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-   -   Musical fidelity A1 all recapped but sounds muffled (http://www.diyaudio.com/forums/solid-state/219667-musical-fidelity-a1-all-recapped-but-sounds-muffled.html)

cretaceous 13th September 2012 06:37 PM

Musical fidelity A1 all recapped but sounds muffled
 
title says it all - I recapped it, replaced power transistors, psu diodes and a few burnt resistors

It certainly now sounds very smooth, but it's muffled - as if a loudness button is on and/or treble is turned down. It's been left on for plenty of spells of 4-6 hours now, but no improvement. It's the same on any input e.g cd or turntable.

When picking the caps I did not choose high quality audio spec - I did not realise it was important. (doh)
The ones I replaced did not have any fancy markings, so I used general electrolytics (but with better temp and voltage specs).
SO.. any ideas what I can change to improve things?

schematic is here:
Musical Fidelity A1 - Technical

Thanks!

Nrik 13th September 2012 09:43 PM

You could parallel the electrolytics in the signal path with some MKT or MKP plastic film capacitors, that usually gives a more open sound and less muffle.

It would be C6, C7, C13 and C14. 100nF on each should do the trick

yagoolar 13th September 2012 09:53 PM

I replaced 10uF input el caps in MF elektra e10 with ERO MKT1813 6uF and then the amp started to sound open and detailed. Find my thread. There are pictures there. I would replace C6 and C7 with styroflex or some good MKT/P. Have you replaced PSU caps?

Thread link http://www.diyaudio.com/forums/showthread.php?t=218584

cretaceous 13th September 2012 10:12 PM

thanks guys.. great I can try some things out
and yes I replaced the PSU caps - bent wire radials to replace unobtainable axials

destroyer X 13th September 2012 10:26 PM

Mistakes in the capacitor values... for sure
 
Mufled sound use to be a result of increased capacitances in key points.

It will not be capacitor type or brand...will be values.... we can install 0.1uf (100n) in the place of 100pf by a mistake... as an example.

Check values first...then try other possibilities.

Do changes in one channel to compare...because we use to believe in things we previously believe...say.... if you try to replace a blue capacitor by a green capacitor..and you believe the green one is better quality one....then you will listen the green one sounding better...the only way not to be fooled by yourself is to compare with the other channel.

If we, humans, had the precision not to fool ourselves we would never believe in UFO and nor religions will be so important in our lives

regards,

Carlos

cretaceous 13th September 2012 10:32 PM

I'm certain values are correct.. I took good care over that!
(I used to do a lot of diy projects many years ago..)

reji 13th September 2012 10:34 PM

what was the condition before recapping?

destroyer X 13th September 2012 10:36 PM

I suggest you to make another check up
 
If you are not an exception... or a kind of extra terrestrial... then you can make mistakes...humans fails.

When we are old and skilled...and when we had assembled thousand amplifiers..then we think we cannot fail...and this is the moment we fail without the believe we can do it wrong way.

Good luck, and good bye.

regards,

Carlos

tvrgeek 13th September 2012 11:14 PM

Is the temp OK Still stable?
Set the bias?
Polarity correct?
Do you have proper equipment to protect against ESD?
Any test equipment? Toss together a simple test rig and use ARTA to look at the response. Couple of zieners, couple of resisters and your sound card.

5th element 14th September 2012 12:36 AM

As Tvr says, measuring the frequency response of the amp is the only way to check if something major is awry. Changing capacitors shouldn't have caused the top end to get rolled off, but as Carlos says, if you've popped a 100n in place of a 100p in a low pass filter, designed to block ultra sonic hash from reaching the amp, then it could have happened. If all you've replaced are electrolytics then it's doubtful that this would have happened, also the cap type is unlikely to create this kind of effect unless you used a faulty part.

Perhaps you're just imagining things, the A1 to my ears was always very smooth and round around the edges, certainly not an overly bright amp by any means. The tl072 based opamp in the preamp can easily be bettered by a suitable opa134, so you might want to give that ago, as even objectively the 72 is a rather poor performer, but it does depend on the application.

Do be careful though if you choose to go the measurement route, the A1 doesn't have any protection to keep it alive if the output get shorted to ground, so make sure the amplifier is off when making any changes to the connections. As you've experienced, those output transistors are a pain to replace, you don't want to have to do it again.


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