I want to build a headphone amp for 600 Ohm headphones using 6L6 tubes in triode mode, but must first address two areas of ignorance:
1. I do not understand how the output load influences distortion in a triode-strapped pentode
2. The 600 Ohm headphones require an OPT with very low ratio, perhaps a line-level transformer, which I know nothing about.
First,
The data sheets for the 2A3 and 6L6 which follow show that the 3rd harmonic (which I want to minimize) increases with increasing load on the pentode, and is reduced by increasing load for the triode:
https://frank.pocnet.net/sheets/127/2/2A3.pdf
https://frank.pocnet.net/sheets/127/6/6L6G.pdf
I typically run my pentodes at lower load than recommended in the data sheet to reduce the 3rd harmonic and increase the 2nd. However if I did that with a 2A3 it would increase the 3rd harmonic. So when a pentode is triode strapped does the shape of the 3rd harmonic curve revert to normal triode shape, or does the shape of the curve still look like those of pentodes?
And the 6L6 data sheet specifies an output load of 5k at 250V in triode mode, and 2.5k at 250V in pentode mode. Is this doubling of desired load typical when triode strapping pentodes?
Second, when searching for a line-level transformer I end up with something like the following:
EDCOR - WSM Series
Is this suitable? I have not used line-level transformers and do not seem to understand them:
I am perplexed by the 10V max. rating in combination with the .5 Watt rating for the 600:600 transformer as it will take 17V to push .5 W through 600 Ohms. I am also perplexed by the offering of multiple 1:1 ratio units. What is the difference between 150:150 and 600:600? I am accustomed to focusing on the ratio and ignoring the resistance ratings. There is an alternative transformer which is rated for 50V, but Wattage is not listed:
EDCOR - XSM Series
For more specifics of my target:
SE architecture to emphasize even harmonics
¼ watt per channel into 600 Ohm load
Triode strapped 829B tube (I am treating it as a pair of 6L6s)(other recommendations?) If running a single 6L6 as a pentode at 240V I would normally spec. a 2000 Ohm output transformer (a little low to emphasize even harmonics), and running a pair in parallel cuts the load to 1000 Ohms. Reducing B+ to 105V should get me close to 600 Ohms (extrapolating from the data sheet, anyone have a formula?). So this reduces the desired load to match that of the headphones (if the load does not need to be doubled for triode mode, and if a low load still reduces the 3rd harmonic in triode mode).
PS with 600 Ohm load with 600 Ohm headphones it seems wasteful to use a transformer when all I need to do is block direct current. Is there a way to apply the resistor-capacitor coupling normally used for pre-amps? I know the efficiency will be bad, but for a headphone (1/4 watt?) that can be tolerated.
1. I do not understand how the output load influences distortion in a triode-strapped pentode
2. The 600 Ohm headphones require an OPT with very low ratio, perhaps a line-level transformer, which I know nothing about.
First,
The data sheets for the 2A3 and 6L6 which follow show that the 3rd harmonic (which I want to minimize) increases with increasing load on the pentode, and is reduced by increasing load for the triode:
https://frank.pocnet.net/sheets/127/2/2A3.pdf
https://frank.pocnet.net/sheets/127/6/6L6G.pdf
I typically run my pentodes at lower load than recommended in the data sheet to reduce the 3rd harmonic and increase the 2nd. However if I did that with a 2A3 it would increase the 3rd harmonic. So when a pentode is triode strapped does the shape of the 3rd harmonic curve revert to normal triode shape, or does the shape of the curve still look like those of pentodes?
And the 6L6 data sheet specifies an output load of 5k at 250V in triode mode, and 2.5k at 250V in pentode mode. Is this doubling of desired load typical when triode strapping pentodes?
Second, when searching for a line-level transformer I end up with something like the following:
EDCOR - WSM Series
Is this suitable? I have not used line-level transformers and do not seem to understand them:
I am perplexed by the 10V max. rating in combination with the .5 Watt rating for the 600:600 transformer as it will take 17V to push .5 W through 600 Ohms. I am also perplexed by the offering of multiple 1:1 ratio units. What is the difference between 150:150 and 600:600? I am accustomed to focusing on the ratio and ignoring the resistance ratings. There is an alternative transformer which is rated for 50V, but Wattage is not listed:
EDCOR - XSM Series
For more specifics of my target:
SE architecture to emphasize even harmonics
¼ watt per channel into 600 Ohm load
Triode strapped 829B tube (I am treating it as a pair of 6L6s)(other recommendations?) If running a single 6L6 as a pentode at 240V I would normally spec. a 2000 Ohm output transformer (a little low to emphasize even harmonics), and running a pair in parallel cuts the load to 1000 Ohms. Reducing B+ to 105V should get me close to 600 Ohms (extrapolating from the data sheet, anyone have a formula?). So this reduces the desired load to match that of the headphones (if the load does not need to be doubled for triode mode, and if a low load still reduces the 3rd harmonic in triode mode).
