distorted sound

[Tannoyed]

Hi

The pioneer output is 30W rms per channel and as far as I can tell the rtel was 20W rms. The RMS figure is the only one that counts, ignore spurious claims about 'peak power', 'transient power' etc..

It is better to have as much power as you can afford. A low power amp will do more damage to a low power speaker than a high power one-as long as you are sensible. A low power amp will run out of steam very quickly and the output stage will be banging between the supply rails squaring off all of your precious hifi signals. These square wave outputs contain a lot of energy at odd harmonics of the fundamental extending well up the frequency range. You won't hear it when it starts but if you put an oscilloscope across the speakers you can easily see when the limit has been reached. Tweeters don't like this sort of thing.

A high power amp will never get near to this point before you notice the speaker cones protesting so you won't end up with a clipped output, just nice clean sinewaves or whatever (unless you like synth music!). A higher powered amp has loads of headroom to cope with the transients (unless you listen to mp3 of course where transients don't exist!).

To answer the question, technically the pioneer should be ok but I haven't listened to it, my knowledge is more associated with repair than subtle nuances! I feel it would be a good buy. ask for a demo to be sure-take your speakers in to try them.

 

[Tannoyed]

They can still be faulty even if they look ok. Try heating each one with a hairdrier and see if the distortion improves. When you have narrowed it down to a few caps get some aerosol freezer and cool each one in turn. If the distortion immediately resumes you have found your dodgy cap. Look for fluid leakage around the caps as well-another clue. Electrolytic Caps usually bulge when they have been fitted the wrong way round, had too many volts applied to them or they have been left unused for years and suddenly asking them to do something becomes a bit much for them, the voltage rating declines with lack of use.

With regard to the use of WD40, be careful. Don't use ordinary WD40. It has to be a contact cleaner-if WD40 comes in contact cleaner form then fine but I have used Servisol contact cleaner for more than 30 years and for gods sake dont twiddle any presets on the pcb. You may upset some finely calibrated biasing arrangement and you won't be able to set it properly again. If you have to, make a careful note of the position of the wiper so that you can return each one to its original position.

For freezer spray or Servisol cleaner look at CPC website. Electrolube is another contact cleaner that works well.

 

[steve_1979]

(unless you listen to mp3 of course where transients don't exist!).

?

 

[Tannoyed]

I was being facetious- no dynamic range ie loads of compression

 

[steve_1979]

I was being facetious- no dynamic range ie loads of compression

To save derailing this (very interesting and informative) thread I've started a new one on the subject here: http://www.whathifi.com/forum/hi-fi/dynamic-range-compression-and-bit-rate-compression

 

[pixelandsoda]

I have opened the amplifier again to look for any bulged caps, but I could not find any. Alhough I found something that looks like a fluid leakage around the caps as per attached image. Furthermore the cap on the left looks as it seats directly on the surface (board). The discolour on the surface around the caps comes from the WD-40 cleaner being applied in this part of the board. Do you have any idea if perhaps this is the couse of the distortion.

[/img]

 

[Vladimir]

Well there is our problem. The caps need to be replaced since they have leaked badly.

If you can find a friend who can solder, this is a cheap fix. If you go to a service tech it might exceede the value of the amp because of the technician's labor, not the actual spare parts going in.

 

[pixelandsoda]

Thank you again. I have a local handyman guy and I will ask him for a quote on Monday. How could you explain the distortion that comes throught the speakers only but not headphones?

 

[Vladimir]

Thank you again. I have a local handyman guy and I will ask him for a quote on Monday. How could you explain the distortion that comes throught the speakers only but not headphones?

Very simple. The leaking caps are in the power amplification section running your speakers. Headphones are powered by a small separate section, which in this case obviosly has no bad parts. That is a good sign, meaning the preamp section sending signal everywhere is still good.

If you guys decide on fixing it, I'd recommend Panasonic FC caps for power sections and Elna Silmic II for signal paths.

