Mcandmars ramblings \ build thread...

[mcandmar]

Forgive me if I missed it, but have you installed the C4S?

I might be making this up, but I believe it helped the noise floor on my S.E.X.

Hi there, C4S is indeed installed, it went in during the initial build.

[physicsmajor]

What Doc said. My amp has a massive difference in noise floor unloaded vs. connected to a good source.

I'd estimate easily 6dB, quite possibly more, in favor of the latter configuration... which is obviously the one that matters.

Even before installing those resistors, hook it up to a good source with a black background and see what happens on the scope. Extra points if you measure the noise floor of the source directly as well, so you can do a true comparison of noise added from the S.E.X. vs. noise amplified through it.

[Doc B.]

Conversely, when you are trying to sniff out what a noise floor may be composed of with a scope you will see the most magnitude in the detail with the output unloaded. Bear in mind I am talking about gear that can be run without a load. High feedback amps and such will probably need some kind of load to remain stable.

[mcandmar]

What Doc said. My amp has a massive difference in noise floor unloaded vs. connected to a good source.

I'd estimate easily 6dB, quite possibly more, in favor of the latter configuration... which is obviously the one that matters.

Even before installing those resistors, hook it up to a good source with a black background and see what happens on the scope. Extra points if you measure the noise floor of the source directly as well, so you can do a true comparison of noise added from the S.E.X. vs. noise amplified through it.

Double checked for sanity sake, listening to the headphones there is zero difference between no volume or full volume, or with or without external sources connected (tried two). The background hum in consistent regardless of what i do at the input stage. Its even consistent between different sets of tubes.

I haven't bothered to setup the scope to measure the various configurations or the sources, but i'm going to bet any noise measured is below the noise level of the amp which would explain why i cant hear any difference.

[najo49]

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

[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