UCD stage impedance

[reins]

Hi,

I would like to exchange the DC-decoupling cap of my UCD180 modules. Bruno mentioned in his application notes that the impedance of the UCD stage is 10k. In another thread it was discussed as 1.8k.

What is the value I should work with when calculating the cap for the high-pass of the tweeter amplifier? 10k or 1.8k?

Thank's
Stephan

[IVX]

1.8k yet.

[Hattori Hanzo]

I did also resize the coupling cap between the input stage and the modulator in order to build a 1st order high-pass filter. I based my calculation on a 1.8k input resistance and it seems to give the correct result.

[reins]

So I will calculate with 1.8k,

Thanks
Stephan

[lucpes]

Bridge the capacitors between the opamp and the ucd input and use 2.2uf coupling caps before the opamp. Impedance is 100k here so you can use higher quality capacitors.

[Hattori Hanzo]

Bridge the capacitors between the opamp and the ucd input and use 2.2uf coupling caps before the opamp. Impedance is 100k here so you can use higher quality capacitors.

I disagree. First you may get DC on the output (at least when using the UCD180 standard version) and second with 100k input resistance you end up with capacitor values way below 0.1uF which means you can no longer use any of the audiophile boutique parts.

[ashok]

"................and second with 100k input resistance you end up with capacitor values way below 0.1uF .............."

Actually with 100K input impedance and a 0.47uF capacitor you would get a -3db point at about 3.4Hz . That's pretty acceptable .
Lots of botique caps available in this range. Even 0.22uF is OK .

[Hattori Hanzo]

ashok

I referred to reins original question (see first post):

What is the value I should work with when calculating the cap for the high-pass of the tweeter amplifier? 10k or 1.8k?

Your calculation seems ok but you have to admit that a 3.4Hz cutoff is probably a bit on the low side for a "tweeter amplifier".

[ashok]

Hanzo .....I'm guilty as charged . I read it out of context .

Talking about tweeter amplifiers . Isn't it a good idea to use a film cap to couple it with the power amp. Eliminates the protection relay contact.

Cheers.

[Hattori Hanzo]

ashok

I generally agree, a film cap would protect the tweeter at least from DC at the amp's output. But then the argument in your first post applies i.e. a HP filter between amp and speaker requires much bigger and more expensive capacitors. Speaker protection is also one of the reasons why I would still prefer to keep the cap between the input stage and the modulator if I wouldn't use it as HP filter.

In my case where the HP cutoff is at 350Hz I can easily use/afford one of the best and most expensive 0.22uF film cap between the UCD input stage and the modulator. The same filter with the same type of cap between the amp and the speaker costs 25x more than a UcD180 module!