Why can't I build my own ARC LS-5 or CJ PV-12?

[river251]

First thing you have to determine is what the preamp has to do. Gain, input impedance, output impedance. You can't pick a tube (or any other part) until you know what the overall preamp has to do.

Thanks Sy, will do. I'll hold off on the kit til I understand that.

Jim

 

[SY]

Once goals are defined, design choices get a LOT more straightforward.

 

[prairieboy]

Hi, thank you all. I picked these two just because of the acclaim they seem to receive from the tube audio crowd. I have a Conrad Johnson PV-5 mated to either a Marantz 8B or McIntosh MC-240 or MC-30s. I gathered this system in the 90s. I have just broken it out and love it, except that the sound is just too "thick" and "congested." I like the fat warm sound of individual instruments like jazz guitar or female voice, but for ensemble playing I would like it to be more clear, so when all the players in a jazz band kick in together it doesn't just sort of mush up and you could still hear individual instruments better. On the amp end I bought a SS amp to try cheaply, a B&K ST-140. It does sound slightly more clear, but not a lot. So I want to focus on the preamp. The PV-12 is supposed to be more neutral than my PV-5 but still with the CJ warmth, and people say ARC is a lot more neutral than CJ and the SP-8 and LS-5 are favored ARC preamps. Thus my choice of those.

Without adding to a discussion of circuits, tube type etc. just curious what speakers you are using to arrive at your opinions?

 

[river251]

Without adding to a discussion of circuits, tube type etc. just curious what speakers you are using to arrive at your opinions?

prairieboy, ProAc One SCs on Atlantis 4-post stands, no sub. I'm in a 11x12 room (bedroom->office). The Snell Es I've had for 20 years just were too bassy for the room.

Thanks,
Jim

 

[river251]

Once goals are defined, design choices get a LOT more straightforward.

Thanks Sy. When you're at my stage, though, it's hard to tell the means from the ends.

Jim

 

[BigE]

If your amps can be driven to full volume with 2 volts or under, then a passive volume control will provide enough 'gain'. In general, the concept of gain in a preamp is not necessary -- it is common to be always "throwing away" (attenuating) the input signal, so the preamp really just becomes a buffer with an attenuator (volume control) and a selector switch.

You need to check two things:

1) The output signal strengths of your sources.
2) The voltage that will drive your amp to full power.

If all your input voltages are high enough to driver your amp to full power, you won't need gain at all. What is the gain of the akido circuit?

A second thing to check is output and input impedances. A rule of thumb is for the amplifier input impedance to be 50 times (or more) than that of the preamp output impedance. What is the input impedance of the amp? (That's on the amp spec sheet.)

What is the output impedance of the akido? Does it vary with tube selection?

That's really all.