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Old 3rd March 2009, 12:50 PM   #1
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Default Musical Fidelity A370

Hi folks,
I've come into possession of an A370 MK II power amp.Sounds fine,but I wondered if anyone has any advice on a foible of the amp.On powering it down,you can hear it takes a while for the supply caps to discharge-not surprising with 100,000 uF per channel I hear you cry!.Just as the caps are discharged,there is some popping/clicking/beeping from both channels.It's independent i.e. each channel does it's own thing.I wondered if this means some of the caps are on their way out? Does anyone have any experience with this amp? Any advice gratefully recieved.

Regards,Ali.
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Old 3rd March 2009, 04:18 PM   #2
AMV8 is offline AMV8  United Kingdom
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Ali

I have two of these amplifiers, a Mk 1 and a Mk2. I think they are great amplifiers. The only problem is the weight and where to put them as they are so large.

There are some things to look out for:
1) Check that your power supply caps are the correct voltage and size. Some models seem to have 50v capacitors fitted in the power supply - both of mine did. The power amplifier runs with 64 volt rails. I recommend fitting capacitors that are over 65 volt and preferably over 75 v.
2) Some amplifiers also have small pcb fitting capacitors fitted in the power supply - again both of mine did. If your capacitors are less than 8cm long then I would recommend you fit higher amperage capacitors.
3) The Mk 1 models run a high bias on the output capacitors and the heatsinks run very hot - around 80 degree C. The Mk 2 models run cooler. I have not done a calculation but I guess the Mk 1 models produce 60/80 watt in class A and the Mk 2 probably 40 watt in class A. Some of the Mk 1 models seem to burn out the output devices - so I would recommend adjusting the bias on a Mk 1 model untill you can just put your hand on the heat sink when hot.
4) Some of the early P270,s used 1.5k and 1.8k 5w resistors to reduce the rail voltage down to 15 volts for the input ic. These resistors ran very hot and really needed changing for 20 watt versions. I do not know if this was the case with early A 370 models but you could check.
5)Early models used an ic only for input circuit. Later models added a pair of input transistors to replace the input stage in the ic. Some people have replaced the ic with better ic's whilst others have replaced the input transistors in later models. To me they sounded ok in standard form.

I found that they biggest change came from replacing all the power suply and pcb electrolytic capacitors with new ones.

If it was mw I would change the capacitors and then enjoy the music.

Don
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Old 3rd March 2009, 04:39 PM   #3
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Hi Don,
Thanks for the informative reply,very useful thanks.Mine is a mark 2 issue 5 if that means anything to you.

1) The power caps are Elnas,Cerafines I think.I'll need to check.Rated at 65v I think.Again,I'll need to check.May I ask what you changed the power caps to?

2)Do you mean small caps used for smoothing? The caps in mine are large Elnas,must be around 150mm in length and 40-50mm in diameter.

3)The heatsinks on mine run just warm,nowhere near too hot to touch.

4) I'll check that in mine.

As for the pcb caps,did you leave in place all the ICW's then,the ones that say "Specially made for MF" and just replace the others? Mine has what look like Black Gates in,at least they look like BG's and are made by Rubycon.

Thanks muchly for your help.

Regards,Ali

P.S. Any thoughts on the noises after switch off? Do yours do this?
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Old 3rd March 2009, 05:15 PM   #4
AMV8 is offline AMV8  United Kingdom
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Hi

You seem to have the larger elna power supply caps - which is good - but I would check the voltage.
On the pcb the caps marked " made for musical fidelity" are poly propylene capacitors of around 8uf. They will be fine. It is only the electrolytic capacitors that it would be best to replace.
If your amplifier is a later Mk 2 and sounds ok you may not need to replace electrolytic capacitors as yet. However they do "age" and I find it is best to replace them after about 10 or 15 years. After that time I find the sound seems less focused and notes are less defined. Changing the capacitors normally clears that up.
I own P270's and A370's. I think they are very good sounding amplifiers. I hope yopu enjoy yours.
Next you will be needing some new speakers to let the amplifier show its capabilities!!!!!

