The bias current varies because of the large amount of thermal inertia between the output transistors and D1.
D1 is used as thermal compensation (among other things) and the idea is that as the output stages warms and so conducts more (that's what transistors do, the characteristics vary with heat) then the diode changes in a similar way but now that change is used to reduce the bias. So after several minutes the whole things settles down to some kind of thermal stability.
A recap would be advised on something so old although it doesn't automatically follow that all the caps are deteriorated, they may well not be. Only specific tests on them would show if that were the case.
20 watts goes further than you might think:
A Test. How much Voltage (power) do your speakers need?
D1 is used as thermal compensation (among other things) and the idea is that as the output stages warms and so conducts more (that's what transistors do, the characteristics vary with heat) then the diode changes in a similar way but now that change is used to reduce the bias. So after several minutes the whole things settles down to some kind of thermal stability.
A recap would be advised on something so old although it doesn't automatically follow that all the caps are deteriorated, they may well not be. Only specific tests on them would show if that were the case.
20 watts goes further than you might think:
A Test. How much Voltage (power) do your speakers need?
Every day is a school day, especially here!! Old lady question if I may (and it’s likely a contentious one) do you put any faith in certain brands or “audio grade” caps? It seems to be quite a hot topic with some swearing by then and others claiming them to be “snake oil”.
I’m trying to maybe find a mid point, something described as audio grade but not silly money. At the moment, it seems to be something along the lines of nichicon FG.
Or am I best just buying the cheapest as it makes no difference and I could spend the money on vinyl!!!
Thanks again for the help and info Mooly.
I’m trying to maybe find a mid point, something described as audio grade but not silly money. At the moment, it seems to be something along the lines of nichicon FG.
Or am I best just buying the cheapest as it makes no difference and I could spend the money on vinyl!!!
Thanks again for the help and info Mooly.
I would always say to buy good quality well known commercial brands. So that is the likes of Nichicon, Rubycon and Panasonic.
I'm not a believer in magical properties of parts 🙂 its goodness or otherwise is down to the engineering of the part.
I don't know the type of cap (Axial or Radial) that your amp uses but radials are by far the most common these days and axials hard to find in decent types. Radials have the two leads at the same end, axials a lead at each end.
You will also find that modern caps are much smaller for the same thing.
When looking at parts try and go for 105C temperature rating rather than the older 85C.
Have a look at CPC in the UK (part of Farnell) as they sometimes have good offers. Never buy off Ebay, too many fakes around... yes caps get faked as well.
I'm not a believer in magical properties of parts 🙂 its goodness or otherwise is down to the engineering of the part.
I don't know the type of cap (Axial or Radial) that your amp uses but radials are by far the most common these days and axials hard to find in decent types. Radials have the two leads at the same end, axials a lead at each end.
You will also find that modern caps are much smaller for the same thing.
When looking at parts try and go for 105C temperature rating rather than the older 85C.
Have a look at CPC in the UK (part of Farnell) as they sometimes have good offers. Never buy off Ebay, too many fakes around... yes caps get faked as well.
Superb advice. Thank you (I’m saying that a lot to you lately). They are radial caps on the my Akai. It’s tough when you’re a newbie to all this and you read marketing ideology around caps sounding like and being made with silk as you see with Elna for example.
I did also ready to keep to the same uF but upping the voltage doesn’t hurt. The temp advice is great, I wouldn’t have looked at that
I’m also glad you shared that link to the wattage experiment as it showed me my speakers were a little unbalanced and running a little higher on the left channel. Not by much but it gave me some tinkering to do. It was the just a case of the balance knob not being seated in the right position when I took it off the shaft to take the faceplate off. So what I thought was centre was actually slightly biased to the left.
I did also ready to keep to the same uF but upping the voltage doesn’t hurt. The temp advice is great, I wouldn’t have looked at that
I’m also glad you shared that link to the wattage experiment as it showed me my speakers were a little unbalanced and running a little higher on the left channel. Not by much but it gave me some tinkering to do. It was the just a case of the balance knob not being seated in the right position when I took it off the shaft to take the faceplate off. So what I thought was centre was actually slightly biased to the left.
Thanks for the kind words 🙂
When looking at caps you can also check the 'pitch' or distance between the pins and try and get replacements that will fit your board directly. Places like CPC show the technical specs of these parts so dimensions are easy to cross reference.
You will probably see caps with 'lifetime' ratings and that can look alarmingly low until you realise that the figure is quoted at not just its maximum temperature but also with the cap passing the maximum AC current allowable. So a quoted 1000 hours becomes more like 50,000 to 100,000 (or more) in use in something like your amp.
If you need to bend legs on parts then do it gently by first preforming them to the correct spacing with round nosed pliers. Don't splay the leads out as they exit the cap as that can only do damage. Form the leads to fit.
When looking at caps you can also check the 'pitch' or distance between the pins and try and get replacements that will fit your board directly. Places like CPC show the technical specs of these parts so dimensions are easy to cross reference.
You will probably see caps with 'lifetime' ratings and that can look alarmingly low until you realise that the figure is quoted at not just its maximum temperature but also with the cap passing the maximum AC current allowable. So a quoted 1000 hours becomes more like 50,000 to 100,000 (or more) in use in something like your amp.
If you need to bend legs on parts then do it gently by first preforming them to the correct spacing with round nosed pliers. Don't splay the leads out as they exit the cap as that can only do damage. Form the leads to fit.
There's also 105°C caps rated to 2000 hours (if that) and others that may be as high as 10000 hours. That's a major difference as well.
If average, that would mean a 20 wpc amplifier provides a crest factor of 10 dB. Enough for moderate to heavily compressed pop / rock material but marginal on anything notably dynamic.
If peak, keep in mind that analogue VU meters are slugtastic and quite unable to follow fast peaks accurately, so will tend to underread.
Minimal amplifier power recommendations for speaker generally assume being able to reach 103 dB SPL @ 1 m anechoic. With a 20 wpc amplifier, that would require speakers (honestly) rated at 90 dB SPL / W / m, a somewhat above average value. I don't see that happening without at least two 6.5" or 8" woofers or one 10".
A more stringent 110 dB SPL requirement would imply 97 dB SPL / W / m, that's quite firmly PA speaker territory.
Now you may be a quieter than average listener and/or at lower than average distance and/or have a very lively (reflective) listening space, all of which would bring SPL requirements down. In a desktop setting, even 82 dB mini monitors may be plenty loud enough.
Average or peak?
If average, that would mean a 20 wpc amplifier provides a crest factor of 10 dB. Enough for moderate to heavily compressed pop / rock material but marginal on anything notably dynamic.
If peak, keep in mind that analogue VU meters are slugtastic and quite unable to follow fast peaks accurately, so will tend to underread.
Minimal amplifier power recommendations for speaker generally assume being able to reach 103 dB SPL @ 1 m anechoic. With a 20 wpc amplifier, that would require speakers (honestly) rated at 90 dB SPL / W / m, a somewhat above average value. I don't see that happening without at least two 6.5" or 8" woofers or one 10".
A more stringent 110 dB SPL requirement would imply 97 dB SPL / W / m, that's quite firmly PA speaker territory.
Now you may be a quieter than average listener and/or at lower than average distance and/or have a very lively (reflective) listening space, all of which would bring SPL requirements down. In a desktop setting, even 82 dB mini monitors may be plenty loud enough.