Output Coupling Capacitors
- Thread starter xcortes
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Nice graphs, but I'm confused. The graph labeled as "6AS7" shows lower -3dB values for a given impedance, with the 5998 graph being opposite. For example, JH16 Pro (18 ohms) is listed as needing a coupling cap of 47uF to get a -3dB point of 25Hz with the 6AS7, yet the same value is listed as yielding a higher (36Hz) point with the lower resistance 5998. Since the 5998 has a lower Rp than the 6AS7, shouldn't the -3dB be lower for any given impedance/cap combo? Maybe the graph labels were reversed?
There is nothing really special going in the charts/tables.
The coupling cap (100uF/160V in the kit) influences the low frequency roll-off in the frequency response of the amp. The formula for calculating the frequency at which the output is reduced by 3dB popped up somewhere way back in this thread. At first I computed the table based on that formula, but that is correct only for a voltage source.
The latest two charts/tables use what I hope is the correct way to calculate the bass roll-off frequency. Any value under 20Hz (better yet 10Hz) and greater than 2Hz is very good. All the values in the tables are frequencies in Hz by the way. The column for the 100uF capacitor has ideal values for just about any headphone on the market, between 12Hz and 2Hz for the 6080 output tube in the kit.
The Bottlehead crew fortunately takes care of this kind of design work before they sell their amps to you and me, so we usually don't have to worry. And it turns out that they got it right yet again. It's just for some very unusual headphones that a different capacitor might make a difference.
I hope this helps.
The coupling cap (100uF/160V in the kit) influences the low frequency roll-off in the frequency response of the amp. The formula for calculating the frequency at which the output is reduced by 3dB popped up somewhere way back in this thread. At first I computed the table based on that formula, but that is correct only for a voltage source.
The latest two charts/tables use what I hope is the correct way to calculate the bass roll-off frequency. Any value under 20Hz (better yet 10Hz) and greater than 2Hz is very good. All the values in the tables are frequencies in Hz by the way. The column for the 100uF capacitor has ideal values for just about any headphone on the market, between 12Hz and 2Hz for the 6080 output tube in the kit.
The Bottlehead crew fortunately takes care of this kind of design work before they sell their amps to you and me, so we usually don't have to worry. And it turns out that they got it right yet again. It's just for some very unusual headphones that a different capacitor might make a difference.
I hope this helps.
tdogzthmn said:
Hi Jim,
They're pretty big. About 4x1.5 inches.
I'm still going to give them another day or two before I do any critical listening. I guess I have about 150 hours on them so far and they've gone through the regular film cap changes (I check them about once or twice a day) and they've seemed to settled down for the most part.
I'll post something in a day or two how they compare to the stock Panasonics.
This is with a stock unit and however they turn out, I'm sure it won't be as positive an improvement as the Speedball is (I seriously doubt that any component change will do that).
They're pretty big. About 4x1.5 inches.
I'm still going to give them another day or two before I do any critical listening. I guess I have about 150 hours on them so far and they've gone through the regular film cap changes (I check them about once or twice a day) and they've seemed to settled down for the most part.
I'll post something in a day or two how they compare to the stock Panasonics.
This is with a stock unit and however they turn out, I'm sure it won't be as positive an improvement as the Speedball is (I seriously doubt that any component change will do that).
Steve,
Ah, thought this was with the speedball installed.
With all the great things people said about the upgrade, I'm thinking the BGs as coupling caps would be quickly taking me into the territory of rapidly diminishing returns and might find themselves better used n something else. Stereomour?
Then again, maybe finding a nice bypass for the stock caps would be enough.
-- Jim
Ah, thought this was with the speedball installed.
With all the great things people said about the upgrade, I'm thinking the BGs as coupling caps would be quickly taking me into the territory of rapidly diminishing returns and might find themselves better used n something else. Stereomour?
Then again, maybe finding a nice bypass for the stock caps would be enough.
-- Jim
Well, after around 250 hours of 24/7 burn in, I can't really say the the Obbligatto caps are full burned in yet. Most of the warts that that sprung up during break in have receded so I think I can say where it'll eventually get to.
With the tube set that I enjoyed the most with the old caps was a Tungsram E80cc and a Sylvania 6080WB the differences with the new caps were pretty subtle. This is with my Audio Technica AD2000's and computer based rig.
