replacement amp

First off, Hi guys/gals, just found your forums, thought I'd drop in and say Hi. I also have a question for those in the know

I have a Pioneer A400 amp that doesnt sound as good as it once did, and Im looking(when funds allow, couple of months) to replace with a similar standard amp. Any comments/suggestions about a worthwhile replacement ??

Cheers

Oh and you can all have a virtual bevvy on me

 

Hi Zero One,

Welcome to the forum.

You have said that your amp is not sounding good. What's different about it's sound? It may just need a service and check up. If you like the sound, this will be the cheapest way forward. One thing it could be is a worn or dirty volume control or input switches. It could also be a failing or failed component such as a resistor or a capacitor. Taking it to a service engineer will mean that there can be a check on the state of your amp.

Also are you sure it's the amp? What is the rest of your system? What's your budget? If you supply some more info, we should be able to get down to the root of the problem.

SCIDB

 

Cheers for the welcome.

The problem(s) I think are from the speaker output stages(no problem via headphones) I'm getting hiss(not too loud) and occasional pops(again not too loud), this is noticable when in between tracks or no track being played, the volume control sounds 'scratchy'(thru speakers) when being adjusted. There is no noticable difference in overall performance, but Im worried that the problems could damage my speakers. Because of the age(and the fact that I bought it for £40 a 1 1/2 years ago) I didnt give much thought to repair, but,does anyone have any Ideas where I could get it checked over/repaired(south england) or failing that are there any schematics available ?

As far as budget for replacement goes, I was looking at around the £300 region.


My 'stereo rig'

Pioneer A400 amp
Pioneer PD-S703 cd player
Mission M33 speakers

My 'AV rig'
Pioneer VSX-808rds amp
Pioneer DV-717 dvd player
Mission M33 fronts
Mission M7c2 centre
Mission 70 surrounds
Mission 70 ASE subwoofer

Thanks

 

yes, be more specific, sometimes you just imagine it, or maybe your ears change.
Then again, the standard things that would be done at a service are the power supply capacitors, as these are 'wet' and they dry out with the heat of the amp. The A400 does run quite warm for a japanese amp. Another thing would be tweaking the bias current. Apart from that there is not much to do, but might be worth spending upto £50 to get it running as new again.

Or you may want a clean break.

If you are in the market for something similar, Rotel are well liked, and there is the Nad 350 or similar.

 

Hiya

Im not technical enough to help with the remedy on the amp.

I will say this, unless you're willing to look at second hand kit I dont think you're going to get a substantial upgrade for 300.

What about the rest of the kit like SCIDB said mate?

Cheers
Chris

NB I know some good repair people in London, but none in Pompey.

 

oh forgot to say, you can take the lid off and spray the volume control inside with wd40/ electrical spray, that will cure the scratchiness.
Don't flood it, just a 2 second max. squirt, and rotate it a few times to clean it. Dead easy, won't hurt it. That's all an engineer will do....try a independant tv/radio shop that does repairs, will be cheaper and they can be very good.

circuit diagrams available from a place called SEME. I had one, but sent it back as they charged £15 for a photocopy.

 

Any other thoughts/suggestions ??

 

If you're not up for second hand stuff, look for ex-demo kit. Cheaper, but you still get a guarantee. Have a look at:

http://www.pjhifi.co.uk/pj/special_offers.html
http://www.noteworthyaudio.co.uk/Offers.html

 

ok..ok... IMO the A400 is a moderately powered clean souding amp.

If you like what it did but just want 'more' how about =
this advert from www.hififorsale.co.uk
NB They also had a Nait1 for 175 quid.

Naim
Nait 3 Int Amp Price: £220.00

RRP: £600


Description
Naim Int Amp Your first step to real Hifi

Contact: larry
Seller: Private seller
Telephone: 01634 305115
Telephone:

Email: [email protected]

Location:
kent
UK


Advert No: 24301
Date placed: 05/11/2003

 

The NAIT3 is a stormer (ignore Wadia Meister and Timpy!) - if you use the CORRECT speaker cable on it and site it well it sounds very good.

Match it carefully though as with thin speakers it might be a bit bright - though with my Rega Kytes, then Elas, it rocks!

 

Well thanks very much on my first thread especially to Lt Cdr Data for the scratchy volume fix, I used a small squirt of 'ultrasolve'(cleaning solvent for mech/elect) and lo and behold no more 'scratching' I also think that this has reduced the hum from the amp(although I think that might be down to the placebo effect). Im going to look into the repair side if it can be done for a resonable price.

Cheers again

 

I've so far replaced the caps in my A400, but Lt Cdr Data (or anyone else) how would I alter the bias current ?

and to what setting ?

I can see 2 variable devices (resistors?) with white dials on the pcb....

Many thanks

 

Right you have to be DEAD careful doing this, as you could blow your output transistors.

This is one of the things Tom Evans does for the a400gte. he sets it to 200ma

the other is 2 multilayer ceramic coupling capacitors

and the final is a dc servo, ( adjusts the dc in the feedback loop)
Erno Borbely is the inventor of this circuit.


open here up.

