Quickie experiments

[cpaul]

Last night I installed the PJCCS and took that opportunity to also experiment with some mods I've read about here.  What follows are my results.  Note, though, that I made 3 significant changes at once so teasing out the source of changes is tricky.  Others should feel free to add their own experiments and results, as it would be nice to have a single place to go to read about them.

System:
Rotel RCC 955 CDP
Zu Audio Oxyfuel interconnects
Marsh A200 amp
Vandersteen 1C speakers

Photo of mods:
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Flh5.ggpht.com%2F_g7Fb8iFSmLM%2FTOE_TwOIZpI%2FAAAAAAAAADo%2F80I_qEZnhFk%2Fs640%2FQuickieMods.jpg&hash=3d2e8d6662903c7309841e41799d6063521b64b5)
PJCCS is obvious.  Soft start caps underneath, soldered to resistor and diode pins.  Blue electrolytic cap in center is for soft shutoff.  Blue mf resistors on one set of inputs is for attenuation of CDP signal, in theory giving me high and low gain inputs (OK, strictly speaking not gain).

PJCCS - OK, not the kind of mod I mean since it's a standard option, but I thought I should just comment here.  This board definitely changes the Quickie noticeably (the other mods are just not likely to be the cause of these changes).  I find music has more sparkle, and the upper frequencies are more clear.  Notes don't smear together as much as with the standard Quickie (not that they smear much with the standard Quickie either).  Bass seems tighter and more punchy.  I don't notice any change in frequency response on the low end but what's there is terrific.  I do have a few small, potential downfalls for this mod in my system though.  One is an apparent drying of the sound.  At times it seems like it has refined the sound so much as to sterilize it a tiny bit.  I've only noticed it occasionally but it keeps popping up.  I don't have a working tube amp right now but I'd guess that effect wouldn't occur with one.  Second is that I am getting some distortion pops at moments on higher frequency notes.  One place is probably less than 15 seconds from the end of Diana Krall's "East of the Sun, West of the Moon" on When I Look in Your Eyes.  I don't know the source of this change.  Perhaps it was always there (e.g. on the recording) and I just hadn't noticed it because detail wasn't strong enough and now it is.  Perhaps it's the amp or speakers distorting or clipping (but it happens even at fairly low volumes.  Another possibly related to PJCCS, though not sure, is that some of my vinyl now shows serious surface noise where there is dust or light scratches (part of the increased clarity and the like).  All in all, though, I'm very happy with the PJCCS.

Input attenuation - I had too much gain using my CDP through Quickie, so I tried a mod http://www.bottlehead.com/smf/index.php/topic,1226.0.html to attenuate that.  In this case I decided to try the -6dB as a test, thinking that would be sufficient to be noticeable but wouldn't overdo it.  I used a 100k as the ground portion, but only had a 47k in-line resistor and not the recommended 49.9k.  I notice perhaps a slight attenuation but the gain is still too much.  And...now I have an annoying hum - not so much that I hear it while music is playing, but I do hear at silent moments.  More importantly, when I reach for the volume control or anywhere near, the hum increases to a serious level.  While the other mods MIGHT be a cause, this mod adds a resistor between the input and ground and I'm guessing that is causing the hum.  As I understand it, that resistor is there to help Quickie maintain it's input impedance.  May try a stronger attenuation, but if the hum doesn't go away, I'll live with the greater gain.

Soft Start/Soft Shut down - I was intrigued by PJ's suggestions for capacitor mods to create a soft start and soft shutoff for the Quickie.  (Soft start for a Quickie?  Is that even possible?)  For soft start, PJ suggested a 1k-5k uF cap across the 4k resistor and diode combination.  For soft shutoff, he suggested 1k uF from the B+ just after the switch to ground.  Before this mod, at power on, if I start up the amp first (giving it time to equilibrate), I get a huge pop from Quickie, with perhaps some weird tube warmup noises here and there.  Amp protection circuit trips.  On shut down of Quickie, I also get a huge pop with those squealy tube cooling noises.  Protection circuit also trips.  I installed one Nichicon 1000uF/50v cap for the shutoff and two random 2200uF/6.3v caps pulled from a computer monitor for startup.  I soldered these across both the 4k and diode in the PJCCS, polarity with neg to ground, didn't use any bleeder resistors.  Results?  Startup is unchanged.  I wonder about the need for the bleeder resistor and what effect that might have at startup.  I also wonder how it would affect the CCS as it seems to bypass the 4k/diode combination, thereby affecting the bias.  On shut down the pop is gone (yay!) but the tube cooling noises remain, and oddly the protection circuit still trips when those noises start.  I may try to fit a 5kuF cap in it's place next to see if the soft shutoff effect can outlast the tube cooling.  If so, I'll report back.  May try to fit some larger caps for soft start, and will change the location (maybe just across the 4k resistor this time?) if someone has a suggestion.  PJ thought the 4k resistors would allow the caps to charge too quickly, reducing the soft start effect.

