I am curious if anything needs to change in this amp to happily drive a 32ohm load. I emailed Nelson about this a few weeks ago and didn't get a response. I wasn't aware of the impending birthday!
My partner wants to drive his Meze Empyrean 2 with a current source, so we picked the F1. We figure that since they are a near perfect resistive load, they should be happy being driven by a current source. We are completely aware that it's a speaker amp and extremely powerful for headphones. If there are replacement output devices that perform much better than the originals, I'd be interested.
I'm most of the way done producing a board for it, completely different from Nelson's design and more like the diyaudiostore boards. I plan to use one of the Universal Mounting Spec revisions. I will share these when I'm done if it's ok with Nelson. I may also make a teeny weeny desktop version that uses Noctua fans.
Happy Birthday Nelson! Thank you for sharing your knowledge and designs with the community.
TL;DR
1. F1 or F1J for use driving 32 ohm planar headphones. Anything need to change for good performance?
2. Making boards for IRF240/9240PBF new parts and DIYAS UMS, will share when finished if ok with Nelson.
3. Substitute devices for better performance?
My partner wants to drive his Meze Empyrean 2 with a current source, so we picked the F1. We figure that since they are a near perfect resistive load, they should be happy being driven by a current source. We are completely aware that it's a speaker amp and extremely powerful for headphones. If there are replacement output devices that perform much better than the originals, I'd be interested.
I'm most of the way done producing a board for it, completely different from Nelson's design and more like the diyaudiostore boards. I plan to use one of the Universal Mounting Spec revisions. I will share these when I'm done if it's ok with Nelson. I may also make a teeny weeny desktop version that uses Noctua fans.
Happy Birthday Nelson! Thank you for sharing your knowledge and designs with the community.
TL;DR
1. F1 or F1J for use driving 32 ohm planar headphones. Anything need to change for good performance?
2. Making boards for IRF240/9240PBF new parts and DIYAS UMS, will share when finished if ok with Nelson.
3. Substitute devices for better performance?
For a headamp use I would try and change the output devices (Q1 & Q2) to something like IRF510, 610, 520.
Less Ciss, still high enough Gm, and importantly low Qg (gate charge).
IRFP240 needs to be run at high enough current (0.7 - 1A) to reach 100kHz and above.
With 510, 610, 520 you'll need 150 - 350mA to drive your cans.
For that you'll need to recalculate the values of a few resistors, which is not difficult.
My 2c.
Less Ciss, still high enough Gm, and importantly low Qg (gate charge).
IRFP240 needs to be run at high enough current (0.7 - 1A) to reach 100kHz and above.
With 510, 610, 520 you'll need 150 - 350mA to drive your cans.
For that you'll need to recalculate the values of a few resistors, which is not difficult.
My 2c.
I have used a stock version of the F1 to drive Dan Clark Aeon open headphones ( 13 Ohm ). No complaints. It works great. Go for it!
How long did you use the Dan Clark Aeon cans with the F1?
A week, a month, a year?
Just curious.
Delivering 2-3 watts to a tiny diaphragm, and 13 ohm is very close to speaker impedances, is going to do a lot of physical things to that tiny diaphragm.
F1 delivers 4A all the time, at 24V, at 8 ohms.
You do the calculations.
A week, a month, a year?
Just curious.
Delivering 2-3 watts to a tiny diaphragm, and 13 ohm is very close to speaker impedances, is going to do a lot of physical things to that tiny diaphragm.
F1 delivers 4A all the time, at 24V, at 8 ohms.
You do the calculations.
I'm not sure that how a current source amplifier works, nevertheless the F1.
Directly from the F1 Manual.
"A given input voltage results in a particular output current."
This is separate from the impedance of the load.
I could be wrong, per usual, but that has always been my understanding of the key characteristic of the F1 and those in that particularly rare category.
Edited to add - Apologies. I think I understand your point now, but if it's 24V output you mean, who in their right mind would try to put 24V into those HPs? Maybe bring it down to a realistic not HP exploding / ear damaging voltage. 🙂
Last edited:
This is directly from F1 service manual, page 7:
"The voltage across R27 – R30 is approximately 0.60 volts DC, and this
figure sets the bias for the gain stages, which is slightly less than 2 amps
per side, 4 amps total."
If you follow the schematic and do the calculations you'll find that this figure is just right.
"The voltage across R27 – R30 is approximately 0.60 volts DC, and this
figure sets the bias for the gain stages, which is slightly less than 2 amps
per side, 4 amps total."
If you follow the schematic and do the calculations you'll find that this figure is just right.
^ Correct, I didn't want to completely remove my error. I own misreading your post / intent. Thus, I edited the post to only include the sentiment re: who would run HPs (even that particular model) at 24V output.
Less than one volt would be MUCH more common and would make the amp quite usable (if proper attenuation was in place, IMO).
Less than one volt would be MUCH more common and would make the amp quite usable (if proper attenuation was in place, IMO).
On a slightly different tangent, I use a ZenLite headamp, which is sort of half of F1, being single ended, which is biased at 0.7A and this is definitely too much for all my cans.
One way to reduce the danger to Low Z cans is to use a circuit like Rob Elliots's Project 100:
https://sound-au.com/project100.htm
The other way is to reduce the output power of the amp, something that I suggested initially.
One way to reduce the danger to Low Z cans is to use a circuit like Rob Elliots's Project 100:
https://sound-au.com/project100.htm
The other way is to reduce the output power of the amp, something that I suggested initially.
