What is the lowest frequency you want to be flat? IMHO 26R should have at least ~1000uF. This gives a -3db of 6 Hz.
If you want real 20-20000 flat, you want 3300uF
RC Low-pass Filter Design Tool
If you want real 20-20000 flat, you want 3300uF
RC Low-pass Filter Design Tool
Well, assuming you didn't read the above, the calculations are
In my experience, output-transformer-less OTL works far better with 100 to 600 Ω headphones than any of the low-value ones you've listed.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
... AND... what KodaBMX says is 100% correct, too. He's dropped the dB loss to -1 dB at 20 Hz by substituting 1,000 µF for the 300 µF that direct calculation provides. And his 3,3000 µF calculation is similarly, 10x bigger value for 1/10 (or -0.3 B) for excellent fit.
C ≈ 1,000,000 ÷ 2πFR
For the –3 dB point. Ideally, you might want the response to be somewhat better, so the capacitor chosen should be somewhat larger … 26 Ω → 300 µF, minimum
50 Ω → 150 µF, minimum
72 Ω → 110 µF, minimum
The real problem at some level isn't the capacitors (though large values can be eye-watering expensive), but rather whether the amplifier can provide the current, especially at higher volume levels. 50 Ω → 150 µF, minimum
72 Ω → 110 µF, minimum
In my experience, output-transformer-less OTL works far better with 100 to 600 Ω headphones than any of the low-value ones you've listed.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
... AND... what KodaBMX says is 100% correct, too. He's dropped the dB loss to -1 dB at 20 Hz by substituting 1,000 µF for the 300 µF that direct calculation provides. And his 3,3000 µF calculation is similarly, 10x bigger value for 1/10 (or -0.3 B) for excellent fit.
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But also... it is important to analyze the entire OTL headphone amplifier, for the same capacitor-versus-low-frequency response.
If, for instance, one stage somewhere is exerting a -3 dB point at 35 Hz, no amount of humungous value final capacitors is going to restore what has been lost.
Sigh...
As is always the case.
GoatGuy
If, for instance, one stage somewhere is exerting a -3 dB point at 35 Hz, no amount of humungous value final capacitors is going to restore what has been lost.
Sigh...
As is always the case.
GoatGuy
Of course an amplifier can not amplify absent signal 🙂
I try to design from a 0db point... It's stupid most of the time but -1 is better than -3, 10x is obtainable (my current HP amp uses 470uF for a 470R load - you can work out the nitty gritty but those who can't or don't want to it's a -3 of 0.7Hz. ) Headphones are ATH-R70x - Professional Open-Back Reference Headphones | Audio-Technica EDIT: Highly recommended for those who need new cans!
The stage will also deliver over 20mA to the load... 🙂
FWIW I use 10,000uF to couple the speakers in my only SS design 🙂 It's single ended BTW
I try to design from a 0db point... It's stupid most of the time but -1 is better than -3, 10x is obtainable (my current HP amp uses 470uF for a 470R load - you can work out the nitty gritty but those who can't or don't want to it's a -3 of 0.7Hz. ) Headphones are ATH-R70x - Professional Open-Back Reference Headphones | Audio-Technica EDIT: Highly recommended for those who need new cans!
The stage will also deliver over 20mA to the load... 🙂
FWIW I use 10,000uF to couple the speakers in my only SS design 🙂 It's single ended BTW
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I do not use headphones anymore.
But all the ones I used, if they had 20Hz applied, would do the following:
Not produce 20Hz sound at any reasonable level.
But when 20Hz was applied, they would:
Produce 40Hz sound; 2nd harmonic of 20Hz.
Produce 60Hz sound; 3rd harmonic of 20Hz.
In those cases, there was more sound energy of the harmonics than the sound energy of the fundamental.
A bass electric guitar can produce 41Hz (open string).
A pipe organ that plays Pedal C produces 32Hz.
There are only a few acoustic instruments that play lower than 41Hz, and most of them are seldom recorded doing that.
If you like electronic music, the low frequency is almost unlimited (only by the recording equipment; playback equipment; the headphones; and your ears which only perceive extremely low frequencies as vibration, and do not perceive them as a musical tone.
But all the ones I used, if they had 20Hz applied, would do the following:
Not produce 20Hz sound at any reasonable level.
But when 20Hz was applied, they would:
Produce 40Hz sound; 2nd harmonic of 20Hz.
Produce 60Hz sound; 3rd harmonic of 20Hz.
In those cases, there was more sound energy of the harmonics than the sound energy of the fundamental.
