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In Reply to: RE: DHT 01a tube preamp posted by mwhouston on February 14, 2025 at 16:29:01
"To limit output I put a series resistor of 330K to the output RCA with 100K to ground at the RCA"
That is a really high output impedance. I can't see how that wouldn't cause frequency response issues (loss of highs).
What cables and amp are you planning to drive with this preamp?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No high frequency problems just reducing output level. Works well. Drives into a few different DIY tube power which also has a 100K input.
Cables are home made which is twisted wire wrap wire - fine copper and silver plated - with two earths. Only one earth terminated at the destination end. Cables are directional. The other earth scavengers holding noise back at the originating end.
| retro-thermionic |
Have you measured the FR of the preamp (connected to an amplifier) with and without the 330k series resistor?
If everything is fine then I have to say that somehow your circuit defies the laws of physics.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I don't need to connect to an amp just load the output of the preamp into 100K. I will check.
| retro-thermionic |
You do need the cable hooked up and the amplifier hooked up.It is the shunt capacitance (in combination with the output impedance of the preamp) that will cause the highs to be attenuated.
The cable has shunt capacitance and the amplifier has shunt capacitance (that is called Miller capacitance). The capacitance from those two add together.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/15/25
Capacitance of my cables is 175pF (0.175nF). Quite low.
| retro-thermionic |
With an output impedance of 330K playing into 175pf the -3db point of the low pass filter will be 2756Hz. The -1db point will be at 1378Hz.That is a massive lose of high frequencies and it is only going to get worse when you add in the Miller capacitance of the first stage of your power amp.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/15/25
Correction, with the 100k resistor in parallel with the 330k ohm resistor the output impedance is 76744 ohms so the -3db point of the low pass filter is 11850Hz. Better but still a lot of high frequencies lost in the audio band.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Is the 100k resistor to ground considered a factor in calculating the output impedance of the preamp or is it considered a factor in calculating the input impedance of the amp? Or both??I'm not very technically oriented but, as I understand it, the dominant factor when it comes to output impedance is tube's Rp in parallel with the value of the load resistor. A load resistor value that's 3x to 5x that of the Rp is common, sometimes higher.
The Rp of the 01A is ~10k which, in parallel with a 50k load resistor, results in a value of 8.33k. Although other resistances may combine with this the resulting value is even lower as long as those resistances are not lower than the Rp. If the 100k to ground is part of the preamp calculation, it only lowers the output impedance. The result of 10k, 50k and 100k in parallel is 7.69k.
Since tube amps generally have an input impedance of 100k there shouldn't be a problem with a preamp whose output impedance is 10k or less. If used with a SS or Class D amp a lower output impedance would be indicated.
Or is the 100k to ground part of the input impedance of the amp? In parallel with a 100k input impedance of a tube amp, it would lower the input impedance to 50k and may create issues. I'm not sure if they would be audible or not, but I imagine they could be measured.
If it contributes to the amp's input impedance it seems like it would be better to use a 33k with a 10k to ground. This would have the same effect on the output voltage and only lower the input impedance to ~9.1k. But this assumes that a tube amp is being used.
It seems like a 33k / 10k combo would also be better if it affects BOTH the output impedance of the preamp and the input impedance of the amp since this would also lower the output impedance of the preamp further. The result of 10k, 50k and 10k in parallel is 4.55k.
A SS / Class D amp typically has an input impedance of 10k to 20k. In which case the 330k / 100k combo would be best.
That's how I see it but perhaps I'm missing something.
Edits: 02/15/25
"The Rp of the 01A is ~10k which, in parallel with a 50k load resistor, results in a value of 8.33k"I will assume your math of correct. Now add to that the 330k of series resistance he said he added. The output impedance is now 338.33k
The output impedance of the circuit is way to high to drive anything without losing high frequencies do to the cable capacitance plus the Miller capacitance of the first stage of whatever power amp it is connected to.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/15/25
Earlier you questioned the value of the 100k resistor to ground but now you're saying that the 330k that's in series actually creates more of a problem??
Perhaps I'm mistaken but I thought that the resistance in series (330k in this case) only affects the calculation of the -3db rolloff point.
If resistance in series is added then, if the amp has a volume pot on the input, wouldn't that also affect the output impedance of the preamp?
Say you're using a 100k pot. Whatever portion of the 100k that's in series with the grid of the amp's first tube would be added to the output impedance of the preamp??
If that were the case it seems like the use of a voltage divider, or an adjustable voltage divider aka a volume pot, would technically create an impedance mismatch in many cases. Perhaps most cases.
You have to look at the circuit from the Pin 2 output of the 01a to the grid pin or input of the component you are driving.