^ Agreed. 200W of dissipation, all that unused available power, along with additional space, and weight seems a bit overkill. However, if someone were wanting to use it for both HPs and loudspeakers, I could get my arms around it. I personally would not build one "stock" solely for HP use, but for the OP, it could be a fun experiment. An "F1 mini" could be pretty nifty.
tl;dr - would it "work"? Yes. Would it be ideal for me, no. But, we live in a fun playground. I look forward to seeing how it works for the OP.
tl;dr - would it "work"? Yes. Would it be ideal for me, no. But, we live in a fun playground. I look forward to seeing how it works for the OP.
I was considering just doing the original F1/J circuit since I don’t want to mess with things I don’t fully understand and ruin the circuit or make it less of a current source. That and it could also be used for speakers.
However, Stanislav’s suggestion of parts made it sound like I can get the same operation with better performance in a smaller package for lower output power. So I am now considering going that route for my partner’s headphone amp, while finishing the regular f1 board for my own/shared use.
I don’t think there’s really a point to an F1 mini, it will instead be an F1 HPA, which will naturally be smaller. Fans are not ideal, and it would be a lot more effort just for a small package.
Output current should be proportional to the input signal, proportional to volume level. And the voltage would be tied to the load and amount of current. So definitely not 24V and 2A all the time, that would only be at max volume I am almost certain. And that would probably be instant hearing damage.
I’m not totally sure how to change out the resistors, I’ve never designed a circuit. My best guess is to find the voltages at different parts of the circuit to determine the operating point of the devices and use that as a loose guide for whatever new devices I choose. As I understand, higher gm means more gain (for this type of amp), while Ciss and qg are ideally minimized to increase bandwidth and lower demands on the preamp. I also imagine I need low power equivalents for the irfp9240, but maybe the ztx550 (using 553) is fine?
IRF610 has gm of .8, I worry that means sub-unity gain, but surely I’m wrong. Not certain how to move forward but I’ll see what I can do.
On a somewhat relevant note, the Moondrop Para is a newer planar with an impedance of 8 ohms, 101dB/Vrms sensitivity. Thought I saw somewhere this was 80dB/mW efficiency. I saw it on sale for $280. This would be super interesting for the F1 and flea watt tube amps!
However, Stanislav’s suggestion of parts made it sound like I can get the same operation with better performance in a smaller package for lower output power. So I am now considering going that route for my partner’s headphone amp, while finishing the regular f1 board for my own/shared use.
I don’t think there’s really a point to an F1 mini, it will instead be an F1 HPA, which will naturally be smaller. Fans are not ideal, and it would be a lot more effort just for a small package.
Output current should be proportional to the input signal, proportional to volume level. And the voltage would be tied to the load and amount of current. So definitely not 24V and 2A all the time, that would only be at max volume I am almost certain. And that would probably be instant hearing damage.
I’m not totally sure how to change out the resistors, I’ve never designed a circuit. My best guess is to find the voltages at different parts of the circuit to determine the operating point of the devices and use that as a loose guide for whatever new devices I choose. As I understand, higher gm means more gain (for this type of amp), while Ciss and qg are ideally minimized to increase bandwidth and lower demands on the preamp. I also imagine I need low power equivalents for the irfp9240, but maybe the ztx550 (using 553) is fine?
IRF610 has gm of .8, I worry that means sub-unity gain, but surely I’m wrong. Not certain how to move forward but I’ll see what I can do.
On a somewhat relevant note, the Moondrop Para is a newer planar with an impedance of 8 ohms, 101dB/Vrms sensitivity. Thought I saw somewhere this was 80dB/mW efficiency. I saw it on sale for $280. This would be super interesting for the F1 and flea watt tube amps!
No need to change the IRFP9240 and the bottom IRFP240. They are current sources. ZTX550 are fine where they are.
Recalculate the resistors around the current sources for your desired current and you'll be more or less ready.
My personal choice for Q1 & 2 will be IRF520 or IRFZ24 - 34 but mostly because I have plenty of them.
Don't forget proper heatsinking. When you think it's enough double it and you'll be set.
Good luck building it and let us know how you go with all that.
Recalculate the resistors around the current sources for your desired current and you'll be more or less ready.
My personal choice for Q1 & 2 will be IRF520 or IRFZ24 - 34 but mostly because I have plenty of them.
Don't forget proper heatsinking. When you think it's enough double it and you'll be set.
Good luck building it and let us know how you go with all that.
Attachments
Would the current sources respond faster/do their job better with lower parasitics and gate charge like q1-2?
I get the bottom irf240 is for bias. I would think I could just make that an irf520 if I use those for q1-2?
Also realized many of the important voltages are detailed in the service manual, as stated earlier.
I get the bottom irf240 is for bias. I would think I could just make that an irf520 if I use those for q1-2?
Also realized many of the important voltages are detailed in the service manual, as stated earlier.
It won't make any difference. The current sources, Q3, 4 & 7 and their surroundings, determine the DC conditions for the amp.
If you want to change them to TO220 cases maybe get some irf540 or 640, cheap, easy to find and reliable. Otherwise stay with irfp240 and be happy.
Keep those 520 for an ACA Mini or a preamp or another headamp.
If you want to learn more about the operation of the circuit check Son of Zen's articles Mr. Pass published long time ago.
Below is a copy of the latest one.
If you want to change them to TO220 cases maybe get some irf540 or 640, cheap, easy to find and reliable. Otherwise stay with irfp240 and be happy.
Keep those 520 for an ACA Mini or a preamp or another headamp.
If you want to learn more about the operation of the circuit check Son of Zen's articles Mr. Pass published long time ago.
Below is a copy of the latest one.