28-11-2016, 10:39 PM
No worries! I believe the modern expression would be "TL;DR", apparently meaning "Too long; didn't read" 
Andy,
That's a lovely workshop! I've very jealous of all that natural light and the views. My workshop is in my cellar, with no natural light and herds of wild elephants above me. Or at least, that's what it sounds like when the kids run up and down the hallway over me (really must fit a carpet one day!).
I've got a much clearer picture of what you've been dealing with now. Certainly not a easy job, but it's getting there. Very impressed with the paint job
Having seen those 15V regulators, I'd have no worries about fitting some kind of heat sink between the PCB and the IC itself. Something like this, perhaps? https://www.sparkfun.com/products/121 - or if there is space, this might be better? https://www.rapidonline.com/to220-heat-s...ting-29625 - these are the sorts of things that I salvage from old gear, as they're always useful. Alternatively, you could DIY something from a scrap of aluminium or copper sheet? Doesn't have to be pretty - only needs to have surface area. Might be wise to insulate the IC from it if there's a chance of something coming into contact with it...
That distortion meter might be better than you think - you quoted 0.1%, but that's the full-scale reading, meaning it ought to measure down to 0.01% with ease. It does rely on the source sine wave being clean, but that's easy to check. Frankly, if it's below 0.1%, it's likely well below audibility for most programme material.
I couldn't get a good freeze frame on the model number, but from a Google image search, I reckon it's the TF2331? Not the A version, which has a red knob at the top-left?
Looking at the manual, it looks like a nice bit of gear. The notch filter claims 80dB depth, which is 0.01%. The rectifier is mean rather than "true" RMS, but that's fine for most practical purposes.
Using distortion meters can be confusing. Setting the 100% scale reading before switching in the notch filter is critical, and how you do that varies from instrument to instrument. And picking the test level is fun. If it's too quiet, the noise of the amplifier can dominate the reading. Too high, and clipping might take place. Some trial and error might be needed. It helps to listen to the output - post notch-filter. On your instrument, there's a set of 4mm binding posts (bottom right) for that. Feed those to a small powered speaker (old PC speakers are ideal for this) - it can't drive a speaker directly. With practice, you can identify the type of distortion from what you hear - with 1kHz as the test tone, you might hear mostly 2kHz from a valve amp, or 3kHz from a solid-state amplifier - just depends. You can also judge the ratio of noise to harmonics - if there's too much noise, then increasing the signal level leaving the amp (and re-calibrating the 100% reading) might help to separate the harmonics from the noise floor. Similarly, if there's lots of 1kHz, that shows that the limiting factor is the depth of the noise floor, not the distortion of the source oscillator or amplifier. Viewing that same signal on a 'scope is also worthwhile. Mains hum jumps out at you if you lock the timebase to "line".
I use an ancient Lindos LA1 for most things. Like the Marconi, it's totally manual. It has a very similar performance (lowest range is 0.1% full-scale). I use it all the time for vintage radio work - it's easily good enough for that. The distortion circuit is fairly straightforward, so if you're thinking of building your own, you might get some ideas from the schematics: http://www.lindos.co.uk/la1-manual.pdf
Measuring distortion is something I do a lot of - both at home and at work - so could say more if it helps. We have a few old-but-nice boxes at work. Newer gear (or soundcard+free software on your PC) gives more detailed analysis. Certainly, plenty to explore
Hope this helps,
Mark
Andy,
That's a lovely workshop! I've very jealous of all that natural light and the views. My workshop is in my cellar, with no natural light and herds of wild elephants above me. Or at least, that's what it sounds like when the kids run up and down the hallway over me (really must fit a carpet one day!).
I've got a much clearer picture of what you've been dealing with now. Certainly not a easy job, but it's getting there. Very impressed with the paint job
Having seen those 15V regulators, I'd have no worries about fitting some kind of heat sink between the PCB and the IC itself. Something like this, perhaps? https://www.sparkfun.com/products/121 - or if there is space, this might be better? https://www.rapidonline.com/to220-heat-s...ting-29625 - these are the sorts of things that I salvage from old gear, as they're always useful. Alternatively, you could DIY something from a scrap of aluminium or copper sheet? Doesn't have to be pretty - only needs to have surface area. Might be wise to insulate the IC from it if there's a chance of something coming into contact with it...
That distortion meter might be better than you think - you quoted 0.1%, but that's the full-scale reading, meaning it ought to measure down to 0.01% with ease. It does rely on the source sine wave being clean, but that's easy to check. Frankly, if it's below 0.1%, it's likely well below audibility for most programme material.
I couldn't get a good freeze frame on the model number, but from a Google image search, I reckon it's the TF2331? Not the A version, which has a red knob at the top-left?
Looking at the manual, it looks like a nice bit of gear. The notch filter claims 80dB depth, which is 0.01%. The rectifier is mean rather than "true" RMS, but that's fine for most practical purposes.
Using distortion meters can be confusing. Setting the 100% scale reading before switching in the notch filter is critical, and how you do that varies from instrument to instrument. And picking the test level is fun. If it's too quiet, the noise of the amplifier can dominate the reading. Too high, and clipping might take place. Some trial and error might be needed. It helps to listen to the output - post notch-filter. On your instrument, there's a set of 4mm binding posts (bottom right) for that. Feed those to a small powered speaker (old PC speakers are ideal for this) - it can't drive a speaker directly. With practice, you can identify the type of distortion from what you hear - with 1kHz as the test tone, you might hear mostly 2kHz from a valve amp, or 3kHz from a solid-state amplifier - just depends. You can also judge the ratio of noise to harmonics - if there's too much noise, then increasing the signal level leaving the amp (and re-calibrating the 100% reading) might help to separate the harmonics from the noise floor. Similarly, if there's lots of 1kHz, that shows that the limiting factor is the depth of the noise floor, not the distortion of the source oscillator or amplifier. Viewing that same signal on a 'scope is also worthwhile. Mains hum jumps out at you if you lock the timebase to "line".
I use an ancient Lindos LA1 for most things. Like the Marconi, it's totally manual. It has a very similar performance (lowest range is 0.1% full-scale). I use it all the time for vintage radio work - it's easily good enough for that. The distortion circuit is fairly straightforward, so if you're thinking of building your own, you might get some ideas from the schematics: http://www.lindos.co.uk/la1-manual.pdf
Measuring distortion is something I do a lot of - both at home and at work - so could say more if it helps. We have a few old-but-nice boxes at work. Newer gear (or soundcard+free software on your PC) gives more detailed analysis. Certainly, plenty to explore
Hope this helps,
Mark
