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In Reply to: Re: B+feedback can be bypassed, .86uf500v works great posted by TUBEVIBB on September 10, 2006 at 06:10:54:
meaning you don't have to pay on a per letter basis, so there is no need to be so cryptic and to use so many abbreviations. I would really like to understand how/where you have implemented this capacitor, and I still don't get it. Is the cap in parallel with the 1K resistor or is it in series? Is the value 0.86 microfarads, as you seem to be saying above, or is it a higher value, which I believe you quoted in a previous post? How did you arrive at the value of the capacitor?
Follow Ups:
I cant get the gist of it either - BUT , thanks to Tre'Perry , Ill give it a shot , to explain Tubevibb better ( I hope ! )Lew - I think hes meaning that there are 2 mods thats he has done on the DIY OTL .
Hes placed a 1k ohm 10Watt resistor between both Positive Rails of one Bipolar supplies . I dont follow why it needs to be 10 watt though .Theres no real continuous potential between each +ve 135vdc rail . Im going to put a scope on this one .
The 2nd mod I think hes now talking about is the , same resistor but with a cap of .86uf @500vdc in parallel ...And Im guessing Parallel , as the term 'Bypassed' has been used . The cap I would assume is a Non polarized type of good quality .
Is this correct Tubevibb ?
Stu
Oopps ...for One - I see you may have read about this much further down Lew . Sorry about that .
Also - my comment should read , " ....between both Positive Rails of the Bipolar supplies . "
I guess that is like an opposite load to the speaker connection , hence is that why its being noted as some sort of feedback path .
Perhaps like a Harmonic balancer built into the crank pulley on an engine .(:-)
The 1K resistor was Ralph's idea a long time ago; he drew the schematic for my amplifiers about 10 years ago, and the 1K resistor is right in there. My amps, which I built in the late 90's, have the 1K resistor. What's novel is the idea of "bypassing" the resistor (or whatever) with a cap. Also, since .86uF is not a standard cap value, I am betting that he meant to write ".68uF", which IS a standard cap value. But I want to hear more from Tubevibb.As to why this needs to be a 10W resistor, I don't know for sure and won't until I dig up the schematic and think about it. You are quite correct that under steady-state (DC) conditions with no signal applied, there is no voltage dropped across this resistor (I once measured it to satisfy myself), so obviously you don't need a high wattage in that condition. But it may be that under AC conditions, an AC potential can develop, but don't quote me til I look at the schematic. Maybe Tre' knows the answer with no further research necessary on my part.
resistor is used to prevent it from being damaged during bias test.The new amps have no need for bias test but we still have a lot of those things on hand... :)
The resistor is a degnerative feedback device. Bypassing it will introduce a high frequency cutoff. If this seems to sound better, it suggests some sort of high frequnecy resonance elsewhere in the system.
Are all bypass caps, all caps.frequncy cutoffs divices at some pont? All i have are resonance,s. I agree thanks for your time.T.V.
All capacitors present an infinite impedance to DC. For an AC signal, impedance is inversely proportional to frequency, according to the equation Z = 1/2*pi*f*C, where the asterisk is meant to indicate multiplication and C is in Farads. Thus, if you put a capacitor in parallel with a resistor, its impedance will be much higher than that of the resistor at low frequencies (so the net impedance of the RC pair will be roughly equal to the value of the resistor alone), and at high frequencies the capacitor will effectively short out the resistor (so the net impedance of the RC pair will approach zero). Where in the frequency range these two different phenomena predominate will be related to the value of the chosen capacitor. This is why I asked how you chose the value for your bypass capacitor.
Bypass caps are usually some sort of high frequency cutoff device, but otherwise capacitors have many different uses!
sounds great but lose,put the 1k in tighten up,but peaky. bypass 0.86 uf 500v peak gone u know. but not for everyone? thank for your time T.V.
"The AtmaSphere type amplifiers sound great but are loose in the bass. (poor dampening factor) When a 1K resistor is added accross the two positive points in the power supplies the sound (bass) tightens up but now there is a high frequency peaky sound. When the 1K resistor is bypassed with a .86uf cap the high frequency peaking goes away. This mod might not be right for everyone or everyone's taste."I think this is what TV is trying to say. I'm not saying that I agree with all of this.
Tre'
A noble effort, and probably essentially correct.
I installed those 1K resistors in my monoblocks from the get-go (~8 years ago) and so have no experience of what they would sound like without them. But there is absolutely no treble peak in the output of the amps, driving my Sound Lab M1 speakers thru a pair of Zeros. I never thought to mess around with that resistor, but one wonders what might happen if one changes the value to say 100R or 10K ohms, etc etc. Obviously that would alter the amount of feedback but we don't now what THAT would do to the sound. My amps, what with the MA2 chassis' and transformers that they use, are so heavy I am loathe to perform experiments of this type, especially when I am quite happy with what I've got.
My acoustat III OR X, An my Martin logan SL3,ROLE TOP DOWN A TUCH!Not peaky,B&w 802m 804m 805n show the peak.An yes lower r500 not r1k well tighen it up more,I am r 800.Bypass caps .86uf500v bypassed 16uf250v SILKY, THE BEST SOUND YEAT!Thanks T.V.
So, you are now using a 500R resistor bypassed by a 16uF capacitor in parallel with an 0.86uF capacitor, for a total of 16.86uF of capacitance. Is that correct? Once again, I would ask how you calculated the amount of capacitance that would be best or if you are just doing this by trial and error (which is perfectly OK, too). To my ears, many of the B&W speakers of the past had a fairly peaky sounding tweeter, with some amplifiers in some rooms, etc. Maybe that is what you are dialing out with your bypass caps.
Yes B&W Do have that peak on top, To my ears all but esl have a puch up at top, I do love my ESL,S. But in tampa fla some time do not work so well. But caps sound same ON esl,s! As for cap uf. By ear. thanks T.V.
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