Working amp witha glitch - or two.

[lazyfly]

Hi folks.

I'm after some advice as what I've read here so far indicates you people know what side to butter your bread on - or something

I've just put together an amplifier from several kits from Jaycar based on Silicon Chips revamped/revised ETI480, the SC480.

Performance
Output Power 50 watts into 8O 70 watts into 4O
Music Power 77 watts into 8O ;105 watts into 4O
Frequency response -1dB at 14Hz and 70kHz (at 1W- see Fig.1)
Input sensitivity 0.875V for 50O into 8O
Harmonic distortion <.05% from 20Hz to 20kHz; typically <.003%
Signal-to-Noise Ratio -114dB unweighted (22Hz to 22kHz); -119dB A-weighted, both with respect to 50W into 8O
Damping factor >140dB at 100Hz & 1kHz, with respect to 8O and without PTC thermistor
Protection fuses plus "Polyswitch" PTC thermistor
Stability unconditional

So that's the amp now for a couple of questions (bearing in mind I'm a novice/beginner, and then some.)

1. The source is fed directly to a 50KOhm dual ganged pot with no preamp. Would the lack of a preamp be the reason for the amp showing up the difference in recording levels between different CD's? I always noticed it on my previous Japanese made amp but it's certainly less subtle now.

2. When I switch the amp off I assume (probably wrongly!) that the capacitors on the power supply are taking some seconds to discharge and thus are still providing the speakers with enough grunt to play music, then, after some seconds, the volume becomes lower and progresses into some nasty distortion for a couple more seconds. At lower listening levels this wouldn't pose a problem to the speakers, it just sounds rather awful. At higher listening levels maybe it could be detrimental to the speakers. How can I prevent this?

3. I was winging it in the layout department and so feel the wiring and layout is not only messy but is possibly detrimental to the sound, too(?) Suggestions on 'cleaning' this up would be most appreciated! For example, the mains wires on the transformer (right in top view) sitting millimetres above the source wiring from the RCA inputs.

SC480 layout

Thanks so much for any advice you may have to give to me. The amp works and works quite well but anything to improve can only be a good thing from a sound and safety point-of-view.

Sincerely

Kendal.

A little headphone amp I cut my teeth on, so to speak.

[Bill Fitzpatrick]

Originally posted by lazyfly

2. When I switch the amp off I assume (probably wrongly!) that the capacitors on the power supply are taking some seconds to discharge and thus are still providing the speakers with enough grunt to play music, then, after some seconds, the volume becomes lower and progresses into some nasty distortion for a couple more seconds. At lower listening levels this wouldn't pose a problem to the speakers, it just sounds rather awful. At higher listening levels maybe it could be detrimental to the speakers. How can I prevent this?

Here's a little known trick that prevents this from happening. It requires a leap of faith but, believe me, it works everytime!

Simply turn off the music! An alternative is to turn the volume all the way down.

[rabbitz]

Originally posted by lazyfly
I was winging it in the layout department and so feel the wiring and layout is not only messy but is possibly detrimental to the sound, too(?) Suggestions on 'cleaning' this up would be most appreciated! For example, the mains wires on the transformer (right in top view) sitting millimetres above the source wiring from the RCA inputs.

Hi lazyfly

I prefer to have the transformers in the front fed by the 240v line that is located at the side of the case. Caps in the middle on the front and power amps at the back. This way you are not getting mains, power wiring near the sensitive audio wiring. Also has a benefit of having short input and speaker wiring which IMO helps the sound.

I can't see in the pic whether that's the volume control on the right but I'd wack it in the back with an extension shaft to the front panel. That way, once again you keep audio leads short and keep them away from the tranny and caps.

Those Jaycar cases aren't bad at all........ I used one, a 3U, and very happy with it. Just used a different front panel.

Agree with Bill on shutdown....... stop the music, turn down the volume, turn the power amp off first, then preamp (if applicable) and source. That way no funny noises or farts.

CD's do vary quite a bit in volume. Maybe your other amp was a bit more compressed.

Cheers

[djk]

Ever wonder why the Dick Smith power supply kit for this amp has a speaker delay relay?

[lazyfly]

Hi fellas, thanks for the tip. I wasn't worried about myself turning it off without turning it down or killing the source but rather someone (anyone) else.

Rabbitz. Thanks for your advice. I'm playing around with the layout at present and will incorporate most if not all your recommendations. Thanks

djk, isn't the speaker delay relay for when you apply power? It doesn't make any audible sound when I switch on, just the current left in the caps when I switch off - or does this relay help there, too?

Thanks again gents.

Sorry for taking so long to reply, I wasn't able to! (forum/user glitch)

[djk]

A good speaker relay circuit will have DC inhibit, a long turn on RC constant, and drop out instantly.

The secret to having it drop out instantly is to power the relay from a lightly filtered half wave supply, and have a reverse biased diode go from the supply to the RC constant.

This will also give you the full RC constant if the power is interupted briefly. Most circuits if the power is interupted briefly don't go through another RC constant because the cap is still charged from the first cycle, so it 'pops' or 'thumps'.