GK-1 Bypass Conundrum

dayneger · « **on:** 28 Nov 2003, 06:45 pm »

Hey Guys (and Gals, if there are any out there),

I'm planning left and right channel active subs for my system in the near future, with a GK-1 to follow. How to hook them up has me at am impasse. Help!

If I recall correctly the GK-1 has a sub bypass to keep the bass frequencies from getting munched by the tube. Hence it would be desirable to hook the subs up to the bypass at line level (I hope the bypass is separate for L&R).

Yet others say it's best to hook up subs at the speaker level--supposed to give the subs more of the same sound characteristics as the primary amp, in my case the 100W AKSA. Better integration etc. (True??)

So, which of the factors is more important to the overall sound quality?

To further confuse the issue, I'm intrigued by putting an active crossover in so my monitors roll off at about 60 instead of their 38Hz F3. Supposed to clear up the midrange. This would only work with the bypass option above, plus would mean a crossover with 4 channels in and out instead of 2 channels in and 4 out.

For icing on the cake, I've read some interesting comments about the benefits of sticking an equalizer in the system to help keep my room better under control. I find it easy to get wrapped up in tiny details with the equipment and ignore the gigantic influence of the room--apparently a classic virus of this hobby. Exactly where in the chain to equilize and how, ??

Thanks!

Dayne

PSP · « **Reply #1 on:** 28 Nov 2003, 07:21 pm »

Hi Dayne,
I can't help you on the main thrust of your question, but I do have some experience using an active crossover to roll off the amps and mains at frequencies ranging from 40 to 120 Hz "to clear up the midrange". (I used a Marchand XM-9 with Marchand's upgraded op amp... to be fair, I did NOT put any other premium parts into the Marchand) )

When I used the Marchand, the midrange and highs had an etched and brittle character--it initially sounded like "detail", but not for long--so I removed it after a few weeks (I'm now using the Marchand only for a low pass to the sub). For many months, I ran my mains (speakers = GR Research Paradox 3, main amps = 55w AKSA/N) full range and filled in from below with the sub at xo=40Hz. Then, after listening to Jens' system I was motivated to get to work on my midrange and highs. I ended up putting a 0.033uf RTX cap in series with each channel of the amp input (this should roll off the amp and mains at ~103 Hz). Even with this, I still had too much low bass coming from my mains, so in adition to the series caps I also plugged the speaker ports with foam to really roll them off. The end effect was not subtle... this produced a huge improvement in midrange detail and significantly better bass too.

So, I would definitely encourage you work at rolling off the bass from the mains "to clear up the midrange"... but perhaps a simple and elegant way to get there would be to use a matched pair of very high quality caps in series with each power amp input (and if you want to try an active XO, be sure that you can return it if you aren't happy with what it does to the midrange and highs).

Peter

AKSA · « **Reply #2 on:** 28 Nov 2003, 08:07 pm »

Hi Dayne,

Yes, the GK-1 has a sub out which bypasses the tube to minimize the active devices in the chain. It's indeed separate for left and right.

Peter is quite right (thank you, Peter!); and his approach - 1st order electronic filter at the amp input and mechanical high order attentuation at the speaker by stuffing up the ports - works very well and is elegant because it avoids the phase shift aspects of high order filters.

The laws of physics preclude use of just one driver to cover the whole audio band, so we have to make do with two, or three. The sub idea is, in my view, the best compromise, provided the mains can be made to roll off below about 100Hz by some means which does not hugely shift the phase of the incoming music signal. It is this which discourages me about active crossovers, which on paper look great but tend to mangle the image.

The advantage of the sub-out on the GK-1 is that it is derived from the level wiper, so moves up and down in level with the main control. The disadvantage is that it requires a reasonably high input impedance, at least another AKSA, around 45K.

Cheers,

Hugh

dayneger · « **Reply #3 on:** 28 Nov 2003, 08:37 pm »

Peter--

Thanks for the idea about how to high-pass the mains without an active XO! Nice, elegant solution. How did you implement it? Ideally it would be relatively easy to switch in and out when playing around with other equipment. . .

Hugh--

I was planning on using those standard 250W plate amps, guess I'll have to see what their input impedance is.

I was reading one of the other links about EQ, sounds like the one of those Behringer DSP-1124 Feedback Destroyers would be the ticket. With my mains rolled off as per Peter's suggestion and the bass signal EQ'd to level the room gain and determine the crossover frequency, I should have better mids and less boom.

In which case I'd better find out what the input impedance of that unit is!

Dayne

PSP · « **Reply #4 on:** 28 Nov 2003, 08:50 pm »

Dayne,
I could have soldered the cap between the amp input RCA hot and the hot input pin on the AKSA amp (and someday I may do that) but right now I have it inline with the cable between the preamp (TLP/N) and the amp (55w AKSA/N). I looked all over for a good quality female cable mount RCA and finally Michael Percy suggested that I do the following:

Cable mount male RCA (I used Cardas GRMO):

dayneger · « **Reply #5 on:** 28 Nov 2003, 09:10 pm »

Hey wait a second--when I designed my AKSA housing I included 3 sets of input jacks on the back hooked up to an Elma switch. Perhaps it degrades the sound somewhat, but it's handy for using my AKSA for both audio and HT duty (or comparing multiple preamps for that matter).

What I'll do, then, is solder the cap on the 3rd set of inputs! Then I have the choice and just have to move the cables to the other jack. Unexpected bonus!

Thanks again for the explanation, Peter! Oh, how much do caps like that run in $? Can you believe the Swiss keyboard has the $ as a direct key on the keyboard?

Dayne

PSP · « **Reply #6 on:** 28 Nov 2003, 10:15 pm »

Hi Dayne,
Glad that there is a simple answer for you regarding implementation of the inline cap high pass filter.

Michael Percy lists Reliable Capacitor RTX 0.033uf for USD $6.50 each + $0.50 each for matching. Of course, you might want to use a slightly different value, depending on the xo frequency you want to hit.

f = 1/(2 * pi * C * Zinput) = XO frequncy (-3db) in Hz.

C = capacitance in Farads, Zinput = amp input impedance, in ohms (that's about 43Kohms, Hugh???).

Peter