Cathode bypass cap and freq roll off

[johnsonad]

I've added subs with a HPF to the system so the power amps are working from 80Hz and up. Am I able to lower the value of the cathode bypass capacitor at this point without adversely effecting the output and if so, to what value? I'm currently using 100uF.

I really don't need to change it as it sounds great but with a lower value I could try different caps and see if they effect the sound.

[Paul Joppa]

I suggest the first experiment to be disconnecting one end of the current bypass cap, i.e. no cap at all. You will likely hear an improvement, but you may or may not pay for it with a loss of bass.

Then, if it not an unequivocal improvement, try some different values - stock was 47uF, try 22uF and 10uF.

At a sufficiently high frequency, the bypass is not needed since the output signal current loop excludes the cathode bias circuit. The bypass is only needed to make up for the falling reactance of the plate choke.

[johnsonad]

Thank you Paul. I was hoping to get rid of it all together and that's an easy experiment.

[johnsonad]

I removed a leg today and shot the system before and after. There is no change at the crossover frequency of 80 Hz and only a couple if dB change at 150 Hz but that's probably due to mic positioning.

One less cap in the audio chain! Thanks Paul!

[Jim R.]

Aaron,

Probably your best bet for taking the LF load off the amps is a passive, line level high pass filter.

Getting the cathode bypass cap out of the amp is nice if you can get away with it, which obviously you can.

-- Jim

[johnsonad]

Hello Jim. The subs have an 80Hz HPF which allowed me to remove that cap. I've since put it back as there seems to be something missing without it. I'll try it again in the future as it's easy to left one leg of the cap.

[Jim R.]

Aaron,

The sub's filter won't do anything to the main amp -- the issue here is to remove the 80 hz and below from even getting into the amplifier -- that's the only way to prevent the amp from amplifying that signal in the first place.

Of course you can't do this with an speaker level sub connection -- you'd have to use a second output on the preamp to drive the subs. Compromises all the way around -- isn't it always the way?

-- Jim

[johnsonad]

Jim, preamp signal goes from the preamp, into the subs which have a built in 80Hz HPF, then to the power amp. Does that clear up the picture?

[Jim R.]

Aaron,

Yes, you can also do it that way, but most folks I know avoid that method as the filters built into the subs are usually of terrible quality. Certainly not up to the level of the rest of your system. But hey, if that sounds best, who am I to argue.

-- Jim

[johnsonad]

Jim, I notice no difference so far though the speakers are new to me. I may try it with the pre driving both sometime in the future.  These Altec 755A's are interesting, enough so that I'm selling my 19's.  I'm not sure completely why I like them but I do and am listening for longer than I did with the 19's which I feel are amazing speakers.

[Jim R.]

I keep forgetting you have the 755As. I have no reference for those, but I imagine that integrating a sub with them is not an easy job. Are you going OB or box? Seems like we're going a bit off topic here.

-- Jim

[johnsonad]

Well it's my topic and fine by me . They are in the publish Altec style box with minimal damping.  They naturally start to roll off around 80Hz so to me, it was a very easy combo. These Rythmik subs sound good to me and fill in what the full ranges don't. I listened to them full range prior for a week and am much happier with the bass.  Using the Dayton v2 mic measurment system it was easy to set levels and adjust for phase. Everyone who has heard the system as compared to the 19's is astonished they so much music is coming from these little drivers. I miss the micro dynamics of the horns for sure but until I find a better horn system, I can live with these 60 year old drivers.

[Jim R.]

Well, if you do ever find that better horn system, you can always send those little guys to me . If you haven't already done so, and feel brave, try isolating the driver from the cabinet, I tried this briefly with one of my Orcas and the difference in clarity was amazing.

-- Jim

[johnsonad]

How did you isolate it from the cab? And you have first dibs on the drivers if I part with them in the future. Better to keep them in the BH family.

[Jim R.]

Well, with the Orcas, I un-isolated them as they already come isolated -- for those of you with Orcas reading this, just leave yours as they are -- this was just an experiment and the Orcas are already isolated.

Anyway, basically you just need some kind of gasket to isolate the basket of the driver from the cabinet -- each driver willl need some experimentation with which material works best, then you have to place the driver in place on the cabinet and exactly ddrill the screw holes right in the center of the mounting holes on the driver flange -- the idea here is that the screws will not touch the driver and conduct vibration to the cabinet. Then carefully position the drivers in the cabinets and put a small o-ring or bonded washer between the screw head and the driver mounting flange -- now, if done right there are no parts of the driver in direct mechanical contact with the cabinet -- which is the goal.

One other benefit is that by carefully adjusting the torque on the screw heads, it can act as a fine tuning mechanism.

I plan to try this on a friend's zigmahornets once the weather is warm enough to work outside consistently.

And for those who want to know, the drivers on the trishnas appear to be isolated already -- and seeing as the horn tunnels are made of PVC plastic and there is no hint of a plastic sound or unpleasant colorations, my guess is that they did a great job of isolating the driver -- and there also appears to be no damping material, again as with the Orcas.

-- Jim