Prototypers wanted

[CYoung234]

Welcome, Craig. And I think the skewedness comes from being a trumpet player, of course you're going to say it doesn't get loud enough. Just kidding, dsavitsk has a thing for low gain and the K501 is pretty inefficient. So much Shosty, but no love for the 5th?

I have a lot of experience with the K501 with K501 and with K601 pads, as that is probably the headphone I have listened to the most, but I sold it awhile back. What I found weird about the sound is the soundstage is big but oddly shaped and the midrange can seem shouty at times because it is brought to the forefront while the bass and treble seem distant. That said, I wouldn't mind owning another pair at some point.

Ha ha . I was often happy I was BEHIND the bell of my instruments (from a hearing loss standpoint anyhow) - unlike the poor french horn players who were sitting in front of me! As far as Shostakovich 5, I suppose I have played it too much. I don't find it as musically interesting as some of the other Shostakovich Symphonies, 1, 7, 8, 10, 13 and 15 in particular.

I will defer to you on this one, as I have only owned these phones for about 9-10 months. I have not noticed that to be a problem so far - the overall balance and soundstage seems good. I listened to Chris' K601's quite a bit before deciding to get a pair of K501's, and prefer the 501's soundstage, as the 601's to me are a bit larger but seem to isolate the instruments a little artificially. Overall though, I still actually prefer my stereo system, but I do not have time to listen to it much when I would not be disturbing others.

[CYoung234]

Guilty as charged. I confess I have not had too much time to play around with different configurations in that regard. I did hook up just the Adcom this morning and listen to it briefly, and it sounds very good and actually has more gain than the outboard DAC. I will try out the CCS resistor change as well and let you know what I think before I give the preamp back to you to finish up. I assume I will need to unplug it before I start messing around, although as an electrician, I am pretty used to working on live power! Just kidding! Of course I will unplug it before putting the resistors in.

In terms of overall dynamics, I think my speakers are slightly better, but that is mostly from memory. I really do not think that comment has much to do with the headphone amp, as I think it is simply reproducing the source. I am actually very happy with the amp - so no worries. It would be interesting to A/B it with the Gilmore at some point. You may want to do that as well once I give it back to you to see what you think.

[Pars]

You can just turn it off and wait a minute or so before putting the resistors in. There is live AC back around the IEC, and also a pair of jumpers on the left side of the board back by the transformer, so stay away from there (or unplug it). Those shouldn't be a problem with the switch off though. It does have a bleeder resistor, so the caps etc. are drained pretty quickly.

[luvdunhill]

Craig, what's your opinion of Solti Mahler 1 / LSO / Decca? Listening to it now on vinyl with this amp and loving it. Need to grab some more Shosti though, this thread has reminded me of that.

[dsavitsk]

While the thread has been bumped ...

I was doing some research on something else the other night, and happened upon distortion spectra for tubes drawing grid current. To my eye, it looked a lot like what we've been seeing on this amp when run hard. That is, I think grid current starts well before 0 bias. I tried biasing the grid a little more negative, and this seemed to help a bit.

My recommendation is that the grid be at least -2.5V, with perhaps -3V being even better. You can do this by swapping out the red cathode bias LED for something in the 2.5V to 3V range (you can search at Digikey by forward voltage.) Or, if you increased the CCS current, then just jumpering to the WE connection should get you to -2.5V with the stock Rk.

The issue is that as bias is increased, plate voltage increases, too. The upside is that, even outside of the grid current issue, the plate curves look a lot more linear at -3V than at -1.8V and this moves heat dissipation from the CCS to the tube. The downside is less room to swing before clipping the CCS. Though, even at -3V and 18mA there is about 50Vp-p. We'll have to see if that's enough. And, increasing the power transformer from 200V to 225V is probably OK, though it will run little hotter.

Anyhow, if anyone gets to do this, please let us know what you think.

[Nebby]

Pardon my lack of knowledge, but what exactly would I need to jumper assuming I'm running the increased CCS current? I just want to make sure I don't make a mistake and blow things up.

[CYoung234]

Craig, what's your opinion of Solti Mahler 1 / LSO / Decca? Listening to it now on vinyl with this amp and loving it. Need to grab some more Shosti though, this thread has reminded me of that.

It has been quite a while since I have listened to that recording. I am maybe a little biased, as I have never been a big LSO fan - I always found the brass section in that orchestra to be a little lightweight and bright for my tastes. More a quality of sound issue than volume. The recording I own is the CSO / Tennstedt live recording, which is pretty good. It reminds me of the live CSO performances I have attended of Mahler 1, mostly at Ravinia. One I would like to have a close listen to one of these days is the Bernstein / Concertgebouw recording, which is praised for great sound, but panned for being too beautiful.

