Musical Fidelity A1 Integrated Amplifier Repair/Upgrade
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- Опубликовано: 20 апр 2022
- Looking at a Musical Fidelity A1 and some of its classic problems which are well documented by Mark Hennessy. We take some reference plots before we do anything then compare with results after our modifications. A power supply upgrade helps with supply ripple and the input amplifier modification reduces the noise floor at higher levels. Noisy volume control and selector switch issues are also exterminated!
Some thoughts after completing this video:
1. The scope noise before modification was horrendous. The input of the amplifier was terminated but its possible that the termination wasn't doing its job due to the precarious input selector switch. This may account for the results being so bad.
2. The non-linearity that I showed on the scope at higher levels before the modifications was not there after the modifications. You can see when I vary the volume control that the output easily goes up to 20v and clips in the traditional way. Possible there was some other fault present before the mod.
These modification and tests take a fair chunk of time. Its unlikely to make economic sense to have these amplifiers modified in this way. The only feasible approach would be DIY.
Marks web page : www.markhennessy.co.uk/mf_a1/
Serial number of this amplifier: 16580 
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I have owned and enjoyed my A1-2008 version since around 2011 or so. Really lovely sounding amp, "in the room with you" sound with both classical and rock. You do need sensitive speakers if you want to rock (I use Reference3A DeCapo I speakers) and it is a match made in heaven.
Mine does run very hot. Too hot in the summer here in Vegas to hold your hand on the top case for more than a 1/2 second. So it collects dust in the summer, as does my T8-LN OTL tube amp. But since Class D is getting better, especially with the PFFB technology, I get by in the summer.
There is one track I always can trust to tell the story of an amp. "She's the One" by Springsteel. The backing rythym guitar just sounds like toneless hiss unless you have a very good preamp/amp. Try it.
I will open up my A1 soon to renew the heat sink thermal compund. Will do no mods or other work unless I see bad caps.
I've had one of those sealed rotary switches in my preamp now for over 2 years, still measures perfect and has a nice amount of physical resistance.
Absolutely fascinating. Love listening to common sense, spoken in an accent that's music to my ears. (from 8000A Malc.)
Awesome video, mate.
I have a refurbished A1 coming my way in a few weeks, which will hopefully give me years of trouble-free listening.
May I ask you what was the total cost of parts and service for the refurbishment?
Cheers pal really enjoy your videos
Class A amplifiers require huge heatsinks and power supplies, they're as much about mechanical engineering as they are electronics because removing the heat is an issue. As a builder of this type of amplifier (specifically the Pass F5 Turbo V3) I used a cold plate to remove the heat from 32 transistors each of which dissipates 25 Watts (it's four channels of class A biased at 3.5 A per amplifier per rail). Forced air cooled heatsinks worked for a while but always ended in smoke & tears (mine), the cooling system uses a central heating pump, two small car type radiators each with a fan, and a 17 litre water tank. The entire system is in the cellar below the listening room and sounds every bit worth the effort it took to build.
Hi, great video, by any chance would you do a refurb on my A1, and any idea on what you would charge.
Noisy volume control, is the main issue.
Kind regards
Neil
Ah sorry, just read your description, you are busy and can't take it on.
If things change don let me know.
Kind regards
Neil
I own an original A1 of the first serie's, modified/upgraded to later version by the importer here in the Netherlands.
Later I modified the volume control to a more traditional style i.s.o, the negative feedback control originally used.
With a very simple modification the volume control is relocated to the output of the preamp IC's.
Last year I checked all the electrolitic capacitors and found only one out of limits.
My amplifier has different distortion when getting to max power, it introduces a kind of crossover distortion.
Also Last year I replaced the A1 by a Nelson Pass F5 clone I built, noticable more power.
Hi, can you guide what was. The simple modification,as I too want to do it.Thanks in advance
Great video! I was wondering if there's any worthwhile mods fir a myst tma3?
