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Look out for Mike “Electricstuff” with fingers in his ears
For more aluminium welding practice I decided to weld up the knob holes in the original Corvette front panel. The front looks OK but you don’t want to see the other side. I then made a new layout using MAD- permanent Marker Aided Design.
Next step was to finish wiring up the Marshall 2204 preamp.
See earlier posts on the Ninja Corvette Hybrid if you’re puzzled by all the extra transistor stuff.
Yeah it makes the master volume Marshall racket 😀 This was recorded at the 1W power output setting.
Got some matching knobs from Thonk. I will get round to making a completely new panel one day using the MAD one as a template.
Soon the phase inverter and power amp were finished and working.
One big difference is that the 2204 uses negative feedback around the output stage while the original Corvette didn’t. So I decided to go with the NFB, and include the transistor output stage in the feedback loop too for an extra challenge.
It was perfectly stable first time! LOL just kidding… It suffered from high frequency parasitics-
And these comically chaotic LF oscillations could be provoked by overdriving it at low frequencies.
After much trial and error I ended up with something like this. The 0.68uF/5 ohm RC snubber killed the HF oscillations, and removing C17 and C18 (this schematic) stopped the motorboating. With these values it was just barely stable with the load disconnected and a 220k NFB resistor (vs 100k in the original 2203 circuit)
Note that when the transistor output stage is in play, the OPT secondary becomes bootstrapped and flies around with the speaker output, so the NFB takeoff point I used sees the output voltage of the transistor stage plus the output voltage of the valve OPT.
Removing C17 and C18 demanded quite a lot of extra current from the bias generator, but it seemed to deliver it no problem, so no changes were required there.
Resistor values were also changed to reduce the current gain of the transistor output stage, due to the increased output of the valve part of the circuit.
I got bored of the Ninja Corvette Hybrid and decided to transform it into something with a little more “FU”.
The plan I came up with was to strip out the valve part of the circuit and replace it with a clone of a Marshall 2204. This is a classic rock amp that I hadn’t had much experience with.
I decided to use 6AQ5 power tubes running off 250V, for a modest apartment-friendly power output. The 3 position power switching would be retained, giving power levels of 1W, 10W and 40W.
The Marshall 2204 circuit has 5 valves, but there are only 2 holes in the chassis…
A new output transformer was also required, as the original one was single-ended. I used the cheapest PP one I could find at TAD. I also TIG welded a bracket for it, as I’ve been watching way too much Project Binky.
I got fed up with the thumb controls on the torch that came with my welder and decided to try a foot pedal. They are extremely expensive to buy so I decided to make one out of an unwanted wah pedal.
The main difference between a wah pedal and a TIG foot pedal is that the switch engages as you first apply pressure to the pedal, telling the welder to open the gas valve and go through its preflow and ignition cycle. Depressing the pedal further then controls the welding current.
To allow this operating mode I threw away the existing stomp switch and replaced it with a microswitch operated by a cam.
The other difference is that it needs a rather strong return spring to avoid igniting the welder by accident. I found this out by trial and error. D: Keeping with the musical instrument theme, I used some Strat tremolo springs from eBay.
I connected it to the socket for the hand controller as a spare plug for this was supplied with the welder. It thinks it is a torch with 1 button and thumbwheel. This seems to work fine, but I will try the foot pedal socket if I ever get round to figuring out the mating plug for it.
And the bead goes on…
The Crown SXA has had black power transformers and silver/rust coloured output transformers for like a decade now 😉 The black Hammerite spray paint is great.
8″ speakers never really did anything for me, so I decided to wedge a 10″ into the Ninja Corvette.
It was just possible by carefully chiselling away parts of the cabinet.
It sounds a lot better than the 8″. Unfortunately I have no idea how to make a good grill for it now!
After a few snarky messages from Mac owners, I thought I had better post a follow-up. I experimented a little at the weekend and found the following:
If I leave my soundcard (Audiophile Firewire) permanently on 96kHz, then:
iTunes will resample to 96kHz internally, with quality that sounds just fine.
