bobkatz

Warning to everyone: the following is a love-letter to Stax: I received my first Pro Bias Stax Headphone yesterday. Ironic that in nearly 40 years of owning Stax phones I never upgraded to Pro before today. Padam sold me an SR5-Gold-ND headshell that had been converted to Pro using (I think) a pair of new Gamma drivers and an OCC gold-plug low capacitance Pro cable. It was clean as a whistle and clearly never used (or hardly used). The headphone pads are soft as new. (By the way, I'd like to buy some replacement pads for my old SR-5s. Audiocubes has them listed on their site but it's not clear from my letter to them if they really have them). I'm still listening to these new Gold phones with a disadvantage, through my old Stax SRA-12S amplifier (has all new electrolytics, though). I'll goose the bias and I'm going to change the coupling caps to 2 uF as soon as the Vishays come from Antique Radio Supply. Anyway, I listened to these new Gold Pro SR5's yesterday and I'm in love. Where the standard SR5s are a little weak in the bass, the Gold Pro SR5s are solid and extended. Where the standard SR5s have a little rise in the upper midrange, the Gold Pros are smooth and accurate. Where the standard SR5s are a little muted in the extreme highs (above 12 kHz), the Gold Pros seem to have a little rise there, perhaps 1 dB up from "correct" or "flat".* This extreme HF rise is very pleasant to the ear and doesn't bother at all. I could easily tone that down with a little RC circuit if necessary, if only to match the presentation on my Revel speakers. Perhaps the rise is due to a combination of the the low capacitance Pro cable, along with my mods to the SRA (removing all excess old coax cable, extra switches and circuitry, installing a new volume control), and perhaps some of it is the high end distortion of a 40-year old amplifier design. Regardless----these Gold SR5s are instantly the best headphones I've ever owned, among the best headphones in the world. They rival the best speakers I've ever owned. Very addictive to listen to. Everything sounds so correct and beautiful and accurate I could mix with these phones and get very close tonally to correct for speakers, except for certain balance issues and stereo separation issues where headphones can't perform. Someone could start a mix on these Gold SR5s and get it 90% there and then take it to us for finishing on loudspeakers with excellent translation. Now I can't wait to hear the Omega IIs I have on order from Woo. How much better can they possibly be than these fantastic Gold SR5s? These Pro bias phones are at least 6 dB less sensitive than the normal bias models I have. Making it impossible to listen to both models at once and pointing out why the 1000 volt swings in the KGSS I've got on order will be the order of the day. The SRA doesn't seem to be clipping, though, probably because I just don't push my earphones higher than about 83 dB SPL average/103 dB instantaneous peak. We must also take into account that I'm playing old SR5s with diaphragms that are AT LEAST 30 years old! I replaced them some time in the mid 80's if I recall correctly. * Compared to what I know these sources to be, having mastered much of the music I auditioned yesterday. I'm very comfortable where the high end sits on these various pieces of music I've mastered, so I can say with confidence that the SR5 Golds have a little rise above 12k.

Just supposition, but how do you think these 507 will compare with the Omega II? Which will sound "more neutral"? I never liked the sound of the Lambda-type and Sigma-type enclosures before (too resonant and boxey), but these appear to be "square Omegas" to my thinking as they appear to sit directly on the ears and not have a large resonant cavity.

Dear Spritzer: You're a gentleman and a scholar! I have a Fluke temperature probe, I'll see what's cooking ). I've got FOUR Siderealkaps left over from a tube project. 3.3 uF 200V "premium audio grade capacitors". These caps are so beautiful. Nice yellow cover over polypropylene I assume. The question is whether 200V is safe. The silver micas that are in the SRA are rated at 500 v. Can I get away with a 200 volt cap? Legally no, but it's so little current... but the cap will be charged to 310 volts or so. There's a bleeder resistor to ground on each stator of 10M. Would you risk it or start looking for a higher voltage cap?

I spoke too soon. After about 1/2 an hour the power transformer heated up to its old splendor, can fry an egg on it. I still think it sounds better. As for the bias, I thought my Fluke meter had an extremely high input R (Z) but I guess not, it must load down that voltage, so (as people have mentioned) it's important to adjust the bias measuring in front of the 4.7 Meg build-out resistor. My next question goes to hotrodding that old SRA-12S. First is on the bias on the old SRD-5 diaphragm. How much is a risk? I used to adjust the bias until the short hairs on my arms stood up on end and then back it off a bit ). (Seriously). Now it's back to 230 volts as I'm getting nervous. The higher bias increased the sensitivity and headroom considerably and I did get (I felt) better, sweeter sound, less transistor harshness. The next thing I've been doing is adjusting the collectors of the output transistors 10 volts higher than recommended (for what it's worth). Of course, this is contributing to the current drain on the little power transformer (. So the collectors are running at 310 volts and again I feel the sound is improved. It sounds a lot more like a class A amp. I have not looked at waveforms with a scope or measured the distortion. It's possible to push those collectors into the stratosphere, but how hot is too hot?! It's definitely "fry an egg" time at the power transformer and V+ resistors and output transistors so basically I've stopped pushing the voltage. Next thought is to replace the 4700 pf silver mica output coupling caps with something bigger in hopes of getting deeper bass? Of course this is all academic now. It's funny I'm concentrating on my old SRA when Justin is building me a KGSS and I'll be buying an Omega II shortly. I have a converted SR5NB (Gold pro) made from an Alpha (Gamma) coming to me also. So maybe I'll let that hotrodded SRA-12S and 3 beautiful SR-5's go, eh?