PS with 600 Ohm load with 600 Ohm headphones it seems wasteful to use a transformer when all I need to do is block direct current. Is there a way to apply the resistor-capacitor coupling normally used for pre-amps? I know the efficiency will be bad, but for a headphone (1/4 watt?) that can be tolerated.
Edcor offers SE transformers with 50 Ohm secondary, it would be fine. Also, NP Acoustics winds for me custom transformers.
Edit: I don't see anymore an option with 50 Ohm in the drop-down menu, but you can ask them.
EDCOR - GXSE10-5K
Edit: I don't see anymore an option with 50 Ohm in the drop-down menu, but you can ask them.
EDCOR - GXSE10-5K
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You don't need such expensive and big valves to get a good sound on valves.It is that you wanted specifically output transformer that i can recommend one of my trials with a home made transformer, otherwise, 600ohms are better driven through a capacitor:
My transformer was made by a local artizan and it was intended to drive anything in bwtween 32...600 oms(separate taps) , it failed a bit on the highs because it had just 1 primary and 1 secondary winding , otherwise the sound was really good.6n1p valves are very cheap and cheerful.My preamp is not really a preamp as it has no gain and it was purposely made just to enhance the harmonic content.If you have high output from your preferred player or dac, you don't need additional gain, just a harmonic generator.
Teardrop headphones amplifier
My transformer was made by a local artizan and it was intended to drive anything in bwtween 32...600 oms(separate taps) , it failed a bit on the highs because it had just 1 primary and 1 secondary winding , otherwise the sound was really good.6n1p valves are very cheap and cheerful.My preamp is not really a preamp as it has no gain and it was purposely made just to enhance the harmonic content.If you have high output from your preferred player or dac, you don't need additional gain, just a harmonic generator.
Teardrop headphones amplifier
These are nice transformers and quite loud enough for headphones, if your ears have not already been damaged. They work well as a class A single ended using half an ECC83/12AX7 valve.
Z1604 | Through Hole Audio Transformer 600Ω 2mW | RS Components
Z1604 | Through Hole Audio Transformer 600Ω 2mW | RS Components
dreamth - I had considered driving the 600 Ohm headphones through a cap., but all of my templates for cap. coupled output stages are cathode-followers or totem-poles. I do not have a template to plagiarize. Does anyone have a template?
JMFahey - I expected my B+ of 105V (clipped by an 0C3) to drag output down pretty far, but could switch to the 0B3 or 0A3 to bring B+ down as far as 75V.
dreamth - while I do need a bit of gain to drive a 600 ohm load from an iPhone, this is intended to be a harmonic generator (thanks for the term!). I had considered some smaller triodes, but I need .02A to push 1/4W through 600 Ohms, and the cathode limit of the 12SN7 tubes in my inventory are limited to a cathode current of .02A (same as your 6N1P), which seemed a little marginal.
Is 1/4 Watt more than I need?
JMFahey - I expected my B+ of 105V (clipped by an 0C3) to drag output down pretty far, but could switch to the 0B3 or 0A3 to bring B+ down as far as 75V.
dreamth - while I do need a bit of gain to drive a 600 ohm load from an iPhone, this is intended to be a harmonic generator (thanks for the term!). I had considered some smaller triodes, but I need .02A to push 1/4W through 600 Ohms, and the cathode limit of the 12SN7 tubes in my inventory are limited to a cathode current of .02A (same as your 6N1P), which seemed a little marginal.
Is 1/4 Watt more than I need?
I have no idea what headphones you have and their sensitivity. My 250 ohms cans are about 60mW at max power, they are quite hungry cans compared to others so i figure that yours should take even less current. I ran my 6n1p at 5.5ma idle current and i was very pleased with it.You just need the right tap in the secondary winding to get your output right.You get the needed harmonic content from the second triode idle current which is set by the BSP transistor ccs underneath it or from the transformer gap size.
My DT 990 headphones are rated at 96 db SPL. I guess 1/4 watt is excessive, but the pursuit of gratuitous power is one of my consistent failings (below is my 871 blower perched on one of my Flatheads (waiting on the manifold), having decided that the 671 was too small !)
