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Old 7th April 2013, 02:19 PM   #11
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[QUOTE=LineSource;3444151]I would never "hot" connect an open amp onto a speaker. Perhaps consider putting an 8 ohm dummy load resistor across the amp output before you switch in the ribbon circuit for test. /QUOTE]

Thank you LS
Can you please briefly elaborate on the 8 ohm dummy resistor idea?
Not sure i understand how this would work.

Thanks again!
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Old 7th April 2013, 02:47 PM   #12
john65b is offline john65b  United States
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On my Duetta Sigs, I notice the tweeters were flopping all over the place too...I was using a DCX2496 active crossover, so I steepened the slope to 18db from 6 and moved the crossover away from 500hz. Worked great.

But then I noticed the foam support about a foot down from top was dust (top 12" of ribbon canted back about 1 inch for vertical dispersion)...I put in a new foam support and took quite a bit of the slack out. Put the crossover points back as original and back to stock. The ribbons now are quite a bit more stable and don't wobble anywhere near as bad as before...
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Old 7th April 2013, 08:51 PM   #13
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[QUOTE=90493m;3444526]
Quote:
Originally Posted by LineSource View Post
Can you please briefly elaborate on the 8 ohm dummy resistor idea?
Not sure i understand how this would work.
I was thinking about:
1) putting just a dummy load(no ribbon) of say 8-50 ohms across the output terminals to place some load on the power supply, close the feedback loop, and allow the DC servo to stabilize. You could measure the DC offset now if you are interested. You could test for turn on/off THUMPS now if you are interested. If you have an O-scope you could look at turn on/off transients .... just for fun.

2) with this dummy resistor still connected to the output, connect-in the ribbon circuit(1.5 ohm resistor plus ribbon = 2 ohm load) in parallel with the dummy resistor load and test if the DC servo continues to provide low offset with this much lower resistance speaker load. You could again test for turn on/off THUMPS by watching the ribbon.

3) When you are confident that the low resistance ribbons are safe from funky amp effects you can remove the 8-50 ohm dummy test resistor for normal operation.

Some feedback circuits, and some DC servo circuits perform differently when a low resistance load is attached. Also, Krell amps typically have several opamps that monitor output current, voltage, and power in order to perform dynamic management.

When your amp can effortly push 2,000 watts into 1 ohm
Be Afraid!
Be Very Afraid!

Last edited by LineSource; 7th April 2013 at 08:59 PM.
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Old 8th April 2013, 02:15 PM   #14
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thanks again LS
Sorry for so many simple questions, im not an electronics expert and dont want to make any potentially expensive mistakes.
Do you think using a lower value on the resistor power rating would serve to act as a fuse against current spikes? Maybe 5w, or less?
i dont see any way around dc offset. hopefully the tweeters can live with 5-10 mv.
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Old 9th April 2013, 05:19 AM   #15
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PERSONALLY;

I would purchase 50 watt (1-ohm or 1.5 ohm) (Caddock or Dale) non-inductive resistors and study the datasheets on heatsink requirements. Caddock have superior sonics, but require more heatsinking. (I constructed non-inductive resistors by gluing household 1um Al foil to plexiglass and cutting a serpentine). You want linear R over wide power.

I would put JUST the (1-ohm or 1.5-ohm) resistor across the Krell output terminals and test over, and over, and over again if the KRELL DC servo, or Krell ON/OFF THUMP, or Krell special bias circuits, cause NASTY currents that could destroy the ribbons. If both Krell amps are solid, I would connect a (1-ohm or 1.5-ohm) resistor in series with each M-T ribbon to the proven Krells. 5mv DC Servo offset is fine. (You are a lucky Dude). The active Xover gain will set the final SPL.

I would use 18 or 24db Xover slopes at 200Hz and 6Khz to minimize ribbon bass BOUNCE, and also to control tweeter beaming. The ribbons are resistive and will provide great transients, even with steep slopes. Shallow 6db/octave Xovers can destroy the ribbons. Sorry, textbook 6db Xover transient perfection are ribbon death.