Usually there is no resistance "in series" with the output, so there are only resistors to ground represent the "load". Typically you will have something like a 1 meg resistor grounding the pre amps output capacitor so that it won't store a nasty voltage if operated without a load. Then there is the input impedance of the thing you are driving. 25K to 250k ohms is typical.
In this case, the impedance contribution of the 1 meg safety resistor is negligible so you are left looking at the load presented by the component you are driving. If you have a 100k component load to drive it will be at the far end of the interconnect. You will be driving that interconnect with the output impedance of the preamp. Lower the better. The very conservative rule of thumb is 1:10, 10K output impedance to drive 100K input impedance. Adding the 330K series resistor adds to the output impedance of the preamp creates a big impedance miss match. You may actually like this tone control, though. If your hearing does not go much above 10khz it might actually sound more pleasing.
People here are pretty knowledgeable, more than myself, and I have learned a lot by listening to them. No one here wants to steer you in the wrong direction or be unhelpful.
"Earlier you questioned the value of the 100k resistor to ground"
No, I never mentioned the 100k resistor. My concern is the 330k series resistance added to the output impedance of the stage.
"...I thought that the resistance in series (330k in this case) only affects the calculation of the -3db rolloff point"
The -3db rolloff point is, more of less, the only concern when it comes to the output impedance of a circuit (a preamp in this case).
"Whatever portion of the 100k that's in series with the grid of the amp's first tube would be added to the output impedance of the preamp??"
If the pot is mounted in the power amplifier, then the output impedance of the preamp is driving the cable capacitance.
The pot's output impedance is the output impedance of the preamp plus the series resistance of the pot with that value in parallel with the portion of the pot that shunts to ground from the wiper. That is the impedance value that is driving the Miller capacitance of the first stage of the power amp.
I was under the impression that mwhouston has placed the 330k and the 100k resistors in the preamp.
"If that were the case it seems like the use of a voltage divider, or an adjustable voltage divider aka a volume pot, would technically create an impedance mismatch in many cases. Perhaps most cases."
I tend to agree.
The stage needs to be able to properly drive the shunt capacitance. When there is no pot at the amplifier, the shunt capacitance seen by the preamp's final stage is the cable capacitance and Miller capacitance added together. Miller capacitance is the capacitance between the grid and the plate of the input tube of the power amp multiplied by the gain of that stage and then the capacitance between the grid and the cathode is added. The -3db point of the low pass filter created by the output impedance of the driving stage and the total shunt capacitance is easy to calculate. If we want 20kHZ to be unaffected by that filter, then the -3db point of that filter needs to be at least 200kHz.
A capacitor is a reactive device, It's reactance (impedance) goes down as the frequencies go up. At the highest of frequencies the impedance of the capacitance reactance will be very low. The device driving that capacitance (the preamp's final stage) has to be able to deliver the current that the reactance of the shunt capacitance will require otherwise the preamp stage will distort.
So a stage has to have a low enough output impedance so that the low pass filter created between it and the value of the shunt capacitance is well out of the audio band and that stage has to be able to deliver the current needed to drive the shunt capacitance's low impedance at high frequencies.
A 330k ohm resistor in series with the output impedance of the preamp will make these requirements hard (if not impossible) to meet.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Hopefully I'm not hijacking the thread, I'm just trying to wrap my head around the interactions and the math.
So, if a volume pot or a fixed voltage divider is used to reduce the output signal (and associated noise in this case) of the preamp then putting the pot after the interconnect (on the amp) would not increase the output impedance of the preamp. Correct?
If the preamp is driving a SS or Class D amp does this affect the placement of the volume pot? I assume that there is no Miller capacitance or the equivalent involved in that case. Or is there?
Would the resistance in series have the same effect on the output impedance if there was no Miller involved, only the cable capacitance?
"So, if a volume pot or a fixed voltage divider is used to reduce the output signal (and associated noise in this case) of the preamp then putting the pot after the interconnect (on the amp) would not increase the output impedance of the preamp. Correct?"That is correct.
"If the preamp is driving a SS or Class D amp does this affect the placement of the volume pot?"
No.
"I assume that there is no Miller capacitance or the equivalent involved in that case. Or is there? "
A transistor has a input capacitance they just don't call it the Miller capacitance but it is very much the same thing.
"Would the resistance in series have the same effect on the output impedance if there was no Miller involved, only the cable capacitance?"
Yes, the output impedance value has nothing to do with the total shunt capacitance value but if there were no Miller capacitance then the total shunt capacitance would be less so the -3db point of the low pass filter would be higher (more out of the way) but that filter would still exist and the output impedance of the stage is still what it is.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 02/15/25
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