just one bulb holder inserted into the live lead. No switches.jp_howard said:
The neutral and earth wires pass straight through to the socket outlet.
I start with a 40W 240Vac tungsten filament bulb. A zero or very low bias current a good build should flash very briefly and go out.
Move up through 60W to 100W as I turn up the quiescent current.
Need to try and source a 150W bulb before they make them illegal.
awpagan said:
Well the maybe change to 47ohm resistor to 68ohm's at least is a definite.
Makes it easier to adjust the bias with the pot.
Also
The 200ohm pot in the LTP, SandyK is right, adjust to 130ohm and take a listen😀
And this is with Q1 and Q2 not matched, they measured Hfe of 515 and 600 one channel and 43*(don't remember) and 503 on the other channel
and what currents (Ic and/or Ie) did they pass when the Vbe was identical for all the transistors?
I'm getting some high frequency hiss on both speakers. It's equally loud on each side. It is loud enough to be just audible from 3 feet from the tweeters - further away is inaudible.
I'll get a suitable adaptor tomorrow so I can plug in headphones, to see if some of this is from the speakers themselves. However some definitely seems to be from the amp, since placing an aluminium sheet on top of the amp reduces the hiss slightly.
Obviously, one way to improve this is to stick my amp in a metal case. However, I'd like to minimise this hiss first (because it may be doing other things to the sound too, and because it'll be harder to hack at the amp when it's in a case).
Is this level of high frequency noise normal? What can cause this noise, and what can be done to reduce it?
I'll get a suitable adaptor tomorrow so I can plug in headphones, to see if some of this is from the speakers themselves. However some definitely seems to be from the amp, since placing an aluminium sheet on top of the amp reduces the hiss slightly.
Obviously, one way to improve this is to stick my amp in a metal case. However, I'd like to minimise this hiss first (because it may be doing other things to the sound too, and because it'll be harder to hack at the amp when it's in a case).
Is this level of high frequency noise normal? What can cause this noise, and what can be done to reduce it?
AndrewT said:
I just used the 4 transistors supplied with the kit.
Measured with an Atlas tranny tester.
yes standard 2mA and 4.5**mA
Vbe was similar or the same, don't remember.
Meaning no drastic changes.
As I said the 4 trannies, yes only the 4 that came with it, I tested and there was the results............... not matched
Didn't expect them to be.
At least they were from the same manufacturer and were BL's
The real result is that the 200ohm pot set at 130ohm's does make a difference, for the better.
Jeremy,
Try moving the amp through 180 degrees, so you can be sure it's not EMI.
Failing that, try a 3.3nF across the speaker output to short any EMI.
If this doesn't clear it, it's likely to be a design or perhaps an implementation issue.
Cheers,
Hugh
Try moving the amp through 180 degrees, so you can be sure it's not EMI.
Failing that, try a 3.3nF across the speaker output to short any EMI.
If this doesn't clear it, it's likely to be a design or perhaps an implementation issue.
Cheers,
Hugh
Implementation yes
Have you rerouted/changed any hookup wires since the last time you listened.
moved the amp or put anything closer to it?
Have you rerouted/changed any hookup wires since the last time you listened.
moved the amp or put anything closer to it?
a semiconductor telling you it's about to die.jp_howard said:
But, from both channels seems unlikely.
jp_howard said:
First thing to do is to make sure the hiss is not due to HF oscillation. You can check this by touching the resistor that's across the output inductor. If it's warmer than the ambient temp then you might have a problem. Amplifiers tend to be better behaved when they're in the Faraday cage so the sooner you can do that the better.
Switch them all off temporarily and listen if your hiss goes away or reduces.
I do mean ALL of them, including the wireless burglar alarm, wireless door bell and anything else that may be emitting RF.
Maybe switch off the heating system, fridge, freezer, etc.
I do mean ALL of them, including the wireless burglar alarm, wireless door bell and anything else that may be emitting RF.
Maybe switch off the heating system, fridge, freezer, etc.
jp_howard said:
Jeremy
I think you have partly answered your own question, when you
said that placing an alumium sheet on top of the amplifier reduced the hiss slightly. Aluminium is not very effective in that application. A steel top cover as supplied with the majority of rack cases is far more effective.
SandyK
I'm guessing the following information is important: disconnecting the source removes the hiss almost entirely.
