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In Reply to: RE: The Official Guide to Converting your Quickie for Headphone Use posted by Caucasian Blackplate on September 23, 2009 at 13:11:17
Bits and pieces are posted here and there but there seems to be some confusion still.The "official" headphone transformer is the Speco, same as used in the SEX amp and in the Paramour for its last several years. The Speco was selected after testing all the line outputs we could find; it is indeed a superior design with better performance than any of the other half dozen or so we studied. (Radio Shack had a pretty good one as well. I don't know if it is still the same design. It had 4, 8, and 16 ohm taps.)
These line transformers have a tapped primary and tapped secondary; the Speco has an 8 ohm secondary with a 4 ohm tap. The primary can be used as an autoformer by feeding signal to the lowest power (highest impedance, 8K ohms) tap which is labeled 0.625 watt, and taking output from the highest-power (lowest impedance, 500 ohms) tap which is labeled 10 watts.
Now pay attention here - we are using this at half the design impedance. Leakage inductance on the Speco is low enough to get away with this without losing noticeable treble, and it maximizes the inductance for better bass, which is especially important at low power levels where the permeability of the core is smaller. So it is nominally being used as 4K to 250 ohms. Because triodes are quite accommodating about load variations, this will work very well with headphones of 160 to 400 ohms, and quite acceptably with 100 to 600 ohm 'phones.
Used this way, the step-down ratio is the square root of the impedance ratio, or 4:1. The Quickie can generate 8vrms at the plate, so an output of 2vrms is available. This is less than the IHF standard of 5v, but most headphones are sensitive enough that this is plenty.
The "odd man out" is low impedance headphones. These are often very sensitive as well as having impedances around 30 ohms. This combination of low impedance and high sensitivity allows them to be used with iPods and the like, which lave low-voltage battery power. The IHF headphone specification calls for a 120 ohm resistor in series so that these headphones don't blast your ears. The resistor cuts the voltage at the headphone to 30/(120+30) or 0.2 times the specified 5 volts. Referred to the 2v available for high impedance phones with the standard Quickie arrangement, the available voltage is 0.4 volts, so a stepdown ratio of 8/0.4 or 20:1 is workable. That corresponds to 3200:8 or 6400:16. Therefor a transformer with a 4K primary and a 16 ohm output (such as the old Paramour upgrade transformer) can be used with good results. If the phones are very sensitive, or have a lower impedance like 15 ohms which some of them do, you can use the Speco on the 8 ohm secondary tap as well.
With regard to DC current, I have checked both the Speco and the old Paramour upgrade transformers, and found that they will tolerate a maximum of 2mA of DC current. That is the Quickie plate current so they can be used in series feed here even though they are otherwise restricted to parallel feed. You may have to reduce the high voltage to 27 volts (three batteries) to keep the current under 2mA if you do this, and you may or may not like the sound. But it's theoretically sound, safe, and educational whatever the outcome.
The plate load resistor (in stock or parallel feed headphone operation) can be replaced with a choke. You want 20 henries or more, a DC resistance no more than 4K ohms, and a rated current of at least 3mADC (the excess allows for bass signals as well as the DC operating point).
Finally, you can raise the plate supply voltage to get more power output. The current will rise so the series feed option is not possible with the Speco, but the other options are available. The maximum specified voltage is 67.5 volts, but I see no reason you could not use eight 9-v batteries for a nominal 72 volts. I noticed yesterday that Costco is selling a 48-pack of AA cells which would also provide a nominal 72 volts. Remember, this is well above the UL "safe" voltage so it's back to being a little careful if you do this!
Edits: 09/25/09Follow Ups:
Just for ha-has (OK, technical curiosity) I checked my two leftover Speco T7010s on my HP 4276A LCZ meter. The 8K to 0 (Brown and Black wires) primary inductance read close to 8H, slightly higher on one, slightly lower on the other. The DC resistance is 298 ohms on both.
