Mcandmars ramblings \ build thread...

[mcandmar]

I would like to know what 6dn7 tubes u have found to bbe the best sounding.n

The general consensus implys there isn't any difference between them.  From the limited selection i have tried i would have to agree, or if there is a difference its very subtle.

Having said that my initial impressions are as follows, RCA, GE, Zenith, National Electric, are almost identical looking inside which imply to me that they are all of the same design/origin. Without the labeling you wouldn't be able to tell the difference.  Sylvanias on the other hand are a little different, and the only two i can honestly say sound a little different.

The coin base variety i have are a very similar construction to the above lot, however they have black plates and shiny silver support rods vs copper.  There is something about them i just don't like, they sound lean to me or less tubey.  I also believe these are the most recent manufacture of the lot. The non coin base are also black plates however they have an unusual looking spacers that set them apart visually from all the other tubes i have seen. These are my favorite for reasons i cant explain, i just keep coming back to them. I also believe these are from a fairly old stock.

Thing is, any difference i hear could just be down to age/use of the tube rather than anything else.  To say for sure i would need to try multiple sets.  My advice, is find the oldest tubes you can that are quite and leave it at that. I have a collection that just wont stop buzzing no matter how many hours i let them burn in, so if you have a nice quite set there really is nothing to be gained by tube swapping IMO.

For example i'm using a set of National Electronics at the moment that quietened down within a few hours use, and neither of them are micro phonic so it looks like these are the set i'm going to stick with for the foreseeable future.

 

[mcandmar]

You can go larger with the heater power capacitor. I suggest you first just parallel the ones you have on hand, to determine how important that is - doubling the capacitance should halve the ripple if it is entirely due to the heaters. If that helps, then larger caps are readily available; a 10v rating is sufficient.

Do be aware that as the tube settles in, the getter will reduce the gas in the envelope and that will reduce heater-to-cathode leakage - this is one mechanism by which letting the tubes settle may improve the situation.

This was tonights fun little experiment.  I am seeing ~1.2v ripple in the default config, with the 2nd 10000uf cap added i measure ~.6v, half as you said. First two scope traces 2v/div, last one is .2v/div.

With the headphones on and the amp running (probably not the best idea) i tried adding and removing the 2nd cap and it didnt make any difference to the background hum.  It may make a difference, but at the moment i cant hear it over the background noise.  Kind of disappointing as that would have been an easy fix.

For fun of it i had a play with PSDII and managed to get a fairly accurate model of the circuit running that gives me very close numbers to what i have measured. I then started playing around with choke/capacitor combinations and found if i replace the RC filter with a Hammond 156B placed between two 10000uf caps it will drop the ripple to .3v which has me interested.  Choke specs are 1.5mH, 5A, .07ohm.  Its cheap and small enough to easily mount above the speaker binding posts.

So for $2 i can add second cap and half the ripple to .6v, or spend $20 to add a choke and reduce it to .3v.  That would be the curve of diminishing returns people often speak of.

Question is, what benefits would a choke have over the resistor in the power supply circuit?  Is that circuit design feasible or have i got the wrong end of the stick?

 

[Paul Birkeland]

Question is, what benefits would a choke have over the resistor in the power supply circuit?  Is that circuit design feasible or have i got the wrong end of the stick?

There is no question that this will not make any difference. 

If you can spend 4 cents on a pair of 120 Ohm resistors, you'll get where you need to go.

 

[mcandmar]

The 120ohm resistors arrived so i connected them up with jumper leads so i could switch them in/out. They do take the harsh edge of the background noise, but the majority of the hum is still present.  It is an improvement, though i would rather track down the source and nip it in the bud there.

The wiring around the headphone sockets was getting too congested with little to no room to add the resistors in so i took the opportunity to replace all the wiring from the impedance switch boards to the headphone sockets.  I used some triple core shielded cable i had which is a smaller gauge so it freed up a lot of room, also much tidier than having six big wires running down to the sockets.

