Wednesday, 5 July 2017

Yamaha CG-100A student guitar

The second guitar I bought for my students is a 1989 Yamaha CG-100A.  I have a soft spot for old Yamaha classical guitars.  This one was a level up from a student model, slightly better woods used and a glossier finish. A good clean up and fresh strings got it back to its original glory.  As with the Musima I grabbed a few weeks ago, the more I played it the more it opened up.

The dimensions of both guitars are almost exactly the same down to the millimetre. I didn't realise there was such a standard for the shape of a classical guitar! The Musima is about 1mm narrower on the neck and the action is a bit lower so it's slightly easier to play. Both shall make good beginners guitars.

The Yamaha is Japanese specification but made in Taiwan, and the Musima is German...sooo Asia versus Euro :-)  I had my wife listen as I played the same songs on both to see what she thought.

Results are Asia has a better bass, it felt like the bass was deep inside and not so much projecting out but booming from within like a subwoofer. It makes the guitar sound powerful, but muddy.  Euro on the other hand has a mid and treble projection that leaps out of the guitar, but the bass is subdued.  My Esteve on the other hand has clear trebles, mids and bass.

But don't take my word for it, have a listen! All captured exactly the same on my Zoom H1, unproceeded, all guitars wearing brand new D'Addario EJ45 strings

Musima 130:

Yamaha CG-100A:

Esteve 1GR11:


So, next stop, see if I can teach two twelve year old girls how to play some classical guitar!

Sunday, 2 July 2017

Fender FM212R amplifier debugging

I do some volunteer work for the school music program, mostly easy stuff like restringing guitars, but I was asked if I could fix an amp. "I'll have a look at it." I didn't live to regret saying it, but it really pushed the limits of what I am capable of.

It's a Fender Frontman FM212R, the last of the Fender analogue amps. Analogue as in it is full of transistors and resistors and capacitors and not a single digital chip inside it. There are always going to be analogue circuitry in an amp, but most amps these days use digital processing for any effects like reverb and drive.

Straight up some potentiometers were noisy. As in, as you twist a knob, there are crackles and hiss and thumps and it's pretty awful. This happens a lot, behind each knob is a potentiometer that generally has a carbon film with a mechanical "wiper" that sweeps across the surface changing the effective resistance. Crackles when there is dust and any other crud building up on the carbon surface. They aren't sealed, so buy a can of electronics "Contact cleaner" that come with a thin straw and squirt some inside the potentiometer (there are generally a few access points) and twist the shaft a lot, most of the time they come good. If they don't it's probably because the carbon film is worn down, in that case, replace them.

I ended up replacing 4 of them, now be careful, some pots are "linear" and some are "logarithmic". Generally, if it is "A" that means it is logarithmic. "B" means linear. This has not always been the case, when in doubt check! A linear pot, when half twisted, will be half way through its resistance. For example an A10k half twisted will be at 5k resistance, whereas a logarithmic will be a lot less than that. Why? It's how our ears work, you generally have to double anything to make it sound twice as loud. If you used a linear pot you'd be at half volume by about position 3, and the difference between say 9 and 10, being maximum volume, you wouldn't hear any difference. Generally, people want to hear an audible difference between volume 9 and volume 10.

Anyway, that was the easy part. The hard part was that the amplifier was distorting.

To diagnose this you need a schematic diagram of the circuit, an oscilloscope to monitor what is going on in the circuit while it is operating, and say an electronic engineering degree and/or several years of experience debugging analogue circuits. I managed to find the schematic circuit (last seen here: http://www.nodevice.com/manual/fm212r/get126250.html), source an oscilloscope, and I do have an electronic engineering degree but was quite lacking in analogue circuit debugging. But I do like trying!

The way to approach this is to inject a waveform into the amplifier, and then use the oscilloscope to check what the amplifier is doing as the signal travels through the circuit. Injecting a waveform is as simple as plugging in your guitar and sounding a note - but what you really want is a continuous signal. I did this with my laptop, an online signal generating website, and a cable from the laptop headphones to the amp. Let me tell you how sick I am of a 440Hz sinewave. I mixed it up from time to time, changing the frequencies, but man, constant pitch is truly awful.

Now a fixed frequency sine wave doesn't tell you the full story - if the amplifier is failing only in different frequency ranges, you might not see the problem. Square waves are great, (insert engineering discussion), they contain all frequencies. The problem is when I injected a square wave and observed the output, it didn't match the square wave at all! This is to be expected as a guitar amp doesn't have to reproduce a square wave. So it's hard to tell what's right and what's wrong.

