right?

 

[hpjun]

Question about the output coupling capacitor.

Using v-caps coupling capacitor calculator. For a 350 ohm load with a -3 db point at 2hz (linear phase at 20hz) it needs a capacitance of ~220uf.

Would a better phase response at 20hz be worth it to get a 220uf capacitor over 100uf capacitor? At stock 100uf the phase isn't linear bellow 45 hz.

There is also much better capacitor options when I go lower capacitance such as a the 47uf mundorf supreme oil, but that will make the phase distorted below 100hz basically all the lower bass.

For people who care about sub bass, I think a 220uf output capacitor is more optimal than stock 100uf value right?

Still deciding on capacitors.

 

[Paul Joppa]

-3dB at 2 Hz is IMHO an unrealistic requirement. Among other things, the last power supply capacitor is in the output current loop, so it should have a higher capacitance than the output cap - stock is 220uF power supply and 100uF output cap.

Also no speaker or headphone can come within an order of magnitude of such a specification. Long ago I read a paper seeking to determine human response to sonic booms by simulating them in a room. The only way to get sound at audible levels below 5Hz was to use an industrial shaker and a 2x4 beam between opposite walls, turning the room walls into loudspeakers.

Incidentally, ultra-low frequency sound has been investigated as a weapon. IIRC exciting the liver resonance at around 5Hz can produce uncontrollable bowel activity if the sound is loud enough to be audible.

 

[hpjun]

From what I understand I should be aiming for 2hz not because I want to hear it, but because the frequency at 20hz won't be affected by distortions in phase (10x the -3db point) so to have a linear phase to 20hz.

If phase distortions doesn't really matter then I could go with a high quality 47 uF cap and get -3db @ 9.68 hz into a 350 ohm load.

 

[Brent]

I was at Rocky Mountain Audiofest several years ago when Bruce Thigpen was demoing his rotary subwoofer.  It was supposedly flat to single digit Hz, IIRC.  I was evidently in the minority made a little nauseous by the demo. I learned I wasn't alone when I saw one of the guys from that room in the elevator, and mentioned that it made me feel queasy. He said a couple other people had told him something similar.

Sorry for the off topic, but Paul's comment brought it to mind and I felt compelled to share. 

 

[Paul Joppa]

From what I understand I should be aiming for 2hz not because I want to hear it, but because the frequency at 20hz won't be affected by distortions in phase (10x the -3db point) so to have a linear phase to 20hz.

That is the argument with which I disagree, on the grounds that there are many much more important and more audible issues, and that achieving such a response is remarkably difficult. Kind of like putting drag-racing slicks on a bicycle.

If phase distortions doesn't really matter then I could go with a high quality 47 uF cap and get -3db @ 9.68 hz into a 350 ohm load.

It's an easy experiment - put the 47uF in parallel with the stock 100uF and then compare with the 47uF alone. If you report your results, the whole community will have more information.

 

[hpjun]

I will try that but the larger stock cap will influence the sound more but I guess it would be a good test to see if there is phase issues that are audible.

47 uf capacitor I am looking at 47 uf premium gold obbligato caps or 47 uf mundorf supreme evo oil or a 100 uf mundorf evo aluminum oil. Not sure if any other caps offer better performance at hopefully just slightly more expensive prices. Please recommend any good caps.

Also I want to know if there is benefit of adding a choke to power supply or upgrading power supply caps for crackatwoa.

I also intend to roll a 5998 or 7802 power tube and mods to run 5687 or e80cc input tubes. I read a bit and learned that the output tubes are arranged in cathode follower. A higher mu from 5998 or 7802 will also increase the local negative feedback and while also lowering output impedance. I am not sure if increasing the feedback while be bad or not when localized. But the lower impedance probably out weighs the additional negative feedback? Is this a correct assumption? Whould like to know what the trade offs are.

Any other upgrades worth while to maximize the performance of crackatwoa? better diodes, better resistors at the signal path? I also thought about rewiring ac heater wires to be tighter. I don't like how the ac tube heater wires coming from the power supply is a little asymmetrical it makes canceling the magnetic fields more difficult so I may need to rewire and see if it does anything but there really isn't any hum at all, although I want to be neat regardless. May be I should be using DC heaters instead.

