Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: garfo on April 10, 2015, 10:38:18 PM
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Good day,
Was wondering if anybody has tried to change the LED with typical R+C and what results has he experienced.
Many thanks,
garfo.
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You can replace each LED with a resistor, and you will reduce the overall gain of the preamp, but it will otherwise still work. Adding a large capacitor across the resistor will restore the gain. (A pair of 470 Ohm resistors are acceptable)
The LED offers the best of both worlds. It gives us the proper bias voltage that we want, with very low AC impedance (like a capacitor), but it sounds way better than any capacitor we have ever tried.
-PB
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Thanks a lot. Do we know how much current does the ecc82 absorb ?
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Approximately 3.8mA.
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Ok, so we shd actually use a lower value than 470R for the cathode resistor. Nevertheless will start with 470R tantalum/1W and 1pct and move from there. What about shunt caps ? Is 150uF ok ?
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A 1W resistor is physically way too big for the job. 1/100 of a Watt is enough power for the resistor, go small so you can get them to fit! Additionally, when you put a big cap across them, the cap is providing the low impedance path to signal currents.
For what it's worth, the LED will sound a lot better than any resistor/cap combination, so I wouldn't spend too much money on parts that you may well end up throwing away.
-PB
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If LED biasing is that performant, why is it so rarely used in SE applications ?
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Good question, but Bottlehead uses it often. In their upgrades they are replaced with constant current sources. But your question would be better aimed at the companies who don't use LED biasing.
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Unfo Mr Kondo has passed away, otherwise I would have asked him... ;)
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In anycase it appears that given 3.8mA absorbed by the tube, the correct value for R should be 390R. This will give more or less the 1.5VDC tension. Ie 390x0.0038=1.482V, owise 470R will give 1.78V.
Am I wrong somewhere ?
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If LED biasing is that performant, why is it so rarely used in SE applications ?
Solid state parts are often "scary" to potential SE customers. It also costs a lot more than just using a resistor/capacitor combination.
Other technology in our amps that you won't find in SE amps:
1. Shunt regulated power supplies.
2. Parallel feed output topology.
3. Constant current source loads.
4. Cree high voltage Schottky diodes
etc. etc.
The biasing diodes give us exactly the bias we want with low dynamic impedance across the audio band (not necessarily the case with a cap). We could save some money by going with a resistor and capacitor instead, but it just doesn't sound as good.
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Money not an issue really, a good cap and a resistor can easly cost twice the led. For your guidance, here in Eu, we can easily buy a pair of HLMP6000 for 50c (sensibly less when buyong more than 1 piece). A good capacitor, like Evox Rifa for example, can go upto 10 dollars.
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Money not an issue really [...] A good capacitor, like Evox Rifa for example, can go up to 10 dollars.
I think money is exactly the issue. We seem to notice sometimes that folks will throw money into parts for their kits that may actually hurt performance. (I did this a lot when I was first building kits)
I'll just reiterate that in our 20+ years of designing and building many different tube circuits, no resistor/capacitor combination has come close to the venerable HLMP-6000. (Even Blackgate caps and Tantalum resistors) You will also notice that in our least expensive tube preamp (the Quickie), we have compromised for costs and used a resistor and capacitor per channel for biasing.
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Thanks Paul. Is the HLMP6k the only LED suitable for biasing Crack's first stage ?
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It's the best one we have found (it maintains its characteristic voltage at many currents, and it's quiet).
There was a builder some time ago who converted to a 5687 driver tube, and that required a different LED (for a different bias voltage).
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FWIW, a few months ago a built a pair of Paramour IIs. I was missing the leds an used resistors bypassed with black gate caps. The amps sounded dark. Then Keto loaned me a few hlpm 6000 leds and the amps have been shinning since then.
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Thats seems to be a very strong statement. Have heard tons of excellent sounding amps with RC on each stage. Certainly the direct coupling of the Crack has a greater influence on the sound that the led biasing. As said beforet, I am only curious to try a good pair of resistore and capacitors and make my own judgment.
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I have also heard very good amps with rc biasing. I'm not generalizing. I'm in the process of building a two pairs of amps with MQ silver iron taht will have rc biasing on the driver.
Still I couldn't believe the difference when going from rc to leds on the Paramours.
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IME a bypass cap on a cathode colors the sound. Running without a bypass cap means you will have some degenerative feedback that will lower the gain. Whether or not the circuit is direct coupled is irrelevant to this, that is, to say that direct coupling or whatever has more influence does not change the fact that there are differences between using LED bias and RC bias. I will suggest that using an LED actually makes the direct coupled design easier since it helps to stabilize the 12AU7s plate voltage. I do think that you are making the correct choice of trying it for yourself and decided whether it works for you. But I'm sorry to inform you that even if you think a resistor and cap sounds better we won't be changing the design ;)
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That's not my aim. The Crack has nice dynamics and runs with power but has a sort of hardness. Not sure wether this is the result of led biasing or something else. I do appreciate the direct coupling a lot (hv all Spectral gear in the main system) that it gives a lively feel to the music presentation. It is certainly a good project but could go beyond expectations and maybe I am starting from the wrong prospective instead of thinking of a better power supply.
