Crack as preamp

[mcrushing]

Hi, all.... tried to re-ignite an older thread on this topic but the forum didn't like that for some reason. Anyway, I tried using my Crack as a preamp and got a puzzling result.
Wondering if Doc or one of the Pauls can explain...

The system: VPI Classic Sig > Kitsune LCR-1 phono stage > Crack (as preamp) > Manley Stingray Integrated > Vandersteen 1Ci's

I'll start by saying there's no logical reason try this. The Stingray/Vandy combo might be a price-point mismatch, but they play well: At 90dB/1m/2.83v and a 6.8 ohm nominal impedance, the Vandy's never hit 9 or drop much below 5, and the Stingray has a one-size-fits-all 6 ohm output tap. So together they play plenty loud/clean even in the 20-watt triode mode. BUT, Curiosity... DIY Instinct... Hifi Insanity... Whatever... Gee, I wonder what a 'lil gain will do.

In my setup, the Crack ordinarily drives a pair of HD600's via the Manley's "Loop Send" output. And because the amp's input section is passive (schematic attached), when I'm on Crack I'm just using the amp's input selector – signal never even makes it to the balance/volume, let alone the actual amplifier, so I leave it off. But on other forums you'll find lots of Stingray owners with active preamps in that tape loop for various of reasons... usually DSP,  extra inputs, or simply b/c the amp's rear-mounted dual mono input switches are annoying.

But of course puts redundant volume/balance pots in the signal path. And why do that, I reasoned, when there's a "Sub Out" jack? Copper is copper...should be able to just drive the amp section from there, using the Crack's volume and bypassing the Manley's, right?  Wrong.

I hooked this up with both amps' volumes fully attenuated, and found that the Manley volume still operates, even with the input following it. So... I know this is only tangentially Bottlehead-related. But what's going on here? Does the Manley's pot affect the gain section's input impedance? Is the schematic (which is from the manual) oversimplified?

I've been running the Crack-as-preamp for a few album sides, and the sound is...  if not better, different, in a pleasing way. But is this setup likely to drive anything too hard?

Thanks!

[Paul Birkeland]

You should just be able to take the loop output and feed that into the Crack, then use a 1/4" TRS to RCA cable to feed your power amp. 

[mcrushing]

Yes that's essentially where I started... the crack has always been on the loop output for headphone listening.

I then decided to see what effect it would have if placed into the loudspeaker signal path as a preamp, so the first move was to replace the HD600s with a TRS to RCA that went to the Stingray's "loop return" input. That works fine, but gives me redundant volume controls. That's when I got the idea (since they conveniently put a schematic in the Stingray manual) to connect the Crack's TRS output to the RCA's labeled "Sub Out" on the amp. Effectively, this uses the jack Manley intended for a powered sub as an input, instead. (The assumed benefit was to "skip" the Stingray's balance/volume controls, and just drive its active stages as if it were a power amp.)

Except what I'm finding is that even if the input follows the Stingray's volume pot, that pot still affects the signal. After experimenting a little more: It doesn't really affect the volume of the signal, it's just that the pot can't be set to zero, or nothing makes it through. So I'm wondering:

1. Why that happens
2. If I'm playing with fire by driving the Stingray's active stage from that Sub Out jack.

Driven by the Crack, the Manley sounds different/good, but with the Crack's volume past 8 o'clock it's LOUD, and I imagine past 9 o'clock could be clipping territory. So this probably won't be a permanent set-up change.... but I'm just curious in learning, electrically, what's going on. Thanks....

[Paul Birkeland]

If the volume pot is turned all the way down, that will short the sub output to ground, so nothing can make it in.

[mcrushing]

Ah, path of least resistance. Got it, thanks, Paul!