As for the above, i suspected the heater circuit due to the 100hz frequency, and people seem to go to great lengths to quieten down these circuits with converting some amps to DC or using linear regulator circuits etc.  It just struck me as being a problematic area from what i have read.

What do you recommend i try next?  open to any suggestions..

 

[mcandmar]

Wait a minute, i've just realized i was supposed to put those resistors in series not in parallel. DOH!

 

[2wo]

You could rig up a temp battery supply for the heaters. See what that gets you...John

 

[mcandmar]

I like your thinking, i wonder how much current a D cell battery can supply as i have four of them sitting here waiting for the quickie..hmm

 

[Paul Birkeland]

As for the above, i suspected the heater circuit due to the 100hz frequency, and people seem to go to great lengths to quieten down these circuits with converting some amps to DC or using linear regulator circuits etc.  It just struck me as being a problematic area from what i have read.

People will indeed go to great lengths to quiet down a filament supply, but not a heater supply.  This is why we use batteries in the Quickie and AC heating on the Crack. 

Regulation comes into play when you want to build something that may get used in Japan (100V/50Hz) or the US (125V/60Hz) on the same transformer.  You can use a regulator to ensure that you have 6.3V available no matter where you might be using said piece of equipment.  In the BeePre, we found the regulator to be so noisy that we had to design an extra circuit to sit after the regulator to clean things up. 

How did the series resistors work?

 

[mcandmar]

People will indeed go to great lengths to quiet down a filament supply, but not a heater supply.  This is why we use batteries in the Quickie and AC heating on the Crack. 

Regulation comes into play when you want to build something that may get used in Japan (100V/50Hz) or the US (125V/60Hz) on the same transformer.  You can use a regulator to ensure that you have 6.3V available no matter where you might be using said piece of equipment.  In the BeePre, we found the regulator to be so noisy that we had to design an extra circuit to sit after the regulator to clean things up. 

Thats a differentiation i was not aware of.  Having googled it i assume you are referring to the difference between an Indirectly heated cathode, and a Directly heated cathode?  6DN7's being indirectly heated, Quickie 3S4 being directly?

How did the series resistors work?

Like a charm. I would say 90% of the hum has been subdued, the tiny bit remaining is only there when your listening for it so not a problem.  I can enjoy listening to the music now without the hum causing a distraction which is great.  Having to use more volume also means i am at the 12 o'clock position for normal listening which is the same as the HD650's via the XLR jack so thats a bonus.

So what effect does a series resistor have sonically?  I'm almost certain it sounds slightly different but i cant put my finger on it.

 

[Paul Birkeland]

Yes, the distinction is directly heated vs. indirectly heated. 

The series resistors pad the output a bit, which is very helpful for super efficient headphones, but otherwise unnecessary.  The damping will be a bit different than without resistors, but being on the 4 Ohm tap already will help mitigate that.

-PB

 

[Doc B]

I think you answered your own question. With the series resistors 90% of the residual hum is removed for sensitive headphones. Thus the background is more quiet, which translates to more resolution and the increased load that the amp sees usually creates a sense of flatter, more extended frequency response. Sometimes people will consider this "less tube-like" due to a misguided assumption that tubes must always be noisy and loose in the bass.

 

[mcandmar]

If anything i am feeling like some of the high end frequency are being rolled off, but i need to spend more time listening to music i know well to say for sure.

Is it worth experimenting with smaller values to find a sweet spot or is there a reason for the 120ohms value?    ...tell me what to google and i'll do my homework

Thank again,

Mark

 

[Paul Birkeland]

The 120 Ohm resistors used to actually be a standard in the audio industry.

The high frequency response should be identical to what was there without the resistors, provided the resistors aren't abnormally inductive.  (By abnormally inductive, I mean actually using an inductor instead of a resistor, lol)

 

[mcandmar]

I'm pretty sure they are resistors   Its probobly just my ears that need to recalibrate..