A henscratched image of my conundrum follows.
The curve of 3rd harmonic vs load for a triode is almost the mirror image of 3rd harmonic for a pentode. So which curve does a triode-strapped pentode follow?
And in the lower graph it seems that the curves for harmonics shift to the right as B+ is increased but I have not seen this quantified, does anyone have a reference?
A henscratched image of my conundrum follows.
The curve of 3rd harmonic vs load for a triode is almost the mirror image of 3rd harmonic for a pentode. So which curve does a triode-strapped pentode follow?
And in the lower graph it seems that the curves for harmonics shift to the right as B+ is increased but I have not seen this quantified, does anyone have a reference?
Attachments
My experience on headphone amplifiers is that a cathode follower sounds better than any design with output transformers. My corrent headphone amplifier is described at post #45 and #46 of this thread: The 6N13S "Artosalo" OTL headphone amplifier The forum member Indaco also built this amplifier with good results. To get some extra harmonics, mostly to make badly recorded music more pleasant, I only need to swap the ECC88 tube with a ECC189 tube (variable transconduttance equivalent to ECC88).
As alternative for your DT990, should you want to stick with the 6L6 idea, just add a dummy load (20W 8 ohm resistor) to the 8 ohm ouput of any 6L6 standard amplifier. Put your 600 ohm headphone in parallel to this load, it will get about 100 mW RMS when the amplifier is driven near to the max. You may need further attenuation to have a good result.
As alternative for your DT990, should you want to stick with the 6L6 idea, just add a dummy load (20W 8 ohm resistor) to the 8 ohm ouput of any 6L6 standard amplifier. Put your 600 ohm headphone in parallel to this load, it will get about 100 mW RMS when the amplifier is driven near to the max. You may need further attenuation to have a good result.
This. I did this myself with a 6l6 amp my friend built for me a long time ago and it worked out great.
Excellent.
Here is my amp prototype for planar magnetic headphones using NP Acoustic transformers:
Wavebourn - Edelweiss-3, high end headphone version, first test | Facebook
However, if to plug in ordinary dynamic headphones, permanent hearing damage is possible...
Wavebourn - Testing headphones... | Facebook
Wavebourn - Testing headphones... | Facebook
One of my best and preferred amplifier for headphones is a 2x30w/8ohm Kenwood amplifier which gets its headphones output internally from the speaker output...It's just that on 250 ohm cans , the volume pot is at about 10%...It has no audible noise though and i don' have many amps that have no audible noise at max output.Most of the stages are built with 2sc1845 and 2sa992...
If I would need something to drive 600ohm I see a few options.
-something with a 12BH7 as kathode follower can work I think
-Use the output of the neve preamp. (see ba283 Neve BA283 preamp section variations and use the output senction only) It happely drives 600ohm to 24dbu without a problem (it can do even more but don't remember exactly). At that point you ears are toast. It's class A/ Single ended and transformer coupled. The output transformer from carnhill is not that expensive also. And it sounds nice
-Use an opamp out of the lme497xx series and add an lme49600 output buffer. This is the cleanest you can get I think.
Don't forget that some headphones like around 200ohms of output impedance. For the tube circuits this will be ok and the neve has an output impedance of around 75ohm I think The lme49600 has a very low output impedance
Last time I needed a headhone output I just used some resistors to pad down the signal of an amplifier. (lm3886) and this worked very nice. It was a monitoring unit we designed for a telefunken M15A. So instead of adding another amplifier I used the lm3886's that drives the speakers. (you can look for that project on our facebook page if you want)
-something with a 12BH7 as kathode follower can work I think
-Use the output of the neve preamp. (see ba283 Neve BA283 preamp section variations and use the output senction only) It happely drives 600ohm to 24dbu without a problem (it can do even more but don't remember exactly). At that point you ears are toast. It's class A/ Single ended and transformer coupled. The output transformer from carnhill is not that expensive also. And it sounds nice
-Use an opamp out of the lme497xx series and add an lme49600 output buffer. This is the cleanest you can get I think.
Don't forget that some headphones like around 200ohms of output impedance. For the tube circuits this will be ok and the neve has an output impedance of around 75ohm I think The lme49600 has a very low output impedance
Last time I needed a headhone output I just used some resistors to pad down the signal of an amplifier. (lm3886) and this worked very nice. It was a monitoring unit we designed for a telefunken M15A. So instead of adding another amplifier I used the lm3886's that drives the speakers. (you can look for that project on our facebook page if you want)
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