VLC will output at whatever the sample rate of the media is, and get resampled to 96k by AUHAL on its default quality setting, which sounds a bit sketchy.
But, VLC can be set to always output at a particular rate, resampling internally. There’s an option called “Frequency” in the advanced audio preferences. According to VLC’s debug stream, it uses a module called “bandlimited_resampler”, and that’s encouraging, because it suggests that the guy who coded it knows the word “bandlimited”. It uses about 6% CPU time, and again, sounds fine. Thanks to the wonders of open source, we can see bandlimited.h here! Ooo, a 1536 point sinc kernel with linear interpolation and single-precision floating point arithmetic.
I got VLC’s “Media Library” to work too, so it can store a playlist, remembering what FLAC stuff I have and the track order of it.
I’ve not tried to play DVDs yet, but hopefully the computer won’t catch fire.
So in conclusion, I’m upgrading this from “:(” to “:)”
Recently I’ve been experimenting with higher definition music downloads. For example, the Studio Masters recordings from Linn Records, or the material from B&W Society Of Sound. Everything went nicely until I got my new Mac.
First of all, the only lossless format supported by iTunes is Apple Lossless. It won’t play FLAC (the open source Free Lossless Audio Codec) properly, even with the xiph.org extensions, Fluke, or whatever. A bummer, as I already had a fair amount of music in FLAC format. Sure, VLC will play it, but VLC can’t understand iTunes playlists.
Second, Linn don’t support Apple Lossless. They supply their music in FLAC and WMA formats only. I guess they’d rather you bought their own stupid, overpriced DS network media players, even if you have a Mac already.
Third, Mac OS resamples! Windows was smart enough (or dumb enough?) to set your audio hardware to the actual sample rate and bit depth of the track that was currently playing. Or at least the setup I used, Winamp Classic with the ASIO driver, would do that, guaranteeing bit-accurate output under all conditions.
The Mac doesn’t. You set the sample rate and bit depth once and for all in the audio control panel, and all the sound gets resampled to that. You can play 96kHz material till you’re blue in the face, using any player application you like, but if the system sample rate is set to 44.1, it’ll get downsampled. Watch a DVD, and the sound is getting resampled from 48 to 44.1.
I don’t know about you, but I wouldn’t trust any resampling algorithm I hadn’t written or at least tested myself. They’re notoriously hard to get right. And a “right” one consumes so much CPU power that there’s a real incentive to downgrade the performance deliberately.
Here’s the official line from the Coreaudio mailing list, found here
This was a comment posed to the list (which was replayed verbatim from Benchmark’s Elias Gwinn)
> […] If the user changes CoreAudio’s sample-rate in AudioMIDI Setup to something different than what iTunes is locked to, CoreAudio will convert the sample rate of the audio that it is receiving from iTunes. In this case, the audio may be undergoing two levels of sample-rate conversion (once by iTunes and once by CoreAudio). (The SRC in iTunes is of very high quality (virtually inaudible), but the SRC in CoreAudio is horrible and will cause significant distortion.) If the user wants to change the sample rate of CoreAudio, iTunes should be restarted so that it can lock to the correct sample rate.
This is the response from an Apple employee:
iTunes uses the AudioConverter API internally but we set the quality to “max” and AUHAL probably uses the default (I don’t know). One SRC at max quality followed by one at the default quality is not so great when analyzing sine tone playback.
So the bottom line seems to be, that if you have material recorded at a higher sample rate than 44.1, then you need to turn your system’s sample rate up to appreciate it. And if you do that, then all of your other stuff will get upsampled, though at least it uses Coreaudio’s best quality algorithm. Or does it? Leopard introduced a new “Mastering grade” algorithm, and it’s not obvious whether that one is used.
One step forward, two back. Come on Apple, twiddle those nose rings and come up with a sexy solution.