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By the way, have just purchased a Sennheiser HD600. Sofar I have been trying the Crack with a pair of Beyerdynamic DT990 Pro (250ohm), I am pretty sure that the 600ohm of the Senn will help the Crack to come up to its best. ;)
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HD600s are 300 ohms, so the loading won't be that much different. I do however much prefer the HD600 to the Beyers myself. I would suggest not changing anything in the amp until you have listened enough to both to decide which headphone you want to use. Then you can start to fine tune the setup.
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Indeed that's maybe a wiser path ;) Thanks quick replies, will post my impressions once the hd600 arrives.
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Here a shot of the power supply wiring.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Foi61.tinypic.com%2F1zx7az8.jpg&hash=da2e96c673bdf6ef9f5625e42bc69308068b7136)
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I'll just mention a couple further technical issues.
Using a cathode resistor to set bias is sometimes called automatic bias because it provides negative feedback at DC to stabilize the plate voltage. Normally that would be the reason to avoid fixed bias such as an LED. But with a current source plate load e.g. Speedball in Crack the cathode current is constant so a resistor cathode bias will not supply the bias stability that it does in a normal resistor-loaded triode amplifier stage, so there is no reason to avoid it.
If the resistor is unbypassed, it can sound comparable to an LED. If the plate load is a current source, and the following load is high impedance as well, this won't affect the gain. But the negative feedback it provides in the audio range causes the plate or source impedance to increase. As long as that higher impedance is not a problem, there is no reason to avoid the resistor. In the new Stereomour II, I have increased the plate current in order to reduce the resistor value to where it does not make the 12AT7 plate resistance too high. I did this to get a bias voltage between the HLMP's 1.55v and the TL431's 2.50v. Engineering is always a matter of tradeoffs; the meaning of the word "best" varies with the context.
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Job done, values 390R/1W and 150uF. Overall sweeter sound, probably slower transients. Will prob get things better with an improved power supply. Suggestions ?
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Foi58.tinypic.com%2F28i9p5l.jpg&hash=e0d9fb308b95c034a618cae6c1bb4644a86c172a)
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Will prob get things better with an improved power supply. Suggestions ?
Give the amp 100 hours to break in the capacitors. Then at hour 101, put the LED's back.
-PB
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Will certainly do that. Was referring to a better PSU because irrespectively from using an LED or an RC, the amplifier sits a bit when reproducing large orchestras and generally when reproducing complex musics. As long as u go with a jazz trio all fine of course, very nice textures and feel.
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the amplifier sits a bit when reproducing large orchestras
I'm not quite sure what is meant by that.
Knowing about your source components and headphones would be helpful. This may be distortion on transient peaks that would happen at much higher power levels with the Speedball. (Sorry, the Speedball removes four resistors and adds even more LEDs!)
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Nothing serious really. I meant to say the ability of the amplifier to resolve large and complex orchestral music into each single elements. At moment I'm on iMac USB into MF V-Dac tru Spotify Premium (of course it is not HD but overall very enjoyable). Of course Im used to the performance of my main system (MacMini firewire into Weiss Int202 into dCS Elgar, then Spectral DMC30/DMA100s into Magnepan MG12QR) which is quite revealing and exciting.
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I'd try better source material. Spotify's ability to resolve complex orchestral music over an internet stream may well be the issue. (You also still haven't mentioned the headphones you're using)
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Beyer DT990Pro 250ohm (waiting for a pair of Senn HD600).
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Went back to LED. Not huge difference to be honest but def more clarity, breath and dynamics. Maybe female voices sound more sexy but otherwise way better the little red light ;)
Thanks everybody for help and for their thoughts ! See u soon.
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I can remember working on a very early Crack prototype a long, long time ago for a musician who wanted a high end headphone amp. I remember sitting down and listening to at least 15 different tube operating points, then I finally found one that just sounded really good. I took voltage measurements, scratched my head a little bit, then realized the operating point was one that I had run into building Bottlehead kits with the same tube.
The biggest improvement to be had in the Crack, beyond the Speedball, is shunt regulation. Unfortunately, you really need a bigger power transformer and chassis to accomplish that.
-PB
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It is probably worth building a Crack II with a sister chassis for PSU only. I'd be a buyer !
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So what IS the accepted load resistor here to use with a capacitor?
I have seen everything from 390 to 470 to up to 1.5k or even 2.2k. A friend wants me to mod his Crack by removing the cathode LED's for 2.2K resistor and 2000µf bypass cap. Just running the values, it seems 2000µf is way overboard where a 470µf will do given 2.2k. Thoughts?
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It is probably worth building a Crack II with a sister chassis for PSU only. I'd be a buyer !
I have done a lot of repairs on audio equipment with umbilical cords over the years, and only once was the umbilical not at least part of the issue. There really is nothing to be gained by doing this.
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A friend wants me to mod his Crack by removing the cathode LED's for 2.2K resistor and 2000µf bypass cap.
I would let your friend know that the designer of the amp said this is a pretty substantial downgrade in performance.
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It is probably worth building a Crack II with a sister chassis for PSU only. I'd be a buyer !
You could just build a Crackatwoa.
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I would let your friend know that the designer of the amp said this is a pretty substantial downgrade in performance.
Yes, and a couple of other friends did the RC mod on their Crack and raves about it. I’m sure it IS different, and I highly doubt “betterâ€. I did the reverse mod for a similar design, the Darkvoice 336 SE and I loved the LED’s. So much better transients and speed. It dropped the hazy slow blunted sound.
But if this person INSISTS on the changes, I just want to say on the same operating point as close as possible.