bypass caps

[aragorn723]

Hi,

Can someone explain bypass caps?  What is the advantage of it, and how are they wired in?  Thanks,

Dave

[Paul Joppa]

Some caps, most notably electrolytics, behave badly at high audio frequencies and above - they have too much impedance in the form of resistance, inductance, and non-linear artifacts. By paralleling them with a better-behaved capacitor, that cap will take over the job of delivering small impedance at high frequencies.

The downside is that at the transition point, where both types are handling the current together, they will interact with resonances and modulation effects. Since there is no available solid data on the bad behavior (and sometimes it depends on the age of the cap), you can never be sure you have made a good choice of bypass - you just have to try different combinations and listen.

[aragorn723]

Has anyone tried bypassing the electrolytics or coupling caps in the Quickie?  I'm thinking about putting in some Russian K75-10 caps for coupling caps and bypassing them with teflon.  How do you calculate the value for the bypass cap? 

Dave

[Paul Birkeland]

How do you calculate the value for the bypass cap? 

Read the post right above this one.

-PB

[aragorn723]

so you just pick one and listen to it??  sounds like the value isn't very important?

Dave

[galyons]

It is a pretty subjective call.  I would try the interstage coupling caps first. No bypass.  Listen and decide if you like the  difference, if any, especially with the K75-10 caps. They are hybrid paper, polyethylene terephthalate, (Mylar) and aluminum foil in oil capacitor and, IMO, have a distinctive sonic signature when used in the signal path. (BTW, this is similar construction to vaunted Audio Note caps.)  If the typical Russian PIO, (K40/KBG) is Irish whiskey, the K75's are Irish whiskey with a splash Bailey's Irish cream.  Think tipped toward a fuller, sweeter midrange.  Usually quite pleasant. In most circuits I have loved them, but, some, not so much. If you think you need a bit more treble presence, then try a bypass.  I'd try a small value film/foil or Russian Silver Mica with the K75's.

As a general rule of thumb,  for power supply/decoupling caps, I use a bypass ratio of about 1/10th the main cap, but this can vary based on the capacitance of the main cap. (Almost always electrolytic)  For signal coupling caps, I usually start with 1/100th.  This is almost always, (read always for me), an improvement in the quality of the B+ voltage.  Film is almost always preferable to electrolytic (I always bypass the last PS cap!)


As PJ said,  you can experience some phase or other anomalies due to the interaction of the caps for signal coupling.  You just have to try, listen, decide.

BTW, give the K75's about 100 hours to fully settle-in.  IME, most PIO type caps take about 40 hours, but the K75's need more. You  can't really go "love or hate" until the caps are burned in.

Let us know how it goes.  There is no right or wrong...just "Do YOU like it?"!

Cheers,
Geary

[Paul Joppa]

so you just pick one and listen to it??  sounds like the value isn't very important?

Dave

I did not mean to say it's not important, only that you can't reliably predict what the best value is. Or the worst value, for that matter. Trust your own ears, don't trust anyone who tells you what value to use!

Actually other people's experience can be quite useful, but in this case only if they are using the same caps in the same circuit, and you already know their ears react similarly to yours. This combination does not happen very often. More frequently you will see posts saying basically "this worked for my ears in my circuit with my caps, therefor it will work for your ears in your completely different circuit with your completely different caps." That's just baloney.

[aragorn723]

It is a pretty subjective call.  I would try the interstage coupling caps first. No bypass.  Listen and decide if you like the  difference, if any, especially with the K75-10 caps. They are hybrid paper, polyethylene terephthalate, (Mylar) and aluminum foil in oil capacitor and, IMO, have a distinctive sonic signature when used in the signal path. (BTW, this is similar construction to vaunted Audio Note caps.)  If the typical Russian PIO, (K40/KBG) is Irish whiskey, the K75's are Irish whiskey with a splash Bailey's Irish cream.  Think tipped toward a fuller, sweeter midrange.  Usually quite pleasant. In most circuits I have loved them, but, some, not so much. If you think you need a bit more treble presence, then try a bypass.  I'd try a small value film/foil or Russian Silver Mica with the K75's.

As a general rule of thumb,  for power supply/decoupling caps, I use a bypass ratio of about 1/10th the main cap, but this can vary based on the capacitance of the main cap. (Almost always electrolytic)  For signal coupling caps, I usually start with 1/100th.  This is almost always, (read always for me), an improvement in the quality of the B+ voltage.  Film is almost always preferable to electrolytic (I always bypass the last PS cap!)


As PJ said,  you can experience some phase or other anomalies due to the interaction of the caps for signal coupling.  You just have to try, listen, decide.

BTW, give the K75's about 100 hours to fully settle-in.  IME, most PIO type caps take about 40 hours, but the K75's need more. You  can't really go "love or hate" until the caps are burned in.

Let us know how it goes.  There is no right or wrong...just "Do YOU like it?"!

Cheers,
Geary

Geary,

The interstage coupling caps are the 150uf, right?  Is it just a simple remove/replace?  Any particular type of caps that would  be good to try here?

Dave

[Grainger49]

Dave,

The Quickie has an output cap (only one stage so no interstage).  That one is 2.2uF.  The cathode bypass caps are the electrolytics.  They are 1,000uF.  This is according to the schematic I have.

The schematic shows that the cathode bypass only needs to be a 6V cap, Bottlehead uses a 35V that is probably also used in another kit.  Mouser and Digi-key sell some nice Polypropylene film caps for a few bucks.   Here is my search for a 1uF.  You could change it to 10uF to get the 100:1 ratio:

http://ca.mouser.com/Passive-Components/Capacitors/Film-Capacitors/_/N-9x371?P=1yznbzsZ1z0wrj5Z1z0x7xm

[mcandmar]

They were changed to 150uf in the Quickie v1.1.  From memory it only sees 2-3v in normal operation, Partsconnexion list 220uf 4v Blackgate caps for $3 each which i always wanted to try in that position, though somehow i seem to forget about it every time i place an order.

[Grainger49]

I have liked the Black Gates I have used.  But never got to use them in a cathode bypass position.  I bought a pair to go in the EF86 cathode position in my Eros and now I can't find them.  Too much crap in my work room!

[aragorn723]

That looks like a good option.  Audio note has an article on these, and apparently uses them in their electronics.  Would it matter that the capacitance is higher than the Quickie value? 

Dave

[Grainger49]

A larger value should be no problem.

[aragorn723]

what would it do, if anything, to the sound?

Dave

[Grainger49]

Increasing the value of the cap will not change the sound appreciably.  In calculations you might get a bass note or two more but in reality the frequency response will probably sound the same to you.

However the change in quality of the cap will make a sonic difference. 

I have long said that caps are like spices.  I like garlic, some others don't like garlic.  I don't like mint, others do like mint.  So it is a try it and see.  It allows you to personalize the sound to your liking once you find the type of cap you like.