Using R+C instead of LED.

[xcortes]

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.

[garfo]

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.

[xcortes]

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.

[Doc B.]

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

[garfo]

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.

[garfo]

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.

[Doc B.]

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.

[garfo]

Indeed that's maybe a wiser path Thanks quick replies, will post my impressions once the hd600 arrives.

[garfo]

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)

[Paul Joppa]

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.

[garfo]

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)

[Paul Birkeland]

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

[garfo]

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.

[Paul Birkeland]

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!)

[garfo]

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.