Then I tried a Tung-sol 5998 with a CBS/Hytron 5814a which I found to be a nice pairing. Wow,.. I still need to re-install the Panasonics to double check but I'm listening to a 24/96 download of Raising Sand with the Pure Music player on my Mac/Metric Halo and this is the best depth and realistic soundstage I've heard with digital. I've used "Trampled Rose" for demo's of my headphone rig for more than a year and this is way-way better than it's ever sounded. I have to admit that I've heard a more analog sound with digital but you don't even want to know what that was done with! But for a "sane" system,.. not bad at all.
Tomorrow when I have time, I'll hook it up to my Tape Project rig and that should be pretty scary.
So, like I said, I'll roll the stock caps back in this weekend but right now, I'm just in hog heaven!
With the tube set that I enjoyed the most with the old caps was a Tungsram E80cc and a Sylvania 6080WB the differences with the new caps were pretty subtle. This is with my Audio Technica AD2000's and computer based rig.
Then I tried a Tung-sol 5998 with a CBS/Hytron 5814a which I found to be a nice pairing. Wow,.. I still need to re-install the Panasonics to double check but I'm listening to a 24/96 download of Raising Sand with the Pure Music player on my Mac/Metric Halo and this is the best depth and realistic soundstage I've heard with digital. I've used "Trampled Rose" for demo's of my headphone rig for more than a year and this is way-way better than it's ever sounded. I have to admit that I've heard a more analog sound with digital but you don't even want to know what that was done with! But for a "sane" system,.. not bad at all.
Tomorrow when I have time, I'll hook it up to my Tape Project rig and that should be pretty scary.
So, like I said, I'll roll the stock caps back in this weekend but right now, I'm just in hog heaven!
I went ahead and switched back to the stock Panasonics and listened today. I still have to say that I'm amazed how good those things sound.
Then I switched back to the Obbligatto PS film caps. (BTW I used some solid copper 18AWG wire I get from Chris Venhaus for the leads)
Once again, these listening tests were done with a pair of ATH AD2000's my digital system and a Tung-sol 5998 CTL top getters (1958 I think) and a CBS 5814a.
Everything seems a touch more defined like a layer of fuzz has been eliminated. Not huge by itself in any particular frequency range.
The soundstage is much more 3D with these caps. Maybe better than I recall it was with the Speedball (It's been a while but I can't remember being blown away by stage depth. Maybe the combo of the 5998/5814 with the Speedball will, I'll have to wait and see).
I think the level of the perceived bass is a touch less but it's better defined just like with the Speedball. The Speedball did add a half octave or so to the lower extension and did a similar job of cleaning up the lower frequencies. The definition of the lows was also better with the Speedball.
So, I'm pretty certain that these caps will stay in the Crack for a while even after I get the Speedball.
They don't seem to add anything bad and seem to do a better job of getting out of the way than the stock caps. Pretty subtle stuff but it may be more pronounced with the Speedball (or less?). We'll see.
Then I switched back to the Obbligatto PS film caps. (BTW I used some solid copper 18AWG wire I get from Chris Venhaus for the leads)
Once again, these listening tests were done with a pair of ATH AD2000's my digital system and a Tung-sol 5998 CTL top getters (1958 I think) and a CBS 5814a.
Everything seems a touch more defined like a layer of fuzz has been eliminated. Not huge by itself in any particular frequency range.
The soundstage is much more 3D with these caps. Maybe better than I recall it was with the Speedball (It's been a while but I can't remember being blown away by stage depth. Maybe the combo of the 5998/5814 with the Speedball will, I'll have to wait and see).
I think the level of the perceived bass is a touch less but it's better defined just like with the Speedball. The Speedball did add a half octave or so to the lower extension and did a similar job of cleaning up the lower frequencies. The definition of the lows was also better with the Speedball.
So, I'm pretty certain that these caps will stay in the Crack for a while even after I get the Speedball.
They don't seem to add anything bad and seem to do a better job of getting out of the way than the stock caps. Pretty subtle stuff but it may be more pronounced with the Speedball (or less?). We'll see.
I feel a little weird replying to my last reply to myself,.. on second though, no I'm not.
Just wanted to let you guys know that I got the Speedball yesterday and installed it. I burned it in overnight so it's probably got around 20 hours on it right now.
So, with the Obbligatto PS caps and the same signal chain as the two previous posts, all is wonderful and killerliciousness!
One of the great things about this combo (other than the price) is the sound seems improved on all genres of music. It doesn't improve a bad recording of course but it also doesn't call a lot attention to the worts either.
IMHO the improved soundstage and imaging is worth the cost alone.
