I have the Boozhound labs MC pre-pre and a Creek phono board built into my Aikido tube pre and the combination of the three gives me way too much gain. I asked Jason of Boozhound about reducing the gain of the pre-pre, and his response was to put a voltage divider between the pre-pre and the phono pre. This sounded simple enough, but what I don't understand is that he recommends high value resistors to do the voltage divider instead of low values.
My question is why? I tried reading an explanation but it had a Java applet that wouldn't run on my computer. If the phono input impedance is 47k do I need to use resistors in that same ballpark for the divider? Is it the parallel load of the input impedance that becomes part of the equation? Is it the current? I just didn't understand the explanation since the diagram didn't work.
Another related question then: If I wanted to reduce the gain for the entire system, not just the moving coil phono, where do I put that? Can I just use a different volume pot? I think the ones I have are 100k. I want to be able to turn the control past 9 o'clock, about as far as I can go now. With the MC phono I can't go past 7 o'clock, and that makes for hair-trigger control.
I couldn't design a circuit if my life depended on it, but I do like to understand what I'm doing.
My question is why? I tried reading an explanation but it had a Java applet that wouldn't run on my computer. If the phono input impedance is 47k do I need to use resistors in that same ballpark for the divider? Is it the parallel load of the input impedance that becomes part of the equation? Is it the current? I just didn't understand the explanation since the diagram didn't work.
Another related question then: If I wanted to reduce the gain for the entire system, not just the moving coil phono, where do I put that? Can I just use a different volume pot? I think the ones I have are 100k. I want to be able to turn the control past 9 o'clock, about as far as I can go now. With the MC phono I can't go past 7 o'clock, and that makes for hair-trigger control.
I couldn't design a circuit if my life depended on it, but I do like to understand what I'm doing.
The best way is indeed to reduce the MC pre-pre gain.
If the Boozhound labs MC pre-pre has the same schematic as the Le Pacific MC pre-pre shown here :
Le Pacific JFET Phono Preamp for MM and MC Cartridges
then reduce the value of R11 ( 47R ), this will lower the gain.
Just solder another 47 ohm resistor over R11 ( = parallel ) as a start.
If the Boozhound labs MC pre-pre has the same schematic as the Le Pacific MC pre-pre shown here :
Le Pacific JFET Phono Preamp for MM and MC Cartridges
then reduce the value of R11 ( 47R ), this will lower the gain.
Just solder another 47 ohm resistor over R11 ( = parallel ) as a start.
Thank you DF96, yes reducing the value of R11 will raise the gain.
Anyway, changing the source resistor ( R11) is the best way to alter the gain without changing the operating point of the Fet to much.
Changing the load resistor ( R9 ) will alter the operating point more and reducing this value to lower the gain will most likely add more distortion.
Anyway, changing the source resistor ( R11) is the best way to alter the gain without changing the operating point of the Fet to much.
Changing the load resistor ( R9 ) will alter the operating point more and reducing this value to lower the gain will most likely add more distortion.
That would roll off the bass at around 150 Hz. You'd need more like a 10uF coupling capacitor then.
Have you tried going straight into the Creek phono stage, with no head amp?
Sounds like the Creek could be overloading due to the MC preamp's high gain.
You could add a 100 Ohm resistor at the Creek's input if you want that value of loading.
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Actually, I had the turntable going straight into the Creek, but then I didn't have enough gain. I would max out and the preamp/amp would run out of steam and get noisy. My cartridge is an Ortofon Quintet Blue, which puts out .5mV.
I was just at my bench checking on supplies and I thought I had more resistors than I do. I'll have to keep looking. I know I had a supply of Vishay/Dale resistors at one point, but I don't remember what values.
I should mention that I'm not displeased at all with the sound, just that I can't adjust the volume control easily. And to make it even worse, I have dual mono controls and they're noisy.
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Then you probably need about 20dB gain in the MC preamp instead of 30dB.
That would set the MC stage's output level to a more normal amount for the Creek input.
One thing you could try is to split the 2.2k into a series 750 and a 1500 Ohms, with the 1500 Ohm resistor connected to the transistor.
Take the output from the junction of the two resistors. This would give you about 10dB reduction of gain.
Yes, that's right. You may have to hack a pcb trace to do this. Be sure that the 1500 Ohm resistor connects to the transistor,
and the 750 Ohm resistor connects to the power supply. Otherwise, you won't get enough gain reduction, only around 3 dB instead of 10dB.
I meant that the 1500 Ohm resistor has one lead connected to the transistor, and the other lead to both the 750 Ohm and the coupling capacitor.
One lead of the 750 Ohm resistor connects to the 1500 Ohm resistor and coupling capacitor, and the other lead connects to the power supply.
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