However, connecting the source (SB Duet connected through an Ebay CS4398 DAC) to my headphone amp (a Silicon Chip "Rockgrottoed" HA) results in no hiss at all, even when trying a variety of headphones.
Turning off all electronic devices (getting to the microwave and fridge switches was a pain!) made minimal difference.
No resistors on the amp are above room temperature by a margin I can detect with my finger.
Rotating the amp does not impact the hiss.
Could it be that the DAC is creating the hiss, but the headphone amp is somehow "cleaning it up"? If that's the case, I guess I'll be asking about improving the DAC next! (BTW, putting the aluminium sheet over the DAC made no difference to the hiss).
Alex: thanks for the tip about not using aluminium for the case - I hadn't heard that before.
However, connecting the source (SB Duet connected through an Ebay CS4398 DAC) to my headphone amp (a Silicon Chip "Rockgrottoed" HA) results in no hiss at all, even when trying a variety of headphones.
Turning off all electronic devices (getting to the microwave and fridge switches was a pain!) made minimal difference.
No resistors on the amp are above room temperature by a margin I can detect with my finger.
Rotating the amp does not impact the hiss.
Could it be that the DAC is creating the hiss, but the headphone amp is somehow "cleaning it up"? If that's the case, I guess I'll be asking about improving the DAC next! (BTW, putting the aluminium sheet over the DAC made no difference to the hiss).
Alex: thanks for the tip about not using aluminium for the case - I hadn't heard that before.
Sorry I should have mentioned: the hiss has always been there. I just hadn't mentioned it before because I figured it might go away as I worked on the other issues.awpagan said:
It's not a big deal, since it's not audible from listening position. Fixing it (and identifying its source) is, however, an interesting learning exercise.
Alex: from your comment about the aluminium sheet, does that imply that all unshielded amps will have a bit of 'hiss'?
Jeremy,
Many DACs have quite a deal of hiss on the output, arising from the digital artefacts of the sampling frequency. This is I believe very true of the non-oversampling DACs. They are usually attenuated with a second order filter, but not entirely removed.
The SCULD has a very good frequency response, only 1dB down at around 220KHz. I saw this briefly on my CRO when I blew up the Zoble resistor (a useful test, method in the madness). IMHO this is too high. The hiss is the non-linear mixing of digital artefacts to give audible beat products.
I mentioned at the time that the input should be fitted with a low pass filter; this is the classic instance where it's necessary. Alex does not agree, but provision can be made on a new pcb if we do one so that it can be left out if desired.
Cheers,
Hugh
Many DACs have quite a deal of hiss on the output, arising from the digital artefacts of the sampling frequency. This is I believe very true of the non-oversampling DACs. They are usually attenuated with a second order filter, but not entirely removed.
The SCULD has a very good frequency response, only 1dB down at around 220KHz. I saw this briefly on my CRO when I blew up the Zoble resistor (a useful test, method in the madness). IMHO this is too high. The hiss is the non-linear mixing of digital artefacts to give audible beat products.
I mentioned at the time that the input should be fitted with a low pass filter; this is the classic instance where it's necessary. Alex does not agree, but provision can be made on a new pcb if we do one so that it can be left out if desired.
Cheers,
Hugh
jp_howard said:
Jeremy
No. The level of hiss also depends not only on the noise factor of the input devices chosen (2SA 970 are quite good in this respect) ,among other things, but also the overall amplification of the amplifier as set by the ratio of the feedback resistors, which are 510 ohms and 12K in this case. (approx.24.5)
IMO, many amplifiers have far too much gain, in an effort to suit all possible input sources.
Alex
I'm doing an audition of the amp with Hugh on Thursday. It'll be interesting to see if we see the same hiss on hiss source and speakers. I rather hope we do, actually - that way I'll be able to see how he goes about fixing it... 🙂
Alex: does the SCHA avoid this hiss by simply having lower gain?
Alex: does the SCHA avoid this hiss by simply having lower gain?
SC ULD2
"I mentioned at the time that the input should be fitted with a low pass filter; "
Hugh
I did not disagree. In an edit a short time later, I suggested a reduction of the capacitor to 100pF. The fitting of a ferrite bead in the input area may be worthwhile too ?
Alex
"I mentioned at the time that the input should be fitted with a low pass filter; "
Hugh
I did not disagree. In an edit a short time later, I suggested a reduction of the capacitor to 100pF. The fitting of a ferrite bead in the input area may be worthwhile too ?
Alex
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