HTH
/ed B in NC
real radios glow in the dark
That's about what I measured at small signal levels. Near saturation the Speco pulls about 100 henries - but the Quickie is never going to get that transformer anywhere near saturation!
Hammond has a couple of high inductance, low current chokes which may be usable as suitable plate loads for the Quickie's 3S4s.
155C, 60H, 8 mA, DCR = 2750 ohms, about $14 each.
156C, 150H, 8 mA, DCR = 3700 ohms, about $15 each.
Since these are both below the nominal 4.02K plate resistor value, one could insert an LED in series between the B+ line and the choke's "hot" side, for a pilot light, or tube activity indicator. Depending on the LED color, it will introduce between 1.6 and 3.6VDC voltage drop, which is pretty much minimal. Don't connect the LED on the plate side, it could introduce some unwanted noise into the equation.
Now, these inductors aren't going to be of Magnequest quality, but at the cheap prices, they may be a good deal for this application, along with the SPECO matching trannies. I'd SWAG that 2.2 to 3.3 uF would be a good coupling cap value , into the reflected 4K ohms. Voltage rating could be as low as 100V, but I'd probably go with 250V (basically because I have lots of such caps in my stockpile).
This topic alone has intrigues me to the point that I put in an order for not just one, but two, Quickies ;-)
Being one who can never leave "well enough" alone, these Quickies are a very interesting and readily modifiable project, which I just can't resist.. A little research indicates that 1S4s can be used with a simple re-conenction of the filament leads (use pins 1 and 7, leave 5 open). Just becuzz....
All of this talk of "quickies" reminds me of a very old, rude joke involving a pastor and a nun, but since this is a polite family-oriented site, I won't post it here ;-)
/ed B in NC
real radios glow in the dark
I was also looking at the 156C (hey, 90 more henries for a dollar). My initial thought, without doing any calculation, was that a CCS (C4S) would necessitate adding a bunch of voltage, as well as adding 1-2 ma overhead, which would drain the batteries.
I too ordered 2 of them. I justified one for my low pass filter and the other for a portable headphone amp to work with my Blackberry. But like you (if I get your drift correctly), I find this to be a very exciting experimenter's platform. It also provides all kinds of incentive to try cheap tricks. I've always gone for high end components on my other builds, and this one is very well suited to trying bang for the buck upgrades, at least for me at this time.
Thanks again for the help before.
Triamp... Take a load off!
"Why use the 3S4" you ask? The new AES catalog shows 1S4 for $2.90, 3S4 for $2.80. 'nuff said? :^)
I've got both 3A4s and 1S4s in my toob-stash, as well as 3Q4s, these olde style battery tubes often showed up for dirt cheap at the local NE flea markets and swapmeets.
As you state, this IS an experimenter's delight!
/ed B in NC
real radios glow in the dark
I have a stash of 1T4's. Though the pinout and construction seem the same as 3s4, Duncan Amps lists them as pentodes with an Ra of 500K. They seem to consume half the heater current of the 3s4's. I'll probably try them when (if) I find the time.
Triamp... Take a load off!
That is immensely helpful.
So I'll be using Speco's for the Senns and maybe the paramour upgrade trannies for the 32 ohm Grado SR 60's , with a switch in between. I'll look around for some plate chokes. I see the old 10H 270 DCR Paramour chokes are out. This little guy is gonna start getting heavy.
I'm planning to add a little mini stereo input jack and a bracket so I can feed it from my Blackberry Tour. It's starting to come together now.
Your post and CB's Speco installation post would be great stickies for a Quickies page.
Thanks again,
Triamp... Take a load off!
If you are looking to choke load the 3S4, you might hop onto the Magnequest forum and ask Mikey how much current his grid chokes can take. The DCR is about right, and the inductance is phenominal.
Thanks. I'm considering the grid chokes for another application. I'm trying to keep this project, as the Kinks put it, "low budget".
I don't know the geography of people here. But if you're going, have fun at the chateau today.
Triamp... Take a load off!
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