Very engaging too,.. (I've been trying to write this post for about an hour now).
Just wanted to let you guys know that I got the Speedball yesterday and installed it. I burned it in overnight so it's probably got around 20 hours on it right now.
So, with the Obbligatto PS caps and the same signal chain as the two previous posts, all is wonderful and killerliciousness!
One of the great things about this combo (other than the price) is the sound seems improved on all genres of music. It doesn't improve a bad recording of course but it also doesn't call a lot attention to the worts either.
IMHO the improved soundstage and imaging is worth the cost alone.
Very engaging too,.. (I've been trying to write this post for about an hour now).
Thanks for the compliments johnsonad.
IIRC you heard my Tape Project/electrostatic rig at the LA CanJam a little over a year ago. Like I said at the time, the amp is in a constant state of flux and the next time you hear it, it should be totally different (in a good way I hope!).
IIRC you heard my Tape Project/electrostatic rig at the LA CanJam a little over a year ago. Like I said at the time, the amp is in a constant state of flux and the next time you hear it, it should be totally different (in a good way I hope!).
Paul Joppa said:
Pauls comments should be self explanatory, but, Im dumb, so I need to ask a dumb question for clarity:
Does this mean that the Headphone impedance should be added to the 120 ohm amplifier output impedance and then this total impedance should be used to reference the chart for the -3db point? So, with stock 100uf output cap, for 32 ohm phones you would reference the chart for the -3db point of 152 ohms (120 ohm Crack output imped. + 32 ohm headphones), not 32 ohms ... so theoretical - 3db would be a little above 10 hz with 32 ohm phones, not 50 hz?
Or, am I once again missing "it".
Thanks.
The load and resistors are in parallel, so they don't add as they do when they are in series. In essence, to get the equivalent parallel resistance, you need to take the invere of the sum of reciprocal values, which essentially means that the total will always be less than the smallest resistance in the series. Simple example would be two 10 ohm resistors: 1/10 + 1/10 = 2/10, inverted is 10/2, or 5. This is the same basic formula for n number of resistors, just add up all the reciprocals and then invert that number to get the equivalent resistance.
From this you can also see that very high resistances in on the order of several k, will have very little effect on the lowest resistance in the series, so the 2.xk resistors across the headphone jacks are negligible.
Hope this helps, I'm no teacher so I apologize for any assumptions I've made on your background.
-- Jim
From this you can also see that very high resistances in on the order of several k, will have very little effect on the lowest resistance in the series, so the 2.xk resistors across the headphone jacks are negligible.
Hope this helps, I'm no teacher so I apologize for any assumptions I've made on your background.
-- Jim
Hi Jim
I should have stated my question differently. I do know that parallel resistors dont add the resistors values together. I used the wrong wording as I didnt mean "plus" in terms of resistors and the math of it. I was wanting clarifiction on Pauls comments reqarding not needing a higher uF cap for lower -3db cutoff with low impedance phones.
Paul stated ... "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."
So what I was asking for was clarification on that. My understanding of Pauls comments are that, the cutoff for most low impedance phones would be what is shown in the 160 ohm line in the chart, NOT the line corresponding to the actual stated impedance of the phones themselves.
For a direct question .. would a 50 ohm headphone have a 32hz -3db cutoff as directly shown by the 50 ohm line on the chart? or would it have about a 10Hz -3db cutoff as corresponds to the 160 ohm line in that chart? From Pauls comments, I was assuming the latter and that's what I was looking for clarification on as to whether I was understanding that correctly or missing what he was saying.
Thanks
I should have stated my question differently. I do know that parallel resistors dont add the resistors values together. I used the wrong wording as I didnt mean "plus" in terms of resistors and the math of it. I was wanting clarifiction on Pauls comments reqarding not needing a higher uF cap for lower -3db cutoff with low impedance phones.
Paul stated ... "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."
So what I was asking for was clarification on that. My understanding of Pauls comments are that, the cutoff for most low impedance phones would be what is shown in the 160 ohm line in the chart, NOT the line corresponding to the actual stated impedance of the phones themselves.
For a direct question .. would a 50 ohm headphone have a 32hz -3db cutoff as directly shown by the 50 ohm line on the chart? or would it have about a 10Hz -3db cutoff as corresponds to the 160 ohm line in that chart? From Pauls comments, I was assuming the latter and that's what I was looking for clarification on as to whether I was understanding that correctly or missing what he was saying.
Thanks