And of course I need to get the twoquiet stepped attenuator as well.

 

[Paul Birkeland]

Please recommend any good caps.

Try these.
http://www.parts-express.com/dayton-audio-dmpc-100-100uf-250v-polypropylene-capacitor--027-447

Also I want to know if there is benefit of adding a choke to power supply or upgrading power supply caps for crackatwoa.

The hybrid shunt regulator in the power supply simulates a 30,000 Henry choke.  So yes, you can add another 10H, but most don't find it necessary.

better diodes, better resistors at the signal path?

The diodes are behind the 30,000 Henry shunt regulator wall, so this goes along with adding a choke.  As far as resistors go, without the attenuator upgrade, there is a pair of 220 Ohm resistors and a pair of 2.2K resistors right in the signal path.  The R1 resistors on the small C4S board and the B side of the big board contribute somewhat sonically, but there is little flexibility in their values, and we have already provided high quality parts here.

I also thought about rewiring ac heater wires to be tighter. I don't like how the ac tube heater wires coming from the power supply is a little asymmetrical it makes canceling the magnetic fields more difficult so I may need to rewire and see if it does anything but there really isn't any hum at all, although I want to be neat regardless. May be I should be using DC heaters instead.

Since the heater pins on the socket aren't symmetrical, the heater wires should go to one side.  (Different for a tube like the EL34, where they are symmetrical)  If there isn't any hum, there isn't any reason to go down this path.  Also, DC heaters for such high current will require an additional power transformer, and likely a regulator mounted to a large heatsink, all of which would need to be in a separate chassis.

 

[hpjun]

Thanks for all the information.

I think I'll go with a 2x 47 uf premium gold obbligato for output capacitors. If not satisfied with the bass phase, maybe i'll get 2 more and parallel them.

Would the power supply caps benefit from an upgrade? I know in the old crack the last ps cap was important, but now with the shunt regulator I am not sure if it's beneficial but just wanted to ask.

I am also going going to order 7802 output tubes, I also might pick up another input tube I am looking at E80CC or maybe E90CC (I noticed on some limited high current cracks used E90CC, does the crackatwoa power supply have enough current for E90CC and 7802?

 

[Paul Birkeland]

Would the power supply caps benefit from an upgrade? I know in the old crack the last ps cap was important, but now with the shunt regulator I am not sure if it's beneficial but just wanted to ask.

You sure can give it a try and let us know.

I also might pick up another input tube I am looking at E80CC

You can use the E80CC, but the R1 resistors on the small C4S board need to be changed (you can search the forum for details)

or maybe E90CC

The E90CC is a higher gain 7 pin tube that would be a bit of a pain to retrofit into the amp.  Some were made for the ECC99 tube, which required a different pair of LED's on the 9 pin socket, as well as different R1 resistors on the small C4S board.

-PB

 

[hpjun]

Thanks!

I think that might be it for the available upgrades. Something else I thought of was shielding the signal wires all the way from input to output, probably with mu metal shielding sheets. I would probably shield the power supply as well if I get it.

I ordered a 7802 Tung-Sol it should drop to impedance from 120 ohms to 42 ohms according to my calculations. 42 ohm out into 350 ohm sennheisers has about a damping factor of 8 which is really ideal, it could probably drive medium impedance headphones although not with a 47 uf output cap.

I also ordered the 47 UF Obbligato Gold Premium Film caps.

I also want to TwoQuiet attuenator but it's expensive. I do like the L/R balance control and the stepped attuenator removes that.

 

[Paul Birkeland]

Thanks!
I think that might be it for the available upgrades. Something else I thought of was shielding the signal wires all the way from input to output, probably with mu metal shielding sheets. I would probably shield the power supply as well if I get it.

Again, you're fixing a problem that doesn't exist by adding expensive metal sheets that can come loose and cause damage to your amplifier.

 

[davve34]

Is it worth adding chokes on the crackatwoa and the discrete bridge rectifier? Or the shunt power supply cleans the power too?

 

[Paul Birkeland]

A choke is pretty pointless in the Crack-a-two-a.  You would be attempting to remove power supply ripple that won't make it through the shunt regulator, and in the process you'll be adding an extra source of magnetic noise that wouldn't otherwise be there. 

The Cree bridge won't hurt anything if you wanted to add that.