[elliottstudio]

Hi all,
 
I've been lurking around here for a while now, have made a few posts in the DIY section as well. I'm mostly a hobbyist but have a small side business building tube audio equipment; I don't make much (if any) $$ doing this, it's basically a self-supporting hobby.
 
I recently got bit by the headphone bug. I've always been a Sennheiser man but never had any thing 'high-end'. Well, that recently changed when I got a decent Christmas bonus and my wife told me to buy something 'fun'. I decided, well, I've never had a good set of 'phones so why not?
 
Long story short, I wanted to buy from my friend's store so I narrowed my search to three models that he carried or had access to:
Sennheiser HD600, HiFi Man HE400i and Audioquest Nighthawks.
 
I read many reviews (most of which I found on this site, thanks guys!) and grew very interested in the HE400i's. After listening to the Nighthawks I quickly eliminated them from the running.. So, it was between the HiFi Man's and Sennheisers. Since my product is on display in his store I was able to audition with my own equipment and decided on the HE400i's, which I got for $299 on Christmas promotion. Lucky me!
 
Cut to the chase: Though my ESA preamp drives them pretty dang good (it's a parallel feed triode Xfrmer coupled SE 1W power amp with an output impedance of 8 ohms) I of course want something more.
 
SO, I'm hatching a new product prototype, which may be a one-off or may make it into "production".
 
My ideas are:
 
Triode based push-pull 5W power amp.
Switchable primary for low or high impedance 'phones.
SE and Balanced outputs
Regulated DC supply (possible external)
and of course, blue LED.... 
 
My questions for you are:
 
1 - Inputs; SE (RCA) Balanced (XLR) or both?
2 - Outputs; 1/4" TRS for SE, XLR or 2X 1/4" (or 1/8") for balanced? If XLR, how many pins is standard (if there is a standard)?
3 - How to cleanly implement a volume control in a balanced circuit? Hot-cold shunt ala Morgan Jones? Balanced stepped attenuator ($$$$)? Any other ideas?
4 - RCA or XLR rear mounted line outs?
 
Any other features I'm missing here? I have the basic circuit already sketched out..

 

[SeaWo|f]

1: SE is probably a safe way to go, balanced gear has a more limited more high end audience. Also balanced gear should also have a SE output as many people that have balanced headphones also have SE and their friends almost certainly do. In fact unless you are willing to mod things many if not most cans can only be run SE.. Single sided non replaceable cables

2: for SE there is a nice locking 1/4 trs people like. For balanced i think the most common is 4 pin xlr followed by dual 3 pin xlr. I haven't really seen amps with other balanced connections but my experience is limited.

3: all I ca say is in the higher end products you tend to start seeing stepped.

4:: rca for SE and three pin xlr for balanced same as input.

My 2c take for what it's worth.. Probably not much

GL have fun

 

[fjrabon]

Ideal:
stepped attenuator volume control.
Balanced & unbalanced input and output. RCA & 1/4" TRS for unbalanced. Dual 3 pin XLR input and 4 pin XLR output for balanced.
What are line outs for? To use it as a pre? Or are you talking about powering speakers too?
Push button muting on the volume control (i.e. Push the volume control in to mute)
Bass boost low pass filter switchable.
Crossfeed switchable.

 

[elliottstudio]

Thanks for the responses!
 
To address a few questions and points:
 
Since the output is transformer coupled it's easy to add a balanced output. Looks like 4-pin XLR (male?) wins. 1/4" TRS will be the locking Neutrik connector. There would be a BAL/UNBAL switch.
 
The line outs would be for using as a preamp or powered speakers.
--Is there demand for an input loop-out so the headphone amp can be used between preamp and power amp?
 
Bass boost would have to have switchable corner frequency. And why not just add tone contorls?
Crossfeed is something I'd have to look into; I've seen a passive crossfeed network but that has too much loss. Any active crossfeed probably involves DSP or at least a solid-state circuit. Both would be options me thinks.
 
