The alternatives for Hughes & Kettner Red Box output capacitor

[bobbysatya]

Hello everyone. I'm planning to build a direct box and I thought the Red Box mk.2 is a good choice to build regarding the simplicity. However, there are a couple of caps that are bit difficult to find in my country. Those are two 1uF/68v caps that are connected to pin 2 and 3 of the output XLR jack. The question is can I replace them with easier to find electro caps? If I can, which way should the polarity be oriented?

P.S. :
Pardon me for not providing the schematic of the circuit. I'm using an outdated phone. The circuit is Hughes & Kettner Red Box mk.2 which should be easy to find on google.

[GibsonGM]

Can you find non-polarized electrolytics? That would solve your problem!  You CAN use two polarized types in parallel and oriented in opposite directions to overcome the directionality, but to my mind that has always seemed a haphazard way to do things - and electros are not as good as film, which is why they specify film (noise...). 

I would try to find some kind of film caps, even if I had to put several in parallel to achieve 1u.  And mind your voltage rating, 63V or higher!
I've taken them from equipment I've taken apart, when I was getting started and had fewer spare parts...

What country do you live in?  Maybe there is someone on the forum nearby who can help you out, you never know...

[bobbysatya]

I live in Indonesia. Non-Polarized high value capacitors are scarce here. I have only one 1uF film capacitor that I salvaged from unused PCB junks I think might use it. I wonder if it's really a necessity to use exact capacitor type as the schematic since the audio signal wouldn't exceed that value.

[GibsonGM]

Well  yes, it is recommended to use the type specified, BUT...I glanced at the schematic and see NO WAY that you will ever come anywhere close to that!!   If you determine the voltage will be lower, then use a lower-rated part!     Can you get 16v or 30V caps?

By not using film caps, you may hear more noise - that is the only drawback that I see.  Anyone else?

[ubersam]

One think I can think of is the presence of phantom power on the balanced line. It could be as high as 48V. That might be the reason for the 63V rating specified. Another thing to consider is the input impedance of the mixing console or whatever the DI will be feeding. It could be as low as 1K (maybe 600Ω?). In which case, I'd want to a bigger output cap, maybe a 10µF.

[slacker]

If you can get 0.47uF (470n) film capacitors you could use two of those in parallel that's close enough to 1uF.

[GibsonGM]

I was coming back to talk about that, uber (phantom power).  Good eye.   

Yes, I'd try to adhere to the design as given - why would someone design something with a 63v spec, for no reason??

[induction]

You CAN use two polarized types in parallel and oriented in opposite directions to overcome the directionality

They have to be in series, not parallel.

[FiveseveN]

ubersam is correct. Are you planning on using phantom power? The cap rating only needs to be higher than the difference between the DI's output DC + signal level and the DC + signal level of whatever input follows. Standard mixer phantom power adds 48 V to the latter DC, hence the rating on the original schematic.
1 uF 60 V film caps are fairly common, you should probably look for those.

[GibsonGM]

You CAN use two polarized types in parallel and oriented in opposite directions to overcome the directionality

They have to be in series, not parallel.

Thanks for the correction... I never ever do that, have only heard it's ok.  Series it is.

[duck_arse]

I don't know what ebay buying is like in indonesia, but here are some things:

http://www.ebay.com.au/itm/10-Values-Kit-100-pcs-0-01uF-1uF-Metallized-Polyester-Film-Capacitors-Pilkor-/351647227518?hash=item51dfcede7e:g:IK8AAOSwrklVezYQ

http://www.ebay.com.au/itm/1uF-63V-MKT-Metallized-Polyester-Film-Capacitors-Pilkor-5-pcs-/261955769610?hash=item3cfdc7d50a:g:gWoAAOSwstxVfdO2

http://www.ebay.com.au/itm/5psc-polyester-film-capacitors-1uF-100V-PILKOR-series-PCMT468-pitch-10mm-/252226903761?hash=item3ab9e51ed1:g:4G4AAOSw-nZTpsV7

[287m]

hey bobby, duniaelektro.com have 1u mkm and 1u nonpolar
or try to call sfe-electronic malang

not the best price but you support local store
hehe

merdeka

[bobbysatya]

I've just learned about phantom power, pardon my ignorance. Now I see the reason why they put high voltage capacitor there. I also find out that the output capacitor values vary from the releases. The mk.I used 470nF whereas the mk.II used 1uF. I wonder if it's okay to use 470nF ones without dramatic change in tone since I have fistful of them laying around.

[GibsonGM]

Could you not try out the 470n caps, and if you like them then you can keep them?  It will not harm anything as long as they are rated over 63V.  If you do not like them, you can get the 1u.

I would expect a more treble tone from them.  If you can find the prior schematics, you can also see if anything else has changed (resistors at/near output?).   

[bobbysatya]

I haven't make a deep comparison between the two. But there are some differences that are easily noticeable. At the input the mk.I used 100nF instead of  much smaller 15nF of mk.II and the resistors that fermed a voltage divider at the speaker in jack are of different value as well 100k/10k combo instead of 470k/10k of the mk.II

[PRR]

Phantom is nominal 48V but can vary. A 50V capacitor may not last "forever". 68V is the usual best-practice rating for Phantom blocking.

Phantom is Positive at the mixer or mike preamp, which can guide you with a polar electrolytic. However electrolytics always leak some, and that could be an issue with this design.

[bobbysatya]

Everyone, I made a mistake in my previous post. The capacitors they put on the mk.I are 2u2/16v tantalums with 1k resistors in series after each of them. If my knowledge serves me right, I'm sure that tantalums are polarized capacitors. Doesn't that mean I can replace them with electrolytics? What do you think?

[287m]

replace tantalum with elco? yes, lets do it

[ElectricDruid]

Everyone, I made a mistake in my previous post. The capacitors they put on the mk.I are 2u2/16v tantalums with 1k resistors in series after each of them. If my knowledge serves me right, I'm sure that tantalums are polarized capacitors. Doesn't that mean I can replace them with electrolytics? What do you think?

Tantalums are sometimes used where their low ESR is important, so you can't always replace them with electrolytics. But in this case, if they've got 1K in series with them, their ESR is probably irrelevant.

Tom

[PRR]

> Pardon me for not providing the schematic of the circuit. I'm using an outdated phone.

It really would help to give pointers instead of blaming your tool.
http://bmamps.com/Schematics/Effects_miscellaneous/H&K_RedBox-MkII.gif

> I made a mistake in my previous post. mk.I

Argh...
http://schems.com/bmampscom/effects_misc/H&K_RedBox-MkI.gif

> planning to build a direct box

A "direct box" is for connecting guitar to Mic input. Usually "no color".

This Red Box is for connecting Line or Speaker to Line Input, and includes a speaker simulator.

Be sure you know what you want.

99% of the time you could omit those output caps. Line Inputs "should" not have Phantom voltage. But stuff happens (some consoles use the same jacks for Mic and Line). Even then, +48V through 6.8K, plus the 1K inside the Red Box, is only 6mA current into the Box's opamps. They can swallow this fine, no harm. H&K's choice to add the output caps is probably over-cautious due to the high cost of Warranty Repairs from idiot customers. You are unlikely to mis-connect it, harm is unlikely, and if it does blow-up an opamp, you can fix it in an hour.

Same for Mk I or Mk II. Just omit (jumper) the caps and party on.