DIY preamp question

[dan85]

Hi all!

Since I want to build a tube bass preamp (an Alembic F2B, to be precise: [Invalid or Expired Link Removed] ), I'd like to know what output level it should have in order to drive:
- a solid-state power amp;
- a tube power amp.

In fact it seems to me that the output voltage could be very high ... even 200V peak to peak with the vol. at max. I would add a voltage divider after the second triode, much like in the Real McTube:
http://www.dogstar.dantimax.dk/tubestuf/graphics/mctube2m.gif

Any advice?

Thanks,

Dan

 

[Loudthud]

Many power amps have a volume control on the input that will act as a master volume control. It typically takes no more than 2V RMS to drive the amp to clipping when the control is maxed. That preamp won't be able to drive a very long cable so the McTube thing is not a bad idea. Some solid state amps have input impedances in the 25K to 50K range and the preamp may have trouble with that. The McTube thing will help with that too.

 

[zombywoof5050]

Some solid state amps have input impedances in the 25K to 50K range and the preamp may have trouble with that.

I'm not sure what you mean by this. Are you implying that it would be better if the input impedance is higher or lower than 25k ohms?

Alembic's website states that "the F-2B does not work well if the power amplifier input impedance is lower than 10 K Ohms", so 25K ohms should be fine. A lot of power amps (QSC for example) only have an (unbalanced) input impedance of 10K ohms, and that worried me when I was shopping for a new power amp for my F-2B. I ended up choosing a Carver because it's unbalanced input impedance is 25K ohms. Just make sure you don't go under 10K ohms input impedance and you should be ok. But the input sensitivity is important too, so make sure you don't get something with too low of an input sensitivity. With input sensitivity, lower numbers equals higher sensitivity (0.775v is a higher input sensitivity than 1.5v).

 

[BassmanPaul]

200V PTP should cause a nice explosion in the power amps input section. Two volts is all you need. SS power amps have input impedances much lower than tube amps. Can be as low as 5K and as high as 50K. In order to drive that input impedance effectively you need something like ten times less output impedance from the Pre. In my designs I look for about 600Ω or less. Most SS power amps have balanced input stages so the pre should be able to drive that too. This is easy to acheive if you use an output transformer from a cathode follower stage.

Paul