Future mods will likely include output caps, volume pot, add an output, perhaps headphone option.  Will report here if I get to any of these.

[corndog71]

You should connect those 47K resistors from the input lug to the ground lug of the pot.  Then between the switch output and the input lug of the pot connect a 68K resistor.

I used a pair of 75K resistors with a pair of 46K resistors.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi576.photobucket.com%2Falbums%2Fss207%2Fcorndog642%2FBottlehead%2520Quickie%2F85816a51.jpg&hash=03b6b42b63b1f4f2923abd008afb2b0533d070fb)

[2wo]

Soft start/shutdown, not withstanding. your amp should always be last on/first off...John 

[cpaul]

Thanks guys.  Corndog, I did it the way I did b/c my CDP has higher output than my phono section(s), and was hoping to tone just one input down.  Based on the way I read the schematic, it seemed like what I did would work.  But then I got the hum and thought differently.  In the end, both have more gain than I need, so I'll try what you suggest.  So you're using a PEC in your?  What do you think of it?  I'm considering an LDR-based pot, but I hear great things about PECs.

2wd, while I agree, my wife and my 7 and 4 year olds don't necessarily agree (they're part of the reason I like the low v Quickie, though it may mean some dead batteries).  I'd prefer to prevent as much strain on the system as possible, even if due to mistakes.

[Wanderer]

If you have not seen them, my comments on a recently installed PJCSS in my Quickie they are here:   

http://www.bottlehead.com/smf/index.php/topic,139.msg8664.html#new

In brief I hear a warmer and more dynamic mid bass and images seem to "sit still better" then with the grid choke mod. Wonder if the chokes may not have been well matched and/or had some frequency dependent variances.

My different use of the Quickie (headphones, no soft start, no attenuator) may account for your hearing different results.     

I did listen to Diana Krall's "East of the Sun, West of the Moon" but from "Live in Paris" on CeeDy. I don't have "When I Look in Your Eyes". No audible issues I could attribute to the  PJCSS-Quickie.

The PJCSS has been all positive for me.   

[corndog71]

The PEC pots are fantastic sounding and very reasonably priced.  I've used them in many projects over the years.  The only problem with the stereo pot is that common problem of slight channel imbalance at the very lowest volume.  By adding the resistors you can use much more of the range and don't have to worry about the lower settings.  Their mono pots are great too.

[Paul Joppa]

Sorry this reply is late; it slipped past me and I was just reminded by another thread. I'm  not sure why the soft start should have failed the fist time. On subsequent trials, the lack of a bleeder would prevent the cap from fully discharging and the start would not be so soft.

Today I did a more thorough analysis - my first posting was pretty casual, plus now we have the experience of developing the Paramount delayed startup circuit.

A value of 56K should give a similar discharge time (20 minutes) as in the Paramount circuit.

1000uF is pretty small, I estimate a turn-on time of 4 seconds. To match the Paramount soft start timing, you'll need the full 4700uF.

In the Paramount circuit we also added a 300K resistor from plate to ground. This allows the output coupling cap to discharge - that may be the source of the turn-on "pop" since film caps hold their charge for a long time.

==============================

This was an interesting design exercise, but let's not forget the effectiveness of a brute-force approach. A switch that shouts the output will silence it completely, and won't harm anything. If you remember to switch it on after things have warmed up, and switch it off before switching off the power, then you'll never have these transients. :^)

[cpaul]

Paul, this is very helpful, thanks.  I'll work on it as soon as I can get the Quickie to the bench (hafta clear off a DIY single-ended mosfet amp I'm casing up - the opposite of Quickie in almost all respects including power consumption, but it's a learning project, and sounds quite nice).  So are you saying a 56,000uF shutoff cap for 20 min discharge?  I'll have to work out the numbers, but for me, even a 1 minute discharge should be fine.  I'll post when I figure it out, but I think 4700 is what I put aside to try next. 

As for the soft start, wouldn't the bleeder resistor only have an impact after it's been tried once (ie already have been charged and therefore not provide the slow start while the cap charges)?  I THINK the soft start didn't work the first time I tried it, but maybe it got charged somehow before I tested it with the amp on.  I'll try the bleeder resistor and report back, and up the startup caps if that doesn't work. 

While brute force works best, I'm trying to protect against inadvertent damage, particularly where my 4 and 7 year olds are concerned.

Happy Holidays!
Carl