But I was in luck. The problem did show at lower frequiencies of sine wave. Not easy to hear, but easy to see on the oscilloscope. You can see in the photo the yellow input sine wave is distorted in the blue output signal (click to zoom). Moving slowly through the circuit, comparing the input signal against the output signal, I found the section of the circuit where the problem. Which was a big section of many transistors, diodes, resistors, and, well, it helped but I couldn't say "it's that part!"

I did some of the basics first - I resoldered a big area of that circuit, in case any of the components had a dry joint. I poked components with a wooden chopstick while it was operating to see it it made a change. No. The distortion sort of came and went, sometimes the amp would play fine. This is the worst sort of problem you can come across; intermittent failures.

I decided that it must be a heat issue. So, with a heatgun (I could have used a can of cold spray, but heatgun was easier) I pointed heat at small sections of the circuit board and watched the output on the oscilloscope. Eureka! A bunch of diodes very clearly showed the distortion changing with heat!

Did I mention I am an electronics engineer? I haven't done electronics for a living for 20 years, but I recognised the signal diodes as the crossover biasing diodes for the class AB amplifier stage. Quick summary - class B amps have matching complementary transistors (or FETs) that deal with half the load each, and the crossover between the transistors creates distortion, so class AB introduces diodes to pre-bias the transistor pair eliminating the distortion.

There was also a diode that I didn't recognise the function of, some sort of signal feedback from a later stage I'm guessing, sadly it was a different sort of diode to the rest being able to take a much higher voltage drop. I couldn't tell which of the 4 were the problem, so I ordered some of each online. You could only buy the diodes in packs of 200, but at $14 delivered for 400 diodes it's not really a problem.

Since a class AB amplifier has a symmetry, I also decided to replace the other set of diodes, even though they weren't showing a problem. As a minimum, the new diodes will be more balanced than a new one and an old one, or, if they failed on one side of the amplifier symmetry it was likely they may fail on the other side too.

I desoldered the diodes and popped the new ones in. Couldn't believe it, problem solved. Played through the amp for at least an hour, perfect.

Let me tell you, I was well pleased with myself, I nearly gave up a few times, it took me two months of stuffing around from time to time on it, trying to keep myself encouraged. On the day I sent it back to the music teacher I said "don't put it into production until you have rebuilt some confidence in it." And here's the difference between a cautious engineer and a music teacher - that very same day he put it into an evening concert...

Thursday, 15 June 2017

Musima 130 student guitar

A family friend asked if I could give her some advice on getting guitar lessons for for twin twelve year old girls. This turned into I will buy everything they need and start teaching them. Which I'm fine with, I haven't had a student in years. Classical guitar teaching, of course, not that I play classical style, but I'm very founded in the ways of classical guitar.

I'm sure to blog more about teaching again, but for now, I wanted to find two classical guitars on a budget.

I've discussed before that I'm much keener on secondhand guitars over brand new budget guitars. Older guitars built back in the days when they were still raping the rain-forests for choice woods, and where the guitar has mellowed with age. Of course I'm glad the rain-forests are being protected now, don't get me wrong, but dollar for dollar I'm sure that your money is better spent in an older secondhand guitar. Perhaps modern day Chinese mass production factories get much better build tolerances, but we've known how to make great guitars long before mechanisation was a thing so It's not like a modern guitar is automatically better.

I purchased the first one which I thought was an exceptionally amazing piece of history – made in East Germany during the 80’s! Yep, a genuine socialist-made behind-the-wall guitar! The cheap fretboard wood must have been in a dry environment for too long, was showing a lot of superficial cracking but nothing to stop it from being playable. Was also missing 3 strings so I could only go by observation and intuition. I had already done a quick google before heading off to see it and it was basically a student level guitar, "Musima 130 Euro Classic", so even at the time it was not a fancy one, but probably sounded better than a brand new student level guitar. $140 plus a carry bag, a bit more than I wanted to pay, but, eh, I was there.

I cleaned it up, restrung it, started playing some rock’n’roll on it – “How you like that frauline? Das ist gut? Bet you’ve never heard anything that before, ya?” It played easily, so good for a student, but sounded kinda flat and dull. So, very East German. (Yeah, okay, a complete stereotype based on nothing but American movies).