 

[VoltSecond]

Replace the transformer supply with a switching wall wart.  I have not tried this in this design, but it should work.  I've used this in other low noise designs with success.
1. Get one rated for the expected inrush current for the cold tubes. Designs with a medical rated version tend to be better.  This current rating is so it doesn't wig out during warm up and so it runs cool in normal operation.
2. Use a series wire wound resistor in the output hot and cold to both drop the voltage, limit the surge current and to act as an RF filter.
3. Put a large cap across the filaments (indirectly heated only.) Without the series resistor, the wall wart may oscillate if you add a cap to the output.
4. Bias the filaments above ground.
5. Put grounded copper tape  around the body of the wart (keep far from 115V blades) or a grounded piece of metal between the body and your circuitry for a shield. (You only really need line of site for this, if the wires and parts don't "see" the wall wart and its power cord, its usually good enough.)
6. Buy a ferrite toroid and wrap the output power lead to the wall wart 5-7 times around it (neatness counts, but don't wind so tight the copper in the lead cuts through the insulation around the lead.)

Switchers start out noisier, but they are easier to quiet down than a transformer rectifier circuit.

 

[mcandmar]

Replace the transformer supply with a switching wall wart.  I have not tried this in this design, but it should work.  I've used this in other low noise designs with success.
1. Get one rated for the expected inrush current for the cold tubes. Designs with a medical rated version tend to be better.  This current rating is so it doesn't wig out during warm up and so it runs cool in normal operation.

Interesting i thought switching power supplies in audio equipment was the work of the devil.  Though my experience is limited to tiny made in China supplies that were absurdly noisy and polluted the entire house with their switching noise.

2. Use a series wire wound resistor in the output hot and cold to both drop the voltage, limit the surge current and to act as an RF filter.

Would that be part of the function of the .1ohm resistor in the existing circuit?  I was reading up on the difference between inductive, non inductive resistors and different winding styles (Bifilar, Ayrton-Perry etc) to cancel out RF.

3. Put a large cap across the filaments (indirectly heated only.) Without the series resistor, the wall wart may oscillate if you add a cap to the output.

I had noticed the Mainline has tantalum caps directly on the tube socket heater pins, but figured i had asked enough stupid questions on said subject to ask..

4. Bias the filaments above ground.

Not entirely sure on what that means.  I have figured out the heater supply winding is center tapped and tied to ground so it swings +/- 3v basically to lower the voltage difference from the rest of the circuit to lower noise.  Or have i got that wrong?

5. Put grounded copper tape  around the body of the wart (keep far from 115V blades) or a grounded piece of metal between the body and your circuitry for a shield.

Do i need a tinfoil hat to go with that?

6. Buy a ferrite toroid and wrap the output power lead to the wall wart 5-7 times around it (neatness counts, but don't wind so tight the copper in the lead cuts through the insulation around the lead.)

Funny you should mention it i was going to start a thread on mains filters to see what other people have done, advise and so on. Will do that shortly as your wisdom would be greatly appreciated..

Cheers,

Mark

 

[Paul Birkeland]

This all presumes that the noise is in fact coming through the heater of the 6DN7, and not just self noise of the tube, power supply noise, or "other".

 

[VoltSecond]

Replace the transformer supply with a switching wall wart.  I have not tried this in this design, but it should work.  I've used this in other low noise designs with success.
1. Get one rated for the expected inrush current for the cold tubes. Designs with a medical rated version tend to be better.  This current rating is so it doesn't wig out during warm up and so it runs cool in normal operation.

Interesting i thought switching power supplies in audio equipment was the work of the devil.  Though my experience is limited to tiny made in China supplies that were absurdly noisy and polluted the entire house with their switching noise.

** They start out noisier, but are easier than many linear regulators to make very quiet.  Most of their noise issues come from common mode noise.

