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!

 

[mcrushing]

Update on this experiment, for anyone interested:

So I plugged the Crack into the Manley's "sub out" jacks and set the Manley's volume where I usually keep it (a bit past 12 o'clock) and listened as the Crack drove it as a preamp for a while. Definitely heard a lot more bloom and heft in the midrange... "euphonic" is audio nerd term, I believe, but gotta admit I liked the weight and shape it gave to a lot of tracks.

However, compared to the Manley on its own, there was a huge loss of clarity. Going back and forth, I recognized a fuzziness when the crack was driving. That must be clipping... meaning too much gain... Oh, well. I relegated the Crack to the headphone duty it was designed for.

That is, until it dawned on me that if signal on the "sub out" node can get shunted to ground by the volume, they must be on the same node. And if that's true – DUH – that means the volume control is still dividing the signal unless it's turned ALL THE WAY UP. So I put the Crack back on the "sub out," cranked the Manley's volume wiper all the way over, and – no fuzziness, more fullness in the mids, and maybe not the same soundstage width... but more depth and more "solid" images. None of it at the expense of clarity.

TL,DR - the Crack makes a nice little preamp! Plus, this setup lets me change the whole system's flavor at will.

May even have inspired some Crack upgrades. I'll post those when I can.