Ruby Amp very quiet

[aloysius]

Hi all, first post.
I just finished building the Ruby amp from ROG. I'm stoked...its my first ever project like this and it worked. I'm learning a lot as I go but I'm a bit baffled. I got myself an 8ohm speaker from Op Shop for $5 from an old stereo. It looks great in wooden box with old brown cloth front. Inside is a 6" speaker. I wired this directly to the Ruby (should have used terminals in retrospect but I always figure solder is better). Even with both Gain and Volume Pots cranked right up its very quiet though. I hooked up my Morrison Tube Squealer pedal to it and it cranked its level and drive right up as well but the sound was still only just loud enough to hear over television.

Could incorrect speaker polarity cause this? Its the only thing I was unsure of when i hooked it all up. The only other thing is I used a 10k Log pot instead of linear for volume....the shop I bought from were out of linear...would this effect it?

Cheers
Joe

[ambulancevoice]

the log pot could possibly be the problem
check the polarity of your capacitors and the pin out of your transistor
are you using a battery?

[JOHNO]

The best why I know of checking speaker polarity is to put a 9v batt across the speaker terminals and if the speaker cone moves outwards thats the correct polarity if it moves inwards well them its backwards. Disconnect the speaker before you do this and dont leave the battery there for a long time as it may burn out the voice coil, though I've never had that happen. I dont think speaker polarity is your problam though but I would certainly check it just to make sure its correct. I really dont think the 10k pot is the problam either because it will still have the same resistance as a linear pot when it is maxed out. So go over the circuit again and make sure everything is correct. Good luck with it, Johno

[darron]

will speaker polarity make a difference though? i thought the sound would just be 180 degrees out of phase?

my ruby wasn't very loud, and i know the polarity was right. i'd say it was a matter of the speaker being only 0.25watt though

thanks for the tip on the direction it should move though. might not put 9volt across an 8ohm load for too long though.

[JOHNO]

If the polarity is reversed the speaker will move inwards rather the outwards when you hit a chord or note. This will cause the voice coil of the speaker to crash into the the magnet at the back of the speaker if the volume is turned up enough. This damages the voice coil and it starts to rub on the magnets in the speaker. This is called "polling". And the speaker will have an annoying squeak or buzz. It is possible to pole a speaker that is hooked up correctly by driving it too hard the same thing happens IE the voice coil crashes into the back of the speaker. What Ive said here probably isn't 100% correct but its close. Anyway I doubt that the ruby amp could pole even a .25 watt speaker. My ruby has a 6" 5w fender speaker out of my champ 600 and it can certainly be heard over the TV.

[darron]

i didn't know that. it's against what i was told. i thought that the speaker went both ways as the audio signal was AC. i'll keep a much closer eye out later.

[frankclarke]

Not pots or speaker phase. Can you simplify it to be a little gem, and see if that helps?
http://www.runoffgroove.com/littlegem.html

[armstrom]

It could be that your speaker just has a really low efficency. The wattage of a speaker doesn't really determine how "loud" it is... just how much heat it can dissipate before it goes boom.
If the speaker you're using has an efficiency rating below 90 dB 1W/1M then it will probably sound "too quiet". Hi-Fi speakers are notorious for having low efficencies (~85dB for many). Because decibels are a logarithmic scale just a few dB in efficency can make a huge difference in "loudness". For example, an 87dB will be half as loud as a speaker with an efficiency of 90dB (assuming the same impedance and amplifier output power).

Most modern guitar speakers have an efficiency in the low to mid 90's (90-95dB) Some are even higher. Try taking your ruby down to your local music shop and see if they'll let you plug it in to one of the passive cabinets they sell. This shouldn't be a problem if you have it packaged up in a tiny box as my experience shows the guitar buffs at the music store all want to hear a tiny 9V battery powered amp drive a 2x12" cabinet

Also, try to ditch the log pot. While it will provide the exact same range of adjustment as a linear pot, most of your "volume" will come rushing in just at the end of the rotation. Normally you want to use a log pot for volume, but presumably the Ruby was too sensitive to the change in input voltage so the guys over at ROG decided to use a linear pot to get a bit more leeway in adjusting the volume. If you want to eliminate the pot as the source of your problem simply jumper the input to the output and use your guitar volume.

[aloysius]

Im going to have another crack at it this weekend. I have build a Jaycar lm386 based kit called the Champ Amp kit with the Pre Amp kit as well...I got 2 speakers when i bought them so im going to hook this 2nd kit up in that. Once that's done I will know if its the amp or the speakers I have a 4x12" kit here i could hook the Ruby up to...but I built it into the speaker box and soldered everthing direct to keep it neat...which means I have to desolder speaker output to hook up to my cabinet. This weekend I rekon I will end up building another ROG amp...either the Little Gem mk2 or another ruby, but in a jiffy box as just amp no speaker.

[aloysius]

Hokay,...turns out it was the speaker I chose...was just not efficient enough. 8ohm 5w. Hooked the ruby up to my 4x12" cabinet and it rocks hard .

Boxed it up in a nice enclosure this evening and have been caining it.

Will do same with my other 386 based amp which will leave me with a totally clean 386 and the ruby with its nice distortion.

Damn...this is going to be another obsesion i can see 

[armstrom]

glad to hear it was just the speaker.