Custom Headphone OPT ordered for the TubelabSE

[sbelyo]

I placed an order today for custom OPT's with Headphone impeadences from Electra-Print.

They're 5K Primary @ 65ma 3W

Secondaries are 32, 120, 300, 400

I'm going to use them with the TubelabSE board. My goal is to try the 45 first, then the 2A3.

Hopefully this will work out good....

[dsavitsk]

Originally posted by sbelyo
Hopefully this will work out good....

It should. I have a pair of E-P 5K:32 OPTs gapped at about 35mA I think for headphones. I am using them in a goofy config right now with a single 7KY6 per side. Good stuff, indeed.

One bit of advice I can offer is to allow some distance between these and the power transformer. Induced hum is more of an issue than with speaker outputs.

Oh, also, if Jack didn't start winding, you might want to add an 8 ohm tap as well since you'll be making a powerful enough amp for speakers - I measured my Grados as needing about 5mW (0.005W) to power them to "plenty loud". You could also buy yourself some 32:8 transformers too, I suppose

[sbelyo]

I thought about the 8 ohm tap, I'm pretty sure I'll never use this amp with speakers. If this design works well I may build a 300B version that will have speaker taps.

I spec'd it to 65ma for 2A3's as well as 45's. That should give me enough power for my K340's and future K1000's.

I suspect this will take me several weeks to gather the rest of the parts.

How do you measure the wattage?

[dsavitsk]

Originally posted by sbelyo
How do you measure the wattage?

The poor man's method: I downloaded a 60Hz tone and played it louder than comfortable. I then used a multimeter to measure the VAC across the 32 ohm headphones. I repeated this with other frequencies (and other meters, and an o-scope) and the numbers seemed pretty consistent. I don't really know if this works, but that's what I did. I'm guessing I'm within an order of magnitude.

[sbelyo]

Originally posted by dsavitsk

One bit of advice I can offer is to allow some distance between these and the power transformer. Induced hum is more of an issue than with speaker outputs.

My plan is to have the power tranny and choke on the left side of the chassis. Then put the board in the middle with the opt's on the right side.

Originally posted by dsavitsk

The poor man's method: I downloaded a 60Hz tone and played it louder than comfortable. I then used a multimeter to measure the VAC across the 32 ohm headphones. I repeated this with other frequencies (and other meters, and an o-scope) and the numbers seemed pretty consistent. I don't really know if this works, but that's what I did. I'm guessing I'm within an order of magnitude.

I'll have to try that... Is there a cheap pc based analyzer that might do that. Like sub $200?

[tubelab.com]

First let me answer some of your other questions:

The 2SK2700 mosfet is extinct, due to ROHS. Toshiba has an unleaded version listed on their website, but no one has them in stock. Many users have used the Fairchild FQP1N50. This works well in most amplifiers, but there have been 2 incidents of these oscillating at high voltages (over 350 volts) with other circumstances (long wires leading to the tube sockets). I have several Toshiba parts to evaluate for use as a better substitute for the 2SK2700. I had planned to evaluate these this weekend, but recent surgery has slowed me down a bit.

If your transformer has a center tap on the 5 volt winding, cut it off or tape it up so that it can not touch anything. It will have the full B+ voltage on it.

The motor run cap can be mounted anywhere you want, but you MUST ground the can. It is not connected to anything inside the cap, but leakage currents can cause the can to be electrically charged if it is not grounded. I have one mounted on the top of the chassis in my "industrial amp".

A few observations about a headphone amp:

I am not a headphone person (my head sweats too much) but I built a headphone amp for a very discerning audiophile using a TubelabSE. The user tried many combinations with several different phones. This amp was a major project that required some modifications to the PC board. Not recommended for a first time builder. The story is here:

http://www.tubelab.com/Headphone.htm

First off 50 to 100 mW can blow the headphones right off of your head. A 45 amp will have more than enough power. A resistive attenuator between the OPT and the headphones may help with the excess power and the hum.

Headphones can be really sensitive, equivalent to a 110 db speaker strapped to your head. Hum will be an issue. The amp above wound up (after the web page was written) with two chokes and a big motor run cap.

The user had me build the amp, and it was working great when it left here, but it was still mounted to a piece of plywood. The user was having a custom case made following the same general layout of the plywood base. The finished amp had some faint hum, so it came back. The hum was fixed by rotating the power transformer 90 degrees and cleaning up the wiring. We added the second choke and the supplemental cap at the user's request.

[MQracing]

Originally posted by dsavitsk


I am using them in a goofy config right now with a single 7KY6 per side.

Beautiful construction on your headphone amp!!!

Looks really sharp. Kudos.

MSL

[dsavitsk]

Originally posted by sbelyo

My plan is to have the power tranny and choke on the left side of the chassis. Then put the board in the middle with the opt's on the right side.

Just breadboard it before you drill. I had about 8" between mine (from center to center) and 90 degree turns, and still got hum. It was so low that it could only be heard at night when the city is quiet, but it was enough to drive me to rip the amp apart.

Originally posted by sbelyo

I'll have to try that... Is there a cheap pc based analyzer that might do that. Like sub $200?

A $5 meter should work fine.

Originally posted by MQracing



Beautiful construction on your headphone amp!!!

Looks really sharp. Kudos.

MSL

Thanks Mike, I appreciate that.

Part of the reason the power supply is separate, aside from the noise mentioned above, is that I have a number of these cases, and I am building a series of amps with the same tubes at more or less the same operating points, but with various topologies. There are so many anecdotes about which topologies sound best, but very few "controlled" experiments to really explore. So, since headphones allow very simple and low powered amps, I thought I'd find out what I really like. So far it's been fun and informative.

[sbelyo]

Originally posted by tubelab.com
A few observations about a headphone amp:

I am not a headphone person (my head sweats too much) but I built a headphone amp for a very discerning audiophile using a TubelabSE. The user tried many combinations with several different phones. This amp was a major project that required some modifications to the PC board. Not recommended for a first time builder. The story is here:

http://www.tubelab.com/Headphone.htm

Thanks for answering my questions....

I read that page on your site at least once a month. I pretty much said to myself that there was no way I'm going to buy a bunch of 01A's just to find 2 good ones. And those transformers are like gold dust. I've seen them pop up every now and then, but they're way too pricey.

My way of thinking was that the 45 should put out 1 Watt and I have 2 pairs of headphones that are pretty hard to drive. I have a class A soild state amp putting out 1 Watt and that just gets the headphones loud enough. I guess I really wanted more power than I needed.

I'm hoping that I can find a layout that will keep the hum in check. I know that I will need a choke for sure, but I may need some help in tweaking the power supply to reduce any hum.

It may take me a while to finish it, but I'll take pictures and keep track of what was done so that you can use it on your site.

[dsavitsk]

Originally posted by sbelyo


I'm hoping that I can find a layout that will keep the hum in check. I know that I will need a choke for sure, but I may need some help in tweaking the power supply to reduce any hum.

You are fighting 3, or maybe 4, main hum issues. The first, and easiest, is ripple in the power supply. For that, as you say, a choke, or two, may be enough. Otherwise, you can increase the PS caps.

The second, which was my problem, is the magnetic coupling between the power transformer/chokes and the output transformer. This just requires distance -- how much will require some experimentation.

The third is from the filaments. AC will probably not work, but might. Again, it will take some trial and error.

Finally, grounding and shielding may be a problem. Tubelab's board should take care of the grounding and hopefully the tubes don't pick up too much noise. These aren't 26's after all.

The real difficulty is that once it is together, if it does hum, you won't necessarily know which of the above is the problem, or if it is only one. It should all be solvable, though, and should yield a really nice amp.