[Jens]

As I said, with the passive filter in place everything measures and sounds great

Sorry, Jens - you've lost me.  If everything measures and sounds great ... why do you feel you need a cap in series with the mid driver? 

Regards,

Andy

Very simple - because there is something to be gained. Without the rolloff the midrange driver is working a little too hard at trying to play the bass notes. It is possible "clean up" sound somewhat when the bass signals are rolled off. Not that the driver cannot withstand it - it's rated at 100 watts continuous and 250 watts peak power! But it will sound a little better (even though it certainly sounds good now).

And the measurements I'm talking about were done when I had my SS preamp, where the cap was implemented 

[Jens]

Got the roll-off implemented a couple of days ago. Everything works fine, and I gained some midrange clarity. I haven't had time to do measurements yet, but my ears tell me that everything matches up fine. They are usually right

The roll-off components (a 1% 47k metal film resistor and .015 uF Vima cap) were implemented in the power amp end of the interconnect running from my GK-1R to my LF55. No hum problems as a result of the change 

Roll-off point (-3 dB) is approx. 250 Hz

Hugh, I was wondering - would it be possible to change the LF55 input cap to create the same effect instead? Or would that create other problems?

I'm asking this because I intend to experiment a little with the input cap anyway

[AKSA]

Hi Jens,

Congratulations!!   

If your RC filter lies before C1 on the LF, then I think C1 could indeed be deleted, as you only need one blocker in this scenario IF the high pass filter is permanently connected and there is no way for unprotected signal transfer through careless hookup, the subject of my post to Kyrill just now.

However, the bias resistor on the base of the LF55/100 is 44K2, so you'd need to reduce this slightly taking your corner frequency up slightly, but it should work fine as long as the resistor is on the LF side of the 0.015uF Wima.

BTW, I'd try something a little better than a Wima here; I've found them to sound a little mechanical.  Try a RTX, or best of all, a teflon.  Some of the Russian teflons are very reasonably priced, and while not as good as the revered Venhaus caps they are much better value.

I would reckon (though this is an educated guess as I'm not a speaker man) that the improved midrange clarity is because the out of band frequencies are now better attenuated, and thus the drivers are passing only those frequencies for which they were designed.

Cheers,

Hugh

[Jens]

Hi Jens,

Congratulations!!   

If your RC filter lies before C1 on the LF, then I think C1 could indeed be deleted, as you only need one blocker in this scenario IF the high pass filter is permanently connected and there is no way for unprotected signal transfer through careless hookup, the subject of my post to Kyrill just now.

However, the bias resistor on the base of the LF55/100 is 44K2, so you'd need to reduce this slightly taking your corner frequency up slightly, but it should work fine as long as the resistor is on the LF side of the 0.015uF Wima.

BTW, I'd try something a little better than a Wima here; I've found them to sound a little mechanical.  Try a RTX, or best of all, a teflon.  Some of the Russian teflons are very reasonably priced, and while not as good as the revered Venhaus caps they are much better value.

I would reckon (though this is an educated guess as I'm not a speaker man) that the improved midrange clarity is because the out of band frequencies are now better attenuated, and thus the drivers are passing only those frequencies for which they were designed.

Cheers,

Hugh

Hi Hugh,

Thank you for your input about my input

What I was thinking was that instead of deleting C1, I could delete the roll-off grid in the interconnect and instead change C1 (and perhaps the bias resistor?) to effect the roll-off in the LF55 circuit, as opposed to outside the circuit, which is what I've currently done. Would this be doable?

I'm well aware that the Wima is not the last cry in caps, but for a test setup (which this is) they are absolutely fine. Ultimately - if I'm still satisfied with the functionality and measurements look good - I would probably go for a Sonicap Platinum or a VH. The LF deserves that!!! 

And you are right about the driver. I use a Focal Utopia Audiom 6W midrange driver. It has a resonance frequency of around 120 Hz and below that frequency it doesn't really have any output. However, when unfiltered, bass signals still reach the driver and cause some cone excursion. This sort of takes the driver's "attention" somewhat away from what it should be doing, namely midrange.

I also believe that removing the low frequency signal from the LF55 will make life just a little bit easier for it, again adding to clarity 

[AKSA]

Hi Jens,

My apologies for not answering earlier.  This has required careful thought and I always delay a little to be sure.

What I was thinking was that instead of deleting C1, I could delete the roll-off grid in the interconnect and instead change C1 (and perhaps the bias resistor?) to effect the roll-off in the LF55 circuit, as opposed to outside the circuit, which is what I've currently done. Would this be doable?

Yes, you could do it, but you cannot change R2, the base bias resistor on T1, which must remain unchanged at 44K2.  Thus only C1 can be changed, which does restrict you a little, and the input impedance of a transistor in common emitter will change slightly with level, so this adds a non-linear consideration, but it should not affect the operation audibly at all.

Xc (capacitive reactance) is given by 159000/F*uF, so for 44K2, the uF is given by 159000/Xc*F, that is, at 1000Hz, 0.0036uF, otherwise known as 3.6nF.  This means the 6dB (half voltage point) with a 44K2 resistor would be 3.6nF at 1KHz.  However, you probably know all this, Jens!!

I hope this helps,

Cheers,

Hugh

[Jens]

Thank you very much, Hugh 

I have to admit that I'm completely illerate when it comes to understanding such calculations - my forces lie elsewhere, i.e. in language and writing 

It appears that for implementing a roll-off in the LF55 circuit, we will need very small values for the caps. From that also follows that my favourite cap, the Sonicap Platinum, cannot be used for this purpose, since it is not available in these values.

However, for something good in these values, the silver micas might be a good choice. Any comments?

Also, I would want to roll off at around 250 Hz - would there be any sonic disadvantages? - and what would the cap value be?