@solderdude , Frans, would you mind comment
"Must offer variable output impedance (the way many manufacturers make their amplifiers sound a bit different) So 3 settings were included" http://garage1217.com/garage1217_diy_tube_headphone_amplifiers_015.htm, please? I believe your answer should cover the above couple of post as well.
Many thanks,
Raul.
Those
bolded words are Jeremy's (G1217 owner) not mine, I only design the schematics (and not just for G1217) and assist in PCB design.
Looks, sales, manufacturing, warranty and pricing is not influenced by me.
My design philosophy is simple.
When you design an amplifier you may want to build something that can drive (almost) all headphones out there.
This means:
- Large output voltage swing (to cater for high impedance headphones)
- Enough output current capabilities (to cater for low impedance headphones)
- Have adjustable gain (to cater for voltage efficiency differences between headphones)
- Have adjustable output resistance
The reason for the latter is simple. Some headphones sound 'better' on higher output R, others on lower output R so an amp that is supposed to drive most headphones should be able to cater for this and set by the user.
0.2 Ohm, 33 Ohm and 120 Ohm (this used to be a standard in the old days) seem logical options. There isn't much difference between 22 Ohm, 33 Ohm and 47 Ohm. Also not much between 0 Ohm and 1 Ohm for instance while 'damping factors' can show huge value differences.
Different designs exist to cater for specific types of headphones depending on output stage type.
The 'gain switch' is not really a gain switch in these designs though but rather an input attenuator (which can be changed in value with plug-in module) as there is no overall feedback in this design to make use of the typical non-linearities of tubes. Local feedback is used of course.
For sensitive IEM's noise thus becomes a problem... hence the output attenuator (with low output R) as a remedy and to protect low power rated headphones being connected to a relatively high power amp.
Jeremy wanted tubes so tubes are used for the designs to introduce 2nd harmonic distortion which is amplitude dependent.
Also made a J-FET version for those not wanting tubes but liking 2nd harm. Added in smaller amounts though.
(yes, these amps don't measure well in the THD and noise dept. but done so by design)
As I wanted to cater for the usage of a lot of different tubes I used some 'tricks' as grid leakage currents, gains, anode currents differ widely as well as an automatic (some designs manual) heater config. options.
My opinion is that measurements are just a tool to check for certain basics and think the owner of an amp should decide if he likes the sound/functionality/looks. Those not wanting this can buy other stuff better suited to their needs/wants. It's a free market and people are free to chose what they want/need.
Don't need to cost a fortune either.
The schematics and some relevant specs are published.
A lot of specs are tube dependent and measured with 'well suited' tubes.
So when rolling tubes the specs may differ quite a lot.
This may even lead to sonic differences which is what owners of said amps are after.
These owners like to deviate from the 'wire with gain' philosophy by choice.
Not particularly suited for most ASR folks.
P.S. Even though I am mentioned on the website of G1217 and occasionally fix amps (if needed) or offer help/advise for E.U. customers I am not making any money of it nor am I on the payroll.
I am in this for fun and to help out a friend (Jeremy) as well as others incl. some other audio manufacturers.
Have a daytime job (designing/building/servicing/supporting) mostly analog (opto)electronics for railway applications so can do this as a hobby
which costs time and money by definition. As long as it stays 'small' it is fun to do.
Used to work in the audio business (service, design, modifications) though.
That's me in a nutshell.
P.S.: I think
@amirm might need to use the AP with a Polaris headamp and see what happens when switching between the 3 output resistors; ideally would be to use some resistors on the outputs, if not real cans. Thank you!
Amir doesn't need to measure any of my amps... they will receive a 'not recommended' label for sure
(really they will as they are specifically designed to perform a certain way which differs from the ASR convention).
On my website there are a lot of headphone measurements that show exactly what happens with the tonal balance of a few headphones.
The
effect differs per headphone so cannot be shown in a single measurement nor demonstrated for just one headphone.