2. Use a series wire wound resistor in the output hot and cold to both drop the voltage, limit the surge current and to act as an RF filter.

Would that be part of the function of the .1ohm resistor in the existing circuit?  I was reading up on the difference between inductive, non inductive resistors and different winding styles (Bifilar, Ayrton-Perry etc) to cancel out RF.

** I was thinking on the order of a volt or two to help limit surge currents.  Normal wire wounds are fine for this application.

3. Put a large cap across the filaments (indirectly heated only.) Without the series resistor, the wall wart may oscillate if you add a cap to the output.

I had noticed the Mainline has tantalum caps directly on the tube socket heater pins, but figured i had asked enough stupid questions on said subject to ask..

** Can't speak about the Mainline, but tantalum caps don't like AC voltages and need ~2:1 voltage derating.

4. Bias the filaments above ground.

Not entirely sure on what that means.  I have figured out the heater supply winding is center tapped and tied to ground so it swings +/- 3v basically to lower the voltage difference from the rest of the circuit to lower noise.  Or have i got that wrong?

** I think PJ is a proponent of this.  The filament is biased above the cathode voltage with a large bypass capacitor to single point ground.

5. Put grounded copper tape  around the body of the wart (keep far from 115V blades) or a grounded piece of metal between the body and your circuitry for a shield.

Do i need a tinfoil hat to go with that?

** Tin foil hats are only needed if listening to DEVO. 

Good wiring habits will control inductive coupling. Metal foil (copper is easy to find at craft stores) makes an easy electrostatic shield.  which helps with the other "work of the devil" noise coupling method: Capacitive coupling.

6. Buy a ferrite toroid and wrap the output power lead to the wall wart 5-7 times around it (neatness counts, but don't wind so tight the copper in the lead cuts through the insulation around the lead.)

Funny you should mention it i was going to start a thread on mains filters to see what other people have done, advise and so on. Will do that shortly as your wisdom would be greatly appreciated..

** Inductance changes by the number of turns squared.  Excluding problems from resonant peaks, the inductance in a noise filter usually has to double to be slightly audible. 1 to  2 turns is a big step.  4 to 6 turns is a noticeable step.  7 - 8 turns isn't that noticeable. Second order effects get to be a problem once the winding goes more than one time around the core.

Cheers,

Mark

 

[mcandmar]

Cap rolling time! 

Having read various capacitor roundups i kept coming back to the Russian Teflons as the best bang for your buck so they seemed like a good place to start. The Teflon varieties FT-1,FT-2, and FT-3 are only available in small capacities under 1uf so i ordered a few FT-3 .1ohm 600v samples for the interstage and burnt them in for a few weeks connected to the speaker outputs of an amplifier playing a selection of frequency sweeps, 0db sine waves, white noise, pink noise, and "How do you say Bass by Bass Mekanik" lol. It was more of a torture test really.

Installation was a bit of a challenge due to the physical size of these caps, but i found with large gauge wiring they stayed in place without moving about. I then added a cable tie to help support the weight to reduce the stress on the solder joints.  The outer casing is bare metal and seems to be insulated from the capacitor but i added a layer of clear heat shrink over them anyway as a safety precaution.

Initially there was a harshness/distortion that seemed most noticeable with cymbals or notes around those frequencies, it's almost as though they are distorting but all the other instruments around it weren't.  It seems this was just part of the long burn in time people say these caps need to settle down, certainly any signs of that disappeared after a week of daily use.

I have been listening to them for about three weeks now and i am still picking up little nuances that have changed. At first i found them very transparent with clearer high end and low end frequencies, but with time i have noticed a couple of things. The tone in the mid-range / low end has much more "something" i can't find the right words for. "Presence and loveliness". For example bass guitars, drums, or any other instrument in that range are much more clearly defined, it's like any piece of music i listen to now i can just concentrate on what the bass guitar is playing without it getting lost in amongst the rest of the instruments. For quite music such as acoustic or Jazz i found wood instruments, double bass or wind instruments, actually sound like wooden instruments, while bass drums/toms have a lovely presence to them. Every now and again listening to music an instrument jumps out at me in a way i have never noticed before. It's all rather fascinating.

