distorted sound

[pixelandsoda]

Hi,

In one of my posts earlier I mentioned about my recent purchase. I have bought Rotel RA-810A amplifier to match Tannoy Mercury MX2 speakers. I was quite pleased with the sound I can get from this budget system untlil I have noticed the distortion of the sound in the left speaker. It's a rattling sound at certain low frequency that comes out from the left speaker. I tried to test it and I swapped the connectors in the amplifier connecting the speakers to the oposit ports and only the speaker connected to the left channel gives this rattling sound. It's clearly not the speaker but the amplifier (or just a left speaker input). Is there any way I can fix it (DIY way) or is it easier just to buy a new amp?

Thanks,

 

[nopiano]

I agree with Vladimir's queries. It is either the LH channel on the amp, or your cartridge mistracking (if your source it a TT).

Just a guess, though. Unlikely to be a CD I'd have thought.

 

[pixelandsoda]

The source is Rotel RP-855 turntable and I have swapped L/R channel in the amplifier and only the speaker connected to the left channel was giving this rattling sound.

 

[Vladimir]

Have you tried swapping L/R channels on interconnects and speaker cables?

What is the source?

Do (can) you have a digital multimeter?

 

[Vladimir]

Try a different source, a CD preferably.

Possible reasons for the distortion:

1) Bad interconnects or speaker cables

2) Badly set turntable / misaligned cartridge

3) Amplifier pots and switches need cleaning inside

4) DC voltage coming out of one dead/dying amp channel, we need a multimeter to confirm this (it might kill your speakers if this is the case).

 

[Vladimir]

PS if you didn't also swap the speaker cables (not just interconnects), the left speaker may be possibly the issue. Swap them L/R (while the amp is turned off) and see if that changes anything.

 

[pixelandsoda]

I just swapped the speaker cable and clearly the left speaker was producing the distorted sound connected to the left channel in the amplifier. When it was plugged to the right channel using the same cable the sound was fine.

 

[Tannoyed]

OK now reconnect the speakers as normal and swap the turntable phono leads over. Does the distortion stay in the same channel or does it move with the connections?

 

[pixelandsoda]

I swapped the phono leads over but it did not change anything. The distortion still remains the same depending on the frequency and the volume level. I have tested it playing the culmination part of the Dire Straits ‘ Telegraph Road‘ song on the vinyl.

 

[Vladimir]

Well its time to get a multimeter so we can check the amp DC offset.

 

[pixelandsoda]

I will have to buy one...would the cheapest do the job?

http://www.ebay.co.uk/itm/Digital-LCD-Multimeter-Voltmeter-Ammeter-OHM-AC-DC-Circuit-Checker-Tester-Buzzer-/360538749861?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item53f1c8bba5

 

[Vladimir]

Yes, it will do the job. Get this electric contact cleaner as well.

http://www.ebay.co.uk/itm/WD40-Specialist-Electrical-Contact-Cleaner-Smart-Straw-400ML-/221233269189?pt=UK_Car_Accessories_Car_Care_Cleaning&hash=item338287c9c5

 

[Tannoyed]

Hello

I assume you mean that the distortion remains in the same channel whatever you do. A DC offset will not do your speakers any good at all so be careful here, however dc offset or not, it seems likely that the amplifier is faulty. Have a look at the speaker cone (bass driver) of the faulty channel and note its position. Now switch the amplifier on and wait a few seconds for any transient events to settle. Have a look at the position of the cone again. Is it being pushed out or pulled in from its original position? This will tell you without spending money on a meter. If it has moved towards one extreme of its travel range and stayed there it will be cooking the speech coils nicely. If it sits in the middle of its travel range they will be ok.

In my view the amp is at fault whether or not there is an offset and will have to be repaired or replaced. If the output stage is faulty (e.g. shorted MOSFET) then your speakers may have suffered. If the input stage (preamp) is faulty they should be ok. Does the distortion get worse as you increase the volume or is it more noticeable at low settings?

 

[pixelandsoda]

Yes, the distortion remains only in one left channel and get worse with the higher volume level.

I have tested the speakers according to your advice and the cone sits in the middle, so I assume they are not faulty.

 

[Tannoyed]

Well if they sit in the middle they will not have been damaged. The amplifier is faulty and needs to be repaire or replaced. How old is it?