We have Ficus trees behind our Apogees for diffusion(and home CO2 to O2 conversion).

Purchase a few 24bit, 96Khz recordings of that are your family favorites. Recording quality is KEY to audio LOVE.
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Old 9th April 2013, 02:58 PM   #16
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thanks again LS for all of your help !!

Greatly appreciated on my end. i am hoping the extra work to direct drive the FR's will pay off. Having a set of fully rebuilt Scintilla's, with big upgrades to the caps, gives me a good reference. in stock form the FR's beat my Scintilla in just about every area important to me. the Scintilla crossovers impart a very nice tone which the FR still lack somewhat but thats about it. Big line sources like FR, IRS V and Dali Megaline have a way of 'breathing' life into the music and have a relaxed quality to the music that is addictive. Some say the image is too big. i find it very life-like.
the adventure continues.....
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Old 10th April 2013, 09:37 PM   #17
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well i tested directly hooking up amp to both MR and TW using series resistors of 1,2 and 8 ohms. there is definitely ribbon movement when amps come on line and it is in proportion to the amount of offset. as you can imagine the ribbon moves in one direction or the other, depending on the phase of the offset and then just stays there until amp is powered off. The MR movement is anywhere from next to nothing, up to about 3/16" with the amp that has 20 mv offset. the TW moves more and its mainly the rear segment that moves, but it moves as much as 1/2 inch and then hits the 'stops'.
At this point i am debating if this is a good idea or not. It will be hard to get all amps down to zero offset. So how much bias can the ribbons tolerate over time?? clearly it will put some stress on them, but i don't know what levels would be considered harmful in the long run.
i guess plan B would be to just use transformers and still tri-amp? Does anyone know if the Speltz Zeroformer will still transfer the DC? Otherwise i would need someone to build custom transformers. I dont know what specs to use however.

Plan C is to stick with the original OEM transformer box and upgrade the transformers AND caps. that seems both expensive and sonically less appealing than plan B.

feedback/comments gratefully accepted.
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Old 11th April 2013, 03:34 AM   #18
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would a coupling cap make any sense in shielding the ribbons from the DC offset?
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Old 11th April 2013, 04:51 AM   #19
Davey is offline Davey  United States
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A coupling cap will prevent steady-state DC from deflecting the ribbons fore/aft, but not turn on/off transients.....especially if it's a large value capacitor.

An autoformer is not an inductive coupling like a transformer, so DC offset (although possibly reduced) will still be passed to the transducers.

I recommend to proceed very carefully whatever path you take. Direct-driving of fragile ribbons like Apogee's is potentially hazardous if power amp (or some other unexpected) failure occurs.

Cheers,

Dave.
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Old 11th April 2013, 06:01 AM   #20
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The servo circuit on my Krell KSA250 amp is constructed with discrete components.(circuit and pictures posted on diyAudio solid state) The main servo DiffAmp inputs are JFETS and there is a trim pot that is connected between the two JFET source pins which sets the DC offset.

I do not know what circuit is used in your amp, but a quick look(Picture) at the main PCB might have the servo trim part marked on the PCB. The DC offset could be adjusted and measured with just the dummy 1.5-2 ohm resistor across the output terminals. Due to thermal drift, any final DC offset measurements would be performed on a warm amp.

As I noted, I use the standard midrange transformer and standard tweeter capacitors on my Full Range, plus an 18db/octave active woof-MT crossover for robust, safe operation.


If you cannot get the DC offset reduced on your amps to ~5mv, you might consider using 3-way electronic Xovers, with 3-way amplification, with the standard mid-transformer and tweet-capacitors. robust, safe operation

After some experience with 3-way Xover and amps, you will have knowledge to decide if a new mid-transformer or difference tweeter-capacitor values are worth exploring.


My DIY direct drive amps used low offset opamps and had ~1mv offset. I would expect ~5mv offset on all of your amps is possible, but might still risk ribbon damage from on/of transients.
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