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Old 13th May 2011, 06:15 AM   #1
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Default Using preamp with high output impedance on F3

Hi, Pass Lab amplifier experts, need your expert opinion on this.

I am using a PrimaLuna Prologue 3 and looking to buy a F3. However, the F3 has kind of low input impedance of 10k Ohm. My preamp has output impedance of 2800 Ohm. The rule of thumb is having the input impedance 10x of the output impedance. In this case, it is only 3x. Is it ok?

I post this question on Audiogon, and got a reply saying he remember that Dr. Pass said it should not be a problem because the F3 requires an extremely tiny current to drive and so even pres with high output impedance are nearly always fine.

I am not saying I don't trust that guy, but just want to get a 2nd opinion, and I would definitely feel more comfortable if I can get other people to double confirm this is ok.

Thanks.
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Old 13th May 2011, 09:26 AM   #2
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Quote:
Originally Posted by dealaddict View Post
I am using a PrimaLuna Prologue 3 and looking to buy a F3. However, the F3 has kind of low input impedance of 10k Ohm. My preamp has output impedance of 2800 Ohm. The rule of thumb is having the input impedance 10x of the output impedance. In this case, it is only 3x. Is it ok?
I post this question on Audiogon, and got a reply saying he remember that Dr. Pass said it should not be a problem because the F3 requires an extremely tiny current to drive and so even pres with high output impedance are nearly always fine.
I assembles several modifications of F3 and tested them with various preamps. With your preamp, I would not support an idea of connecting it to F3. Output impedance of your preamp probably frequency dependant, and this will affect your passband with 10K input impedance of F3, apart from other possible negative artefacts.
In other words, F3 is good with pres posessing around 100 Ohms output impedance.
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Old 13th May 2011, 09:54 AM   #3
juma is offline juma  Serbia
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dealaddict, it's not just the matter of input resistance - tha capacitance of LU1014 (about 1nF) is a factor too...
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Old 13th May 2011, 10:06 AM   #4
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Originally Posted by juma View Post
dealaddict, it's not just the matter of input resistance - tha capacitance of LU1014 (about 1nF) is a factor too...
Can you please elaborate? Is it good or bad? Thanks.
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Old 13th May 2011, 10:28 AM   #5
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Originally Posted by dealaddict View Post
Can you please elaborate? Is it good or bad? Thanks.
One of F3's specific features is that its input impedance is formed by a resistor (10k) connected to the gate of the input j-FET. For example, consider that the gate has 1000pF parasitic capacitance. These resistor and capacitance form low-pass filter, with cut-off (-3dB) frequency = 1 /(6,28*R*C) = 1/(6,28*10000*0,000000001) = 16kHz. This would be the F3's passband. Fortunately, the effect of the gate parasitic capacitance is suppressed partially by the fact that j-FET is cascoded, and by NFB. Actual F3 passband, according to Nelson Pass, is 80kHz.
But, if you add some 2kOhms of the preamp output impedance, to the mentioned 10k F3's input resistor, this does not improve the passband. And, if output impedance of the preamp is frequency dependent, for instance 2k at 1kHz and 2,5k at 20kHz, this will decrease the passband even more.

Last edited by VladimirK; 13th May 2011 at 10:34 AM.
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Old 13th May 2011, 11:01 AM   #6
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Old 13th May 2011, 11:07 AM   #7
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Quote:
Originally Posted by dealaddict View Post
Can you please elaborate? Is it good or bad? Thanks.
It's bad - the impedance is a complex sum of thermogene resistance (which is linear in frequency domain) and reactance (capacitive and inductive i.e. frequency dependant, non-linear resistance). With such a high capacitive component, the passband of the amp is compromised. That's one of the reasons why F3 has a softer lower end (output capacitor) and "smooth" highs - with such a frequency characteristic phase non-linearities also come to play (for example, at 10kHz signal is delayed about 20 degrees compared to 1kHz, and the same thing happens in lower frequencies).
Although, such a "loose" phase/frequency properties of the amp are ideal for people who love playing with different interconnect and speaker cables - their influence will be more noticeable...
And if you don't like playing with cables, use the buffer (B1 for example) between your preamp and F3.

Edit: I see that ZM already recommended the use of the buffer.

Last edited by juma; 13th May 2011 at 11:10 AM.
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Old 13th May 2011, 11:13 AM   #8
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Will adding the buffer not change the sonic signature of the amp? Aren't the impedance mismatches you speak of common in the tube world?
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Old 13th May 2011, 11:18 AM   #9
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Originally Posted by buzzforb View Post
Will adding the buffer not change the sonic signature of the amp?
If you make it properly (without input/output caps, good PSU...) it won't.

Quote:
Originally Posted by buzzforb View Post
...Aren't the impedance mismatches you speak of common in the tube world?
Yes they are, and that's one of the reasons why some people love the "tube sound" and some can't stand it...
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Old 13th May 2011, 12:54 PM   #10
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Quote:
Originally Posted by juma View Post
It's bad - the impedance is a complex sum of thermogene resistance (which is linear in frequency domain) and reactance (capacitive and inductive i.e. frequency dependant, non-linear resistance). With such a high capacitive component, the passband of the amp is compromised. That's one of the reasons why F3 has a softer lower end (output capacitor) and "smooth" highs - with such a frequency characteristic phase non-linearities also come to play (for example, at 10kHz signal is delayed about 20 degrees compared to 1kHz, and the same thing happens in lower frequencies).
Although, such a "loose" phase/frequency properties of the amp are ideal for people who love playing with different interconnect and speaker cables - their influence will be more noticeable...
And if you don't like playing with cables, use the buffer (B1 for example) between your preamp and F3.

Edit: I see that ZM already recommended the use of the buffer.
I would not be so technocratic, F3 is an excellent amp, and all the phases etc. are important only so-so. Much more important, how an amp deals with signal microdetails, and "short" schematics is important for that.
For estimating the F3 sound, I would propose to listen to it with Parasound JC2 preamp, and then make conclusions about F3.
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