A bass electric guitar can produce 41Hz (open string).
A pipe organ that plays Pedal C produces 32Hz.
There are only a few acoustic instruments that play lower than 41Hz, and most of them are seldom recorded doing that.
If you like electronic music, the low frequency is almost unlimited (only by the recording equipment; playback equipment; the headphones; and your ears which only perceive extremely low frequencies as vibration, and do not perceive them as a musical tone.
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Thanks for all your feedback to my question. It explains a lot. So I am using a 470uf capacitor with my 72 ohm headphones and they sound great. It seems like I should probably experiment with other values to see how the sound changes.
kodabmx,
Thanks!
Good to know about the 5 string Bass' lowest note.
An old Electro Voice 30" Woofer that had an f0 of 16 Hz, and a magnet weight of 9 lb. (yes, the magnet; the rest of the magnet structure added even more weight), and a cast frame, might qualify for that setup.
If I remember properly, an 88 note piano goes down to 28Hz.
But live, you hear more harmonic energy than fundamental energy, partly because most pianos are not that efficient at sound reproduction there, and partly because of the Fletcher Munson curves of the ear.
Your brain is smart, the ear hears the harmonics and the spacing of them, and the brain extrapolates the fundamental for you. That is the first sound processor.
Thanks!
Good to know about the 5 string Bass' lowest note.
An old Electro Voice 30" Woofer that had an f0 of 16 Hz, and a magnet weight of 9 lb. (yes, the magnet; the rest of the magnet structure added even more weight), and a cast frame, might qualify for that setup.
If I remember properly, an 88 note piano goes down to 28Hz.
But live, you hear more harmonic energy than fundamental energy, partly because most pianos are not that efficient at sound reproduction there, and partly because of the Fletcher Munson curves of the ear.
Your brain is smart, the ear hears the harmonics and the spacing of them, and the brain extrapolates the fundamental for you. That is the first sound processor.
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I've just been modelling the frequency response. The Broskie original design for my P-SE OTL headphone amp uses a 330uF with a 10uF in parallel. I can see the rolloff starts at about 100Hz and is down quite a way from memory for 20Hz.
Given his approach uses an elevated cathode (rather than sitting at zero volts), it offers an initial opportunity to use an electrolytic 450V 330uF (or 470uF 450V for £5). I'll model the response with 470uF too.
100uF 500V polypropylene film caps seem to be about £20 each, so an option is to stick with the 330 EL but then add a third 100uF PP along side the 10uF PP, only issue I can see is that the internal resistances presented may change.
At a later date I may simply parallel 100uF PP caps to replace the electrolytic.
Current-wise I could retune it. Idling 3x25mA with 3x60mA max, but I could add the fourth triode in idling 100mA and 240mA peak current. In theory... However I've set this up far less than that and have current limiter on the B+ supply, principally to cope with an output cap short.
However I'm learning a lot from this thread, so thank you.
Given his approach uses an elevated cathode (rather than sitting at zero volts), it offers an initial opportunity to use an electrolytic 450V 330uF (or 470uF 450V for £5). I'll model the response with 470uF too.
100uF 500V polypropylene film caps seem to be about £20 each, so an option is to stick with the 330 EL but then add a third 100uF PP along side the 10uF PP, only issue I can see is that the internal resistances presented may change.
At a later date I may simply parallel 100uF PP caps to replace the electrolytic.
Current-wise I could retune it. Idling 3x25mA with 3x60mA max, but I could add the fourth triode in idling 100mA and 240mA peak current. In theory... However I've set this up far less than that and have current limiter on the B+ supply, principally to cope with an output cap short.
However I'm learning a lot from this thread, so thank you.
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So I’m at the decision stage.
The cathode rail for output is quite low ~12-30V, B+ is ~150V possibly to 180V. Idle of 100mA with 240mA capable. Broskie’s design has a 330uF bypassed with a 10uF.
I have a few options and I could do with picking some brains.
* 330uF 100V bi-polar electrolytic (there’s also ~70V mundorf and nichicon 50V)
* 330uF 450V electrolytic nichicon
* 330uF 450V polypropylene
Then bypass with. 10uF polypropylene such as Auri XO.
I was thinking of starting with bipolar electrolytic bypassed. However at the back of my head the idea of PP from the start is gnawing away - is there that much difference?
The cathode rail for output is quite low ~12-30V, B+ is ~150V possibly to 180V. Idle of 100mA with 240mA capable. Broskie’s design has a 330uF bypassed with a 10uF.
I have a few options and I could do with picking some brains.
