Using R+C instead of LED.
- Thread starter garfo
- Start date
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
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
Paul Joppa
Moderator
Approximately 3.8mA.
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
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
Grainger49
New member
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.
Solid state parts are often "scary" to potential SE customers. It also costs a lot more than just using a resistor/capacitor combination.garfo said:
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.
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)garfo said:
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.
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).
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).
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.
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 
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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