Don
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Old 3rd March 2009, 05:40 PM   #5
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Thanks Don,I'll get the electrolytics changed.I'm a valve amp freak really.This amp will be used mostly for satellite tv duties,though I do have a cd player plugged into it.I've tried the amp in my main system which uses ER Audio DIY electrostatics,and it sounds very good indeed,quite close to my 845,300b and 813 amps.Frankly it sounds a lot better than I thought it would! Very good soundstaging,on a par with my valve amps,which was quite a surprise to me.The bass is pretty good too!
Thanks again for your help.Any comments on the popping noises?

Regards,Ali.
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Old 3rd March 2009, 06:09 PM   #6
jez is offline jez  United Kingdom
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The strange sounds on power down are normal. It's due to the - & + supplies discharging at different rates. We sometimes used to add resistors across the rail not powering the LED to try and cure this....
There are no smoothing capacitors on the actual boards. They are supply decoupling caps. It would indeed be worthwhile to replace all electrolytics. Mk1's had 5 pairs per channel of 22,000uF Elna's of IIRC 68V rating. All the original caps were bog stock, not black gates or anything. When replacing the caps use 105 C types, not 85 C as originally used. MkII's were basically built down to a cost compared to the earlier ones and again IIRC some only had 3 pairs of caps. I would not mess with the op amp type if I were you. It's inside the feedback loop of the amp and the whole amp is compensated for the original LM318's.
The amp should run too hot to touch for more than 5 seconds or so. If yours is only warm then the bias may have been turned down to make life easier for the components.....hence the recommendation of 105 C caps.
I used to work for MF and repaired these every day
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Old 3rd March 2009, 10:12 PM   #7
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Great thanks for that.Would you recommend also changing the supply caps then? If so,what would you recommend in place of the Elnas,and would this improve the sound?
Please excuse the silly questions,but I've no knowledge of sand amps,I've only used valve amps for years.Another thing is I can hear a low-level buzz/hum if I listen at the speaker cone.Would this be normal for an amp like this? I realise it may be down to earth loops etc.The buzz sounds like 50 hZ.Would it also be worth changing the polyprops to improve sound?
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Old 4th March 2009, 09:59 AM   #8
jez is offline jez  United Kingdom
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I was including the smoothing caps in "replace all the electrolytics".
The brand is irrelavent but ideally, use low ESR 105C rated type. This will not be cheap.... I'd leave the polyprops alone.
Check all the soldering on the pcb, especially on the large wirewound that is vertically mounted.
There should not be any hum at all. After the first bank of four 22000uF caps there are a pair of 0.47R 11W wirewounds feeding the the next 6 22000uF caps. This forms an RC filter which results in very low hum (the high PSRR of the op amp which is itself fed from constant current sources kinda helps as well!)
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Old 4th March 2009, 10:10 AM   #9
AMV8 is offline AMV8  United Kingdom
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Ali

When I rebuilt my amplifiers I used Kendeil capacitors in the power supply. I used a total of 8 ( that is 2 per rail per channel ) hey are each 20,000 uf, 100 volt and 30 amp capacity. They are metal cans with screw connections on the top. I used elna 105 degee caps to replace all the electrolytics on the pcb's.

You will find that everyone recommends different makes of electrolytic capacitors. I still beleive the best were the ALS20A, the Seimans capacitors and more recently the Kendeil capacitors. However good metal can electrolytics are expensive. I will be surprised you buy the Kendeil caps for less than 25 per capacitor. I guess that is why most manufacturers do not the best quality. The large metal cans simply hold more energy than the smaller pcb connection capacitors. The effect on the sound is that the base in particular is sharper and transients are stronger. Why; because both of these require a lot of energy that smaller capacitors can not deliver.

I think you will find that the base is much better than that from a valve amp - but I suspect some others may not agree with that.

Don
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Old 4th March 2009, 10:25 AM   #10
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Most helpful,thanks very much Gents.I'll look into the hum.It may just be a cable issue.Time to start looking for suitable caps then!

Regards.
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