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Old 19th June 2008, 07:45 PM   #11
GordonW is offline GordonW  United States
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This sounds suspiciously like a case needing a Zobel network.

Probably something like 22 ohms and .47uf in series... that will probably take care of any impedance rise from the speakers. Wire it across the amp secondary...

You can experiment with other values... just keep the resistance at or above the impedance of the speaker load, and I would think the appropriate cap value would be for equal Z to the resistor, at about 10KHz-20KHz or so (you can use a first-order crossover calculator to find this- just plug in the resistor value into where it asks for "tweeter impedance", and the frequency you want (somewhere in the range above, or maybe higher or lower, depending on what works best)...

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Gordon.
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Old 20th June 2008, 07:33 PM   #12
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Okay, Mr. Zobel Kept the amplifier from Oscillating Audibly, but it still sounds terrible. the frequency response is alot worse, almost no bass at all. I'm still thinking about what eles to try, I'm thinking maybe the operating point of the Pentode itself may be causing this.
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Old 21st June 2008, 12:00 AM   #13
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Hi Alex !
Make sure that you take the feedback signal from the right side of the OPT. Else you will get positive feedback.
Thorsten L.
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Old 21st June 2008, 05:26 AM   #14
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Quote:
Originally posted by alexmoose
I'm thinking maybe the operating point of the Pentode itself may be causing this.
I don't think so. Over at DiY Tube someone was having the same problem. This time with a commercial amp and Aspen full range speeks. For whatever reason, those FR drivers seem to be a very difficult load for any VT amp that incorporates gNFB. Alas, no solutions over there either.

You might try the Zobel on the primary side of the OPT. Make R equal to the primary load, and Xc= R at 20KHz or so and see what that does in terms of stability and sonics.
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Old 22nd June 2008, 03:06 AM   #15
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If Global NFB isn't making it happy, Perhaps I should attempt Local? maybe some Plate to grid feedback? i've been dying to try it after the high praise George (Tubelab) gave it, I'm just afraid to go it alone on implementing it.

Perhaps Cathode feedback would help? I used to use it on my JBL e60s to keep the bass under control (not a problem when the amp is crossed at 100hz), but perhaps it will allow help to tame the highs as well (the curse of the Midrange shout).


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Old 22nd June 2008, 03:49 AM   #16
zobsky is offline zobsky  India
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can you use direct coupled plate to plate feedback as in the RH amplifiers?. Should be plenty easy, but check with Mr TubeLab first.

http://www.tubeaudio.8m.com/RH84/rh84.html

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Old 22nd June 2008, 05:48 AM   #17
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Haha, you know something funny? I built that exact same EL84 amplifier to be used as a headphone amp. run in triode mode, however, now I will definitely drop that resistor in there, and see how it sounds as kind of a test amp. But yes, that is exactly what I was talking about.


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Old 22nd June 2008, 06:30 AM   #18
Jeb-D. is offline Jeb-D.  United States
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If your speaking in regards to Pentodes high output impedance. I have actually had really good results with Pentodes driving Fostex. I used the Pentode, Pentode wired with no feedback. It behaves like a current source amplifier. You can then tweak the frequency response by altering the impedance plot. This is done by shunting the driver with various passive components.

The driver I used was FE167E. The end result sounded better than any of my voltage source amplifiers pushing the same speakers. Since your speakers are Bi-Poles the bass boosting characteristic may or may/not be beneficial to you. Shortly after experimenting with current source amplifiers I found that Nelson Pass did some similar testing
http://www.passdiy.com/pdf/cs-amps-speakers.pdf

My approach was slightly different. I can explain if anyone wants to experiment with it.
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Old 22nd June 2008, 07:22 PM   #19
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hey Jeb-D,

Sounds like we have much to discuss! What kind of enclosure were you using to drive the 167e? and how did that affect your end result

I would love to hear how your methods differed from Mr. Pass's, as well, what kind of Pentode amplifier did you you use? push-pull? or Single ended?
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Old 22nd June 2008, 10:40 PM   #20
Jeb-D. is offline Jeb-D.  United States
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It is a little push-pull amp, but the principal should work well with single ended too. The FE167E's are in a simple bass reflex.

We all know that full rangers dominate in the mid-range. The main idea of adapting the driver for a current source amplifier is to flatten the driver impedance, but not completely. Since the drivers impedance increases in the low and high frequencies (the two areas full rangers fall short). We can get the amplifier to pump out extra juice in those regions.

I didn't find the notch filter that Nelson used, necessary for the FE167E. I used a Zobel instead to flatten out the impedance rise in the upper frequencies. For the Zobel capacitor, 6.8uF seemed to work well for this driver. You can adjust the "treble" by adjusting the resistor value in the Zobel. Don't use a resistor that is smaller in value than that of the drivers nominal impedance. You can go higher in value, but not lower. This Zobel made the largest difference out of all the circuits tried. It sounded horribly harsh before this was added.

The circuit I used to flatten the drivers bass resonance was the same as Nelson's, except with a capacitor in series to bring the impedance of the network back up below the resonant frequency. I have not got to do any with/without cap comparisons yet. The "Bass" can be adjusted with the resistor value.

(This part not recommended for bipoles)
I also tried adding 3dB of Baffle-step compensation (put the BSC circuit before the amp), and tuned the driver impedance so the bass was +3dB on top of that (+6dB total). So, the bass was getting full-space compensation, and the lower mid was getting somewhere in between full and half space compensation. I found this approach to give the better sonics at some listening levels and speaker placement, but worse in some. If the speakers are a few feet from the wall, I'd recommend this(but not for bipoles), but if it's fairly close to the wall then I think this circuit won't be worth the trouble. Just tweaking with the two resistors of the circuits listed above will produce good results.


The good thing about these type of circuits is that they are all in parallel with the driver, so the speakers can still be driven just fine by a voltage source amp
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