Should I remove These Caps?

[Magz]

Hello all,

I've done extensive upgrades to the EQ unit for my Infinity RSIIb speakers (schematic below), including changing the output stage to a DC-coupled opamp buffer and eliminating the output coupling caps (big improvement in clarity and detail).

I'm wondering if I could also eliminate the two input caps circled in red in the schematic. I know they will block DC, which is useful, but the ONLY source this EQ unit will ever see is my tube preamp, which has a parafeed cap and output transformer so that it will not ever output DC. So I guess the question is whether these two caps have any function other than blocking DC? The way I see it the .029 also serves to roll off the LF extension to prevent over-zealous boost application from blowing the woofers, so maybe that's a keeper; I have already increased it to 0.1uF with no ill effects and much better deep bass - is complete removal the next step?

Any feedback on this is appreciated.

[HVfanatic]

In my opinion both influence the frequency. 0,029uF is a frequency dependent voltage divider. The 0,18 uF interacts with the 100K and the 360K. No good idea to change. Next problem removing the 0,018 cap: DC balance of VN40F. Did you simulate the circuit?

[Magz]

Originally Posted by HVfanatic

In my opinion both influence the frequency. 0,029uF is a frequency dependent voltage divider. The 0,18 uF interacts with the 100K and the 360K. No good idea to change. Next problem removing the 0,018 cap: DC balance of VN40F. Did you simulate the circuit?

As I mentioned, the output stage has been completely changed, to an OPA827 unity gain buffer, so there are no more VN40AF mosfets in the circuit. The OPA827 has an input offset of less than 120uV, and allows me to remove the output caps completely - HUGE improvement in clarity.Because of the above, all I can see the .018 uF cap doing now is functioning as a high-pass at about 19 Hz (cap RC filter with 360k plus 200k from the 1Mohm pot where it is usually set). I upgraded the .018 cap to a Jensen copper PIO which gave a nice improvement over the stock mylar but I'm thinking I may as well just remove it altogether if its only function is blocking DC.

Since the bass boost circuit has DC gain I may keep the other cap in there just for safety. I have already increased the .029 value to .1 and noticed increased bass depth - the 100k pot then needed to be set at a lower value to get the overall bass level correct.

[Magz]

Thinking about this a bit more it does seem that the .018 has an additional function in shaping the sound above and beyond blocking DC, so it will stay, as will the .029.

Thanks for the sounding board.

[kevinkr]

Yeah, you need both those caps.. Doesn't mean however that you couldn't replace them with better caps depending on what is in there already. I tend to prefer precision teflon film & foil types for a variety of technical and personal reasons - not that I want to get into a cap debate. (I REALLY don't..)

[Magz]

Thanks, Kevin.

I have replaced the stock generic mylar radial caps - the .018 was replaced with a .016 uF Jensen Copper PIO and it added a nice little bit of bloom to the midrange. The .029 mylar was replaced with a .1uF Solen MKP, which gave increased bass depth and impact - that's about as far as I could go with that cap since I need to adjust the mid-bass pot up to almost full scale to compensate for the larger cap there; the bass level pot also needed to be turned down from 12:00 to 10:00. The EQ profiles are pretty close to stock but with added low end extension once the pots are adjusted.

I was hoping to be able to get rid of one or more caps in addition to what I already ditched, but alas, I guess I need what's left.

Thanks to all who helped!

[kevinkr]

Sounds like you are pretty much set as long as you have sufficient adjustment range to achieve proper balance..