You switch it on, play music thro it for 10 mins, get it warm.

no kids or pets when doing this, don't touch anything metal inside, and use rubber gloves if you are ham fisted.

Use a multimeter on the dc volts setting, very low reading, place the leads across the big white resistor close to the ouput stage. and measure the voltage across it.

You ideally want about 150-200 milliamps flowing through this resistor, and the japs run them quite low, maybe 20-50 milliamps.

You get the current by dividing your measured volts by the resistor value. (simple ohms law)

say the resistor is 0.33 ohms, and the volts is ( v ), then this is the dc bias current that keeps the transistors slightly on to prevent crossover distortion.

your current= Volts ( v ) / resistor value.

So work out the new volts for the new current of 150 milliamps, this is

volts= current x resistance.

=150 milliamps x value of white resistor.

You need to tweak the little variable resistor with a philips no 2 screwdrive, insulated handle, stanley very good, VERY VERY carefully indeed, increase a little literally a millimeter at a time, DONT turn it all the way or its goodbye amp. Little minute turns WHILST the voltmeter is clipped on the resistor, and you will see the voltage rise, tweak until it gets to the desired value above, and repeat for the other channel.

You amp will run hotter, more in class A, and the sound may improve. You can do this with any amp with a pot but you need to be sure the pot is for this, and not dc offset, or anything else.
In the a400 its ok.

You must be careful, as if you do it too much, current= fried, and you will set your transistors on fire if you are to heavy handed and turn it all the way.

Also, don't go above 200milliamps, as the heatsinks can't take a large class A current.

Write again if in doubt and be careful, if you are, its easy.

If you go ahead, let me know the value of the white resistor, and the measured volts across it first, to be on the safe side.

And it have to settle a little, you may need to turn the pot, leave a minute, turn a little more, or back, it hovers until it settles.

Its only a 5 minute job and dead easy.

 

Many thanks for the advice. I'll be looking to buy a multimeter later on today and will open up my amp to see if I can locate those big white resistors. Is it easy to see which pot is related to which resistor ?

Also, regarding the coupling caps that Tom replaced in the A400, you wouldn't happen to know which ones he used would you and of what value ? The standard ones are 50v 10,000 uF.

 

He uses multilayer ceramics, they are not 10,000uf, those are the smoothing caps.

They are about 2.2ufarad he uses, somewhere near the opposite side to the main output transistors, but I don't think the values are the same.He uses smaller as they are the largest you get in the multilayer ceramic.

I will have a look for you later.

I forgot to say he does one other thing, he reduces the feedback by 6 dbs, by changing a resistor.

You must not increase feedback, as it will turn your amp into an oscillator and fry your speakers.

But it can be reduced, tho don't mess around with it. It is a balance between sensitivity, distortion and stability, lowering will make it distort a little more. The feedback in teh a400 is complex, so do not play.

 

Well the value of the big white resistor was 0.33. Voltage reading was around 20 mV(?) I think... Interestingly the long adjustment groove on the pot dials were initially aligned straight i.e. parallel to the left/right side of the amp case. After adjusting them I found that the pot nearer to the back of the amp needed turning a greater amount to have the same affect as the other channel's. For the voltage, I went for a reading of between 60 - 66 max which returns a max current of 200mA. Was a bit tricky trying to set it as the values varied on how much the amp had been used.

The sound has improved - more detail and bite, so it's a change I would recommend to others. Thanks Data.

How about the coupling caps ? There are electrolytic caps nearest the transistors between the big white resistors which have a value of 2.2 uF 50v - would they be the ones ? If not where can I find them, as I'd be keen on upgrading these if they are known to improve the sound.

....after reading your post again, am I right in guessing that the coupling caps on the A400 are already multilayer ceramic ?

 

+ any others worth changing ?

 

probalby not, evans' cost about £2 each, best dielectric you can get from rs or farnell.

the ones he replaces are c201 and c202 on the board, that is all.

no idea what the stock ones are as don't have the amp now

that's about all I know, good modding

not sure if multilayer ceramic are a good idea, the std advice is the best you can find, ansar polyprops are good from cricklewood electronics, london.

If amp gets too warm back off to about 150 ma, use your nouse. ATB

 

Well the existing coupling caps, c201 and c202 as you mention, are Elna 2.2 uF 50v - a metallic maroon colour. I'm wondering whether I should replace these with Black Gate electrolytic caps, mulitlayer ceramic or polyprops ??? Which would sound better? I like the idea of adding Black Gates which I'll be adding elsewhere in the amp but may be just for the coupling caps, alternative types may better - hence Evan's decision to use multilayer ?

Also one of the caps I'm looking to replace is 330uF 10v but I can only track down 330uF 16v - would this part be usuable despite the voltage difference ?

Any ideas?

Thanks,

Dav

 

I think evans philosophy about the ceramics is they are small and cannot vibrate.
I would have thought the ansar polyprops, from cricklewood,
or rs or maplin polyprops would be best.

With other caps, its fine to get a higher voltage, but not the other way, or the cap may be destroyed.

As a rule, electrolytics are not liked, even black gates, and are held to be inferior to plastic ones, paper or even teflons.

whether this is true or not....

you may also like oscons, or silmics.