[luvdunhill]

I always found the brass section in that orchestra to be a little lightweight

I was thinking the whole time, where's the rest of the orchestra (admittedly from a volume standpoint)

Doug: I asked about this a while back, but can you give a voltage and current so that we can model the LED Vf before dropping it in?

[dsavitsk]

Doug: I asked about this a while back, but can you give a voltage and current so that we can model the LED Vf before dropping it in?

Not sure I know what you mean? The current is the CCS current. I'd set it anywhere from 12mA to 18mA to experiment. Voltage is the Vf of the LED. In an ideal world, you'd pick an LED with the lowers dynamic impedance you can. In the real world, this information is rarely clear from the datasheets, and LEDs don't really differ that much, so worrying about it is likely not that important. You know the old saying, that in theory, theory and practice are the same, but in practice, ...

[Beefy]

Not sure I know what you mean? The current is the CCS current. I'd set it anywhere from 12mA to 18mA to experiment. Voltage is the Vf of the LED. In an ideal world, you'd pick an LED with the lowers dynamic impedance you can. In the real world, this information is rarely clear from the datasheets, and LEDs don't really differ that much, so worrying about it is likely not that important. You know the old saying, that in theory, theory and practice are the same, but in practice, ...

I've read a fair bit that the 'best' LEDs for biasing tubes are older style, low illumination red units, Vf approx 1.5V. As you get up into the green and blue units, they become less suitable.

Would there be any potential benefit in two red units in series?

[dsavitsk]

Pardon my lack of knowledge, but what exactly would I need to jumper assuming I'm running the increased CCS current? I just want to make sure I don't make a mistake and blow things up.

You are not likely to blow anything up. You should have an email with the jumper information. If not, send me an email and I'll send it to you.

There are 4 jumpers, the two closest to the tubes switch between connecting the cathode LED and Rk, while the two by the transformers switch between connecting the transformers' secondary returns to the cathode and connecting them to ground. For the WE connection, you want the resistor connected, and the transformers connected to the cathode. For the LED connection, the LEDs (D3L and D3R) should light up and the transformers should be grounded.

Please note that these are not the CCS LEDs which you should leave alone.

[luvdunhill]

I figured given voltage and current, the Vf of the LED would change. So, if I had a supply of 100V and 18mA current through the LED this would be different than if I had a supply of 5V and 18mA. Now I'm doubting that gut instinct, as that wouldn't make the greatest voltage reference huh

[dsavitsk]

I've read a fair bit that the 'best' LEDs for biasing tubes are older style, low illumination red units, Vf approx 1.5V. As you get up into the green and blue units, they become less suitable.

Would there be any potential benefit in two red units in series?

This is SY's claim I think, and I believe even he has backed off it -- maybe he ran out of his old LEDs See the part about theory and practice. Bottlehead, with their CCS kits, used to include some "special" LEDs that had super low impedance and that they claimed were the only ones that really worked -- they no longer do.

Good red LEDs theoretically have a reported impedance in the 8 ohm range. Bad ones are more like 12 ohms.* I think anything you find will be in the high range. Green and Blue and whatnot might be as high as 12 to 14 ohms, but they are still generally lower than 2 reds in series. This impedance is multiplied by mu and added to rp. mu here is 38 and rp is ~3500, maybe more like 3K at high current. So, we are adding maybe 500 ohms to that due to the LED bias. To my ear, and by measurements I've done, this is more than made up for as opposed to a resistor and a cap. But, importantly, the difference between adding 400R and 500R here is pretty insignificant.

Oh, and while you could theoretically put two in || to lower the Z, my experience is that you need a pretty high current before you hit the linear part of the Z curve, and we don't have enough for two here.

* this is my understanding from what others have claimed, and from looking at datasheets. I have not tested them directly ... probably should

Edited September 16, 2010 by dsavitsk

[Beefy]

I figured given voltage and current, the Vf of the LED would change. So, if I had a supply of 100V and 18mA current through the LED this would be different than if I had a supply of 5V and 18mA. Now I'm doubting that gut instinct, as that wouldn't make the greatest voltage reference huh

Well, that is the whole benefit of using an LED isn't it? That the voltage across it is always the same regardless of current.

Good red LEDs theoretically have a reported impedance in the 8 ohm range. Bad ones are more like 12 ohms. I think anything you find will be in the high range. Green and Blue and whatnot might be as high as 12 to 14 ohms, but they are still generally lower than 2 reds in series. This impedance is multiplied by mu and added to rp. mu here is 38 and rp is ~3500, maybe more like 3K at high current. So, we are adding maybe 500 ohms to that due to the LED bias. To my ear, and by measurements I've done, this is more than made up for as opposed to a resistor and a cap. But, importantly, the difference between adding 400R and 500R here is pretty insignificant.