 

[Tannoyed]

If the distortion gets worse as the volume is increased the fault probably lies in the output stage, I should imagine the signal is thumping against one or both of the supply rails. If the amp is old have a look at the electrolytics, especially the small surface mount ones. Any small cap near anything hot will have a short and sweaty life. They go leaky both electrically and physically. Try playing a hairdryer over into the vent slots to warm things up a bit. If there is a faulty electrolytic this will reduce or remove the distortion. After that it is a matter of using aerosol freezer and heat to narrow it down to the guilty one! Freezing will make it worse. The value falls and the ESR rises with reduced temp, especially with a faulty one.

Some caps are used for coupling, others for decoupling. An electolytic capacitor is an aluminium can with a rubber bung in the end to hold the electrolyte paste inside. Inevitably the liquid dries out with time, especially at elevated temp. Modern equipment uses very small electrolytics, many of them surface mounted. These are extremely unreliable components, not least because they were subjected to considerable heat when they went through the reflow oven when they were soldered to the PCB!

Small comoponents contain less fluid so the loss of a given quantity is a greater percentage of the total when it is a small component than when it is a large one. I have a valve radio made in 1958 still going strong on its original (and enormous) main smoothing electrolytic. There are other problems associated with lack of use. A small current is used to maintain the oxide layer between the two aluminium plates inside. This becomes very thin if the device is not powered, reducing its voltage rating. If it gets too thin and power is applied it can blow up, with spectacular results!

I digress again!

Check the caps!

 

[pixelandsoda]

It is a Rotel ra 810a and I assume it is 25 years old. I just bought it online a couple of weeks ago as a budget solution to run with my turntable.

Would you advise of getting something brand new instead of fixing the current one? Is there anything off the top of your head what could suit my needs and match the Tannoy Mercury MX2 spekaers? It's a shame anyway, because the sound produced by the old rotel (except the distortion) was impressive.

 

[Tannoyed]

If it's that old and you can do it check the capacitors as I described. I just bought a Yamaha AS500 which is to my mind excellent (£200 from Superfi). This may be beyond your budget but it's a lot of bang for the money.

 

[Tannoyed]

More thoughts:-

Avoid old electronic equipment unless you are prepared/able to repair it, no matter how shiny it may be. A 25 year old amp will probably contain a lot of flaky electrolytic caps. Heavy use will have generated a lot of heat with associated damage(see earlier post), storage will have caused the oxide layer to become thin resulting in failure soon after powering up (see earlier post). These are easy/cheap to replace (Farnell or RS or better still CPC supply these and many other things cheaply). You will need to be able to solder/desolder without damaging the PCB. A 25 year old amp will probably consist mainly of through-hole components soldered using lead solder. The job becomes a whole lot more difficult with more recent stuff where surface-mounted components are used and they are attached with lead-free solder.

Equipment which has seen moderate use throughout its life and which has not been left switched off for years stands a chance of being ok.

The fault may be another component, possibly an obsolete one, or the effects of soldiering on with failing electrolytics may have caused other components to fail. By the sound of it you would be better off buying new from say, Richer sounds who do quite good deals on budget stuff. If you can trace faults to component level and have a circuit diagram the vintage path is fine, otherwise play safe and buy new.

 

[pixelandsoda]

What do you think about Pioneer A10 amplifier. Would the sound be as good as my old Rotel?

 

[pixelandsoda]

Just a quick update with regard to the previous comments. I just received WD-40 cleaner. Could you please advise on how to use it without making any damage to the amplifier?

Thanks,

 

[Vladimir]

Just a quick update with regard to the previous comments. I just received WD-40 cleaner. Could you please advise on how to use it without making any damage to the amplifier?

Thanks,

Hey pixel, good to hear you haven't given up on your amp yet.

Read my reply post in this thread, it has basic directions with video links.

http://www.whathifi.com/forum/hi-fi/fixing-input-selector-old-yamaha-amp

Be sure to wait for the spray to evaporate before you close up the amp and turn it on. Give it a generous hour to air out with the lid off.

 

[pixelandsoda]

Thanks Vladimir, I managed to clean all the pots and switches according to your guide, but the distorion remains unchanged...

Have you got any other tips how to repair it?

Many thanks

 

[Vladimir]

Do a visual inspection and see if there are any bulging caps (like on this photo).

 

[pixelandsoda]

I have closed the cover already, but looking through the cover grill they all look fine.