Having re-read a few roundups i found people commonly refer to MKP capacitors are being plastic sounding, i understand exactly what they are talking about now. And the FT3 are commonly referred to as being smooth but revealing, have to agree with that too. For me the FT3 are preferred, but i can see how some people might find the greater mid/low end presence not to their liking. And they do make the overall sound very smooth sounding without losing any detail, but for some a more forward trebly presentation may be more their cup of tea.  Having said that i would prefer more high end detail from the S.E.X. amp as i always found it had a very neutral presentation. The FT3s in no way reduced the high end, if anything they unveiled it a little, but it's the magic of the mid/low range is what does it for me. I am very curious to try something better in the future as the difference they have made was very surprising to me, i was expecting a more subtle change. 

Next i am going to look at replacing the Solen output caps, i had a short list of candidates drawn up and then had a moment of weakness in the post holiday sales and ordered a pair of 1.5uf Mundorf Silver/Oil caps.  I was really looking for something with neutral and clarity in its traits, and the silver/oil seem to fit the bill, we shall see..

 

[mcandmar]

Cap rolling part II: Output caps

This is where things get a little more complicated.  It has become obvious to me that i could re write what i wrote in the above post. A lot of it is still true however it has become apparent that my view of the FT3, or the role of the interstage capacitors in general was viewed through the window of the output capacitors.  In other words replacing the Solen output caps has removed that restriction allowing me to see the full picture.  In hindsight it would have been more enlightening to upgrade these capacitors in the opposite order starting with the output capacitors and then moving onto the interstage caps.  Certainly if it wasn't such a chore to solder/unsolder them i would do exactly that out of interest. If you are thinking about cap rolling the S.E.X amp i recommend starting with the output stage as i suspect that's the stage where the greatest benefit will be found.

My choice for the output caps were 1.5uf Mundorf Supreme Silver/Oil as they have a reputation for being neutral and highly detailed. Physical size is 36mm x 42mm which as you can see i only just managed to squeeze in next to the C4S board. Anything larger would have been a problem.

So how do the Mundorf caps sound?  I don't really know as i am unable to directly attribute what i am hearing to either pair of capacitors so i can only really speak of the sonic difference from my previous post with just the FT3's. In the above post i commented that i wished for more high end detail, and that some might find the mid/low end a little overly pronounced. Those comments have been rendered redundant. It is obvious to me now that what i was hearing was the limitations of the Solen output caps. The overall balance from the lowest of the low frequencies to the highest of the highs is just perfect now, i wouldn't change a thing.

I also don't find it as simple as saying there is more refinement, smoothness, clarity, or high or low frequencies. The change i am hearing is so pronounced and all encompassing all i can say is the sound is superior in every way, or perhaps i have just simply run out of audiophile vocabulary to describe it. If i had to pick just two buzz words it would be clarity and refinement, coupled with the tone and presence i experienced with the above FT3 caps.

The best analogy i can come up with is it's similar to finding that sweet spot in the middle of the room when your head is in that perfect position in relation to the speakers and four walls.  Everything just comes into focus giving you the full sonic picture with all its detail and texture, that's basically the difference i am hearing. I have found that unveiling has brought me closer into the music, and consistently sucks me in well into the early hours of the morning when one more track turns into one more album. Overall the combination of these four caps have brought this amp to a whole other level, and for me well into uncharted territory that i have never experienced before.

Interestingly i have little desire to experiment any further, or upgrade any components, though it does make me wonder where you can go from here with caps like, Jensens, Deulends, Jupiter or VCaps.  For the moment i am more than happy to be sucked in and wowed by this amp, even if the sleep deprivation is starting to catch up with me. And they say crack is highly addictive.. 8)