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Old 21st August 2011, 03:15 PM   #1
jiinyus is offline jiinyus
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Join Date: Aug 2011
Default Hopefully an easy one for you...

I posted this in the introductions forum and was told I may get more help elsewhere, so here's my question guys...

Hey everyone,

after looking through the forums for countless hours now, I've resigned to registering and posting my own problem as I can't help myself. Hope you guys can if it's not too much trouble. Also, I don't even know which section this would come under - all this new stuff I'm learning is making my head spin and I'm thoroughly confused. If someone could move it to the right section I'd be very appreciative.

Anyways, on with my problem. A year or 2 ago I was the recipient of two Radio Shack Optimus speakers. These ones to be precise (you may notice a few links in this post, I'm just trying to provide as much info as possible ):

Radio Shack 12-1736 Optimus 10" 2-Way Stereo Box Spkrs | eBay

I have been using them occasionally with a housemate's Denon PMA-350SE and Technics SU-600 (eBay link for the Denon and service manual for the Technics below).



They sound great, so I was swayed to set them up for daily use. But obviously I needed to get my own amp. I invested in this:

New QTX Sound SA-2 / SA2 DJ Stereo Amplifier 100W | eBay

I know it's not the best one out there, but I purchase it mainly due to budget constraints and the fact that it went very well with my other blue, blinking gadgets so I took it. I've hooked it up to my PS3 and PC and get absolutely superb sounds out of everything, but it's much too loud. I'm talking, raise it up 2 marks from its absolute minimum and that's starting to get relatively loud. Not unbearable, but I would like the option to have it a couple decibles lower. I've adjusted all volume settings elsewhere, and have managed to adjust the PC to a comfortable setting, but when playing stuff through my PS3, I'm still deafened. My PS3 is connected to my TV through a HDMI cable, then there's an RCA cable from the Line Out ports on my TV to the amp. Silver Audica cables run from the amp to the speakers, but as they're also inherited, I have no idea of the model number or anything. Apologies.

So, my questions are as follows:

First off, is this even a suitable amp for my speakers? Is my setup OK in general?
Secondly, when I power on my amp I get a loud hum, which fades and then no unwanted noise. When I turn them off, I get a minor thump sound. I looked this up and I'm just lost again. One forum told me it was absolutely normal. Then I stumbled across this:

speaker thump /amp-on question

Can someone reassure me on this matter or persuade me to go along with what was said on these forums as I'm trying to refrain from using them at the moment.

Thanks in advance for any help you guys might provide me with. I've completely given up with trying to teach myself anymore!

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Old 21st August 2011, 05:15 PM   #2
ingenieus is offline ingenieus  South Africa
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Join Date: Apr 2011
Location: Pretoria
The link does not work.

Sounds like the amp has too much gain for your application. You could modify the circuit, but it would be easier yo have some sort of attenuator between your TV output and your amp input. You could also investigate if your TV allows you to set the level of this output.

The thumps are quite normal. The only way to get rid of them is to place a relay on the speaker output with a control circuit that disconnects the speaker during power on and off. The thumps will still be there, you just won't hear them.
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Old 22nd August 2011, 01:19 AM   #3
jiinyus is offline jiinyus
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Join Date: Aug 2011
Sorry, copied this from a previous post and forgot about the links.

My speakers: Radio Shack 12-1736 Optimus 10" 2-Way Stereo Box Spkrs | eBay
eBay link for the Denon: DENON PMA-350SE AMPLIFIER 50 WPC SOUNDS GREAT VGC | eBay
Service manual for the Technics: https://rapidshare.com/files/4036527...ics_su-600.pdf
My amp: New QTX Sound SA-2 / SA2 DJ Stereo Amplifier 100W | eBay
Speaker thump/amp-on question: speaker thump /amp-on question

So my amp is 100W and speakers are 450W Max per pair. That's bad, right?

Thanks for the help,
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Old 22nd August 2011, 01:34 AM   #4
Michael Bean is offline Michael Bean  United States
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Did you see my post in your other thread? (#6 Here: http://www.diyaudio.com/forums/intro...ml#post2680251 )
By the way, that amp is only rated for 25 watts per channel into 4 ohms, peak power ratings are, for the most part, irrelavent marketing hype.


Last edited by Michael Bean; 22nd August 2011 at 01:38 AM.
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Old 22nd August 2011, 02:16 AM   #5
Bigun is offline Bigun  Canada
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Location: Waterloo, ON or Herefordshire UK
Hopefully an easy one for you...

I think that's a very neat looking amplifier - it would fit nicely on my desktop

I add my vote to the earlier suggestion for a passive attenuator (i.e. resistors) between the PS3 output and your amplifier. It needn't be complicated but it would be worth making a nice job out of it. My thought would be a small box, fit some RCA sockets to it so you can connect everything up with regular audio cables.

Inside the box you want a 3 resistors on each channel in a 'T' arrangement which you could wire directly to the back of the RCA sockets if you mount them appropriately close enough together in the connecting box.

The 'T' arrangement means you don't care which way the signal is traversing through the attenuator box - there's no difference which way round it is.

Some useful links:


Passive Attenuator Design
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
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Old 22nd August 2011, 04:27 PM   #6
ingenieus is offline ingenieus  South Africa
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Join Date: Apr 2011
Location: Pretoria
Or you could just buy one, if you don't feel like DIY.

An externally hosted image should be here but it no longer works. Please upload images instead of linking to them to prevent this.

Ask Google/Yahoo/whatever about "RCA audio attenuator" for a convenient supplier.

PS: a 100W amp driving 450W speakers is not a problem.

Just make sure about that "max" rating. It is quite meaningless. Look for the RMS value for both amp and speakers.

PPS: your amp is rated as 50W RMS into 4 ohms. Your speakers are 150W RMS and 4 ohm. No problem here.

Last edited by ingenieus; 22nd August 2011 at 04:35 PM.
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Old 23rd August 2011, 01:43 PM   #7
jiinyus is offline jiinyus
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Join Date: Aug 2011
Thank you for all the help guys, I really appreciate it. I have slowly been reading over all this and am now debating whether to build an attenuator or take the easy way out and just buy one. I would like to try it myself but I'm not sure how much I trust my DIY skills. Either way, I'll let you know how I get on. I'm just relieved my equipment isn't going to blow up any time soon. Was really stumped on those power ratings, still not 100% sure I understand but I trust you lot. Also, I've just invested in:

Cambridge Audio SYMPHONY 200 | Speaker Cable Per Metre | Richer Sounds

Anyone got an opinion on Cambridge Audio? I just don't trust that single strand silver wire I have hooked up now.

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