After I had been playing it for around an hour to stretch the strings in, I was sure that it had “opened up” and was sounding a little bit brighter. Perhaps I was getting used to the dull sound but no, I’m quite sure Frauline was indeed brightening up. 30 year later - another part of East Germany reunified.

The next guitar I've lined up to look at next week is a mid 90's Yamaha mid-range classical. I'm a fan of Yamaha classical guitars - so long as it's not a C40 - which is in the class of Chinese mass produced student guitars I mentioned earlier :-)

JAW

Sunday, 28 May 2017

Commando High Score Recording

Although I arranged this a year ago, I hadn't got around to recording it.  In fact I hadn't recorded anything for nearly a year.  Partly because it takes me several hours, and partly because there isn't much need for more guitar videos in the world.  I feel like my music comes across better live, so I prefer to share my songs at gigs these days.

I have a few songs to record yet but this one decided to come out.  It's 1985 Rob Hubbard, nah, not the scientology bloke, the music composer.  My favourite of his is "Monty on the Run" but that is too much to take on.  This piece however, which was used in Command High Score and another that I can't remember, was somewhat simpler.  The SID chip it was composed for used three instruments, a bassline, a looping middle rhythm part and the melody.  I tried a few ways to put the middle into it but it never quite sounded right.  So I left it as a simpler bass and melody.

To keep video recording simple I zoom in on the guitar and keep me out of the picture.  I use my trusty Zoom H1 condenser microphone for the audio.  My DSLR camera is not easy to self-record but I don't think any are - well at least not the ones that have a backscreen that can't flip forward. So if you can't frame yourself, and set your focus, what do you do?  After much scouring of the internet to come up with a solution, I still simply use a mirror behind the camera.

So as you can see in the photo below, I've got the camera on a tripod with a mirror on another tripod behind it, and on the bottom left the microphone.  Standard deal - take video and audio separately, synchronise by eye and ear, tweak the audio EQ and the colour saturation, add a logo, job done.

Friday, 12 May 2017

Two recent gigs

I've played at South Beach Tacos in Fremantle twice now which has been a good experience.  Fremantle has more, for want of a better term, "hipsters", who as a complete generalisation get more into live music.  So in playing my stuff I get more connection with the audience, which inspires my "A game" which is good for everyone! I take  my active speaker, a stand for it, power cable, lead and guitar.  Minimal - though the speaker is heavy so I put it on a trolley if I'm going to be walking for a bit.  I sit with the speaker partially behind me so I can hear what is coming out of it, I play better when I'm playing to the sound that we are all hearing.

Speaking of playing to the sound you are producing, I played at my mate Shaun's new open mic at The Windsor in South Perth.  Shaun provides the sound system, you just plug in.  He's got a few speakers on poles directed at the crowd, and a big sub, and a foldback speaker for you, all through a mixer. Which is a pretty standard thing to do from what I've seen.  The problem being the foldback was giving a sound hard to play off; I felt that some notes were resonating, the bass wasn't coming through properly, the crowd was loud, etc etc.  They told me afterwards it sounded great, I guess it's just something I will have to learn. When I'm playing I'm very much playing off what I hear from the guitar, so if what I'm hearing is not a good indication of what is actually happening then I'm distorting what I'm doing to "fix" it.  I'm not sure if I'm making any sense...well picture playing the guitar with 100% block out earmuffs on.  Sure I'd be playing the right chords and all that, but I would be losing subtlety.  In the engineering game we call that open loop, as compared to adjusting for feedback which is closed loop.

Maybe I should record myself playing closed loop (playing to what I'm hearing) and playing open loop (playing without hearing what you are playing) and see if there is any difference.  Ha.

While there, I had an interesting conversation with a professional gigging musician who answered some of my questions with his insights. For example, I said "Good on cafe owners for getting me in to play live music, but I'm not attracting more business, how are they getting value?"  He said "When venues ask how many people I'm going to bring to a gig, I say 'zero'.  This isn't the 80's anymore, it's not about bringing a crowd with you. But what I do give is a better experience to the people there.  I'll turn somebody who might not come back for 6 months into someone who will come back in two weeks.  Someone will buy another cup of coffee or a beer or a bottle of wine a listen a bit longer.  That's the value I'm offering to the venue."

He waxed philosophically for quite some time further, my brain popped, I should have been taking notes :-)