Using Speco's to drive 32 ohm headphones?

[Len]

Hi,

I know this has been touched on before, but I didn't know where to ask this:

I am now setting up a Quickie as a dedicated headphone amp. All the hardware is mounted. I am using a the Speco's and a pair of Magnequest BCP-14 plate chokes, rated 100H at 10ma. DCR is about 2Kohm, so I put a 2Kohm mills before each BCP-14.

I have two headphone jacks installed, a large one for my Senns and a mini for the Grado SR-60's. Input is another mini to run off my computer soundcard or Blackberry. One pair of RCA's is paralleled with the input, another with the Senn output, and the third left unconnected for now. I plan never to have more than one input and one output hooked up at a time.

Since the 500 ohm outputs go to one jack and the 8 ohm outputs go to another, I would like to use the original input selector switch to choose the Speco PRIMARY impedance if feasible.

So the questions:

1. Would it make sense to use the Speco 4Kohm tap for its input when using the 8 ohm as output? If so, then we would be getting  less of a stepdown for an 8 ohm output to run the 32 ohm phones, which would be better than using that tap at 4 ohms I think. Also we would not need to rely on the Speco's leakage inductance figure to run it at a point for which it was not designed, at least for the Grado's. Stepdown would be 22:1 instead of 32:1.

2. If we do this, would it help to then load the 8 ohm tap with an 8 ohm resistor to ground, to "nail down" the 8 ohm output, thus ensuring the 4K output impedance? I figure we can't do this for the Senns, since we would be loading a 500 ohm tap with a 250 ohm resistor (but of course I could be wrong).

3. If we could do this, I'm thinking we wouldn't even need to use a compromise value for the parafeed cap, since the output impedance would be 4K in both cases. Is that right?

 

[Paul Joppa]

...DCR is about 2Kohm, so I put a 2Kohm mills before each BCP-14....

1. Would it make sense to use the Speco 4Kohm tap for its input when using the 8 ohm as output? ...

2. If we do this, would it help to then load the 8 ohm tap with an 8 ohm resistor to ground, ...

3. If we could do this, I'm thinking we wouldn't even need to use a compromise value for the parafeed cap, since the output impedance would be 4K in both cases. Is that right?

I have a white paper in draft form now; its' not ready for release so I'll just make some preliminary comments.

First, lose the 2K resistor - a lower resistance choke will just give you a hair more power.

Second, the Speco has limited inductance and going to the 4K tap will cut that in half as well as the impedance. Try it with clip leads and see, but I suspect the increased gain will not compensate for the loss of bass.

Don't bother with a loading resistor, it's not necessary and increases distortion slightly.

The parafeed cap value is hard to guess; I say try it w th the stock value first. The difficulty is that the effective load impedance in the deep bass is strongly sensitive to the transformer losses equivalent resistance - eddy currents and magnetic hysteresis in the core - and these are not specified as well as being a function of signal frequency and voltage level.

An experiment you might want to try is to use the 2K resistor as the cathode resistor in place of the stock 1K. This will re-optimize the circuit for 8K impedance, giving a hair more power but lower gain.

 

[Len]

Thanks. You've answered my questions, given me a few things to try and a lot to think about.

 

[Len]

I built the headphone amp this weekend, and was very surprised by how wonderful it sounds! It definitely wakes up the Senns, and the Grados sound better than I've heard them, too.

I'm not used to headphones these days and found it a little weird to have deep bass only in my ears, without it vibrating my big belly.

These are built with a mini jack in parallel with the RCA inputs, no selector switch. This way I can have the output from the soundcard go to the jack, with output to my EL84 computer amp when the headphones are unplugged. The jack also allows direct input from my Blackberry.

8 ohm Speco taps go to a mini jack for the Grados, 500 ohm taps go to the large jack for the Senns and are paralleled with the output RCA's.

Chokes are Magnequest BCP-14 100H at 10ma. Volume control is an Alps Black Beauty 100K log taper. Parafeed caps are Auricap 3.3uF.

I took PJ's advice and tried 2K cathode resistors. I used the original input selector switch for them. Center post to ground is the original 1K, one position puts a wire in series with it for a total of 1K, other position puts another 1K in series with it for 2K.

Sound with the Senns (unfortunately only 580's) is great in both positions from the 500 ohm taps. 2K position gives more bass and roundness to the music, kind of what changing to cobalt B7 output transformers from standard ones did on my preamp a few years ago. 1K position sounds more delicate. I did not need to move the volume control when switching positions.

Grados have enough volume off the 8 ohm taps, at least in a quiet environment. I drove them directly from an Audio Alchemy CD player, and only cranked them to about 3 o'clock. They don't sound as full as the Senns, but they sound pretty good.

I didn't get a chance to try the Blackberry earbuds with the 8 ohm taps. If they sound great like the other phones, I'll post.

I don't know how the guys at Bottlehead come up with such great designs. I haven't listened to phones for years because I didn't really have anything great to drive them before. Now I'll be listening a lot at night, and I won't have to close every door in the house!

I used some decent parts from my parts drawer. Even so, it amazed me that I purchased a kit for a hundred bucks that produced a headphone amp so good that the model of Sennheiser headphones is the limiting factor in sound quality!

Kudos to all. Very highly recommended.

 

[Len]

Here's a blurry closeup of the underside of my Quickie headphone amp.

Not pretty, but great sounding.

 

[Doc B]

Nice work Len! Thanks for the thorough report and the kind words. Like Eros, the Quickie is a design that almost didn't go into production. You make me glad we decided to go ahead with it.

 

[Len]

...the Quickie is a design that almost didn't go into production...

Oh, come on, Doc. It's a great kit for so many reasons:

1. No power cord, and it's small and light. When you want to work on it, just unplug the inputs and output, flip it over in its chassis and go. It takes seconds.

2. No high voltage, inexpensive tubes. It allows you to try quick tweaks without fear you're going to weld an expensive part.

3. Battery power. When you try something, you can just see what it does in the audio circuit, without always double-thinking whether you need to mod the PS. And it's not at the phono stage level, so battery (chemical) noise is a non-issue.

4. Plastic top. No drill press, no metal chips in the circuit, no punches. I just look at where I want a hole and use a slow cordless drill to put it there. I just do it by eye. I don't even measure. And it ends up looking like stock.

There have got to be a lot of reasons I missed. This is the ultimate beginner's kit: easy, low voltage, huge improvement in sound over what's out there, portable, low voltage, clean PS already supplied, etc., etc. And I'm always a beginner. (For example, I didn't know you could get good sound with a 5 dollar output transformer). It's so easy to mod it's like a perfboard kit. It adds a lot of understanding, so when one mods a Foreplay or phono stage, there is more knowledge behind that mod. I think it makes the whole process more efficient. I'm glad you got it to market. It's let me do things with my system that I couldn't do before.

I hope others develop this platform, to see how far it can go.