Thanks again.. I'll re-read that chapter and re-think my Vbe.
I have several metres of 150x6mm aluminium bar and aluminium flashing from which I could use to fashion some fairly serious heatsinks to bolt to those 50W resistors you linked to. The price is certainly more attractive than the ones I was considering.
I perused Pete's website and found the Ebay listing. This looks like a fairly serious test instrument; constructing it would be a project in itself! Would you say this is more useful for this type of work (audio testing / diagnostics) than a cheap oscilloscope?
I have several metres of 150x6mm aluminium bar and aluminium flashing from which I could use to fashion some fairly serious heatsinks to bolt to those 50W resistors you linked to. The price is certainly more attractive than the ones I was considering.
I perused Pete's website and found the Ebay listing. This looks like a fairly serious test instrument; constructing it would be a project in itself! Would you say this is more useful for this type of work (audio testing / diagnostics) than a cheap oscilloscope?
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Hi,
I think you're doing a great job in digging into the details to learn as much as you can.
My 2c - The most useful thing you can get is a good oscilloscope. I much prefer one with analogue controls and of course it needs at least two channels.
Quick thought: For a simple but more accurate Vbe multiplier, look at my TGM5 thread. The design of the Vbe multiplier is by Haggerman. The primary benefit is a reduced dependence on fluctuations in VAS current. Of course fine-tuning the thermal response to suit your heatsink etc is an option if you want to invest the time.
I think you're doing a great job in digging into the details to learn as much as you can.
My 2c - The most useful thing you can get is a good oscilloscope. I much prefer one with analogue controls and of course it needs at least two channels.
Quick thought: For a simple but more accurate Vbe multiplier, look at my TGM5 thread. The design of the Vbe multiplier is by Haggerman. The primary benefit is a reduced dependence on fluctuations in VAS current. Of course fine-tuning the thermal response to suit your heatsink etc is an option if you want to invest the time.
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That's a hard question and it depends on how broad your applications will be. For general use in audio test and measurement, a digital oscilloscope of 50 Mhz bandwidth or better is necessary. Note that analog scopes still display well beyond their stated bandwidth, just not with full amplitude and clarity, so a cheap 20MHz analog scope is ok at HF too, if a bit faint and smaller amplitude than it really is.
Generally, cheap digital scopes are ok because they are small and light - great if you don't mind stabbing crumbling little menu buttons and jaggy traces don't worry you either. Upmarket types like the Rigol 2000 series and newer, use larger, 7"screens, higher sampling rates which give smoother traces and more representative results which is good - at a price. Go to Youtube for David L Jones' EEVblog and look at his hammed up reviews of these and so many test items.
He's a nut case but speaks fairly enough. I also find it a matter of personal convenience. If you need your PC to refer to something for guidance or for other uses while you work, tying it up as the the 'scope or any number of instruments can be particularly annoying but still necessary if you want to process or export the results.
For most proof tests though, a soundcard is a very flexible device that can be used for many more instrument and audio programming applications than amplifier testing. I would look at that first anyway, because it is cost effective, interesting and you only need spend extra on software and the sort of interface controls and connections that you require. Though it's no longer current, I have an ESI Juli@ soundcard with direct or attenuated input via balanced inputs and diode limiter. Several guys here had used and recommended them and the results are superb for quick tests at the desk which suits me fine as I don't need to use the workbench.
For USB soundcards, the recent Quantasylum is great value as the software for distortion/noise testing is included - not much good at other stuff, though. http://www.diyaudio.com/forums/equipment-tools/231401-quantasylum-qa400.html. Note there is a dumbed down interface here by the OP and there is much more in this thread
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