Stepped attenuator could also be an option for those who want to pay premium.. IF this thing becomes more than just another project.
 
Anyone else?

 

[SeaWo|f]

Xlr female is typically on the headphone out on the amp. As for the additional features I've never looked for them and have no idea about demand.. But pass through and pre amp functionality is pretty common. On my last amp purchase, schiit ragnarok and my next amp purchase decware taboo mk iii the ability of the amp to power speakers was/is a factor.

 

[elliottstudio]

Yes of course.. female. I work in pro audio and XLR outputs are always male but we're not talking about established standards when it comes to stereo equipment!
I think maybe a set of binding posts for speakers (very high-efficiency) would be fun and run them directly off the amp..
 
I have casework from a preamp project that could be used. It would be a full sized component to start with for ease of construction / modification and if it is viable maybe design more purposeful casework with a smaller form factor.  Or not...

It would look something like this..

 

[elliottstudio]

Well, after a long delay in finishing another project I got around to building the prototype headphone amp as discussed above. I went with single-ended input and single-ended output, and can have custom output transformers wound to accommodate both single-ended and balanced output. The circuit is essentially just a miniature power amp; push-pull ECC99's transformer loaded with a phase splitter input. I eschewed balanced inputs because I figured most users sources would be unbalanced, ie; sound-cards, record out circuits, etc. It probably outputs around 5W into 30 ohms which is more than adequate for most situations, and I'll include a HI-LOW switch that will select output transformer primary windings to match to high-impedance headphones.
 
The prototype sounds rich and powerful driving my HE400i's and various Sennheiser closed back HD series cans I have laying around..
 
I'll post some pics later tonight when I have more time!

 

[Monsterzero]

looks sweet man!

I dunno anything about the science behind the magic of headphones and amps,so what I'm about to say might not even be possible.

I would like to see an affordable amp that can drive both  alow impedance high sensitivity headphone and a 600ohm can.

My WA2 sounds amazing with my Beyer T1s,but when i try it with a low impedance amp it sounds bad.I knew it would as Ive read a lot,but I dont really understand why.

If someone can come up with a powerful jack-of- all- phones amp Id be interested.

 

[elliottstudio]

thanks Monsterzero -
 
Unfortunately, when it comes to tubes nothing is "affordable" that can 'do it all', unless it's made in China! That said, it's a pretty simple circuit with a big-ass power supply. This amp will be able to drive low and high impedance 'phones for sure.

Pictures to come - have to get them off my phone -

 

[elliottstudio]

Here's pics of the prototype. The casework is an old Carver preamp chassis I picked up at surplus years ago and has housed many projects in the development stage. The power supply is a standard ESA external regulated that I use for all my line-level products (not shown)
  I will add the balanced out option, maybe switchable inputs, pre-outs, pass-throghs and the HI / LOW switch in the final product. This is just proof of concept to see where the component topologies lie before designing a PCB.
 

 

 
The final product will use better quality components as most all of these are from the parts bins..

 

[elliottstudio]

Here's some preliminary measurements for those who care about such things. I used REW V5.1 to generate automated sweeps into a 38 ohm load, sampled into my Creative Labs external USB soundcard.
 
The response is flat and the distortion is low. This is with the attenuator wide open so is worst case for noise. The distortion cursor is at 999Hz as can be seen. The first graph is wide-open full tilt boogie  - a level representative of full scale digital from a CD player. The rise in distortion at low frequencies is likely due to transformer core saturation. I could probably reduce some of the 60Hz line harmonics by installing the top and bottom cover and moving the external power supply to a more remote location but you get the idea.
 

 

 

 

[elliottstudio]

Next I'll bring the unit to my buddies Hi-Fi store to test with his variety of headphones (mostly Sennheiser and HiFi Man). I think it's ironic that the last power amp I built was single ended and the headphone amp I built is push-pull!