* 330uF 100V bi-polar electrolytic (there’s also ~70V mundorf and nichicon 50V)
* 330uF 450V electrolytic nichicon
* 330uF 450V polypropylene
Then bypass with. 10uF polypropylene such as Auri XO.
I was thinking of starting with bipolar electrolytic bypassed. However at the back of my head the idea of PP from the start is gnawing away - is there that much difference?
32ohm target for future proofing but the headphones are 55ohm.
Those are max mA, so probably running ~60mA normally.
There can be significant differences, try both types if you already have them. Just leave the leads
long enough to be able to reuse them until you decide. Forego the bypassing at first.
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B.T.W. : Did anybody notice what Frank Berry wrote in his response #9 :
"Whatever you use, make sure that you add a resistor (10k, perhaps) across the output to prevent blowing out your headphones
if you plug them in after powering up the amplifier."
"Whatever you use, make sure that you add a resistor (10k, perhaps) across the output to prevent blowing out your headphones
if you plug them in after powering up the amplifier."
Then I'd be looking into 2200uF or so. I like my amplifiers flat from 20Hz which implies -3db @ 2Hz. A 2200uF cap will give you a -3db @ 2.26Hz on 32R headphones.
FWIW: This is the cap I picked to try in one of my headphone amplifiers. The specs are good enough, and the price is right. I bypass with a small CBB or MKP (or MKC but getting low on those!) anyway. SamYoung Electronics|SamYoung Electronics TLS 250V2200 35*50|Aluminum Electrolytic Capacitors - Leaded|LCSC
Your 330uF cap will be flat from 150Hz, -3db @ 15Hz on 32R headphones. The bass will suck IMHO.
Also, since you're using low Z phones and a large cap, I'd use a 1k terminating resistor (1k in parallel with the output) per channel to let the caps charge/discharge faster. For High Z headphones this won't matter as you have the current to waste.
Koda
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Broskie’s original design has an 11K after but modelling I did see a large spike on connect/disconnect at startup. I’ve altered the design to put a 32ohm resistor for the mute circuit but I will look at a smaller value than the 11K as long as there’s no impact to frequency response.
I noted in a later blog post he said he used a 220uF for each cathode rather than one 330uF but to me that adds a yet another phase shift on the feedback loop. Let’s get the thing done first then I’ll worry about tweaking!
Will have a look at a larger output cap and do a quick model. The freq response was reasonable but I did note a early roll off.
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This is a new request for help. I am hoping someone can help me solve an issue with hum in my headphone amp.
First, the amp is a 2 tube headphone amp using a 6AS7 and a 6922. This is the link to the circuit:
A Single-Ended OTL Amplifier for Dynamic Headphones. – HeadWize Memorial
These are the details of the hum:
1) the hum is fixed and always there, even with the volume set to 0
2) the hum doesn't change as I change the volume
3) I have narrowed the issue to the filament power supply
4) I am using a DC filtered power supply for the filaments similar to the one in the writeup
5) the hum is very loud
6) if I remove the DC power supply and drive the filaments with 4D batteries, the hum is completely gone.
I hope someone can help me - I have tried everything I can think of and can't solve this issue
First, the amp is a 2 tube headphone amp using a 6AS7 and a 6922. This is the link to the circuit:
A Single-Ended OTL Amplifier for Dynamic Headphones. – HeadWize Memorial
These are the details of the hum:
1) the hum is fixed and always there, even with the volume set to 0
2) the hum doesn't change as I change the volume
3) I have narrowed the issue to the filament power supply
4) I am using a DC filtered power supply for the filaments similar to the one in the writeup
5) the hum is very loud
6) if I remove the DC power supply and drive the filaments with 4D batteries, the hum is completely gone.
I hope someone can help me - I have tried everything I can think of and can't solve this issue
DC filtered similar to but not the same. We'll need the exact schematic...
When I did a similar amp, I just used filtered DC, no regulator and I had no hum but the power supply was in another chassis. It could just be magnetically induced by the transformer. Have you tried moving the heater transformer away from the rest of the circuit to see if that helped? Assuming you are using two transformers like in the linked schematics, are they both in the same phase? I have fixed hum by flipping the plug around so to speak.
When I did a similar amp, I just used filtered DC, no regulator and I had no hum but the power supply was in another chassis. It could just be magnetically induced by the transformer. Have you tried moving the heater transformer away from the rest of the circuit to see if that helped? Assuming you are using two transformers like in the linked schematics, are they both in the same phase? I have fixed hum by flipping the plug around so to speak.