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.
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.
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.
dealaddict, it's not just the matter of input resistance - tha capacitance of LU1014 (about 1nF) is a factor too...
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.
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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.
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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?
If you make it properly (without input/output caps, good PSU...) it won't.
Yes they are, and that's one of the reasons why some people love the "tube sound" and some can't stand it...
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.
There's no doubt about that, it's just that it's a bit picky when it comes to choice of input signal source and speakers' sensitivity and impedance modulus... 😉
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thanks for the replies. It seems F3 is no different from other amp that impedance matching is important. I thought F3 is somehow special that impedance matching is not important.
So, about the comment that only very little current is required and so impedance is not important ... any comment about this? How is the amount of current required related to this discussion?
So, about the comment that only very little current is required and so impedance is not important ... any comment about this? How is the amount of current required related to this discussion?
The actual F3 has a nearly 0 ohms input - it depends on the input resistor
in series to create the input impedance.
It doesn't care much what that value is, and adding 2800 ohms just adjusts
the gain down a bit.
The only consideration is whether your source is happy with such a load.
Some are and some aren't, and it doesn't have a lot to do with the
output impedance. It depends on the type of circuit design.
😎
in series to create the input impedance.
It doesn't care much what that value is, and adding 2800 ohms just adjusts
the gain down a bit.
The only consideration is whether your source is happy with such a load.
Some are and some aren't, and it doesn't have a lot to do with the
output impedance. It depends on the type of circuit design.
😎
What an honor to have the designer to answer my question!
Can you please elaborate on "depends whether the source is happy with such a load"?
Do you mean ... my preamp may not be able to "drive" the F3? And what would happen to the sound if my preamp is not happy with the load? The ultimate question is, how can I know if the preamp is a good match with the F3 or not?
Sure.
iTunes as player USB out to DAC
Maverick TubeMagic DAC (2V output)
PrimaLuna Prologue 3 preamp (2800 Ohm output impedance, 12dB gain)
Currently using McCormack DNA-1 deluxe power amp, too much gain (10k Ohm, 30db gain)
Zu Omen Definition speaker
6moons has quite a detail review on the Prologue 3
6moons audio reviews: PrimaLuna ProLogue 3 & 5
Gain 12dB (4x)
Frequency Response 4Hz - 110kHz +/- 3dB
THD <0,2% at 775mVrms output
S/N Ratio >93dB(A) ref 775mV
Input Impedance 100 kOhm
Input Sensitivity 150mV for full output power when coupled to ProLogue Four or Five
Output Impedance 2800 Ohm
Power Consumption 46W
Tube Complement 2 x 5AR4
2 x 12AX7
2 x 12AU7
Isn't the answer "it doesn't have a lot to do with the
output impedance. It depends on the type of circuit design." ?
Athough 2800 ohms is quite high and that might just as well be a killer? Do we need to go research your gear for you? Do you have manuals? What is the source output circuit? A follower? A transformer?
output impedance. It depends on the type of circuit design." ?
Athough 2800 ohms is quite high and that might just as well be a killer? Do we need to go research your gear for you? Do you have manuals? What is the source output circuit? A follower? A transformer?
Isn't the answer "it doesn't have a lot to do with the
output impedance. It depends on the type of circuit design." ?
Athough 2800 ohms is quite high and that might just as well be a killer? Do we need to go research your gear for you? Do you have manuals? What is the source output circuit? A follower? A transformer?
Did you always know as much as you do now? I am a newb as well and somtimes there are those who perhaps do not have a complete understanding of all that goes on in these wonderful amps. Perhaps you could have suggested trying to find a schematic or maybe you have already know it and are wanting to help and are willing to share ....like Nelson.
Try listening to it 😉
Since you are, as you say, a newb, try acquiring some listening experience. Visit some acoustic music concerts, public lessons in music schools and similar events. Form your impression how real instruments and orchestras sound live. Visit different hi-fi equipment dealers' showrooms and listen to the stuff they sell. All that should help you to evaluate your home system with more certainty...
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