Thanks for that, it helps immensely

[dsavitsk]

I figured given voltage and current, the Vf of the LED would change. So, if I had a supply of 100V and 18mA current through the LED this would be different than if I had a supply of 5V and 18mA. Now I'm doubting that gut instinct, as that wouldn't make the greatest voltage reference huh

The LED sees the current, but it knows nothing of voltage. If you supply 18mA via a current source, the LED will drop Vf and the CCS will drop the rest of the voltage. if you have a voltage source of 100V, the LED will burn up.

Edit: By the way, you can think of the tube as an adjustable diode here. At a particular bias, it has a Vf and a fairly high dynamic impedance. And, as with the LED, the CCS will drop any voltage not dropped by the tube + LED -- maybe someone should try using a TL431 as a little sand tube .

Edited September 16, 2010 by dsavitsk

[Nebby]

You are not likely to blow anything up. You should have an email with the jumper information. If not, send me an email and I'll send it to you.

There are 4 jumpers, the two closest to the tubes switch between connecting the cathode LED and Rk, while the two by the transformers switch between connecting the transformers' secondary returns to the cathode and connecting them to ground. For the WE connection, you want the resistor connected, and the transformers connected to the cathode. For the LED connection, the LEDs (D3L and D3R) should light up and the transformers should be grounded.

Please note that these are not the CCS LEDs which you should leave alone.

I see now, when you referred to the "WE connection" I was looking desperately for something labeled "WE" in the schematic. When I didn't find anything, I got confused Thanks!

[Pars]

Oh goody! I can put blue LEDs in

[dsavitsk]

Good red LEDs theoretically have a reported impedance in the 8 ohm range. Bad ones are more like 12 ohms.* I think anything you find will be in the high range. Green and Blue and whatnot might be as high as 12 to 14 ohms, but they are still generally lower than 2 reds in series.

Oh goody! I can put blue LEDs in

Well, I think I spoke too soon. I picked up some 3.2V blue LEDs, and impedance seems to be in the 40 ohm range. So, maybe 2 red LEDs in series is better. Best might be 4 reds in series/parallel.

[kevin gilmore]

There is a reason stax always used red led's.

led's don't share current well, so putting them in parallel, usually does not work.

[Beefy]

led's don't share current well, so putting them in parallel, usually does not work.

I suppose that if the Vf aren't perfectly matched, you can have problems......

[dsavitsk]

If you put a small resistor in series which each, then they tend to do OK.

[url=http://syclotron.com/?page_id=31]SYclotron Audio

[Pars]

Well, I think I spoke too soon. I picked up some 3.2V blue LEDs, and impedance seems to be in the 40 ohm range. So, maybe 2 red LEDs in series is better. Best might be 4 reds in series/parallel.

Forgive the stupid question, but why does the LED impedance matter in this application? From your previous post, mu (38) * 40 ohms is 1520, added to an rp of 3500 = 5020 ohms. Two red LEDs in series, assuming 10 ohms, would be 20 * 38 = 760 + rp (3500) = 4260 ohms. Higher, but not huge? I'm sure I'm missing something here?

[CYoung234]

It has been quite a while since I have listened to that recording. I am maybe a little biased, as I have never been a big LSO fan - I always found the brass section in that orchestra to be a little lightweight and bright for my tastes. More a quality of sound issue than volume. The recording I own is the CSO / Tennstedt live recording, which is pretty good. It reminds me of the live CSO performances I have attended of Mahler 1, mostly at Ravinia. One I would like to have a close listen to one of these days is the Bernstein / Concertgebouw recording, which is praised for great sound, but panned for being too beautiful.

Okay - I checked out both recordings from the library to refresh my memory (the LSO / Solti and Concertgebouw / Bernstein). Things were pretty much the way I remembered them with the LSO / Solti recording. I will say that overall it is not a bad recording. I should clarify that my problem with the LSO is mainly stylistic - the principal trumpet player plays with a short pecking style that I find rather annoying. Listen to the first movement to see what I mean. Now, compare it with the other recording noted here - the Bernstein / Concertgebouw recording. Here, the principal trumpet players notes, while still short, have body and pop to them even at very soft volumes. Next, get a copy of the Tennstedt / CSO recording to hear Herseth play it.

I will say that I personally really like the Bernstein recording overall - critics jump all over his fast tempo in the 3rd movement, but I actually liked it quite a bit. The sound is very good as well.

Edited September 25, 2010 by CYoung234
becuz I can't spel

[luvdunhill]

well, vinyl release isn't cheap so I suppose I'll go for one of the little silver discs. In fact, I might get the entire Bernstein / Concertgebouw / Mahler collection, despite it being "DG Digital"

[fishski13]

... despite it being "DG Digital"

the DGs in my collection have a fairly dry, chalky house sound.