So I have a solid state DAC, Preamp, and Power Amp. I am adding a tube buffer (Musical Fidelity X-10D) (2x 6922EH) for some of that valve “magic”. I have it attached to the preamp tape loop. It sounds great but occasionally there is distortion. I suspect the DAC’s output is over driving the tube buffer stage. The distortion is not present when I do not engage the tube buffer (turn off Tape Monitor). I will mention, all connections are RCA single ended. I know the DAC output is “hot” at 2.127V. When I drop the DAC output (has digital “preamp” volume control) 4-6db the distortion seems to go away.
My questions (go easy on me, I’m totally new to tubes). Is the line level output voltages to high for the tube buffer input? It was advertised to be used between a CD player and a preamp. Is the DAC output too high with hot recordings? What is the preferred connection to support all my inputs (digital and MC phono)? Should I skip the Tape Loop/Monitor set up and instead place it between the preamp and the power amp? With this configuration, voltage will be attenuated by the active preamp and would unlikely every hit the same voltage levels of input unless playing VERY loud. I would rather not run the digital volume control on the DAC and give up the bits that it uses, and when playing DSD, there is no digital volume control (1-bit signal).
Thanks for the guidance. So far, aside for when the signal gets distributed, the sound with the tube buffer in place is very pleasant.
Andy
My questions (go easy on me, I’m totally new to tubes). Is the line level output voltages to high for the tube buffer input? It was advertised to be used between a CD player and a preamp. Is the DAC output too high with hot recordings? What is the preferred connection to support all my inputs (digital and MC phono)? Should I skip the Tape Loop/Monitor set up and instead place it between the preamp and the power amp? With this configuration, voltage will be attenuated by the active preamp and would unlikely every hit the same voltage levels of input unless playing VERY loud. I would rather not run the digital volume control on the DAC and give up the bits that it uses, and when playing DSD, there is no digital volume control (1-bit signal).
Thanks for the guidance. So far, aside for when the signal gets distributed, the sound with the tube buffer in place is very pleasant.
Andy
What voltage does the tube run off? Part of the "magic" of tubes comes from the fact that classic tube circuits are powered by high voltages (typically 200 volts give or take) and the signal is way below the rails at all times.
Transistor circuits that run off say 12 volts (a typical supply voltage) run way closer to the rails. This can decrease linearity. "Modern" tube circuits that run off 12-24 volts will almost certainly have decreased linearity over their classic counterparts that ran off 10 times the supply voltage. Exacerbating this is the fact that many modern tube designs do not have global feedback; they are trying to emulate the "classic" sound but they will always fail at this unless they use classic designs and classic parts (like 12AX7 which was in virtually every consumer audio tube circuit).
What you have may be poorly designed. Products like this exist to satisfy fads and have little to no practical use. It may be better put to use as a guitar overdrive stage.
Transistor circuits that run off say 12 volts (a typical supply voltage) run way closer to the rails. This can decrease linearity. "Modern" tube circuits that run off 12-24 volts will almost certainly have decreased linearity over their classic counterparts that ran off 10 times the supply voltage. Exacerbating this is the fact that many modern tube designs do not have global feedback; they are trying to emulate the "classic" sound but they will always fail at this unless they use classic designs and classic parts (like 12AX7 which was in virtually every consumer audio tube circuit).
What you have may be poorly designed. Products like this exist to satisfy fads and have little to no practical use. It may be better put to use as a guitar overdrive stage.
I have a feeling it runs from a 12VAC supply to make +-36V or so?
I found this schematic... Not sure if it's correct for the OP though.
I found this schematic... Not sure if it's correct for the OP though.
The second section of the ECC88 lacks biasing network. Also, why a negative supply in a very simple circuit?
The only thing I can think of is it's slightly safer that a single 72V supply - you would have to touch both rails to feel it.
The second ECC88 is using a 1M8 grid leak resistor which makes me think it's using grid leak bias? Also, with a low B+ you'd expect a lower bias point.
The second ECC88 is using a 1M8 grid leak resistor which makes me think it's using grid leak bias? Also, with a low B+ you'd expect a lower bias point.
It uses global feedback.
Given the value of the bias resistors, I'd say it runs off a lower voltage.
Given the value of the bias resistors, I'd say it runs off a lower voltage.
Grid leak is for hi mu triodes or pentodes with low screen voltage, not for medium mu. Also, 4.7M or more is needed, usualy 10Mohm.
NFB can't modify the distortion in the grid cathode diode when directly biased by the signal. When anode current saturates, the loop goes open, so negative feedback takes no action.
NFB can't modify the distortion in the grid cathode diode when directly biased by the signal. When anode current saturates, the loop goes open, so negative feedback takes no action.
Yes. Unless doing so puts an unbearable amount of background noise into the system. If your power amp has level controls at its input, even better that you could reduce its sensitivity.
I imagine the TubeBuffer wants a "just right" level going into it; if tape loop too hot, maybe preamp with power amp inputs wide open too cool, but with power amp turned down and preamp outputting just a little more than usual to compensate, the TB is happiest.
If your power amp doesnt have level controls, then the noise with the TB inserted would determine the feasibility of connecting it that way.
Also, of course, be aware of power on/off issues as I expect the power amp will be up before the TB's tubes come on - any electrical humps, bumps or jumps coming from the TB at power on will be amplified by the full power of your amp! Not so withn the tape loop...
Thanks for the input. Some great information here. I played around with more hook up options. When inserting it between preamp and amp it sounded good but introduced noise. Inserting it directly after my DAC before the preamp produced exactly the same results as having it on the tape loop. The issue is only really a problem with some bad recordings that distort already and are exaggerated by the buffer. Dialing back the DAC’s output 4db seems to be the sweet spot. I have it back on the tape loop and I am happy with the results.
This unit has a linear power supply designed inside the unit. Here are some more details on it.
1: uses a better parallel Class A regulated power supply and boosts the voltage to +/-36VDC
2: The LT1085 regulator is used to supply power for the two tubes , which avoids the failure of the internal resistance of the filament due to the different internal resistance of the filament.
3: Use VISHAY glossy glass-sealed rectifier
4: The main filter capacitor is Nikko KZ 1000uF
5: Transistor for power supply, Toshiba A970-C2240, A1358-C3421
6: Coupling capacitor adopts ERO series
This unit has a linear power supply designed inside the unit. Here are some more details on it.
1: uses a better parallel Class A regulated power supply and boosts the voltage to +/-36VDC
2: The LT1085 regulator is used to supply power for the two tubes , which avoids the failure of the internal resistance of the filament due to the different internal resistance of the filament.
3: Use VISHAY glossy glass-sealed rectifier
4: The main filter capacitor is Nikko KZ 1000uF
5: Transistor for power supply, Toshiba A970-C2240, A1358-C3421
6: Coupling capacitor adopts ERO series
I had pretty much the same issue with the SP1 tube used in the Pass B1 buffer. Probably not the best idea but I put a volume control on the input. I also wonder if the tubes you have are part of the problem. Just shooting from the hip.