 

[Tannoyed]

Hold on a minute! This new revelation changes everything. Headphones are usually driven via resistors from the speaker output (typically 2 off 330 R resistors of about 2W rating). It is possible that they may be driven from an earlier stage and Vladimir's comment would be valid in this case, but on a cheap amp like this they will have taken the easy/cheap route I am sure. If the headphone sound is clean then the speaker sound should also be clean. The fluid you mention looks like an over exuberant application of switch cleaner (WD40). Also I should tell you that radial lead electrolytics are usually stuck to the pcb with an adhesive that looks a bit like evo-stick. This is perfectly normal. They do this to enable the product to pass various mechanical shock and transit tests. The information you have provided is inconsistent and I am now baffled. Are you sure that both speakers exhibit the same fault when connected to the faulty channel?

 

[Overdose]

Wouldn't it be cheaper to use a tap from the preamp section for the headphones, rather than have to install what amounts to a couple of power resistors on the output?

Anyway, the cost of the caps and a couple of beers for the work is cheap enough to test the theory, but for us commentating on this, nothing can be determined for certain without the wiring diagram.

 

[Tannoyed]

They would have to use an extra driver/buffer stage and this would cost more. 2W wirewounds are dirt cheap and the switching can be included with the speaker switching arrangements. I looked for a cct diag for this and found one for an adjacent model where two wirewounds are indeed used, fed from the ouput stage. This has been the case with most amps I have looked at. I think this chap should give up and buy new. To really know what is going on some measurements and tests are required by someone competent and much as I dislike giving up I don't think the OP has sufficient knowledge to carry out the work or to assess the fault. He will be wasting his beer money in my view but by all means give it a try.

 

[Tannoyed]

I have now found the circuit diagram and without question the headphone sockets are connected directly to the speaker output via 2 resistors (1W 330R). If the speaker sound is distorted and the amplifier is at fault then the headphone sound will be similarly distorted-there is no doubt about that.

It is a conventional push pull bipolar output stage Do NOT Adjust RV601 or RV602, otherwise the biasing will be affected and the output transistors will cook!!!!!!

 

[Vladimir]

Hmmm. Makes no sense why would he spray WD-40 on the caps and the PCB? We clearly said just the pots and very sparsely. That specialist electronic contact cleaner WD-40 evaporates within minutes. But it does look like its from the spray. Leaking caps look like they just had a "number 2" and on this photo looks like they did a "number 1."

This is more typical viscous product from electrlytic caps.

The headphones are driven by the same power stage but show no distortion. Maybe the caps aren't stressed enough to come at the point of distorting. Not much ripple current going on with headphones.

It really sucks we still don't know the DC offset.

 

[Tannoyed]

Good point

Agreed, the headphones will demand far less current. That suggests to me a high supply impedance and points us in the direction of the power supply/main smoothing electrolytics C905 and C906 or the series regulator transistors Q901 (+19.4V) and Q902 (-19.4V), and associated components. What a pleasure to see such a conventional arrangement! Happy days!

pixieland whatever your name is, if you can do it measure the voltage at the emitters of Q901 and Q902 as the volume is increased. They should stay steady at 19,4V with respect to OV (the case metalwork). Any sAGGING INDICATES REGULATOR TROUBLE. Is there any hum on the sound (100Hz ripple)?

More probably the main smoothing blocks are dodgy because these regs only feed the low signal stages. The unregulated supply goes straight to the output stage. Measure across C905 and C906-you should get +/-37V. This will drop a bit under load but not drastically. If it does change these caps (6800uF 50V)

 

[Vladimir]

There is the possibility that someone poured coffee or something sticky on top of the amp (not saying the OP did it, but someone before). Maybe that area from the sticky stuff and all the dust at higher currents becomes conductive, maybe cleaning it is all that it needs. Just thinking outloud.

Problem is it is right at the main caps that hold voltage and it is not safe for the OP to clean this himself. A technician would just short out the caps with a resistor to drain the voltage left and clean everywhing with a brush and alcohol.

 

[Tannoyed]

Well strictly speaking it exceeds SELV (safety extra low voltage) spec but I would happily grab hold of 37V! The only dangerous bit is the unisolated mains supply so don't touch anything before the transformer! As I said probably best to buy new-We can't really tell what's going on from a range of 100 miles or so and this chap is not experienced but the pointers might help someone who is.