Thanks, Spritzer! By the way, I just finished replacing all the Electrolytics in my SRA-12 with Panasonic LOW ESR types. It's amazing how much smaller and compact electrolytics have gotten in the years. But what really was amazing was that the power transformer went from too hot to touch to practically stone cold! So the old electrolytics must have had considerable internal leakage. Just that fact alone made it worth replacing the caps. I had done a replacement sometime in the 80s or 90s and it was clearly due again. I think it sounds better, too ). BIAS on the normal phones. I had replaced the series R in the L pad (****NOT**** THE 4.7 MEG BUILD OUT R) with a 100k ohm rheostat many years ago so I could goose the bias. Now that the electrolytics are so much more efficient, I'm getting 233 volts in FRONT of the 4.7 Meg resistor. But even with no load I see about 160 volts on the load side of the 4.7 meg. But I don't think there is any leakage to ground on the load side. Is this normal? Must be because the DC is coming from such a high impedance source that even a length of wire load on the resistor will drop the voltage. Is this normal? Does this make any sense? Plugging in the cans the 160 volts barely drops so the cans load the resistor negligibly.

I wish there were a way to make a sub-topic. Replying to the thread can get tiresome to search! My question is about the resistors that are INSIDE the phones. If there is a 5M ballast resistor already inside the amplifier, what is the purpose of the resistors that are inside the cans (no one says "cans" around here ))? At least the last time I replaced the cable on my SR-5's, I found a resistor inside each phone. Next time I have one open I'll measure it!

Thanks. I already have the service manual. It "was" an SRA-12S which I proudly bought new in 1972! Now that it's been stripped; it's just a Stax amplifier, no preamp or booster section. I've changed the volume control to my own hand-built stepped volume control, a few other goodies. I'll have a look for the restoration thread, it will be interesting.

I'm not so sure of that. You're looking at an unregulated voltage and it could be off by any amount depending on the line voltage and other variables. Perhaps looking at the raw ripple on an oscilloscope and/or measuring the residual AC RMS level on the DC line. But at the Prodigy forum I learned that a cap may have good capacitance but its ESR could have been shot and this may not show up by looking at the ripple, or would it? Bottom line is various authorities I've consulted seem to indicate that the only sure way of knowing if an old filter cap is bad if the DC and ripple measure good is to replace it and then listen! (how primitive) BK

Dear Espen: The HD650s are great phones, but not much isolation if you are in a noisy location. But it seems that good sounding phones goes together with poor isolation. Anyway, if you are having any issues with the 650's and the ULN-2, try this solution. It's got great sound, will drive any dynamic headphone and will serve you well on location: CEntrance -> DACport I have a ULN-8 and a pair of 650's. When I get a chance I'll report how it works with it. Best wishes, Bob

Thanks! So the only way to tell if there's a filter cap problem is by listening... and change the caps and see if the listening improves? (There's no hum; the regulators and I assume the caps are doing their static job). I assume that bass performance is the place to listen for an improvement or degradation. I wish there were an objective way to assess bad or old filter caps. Bob

Speaking of caps. I replaced the main filter caps and the low voltage filter caps in my Stax SRA-series amplifier about 20 to 25 years ago, so theoretically it's overdue for a new set (some say every 10 years!). The amp has been performing very well, however. I have an LED on it that takes 6 seconds to go out on power off so there's a decent amount of reserve capacity. I just measured the current (pun not intended) caps in situ and they all measure at least 30% higher than the specified value. They were all good-model electrolytics. My question is : If the caps measure fine is there any need to replace them, even after 20 years?

What's my Stax? I've got three pairs of 30 to 40 year old Stax phones that I first bought in 1971 (when money was very tight and I was very young. Now money is still tight and I'm much older )). I have two SR-3's which I retrofitted with SR-5 diaphragms in the late 80's and one original SR-5. The driver amp is my own custom-modded SRA model, removed all the preamps and doodads and kept the driver amp, but I goosed the amp's bias and also the headphone's bias with my own bias control, so I'm running the phones "hot-rodded" with between 200 and 300 volt bias. Basically I adjust the bias until the hair on my arms starts to stand on end and then I back it off ). I know it's risky but there's a resistor in the phones and life is too short. The higher the bias the higher the phones' output and thus the cleaner the amp runs before it runs out of headroom. And now I have a little spare cash so I'm going to splurge on a total luxury item that I totally do not need: A pair of Omega IIs and a Gilmore-style solid state amp. I think the Omegas are the best-sounding headphones I've ever heard and I've been salivating over them for as long as they've been around, but kept putting my money into loudspeakers and amps and my mastering gear. Does anyone have a spare headband for sale for an SR-3 or SR5? My SR5 headband is cracked and cannot be repaired with superglue or anything short of custom-made parts. (To help finance this I'm selling a beautiful Pass X250 amp if anyone is interested)