Inside my QSC 1400, and getting it to drive!

I picked up a QSC 1400 at a tag sale in Oakland a few years ago for 20 bucks. I didn’t really ask too many questions. It’s an old, solid-state Series One amp from the 1980’s, but I’ve worked with a a few newer model QSC amps in live settings, and I know they’re work horses–even if this QSC wasn’t in total working order, it’s definitely worth the money.  Should have a ton of power: 200W per channel at 8 ohms. Pretty sweet.

Unfortunately, until recently I’ve tried pairing the amp with a few different speakers to pretty middling results. A while back I was playing with a show, and I thought I’d take advantage of Guitar Center’s no questions asked rental return policy and with some Peavey SP-12’s, and found that I wasn’t able to get any power. I also recently picked up some vintage Polk 5B Monitors (with sweet Peerless tweeters), and similarly haven’t gotten very much power out of them. So I decided to open up the QSC and see what was going on since at the very least I could tell she was dusty and needed a clean.

In spite of this hasty, desperate desire to dismantle and debug, I took a look at  at the manual, and realzied the  problem is mostly likely an impedance matching issue. So far, I had tried running the signal off my MacBook, iPod and a low-impedance mic to crappy results. Since this is a high-performance amp that usually runs with at least a mixer in the chain, it’s got 10K (high-impedance) inputs. When you plug the low-Z signal into the high-z input the resulting voltage that get’s amplified is too weak to really drive the speakers. Unlike the consumer receivers you buy at Radio Shack, the QSC has no preamp stage.

I called my homie (and fellow audio nerd) Caley who plays in Extra Life and asked him if my hypothesis was correct. (I only mention this because the advice he gave me was spot on.) While he suspected that the output from a computer or iPod should be enough to match the input requirements of the QSC 1400, a good test would be to insert a mixer or preamp into the chain. If the QSC was clipping and the output was still weak, the there’s a good  chance the amp was at fault.

I picked up the condescendingly-named, super-budget  M-Audio Audio Buddy mic preamp ($50), which says it’s output impedance is 500 ohms. While this means it’ll bridge at 5K (and not 10K), it should be more than enough impedance to make the QSC sing. Presto. The Polk’s were pushing air nicely at all frequencies with almost no gain out of the Audio Buddy or QSC. I already pissed off my roommate.

I’ll actually go through this chain with a voltage meter and test this hypothesis for real. More to come.

Anyway, photos of the QSC’s guts after the jump, including shots of some resistors that could use replacing …

I was a little nervous opening up the case. A friend of mine was once restoring an old vintage synth he found in his basement, and when he opened it up, found a gnarly creeping plant growing in side.

Luckily, it was just a dusty mess inside, but it really could have been in worse shape. A bunch of the resistors need to be replaced. R18a and R27b are the most glaring examples. They’re apparently still partially functional (the amp still carries) the signal through), but they’re clearly worse for wear.

Another view of fried R18a.