500C Pre-Out?

[jonnyz123]

Hey all. Does the RCRDR out on my 500C act the same as a pre-out? I need to drive a second pair of speakers and didn't want to drive all four concurrently off the 500C. Or, could I do that safely?

Along those lines, are there power amps that pair naturally with the 500C? It will just be driving a pair of Canton Karat 920DC bookshelves (60W-100W, 90db) so I don't need anything massive.

Thanks and advance.

 

[larryderouin]

In a word NO for the Recorder output. It's before the Volume control and Balance control. If I'm reading th scat right it's also before the Hi filter, and both tone controls. Depending on the Amp you are using, there will be an impedance mismatch, which isn't always good. Solid State units are usually LOW Impedance, while Tube Gear tends to HIGH Impedance. Solid State stuff plugged in to tube gear tends to load down the tube gear excessively and the sound ain't that good. You could modify the Space expander jacks to operate as a PRE-OUT MAIN IN like a Solid State unit but it's a lot more involved than just moving some wires. You'll need to build an opamp buffer between the Volume control and the amp section of the 500c, and a couple other mods. Look up "improving the 400" by Dave Gillespie (it's in the Stickies Sub-Forum in the 1st page or under "Technical Threads for FISHER's" Thread there. Most everything that pertains to the 800c will also work with minor differences on the 500c. Anda lot of th mods to the 400 would also work on the 800c/500c.

As for the speakers, tube gear is a bit more flexible and not as rigidly Anal like Solid State gear is. As long as the speakers are 8 ohm (all 4 of them) and the impedance lead is on 8 ohms, no worries. You'll loose a couple watts if the volume control is set to 11 but otherwise it shouldn't hurt it. Currently I'm running an 800c (standalone) with Heresy's and NLA Advents in different rooms, and the spkr selector on A&B. It run 8-10 hours a day 7 days a week and except for the trannies getting a couple degrees hotter, and a slight loss of volume for a given setting, it runs fine. Word of CAUTION!!!! If your speakers are 4 ohm (all of them), and with both sets running at the same time you could be "down in the weeds" with impedances down around 2 ohms. Not good. Also it's NEVER a good idea to mix different impedances of speakers ie: a pair of 4 ohm on "A" and a pair of 8 ohm on "B". Make them all the same impedance and set the selector lead on the 500c to 8ohms. If you have some good 16 ohm drivers, you'll be somewhat surprised at how good they DO sound.

Good Luck and hope this clears up some of it.

 

[jonnyz123]

Thanks much @larryderouin ! That does clear things up. I had been running all four (8 ohm) speakers for a while, with zero issues. Then someone more experienced said that I was over-taxing the unit and that it was a bad idea. I think I'm going to take your word for it though. Noted on the impedance, very helpful.

 

[larryderouin]

The main difference is the fact that there is a transformer btwn the output's on a tube unit while the Solid state gear is direct coupled (with a few excetions that are cap coupled but even that falls into direct coupled for our purposes). Transistors are delicate and don't like anything out of their comfort zone (2 pair of 4 ohm speakers in parallel which gets you effectively 2.3ohms). Transformers can handle the change more gracefully and all that, but it's still not a good idea to run 2 pair of 4 ohm speakers at the same time. Now if the unit is designed to run "down in the weeds" (and a few very expensive rigs are) then there is no real problem. But it's always safer even in this instance to run 2- 8ohms than 2-4ohms.

 

[Tim D]

If the speaker pairs are of the same make/model of speaker (I.e. do you have two pairs of "Canton Karat 920DC bookshelves") you could run the two 4 ohm pairs in series. If they are different make/model then maybe not because the impedance and efficiencies wouldn't be a good match and the SPL wouldn't be evenly split.

 

[gadget73]

The bigger problem with impedance mismatches is the output stage ends up operating inefficiently. usually it produces less power and more distortion when you run it like that. Generally it won't damage anything, though it is hard on the tubes if you ask for a lot of power output. Running it at a couple of watts won't bother it much.

 

[thornev]

Tim - How do you get stereo running 2 speakers in series? Won't the last speaker in the series get the same audio signal as the first speaker in the series?

 

[Tim D]

Hey thornev,

Maybe I didn't say it right. You would have two 4 ohm speakers in series for the right channel (for 4+4=8 ohms) and two 4 ohm speakers in series for the left channel (for 4+4=8 ohms).

So when I say 8 ohms, though that is an average and speakers don't represent a flat impedance across the entire audible band. So doing this should be fine as a load for the receiver, but I'm not clear that would give the best audible results because it would exaggerate the impedance swings over the frequency range. I.e. A speaker that has 4 ohms at one frequency might actually have 12 ohms at another. If you add them in series suddenly at the one frequency you have 4+4=8 ohms and at the other frequency you have 12+12=24 ohms! See what I'm saying?

Again, as a load it would work, but you'd need to try it with two matching pairs of speakers so that the SPL is the same across all of them. Also you'd need to see how it sounds because the impedance swings by adding them in series could make an audible difference.

I invite others to chime in........

 

[larryderouin]

I've got a paper from I think, Dynaco for use with the A-25's stacking in series/parallel configurations. Stack 2-4-8 of the A-25 speakers and get a Wall of Sound. They showed you how to set them up and all.