Output Coupling Capacitors

[Grainger49]

I saw them and assumed they were just like the almost identical 100uF that I have.  I see now, reading, that they are not the standard oil motor start/run caps.  But they look a lot like them, just a lot more expensive and a better dielectric.

[pro_crip]

I just bought 2 for the heck of it. There's only 8 left in stock if you're interested

Rich

[dmannnnn]

I just bought 2 for the heck of it. There's only 8 left in stock if you're interested

Rich

Let us know how they sound.

[dumbo]

Bought these to try.  Nubie question: Do they need to be installed with a specific orientation?

[Grainger49]

IIRC, for power supply applications the outer wrap should be at, or closest to ground than the other lead.  For a coupling cap application the outer wrap goes to the input, or toward the tube. 

You will most likely have to contact the manufacturer to find out which lead is which.  But post back when you get an answer.

[ironbut]

I just don't have any restraint anymore!

I ordered a pair of those new(ish) Obbligato film caps that they're marketing as power supply caps. If you haven't seen them, they come in two sizes, 47uF 500V and 100uF 400V.
I got the 100uF caps the other day but I figured I'd stick to my guns and wait to install them after getting the Speedball .
Well, I guess if they're in the house, they're fair game right? Well, at least that's what I'm trying to convince myself.

I'm listening to Esperanza Spalding right now. She's kind of a hot bass player vocalist I saw at Monterey last year and her album kicks some pretty serious butt.
It's been in pretty heavy rotation around here and with the change in caps, I'm hearing some pretty nice changes so far. Pretty lively sounding, fast, open and doesn't take anything away from the stock sound but just improves on it. Unless they take a turn for the worse, I'm thinking they'll be taking up permanent residence in my Crack (ouch!). Just like the film caps other folks here have tried, these are big suckers but I think there's going to be plenty of room with the Speedball's heat sinks and all.
Not crazy money either ($39 each).

I did some searching around but didn't find word one about these caps. I can't wait to try them in a power supply. I've got some other caps I'll try bypassing these just for fun but these are sounding pretty nice ala carte.

[Grainger49]

Hmmmm interesting use of these.  I have used the PP Film in Oil Obbligatos in my Paramours.  I only needed a 10uF. 

Very imaginative!

[sharpi31]

Waiting eagerly for my Crack kit to arrive. Won a pair of HD800s and can't wait to hear them with the Crack.

For anyone looking for coupling caps in the UK, I found these 50uF/250V polyprop caps at Farnell (the price is super cheap):

http://uk.farnell.com/parmar/r1008hp500p25/capacitor-50uf-250v/dp/3851850

You'll need two in parallel for each channel, but even then the price is great. No idea how they sound, but can't be worse that an electrolytic.

[grufti]

That Parmar cap is nearly 10 cm long [4"]. It has a 5 cm diameter [2"]. Four of those will be hard to fit.

[Doc B.]

For those of you who have not seen it, grufti made a very nice chart showing the low frequency cutoff you can attain with different output cap values and different headphone impedance ratings. Depending upon the cans you choose 50 uF could be a reasonable value, and it should work just fine with HD800s, getting down to around -3dB@11Hz:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg808.imageshack.us%2Fimg808%2F6843%2Fcouplingcap.jpg&hash=76ad0b3ab4e7303f822fd7592a3e0234b50abd76)

I would be interested in hearing feedback from folks using lower impedance cans in the 32-70 ohm range who have compared the stock 100uF caps with higher values like 300uF-500uF.

[Dr. Toobz]

For those of you who have not seen it, grufti made a very nice chart showing the low frequency cutoff you can attain with different output cap values and different headphone impedance ratings. Depending upon the cans you choose 50 uF could be a reasonable value, and it should work just fine with HD800s, getting down to around -3dB@11Hz:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg808.imageshack.us%2Fimg808%2F6843%2Fcouplingcap.jpg&hash=76ad0b3ab4e7303f822fd7592a3e0234b50abd76)

I would be interested in hearing feedback from folks using lower impedance cans in the 32-70 ohm range who have compared the stock 100uF caps with higher values like 300uF-500uF.

Just curious - with a 5998 instead of the 6080/6AS7, and a 500 or 600uF output cap, it seems like one could possibly drive a small 8 ohm speaker, no? How much power would you be able to put into something like a dinky desktop monitor? I have the Crack next to my computer and have wondered about this - little desktop speakers probably would need no more than 250mW or so and certainly wouldn't even come close to the -3dB cutoff in the bass frequencies. (Another idea I've had is to make a sort of "FrankenCrack" - OTL as usual for headphones, and add a MOSFET chip output stage from the 12AU7 for small speakers).

[Paul Joppa]

The chart is for a voltage source/cap/load but the Crack has a 120 ohm output impedance. The cutoff in the bass goes with the total of 120 ohms plus the headphone impedance, so the 160-ohm line shows about what you can get with any low-impedance phone. You do not need a huge capacitor unless you do something (different tube, feedback, etc) to lower the output impedance.

For speakers, the output is limited to about 20mARMS, which would be about 3mW into 8 ohms. The damping factor would be about 0.07, so the bass design would be quite different from the usual. Being current-limited, you get power proportional to speaker impedance so a whole bunch of drivers in series might be workable. Or get a 600:8 ohm transformer; you should see around 250mW then. A 25-v line transformer would do the job and be quite cheap.

[Dr. Toobz]

Sounds like the MOSEFT hybrid idea might be more promising if I want to squeeze in occasional speaker duty. But I may put bigger caps in there anyway if I end up getting AKG's (K-702; 62 ohm impedance), and will post any observations at that point.

[grufti]

Very true.

I have continued to work on the chart in the meantime and I'll post two new versions of it later on today. One for the original tube configuration with the 6080/6AS7G and its 120 Ohm output impedance. And then a second chart for the 5998/7236 output impedance around 75 Ohm.

It may even be possible to combine the two into a single table. I'll give it a try.

The chart is for a voltage source/cap/load but the Crack has a 120 ohm output impedance. The cutoff in the bass goes with the total of 120 ohms plus the headphone impedance, so the 160-ohm line shows about what you can get with any low-impedance phone. You do not need a huge capacitor unless you do something (different tube, feedback, etc) to lower the output impedance.

For speakers, the output is limited to about 20mARMS, which would be about 3mW into 8 ohms. The damping factor would be about 0.07, so the bass design would be quite different from the usual. Being current-limited, you get power proportional to speaker impedance so a whole bunch of drivers in series might be workable. Or get a 600:8 ohm transformer; you should see around 250mW then. A 25-v line transformer would do the job and be quite cheap.

[Doc B.]

Or get a 600:8 ohm transformer;

Better yet, get a S.E.X. amp. That's what we designed that amp to do - headphones or speakers. You'll see a lot more power output into 8 ohms. On another forum someone was asking about comparisons of Crack with a $1200 headphone amp that a competitor makes. If that is a reasonable budget and one is willing to build their own gear one could buy a Crack, a Speedball, a S.E.X. amp and the MQ iron upgrade for S.E.X. for less than the price of the competitor's amp, and have all sorts of fun.