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Old 12th January 2009, 06:52 PM   #11
wicked1 is offline wicked1  United States
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Thanks!
Ty, do you know, is it really important to figure out the resistance of the amp, if I want to make a passive input filter, or is there a general figure I could use for a 300b se amp?

Like I siad, it doesn't need to be perfect. I can adjust the subs to fit. I'd like it at about 80hz +/- 20hz
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Old 12th January 2009, 07:15 PM   #12
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I believe for a high pass filter, Fc = 1 / (2*pi*R*C). R is shown as 121K on the Tubelab schematic. For simplicity sake, I'll assume you do not have an input volume pot. Solving for C in your case gives C = 1 / (2*pi*121000*80) = 1.64E-8, or 0.016 uF.

Something like one of these might work. Just stick it in series with the input.
http://www.mouser.com/Search/Product...xfzed3qQ%3d%3d
http://www.mouser.com/Search/Product...vhjUrgGA%3d%3d

Somebody correct my math for me...
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Old 12th January 2009, 07:29 PM   #13
Jeb-D. is offline Jeb-D.  United States
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A high pass before the amplifier would be best. Filtering the low frequency out of the music will give your amplifier some more headroom before clipping. That's because the high frequencies ride on the low ones, like AM.

As Ty recommended, a simple coupling cap on the input of the amp will do the job with excellent fidelity. Since it's a first order filter, I'd recommend setting cutoff to 100Hz since the slope is not steep.

I wouldn't recommend a synthesizer filter. Synthesizers typically use a low-pass VCF. You will need a high pass that isn't voltage controlled. Fidelity usually is not of concern in the world of synthesizers (there built to sound cool, not for sound reproduction).
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Old 12th January 2009, 07:48 PM   #14
wicked1 is offline wicked1  United States
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"R is shown as 121K on the Tubelab schematic."

OH! I thought I had to calculate the resistance of the amp itself, from input to to output of the transformers. Really, I think the pages I was reading were talking about doing that, and it didn't sound easy. They were feeding test tones, and using the frequency of the test signal in their calculations. I didn't put a lot of though into it, but from skimming the pages, it seemed overly complex.
However, like I said, I don't care if it's perfect. I just want to cut some of the lows.

SO, to the workbench!
Thanks for all of your help.
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Old 12th January 2009, 08:04 PM   #15
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The input impedance of the grid is very large - I'm certain it is safe to ignore it for all practical purposes. Your source is effectively working into the grid resistor - R8 (and R19), or 121K in this case.

You'll need a non-polarized capacitor of some sort, most likely a film type. I suggested a Vishay Sprague Orange Drop - they get good press. You don't really need the 200V rating here, but I don't think they come in anything less. Generally, people seem to say the 716P is a better cap than the 715P. I'll remain silent on the issue, though I will say I've used the 715P in my own stuff.

You could also use any sort of exotic fancy stuff which pleases you as well. The 0.015 uF part should give a 87 Hz corner frequency, which ought to be close enough.
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Old 13th January 2009, 02:48 AM   #16
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Default One channell or both?

Hey.
This degradation... is it one channell or both.. how is your rectifier?
What brand of 300b? My friend kept an eye on his 300b's with his AVO and found they degraded quite quickly.. which is another reason why I ditched my plans to build a pse version.. 300'bs too expensive and somewhat unreliable.. yes I know they sound great but maybe your tubes are calling it a day... dont suppose you have a few lying around to test?
Nick
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Old 22nd January 2009, 07:56 PM   #17
wicked1 is offline wicked1  United States
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So, I did put a capacitor on the input, and WOW.. It has made a HUGE difference. I haven't changed anything else, w/ my subs or anything..
From my listening position, I hear just as much tone, like, everything just as it should be, but that distortion is gone! (makes sense.. the bass still comes from the subs, but isn't being pushed through my tube amp now)

I'm using a Zune as my audio source. (it has a GREAT DAC, as far as most MP3 players go.. better than Ipods, that's for sure)
I started noticing this problem after an update to the zune. Maybe they changed something that is letting all sorts of super low frequency stuff through, I dono... but this has certainly helped.
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