I remember these! They sounded good but ran really really hot 🔥. That type of wafer switch for input selection was also used in in old Sansui and Kenwood receivers; Sansui's usually had 3 or 4 of them in a row inside a slotted metal frame, and they used it to switch the tuning dial and input indicator lights as well as input selection and Dolby FM modes. I have taken them apart and cleaned them, which is indeed kind of a pain (especially if there are 3 wafers in a row) ---- but the plastic rotary switch you replaced it with really isn't that much better, and I had to replace one of those on my NAIM preamp.
Interesting, how many wats is it?
@@neilstern7108 , I think it was just 25 or 30 watts per channel if I remember correctly.
@@goodun2974 heck I have about 50w per channel and I can play it all day. That amp must have shorts or lots of resistance. Thanks for your reply.
@@neilstern7108 , its a class A amplifier design, and Class A designs always run hot. In a commoner, everyday Class AB "Push-pull" amp circuit, one output transistor ---- or a group of them ---- handles one half of the audio signal and the other transistor or group of them handles the other half of the audio signal, with some overlap, (like a relay race where a runner hands off the baton to another runner and they are briefly both holding it). Push-pull amplifiers are more efficient and don't waste as much electricity in the form of heat because one transistor is turned off while the other one is turned on, and vice versa, and so there is a rest period for each device in which they are not generating heat or are cooling off slightly. However, there tends to be some *crossover distortion* at the point where one transistor (or a group or "bank" of them) is handing off the work to another (when the handoff is about to occur, and/or when both runners are holding the Baton simultaneously). In a Class AB amplifier, all the output transistors are turned on 100% completely all of the time, and can conduct both directions of the audio signal without handing it off to another device or group of them, but this means that the transistors have a lot of current flowing through them even when they aren't passing an audio signal, and therefore they generate a lot of heat. (imagine your car idling at 3000 rpm when you're at a stoplight with your foot on the brake). A well designed Class A amplifier can have very low distortion, or at least a different, perhaps less audible type of distortion then a class AB push pull amp (and it might also have better "damping" and might handle the ever-shifting electrical-impedance load of the speaker better; speakers aren't a steady state resistance and the load varies with music frequencies); but Class A amps will tend to be fairly low powered yet require a big power supply and transformer to keep up with the demand for excess current, and big heatsinks to get rid of the excess heat. Neither design is inherently better or worse; there are almost always engineering and design trade offs involved, and considerations must be made to what the amplifier will ultimately be priced at. For what it's worth, I've heard good sounding Class AB push pull amplifiers and bad sounding or impractical/underpowered/overpriced Class A amplifiers.
@@neilstern7108 , PS, It might be helpful for you to understand that transistors, if allowed to get too hot, can go into a "thermal runaway" condition, whereby, as they get hot they conduct more current, which causes them to get hotter, which causes more current, which causes more heat, until 🔥 💥! Transistors are a little bit like electronic switches but they require a "bias" voltage to turn them on, and there is a range of biasing (quiescent or steady state current flow through the transistors when no signal is passing through them) within which the transistor will behave in a fairly linear fashion and will likely have acceptably low distortion; but if the bias is set too cold, the amp will both physically run cool to cold and likely sound bad, with measurably higher distortion; and if it is biased too hot, closer to or into Class A mode, it might self destruct unless designed to be run in that mode, ie: it having sufficient heat sinking and heat sensing/automatic bias-feedback circuits to prevent self-destruction. In the case of the amplifier shown in this video, the mechanical design traps a lot of heat inside and so it tends to prematurely degrade and shorten the lives of other components such as capacitors and even solder connections. This might have been a case where the marketing department decided that they needed to have a Class A amplifier in their product line strictly for purposes of covering all the marketing bases by appealing to a certain segment of the audiophile crowd who are hardwired to assume that Class A "always sounds better". Had they not gone for a slim, low profile mechanical lackage, and had they incorporated vertical heatsinks and slots or vents for increased airflow, perhaps it would have run cooler and lasted longer before needing service.
That so called active volume control is similar to the old Cambridge Audio P40/50 where the pot is placed in the feedback loop. Noise was caused the slightly different voltages across the pot. Same effect. nothing wrong with the pot just noise due to DC.
Could you please tell me what is the AC output voltage of that toroidal transformer. I am trying to rebuild one unit that someone ruined with two small classic transformers just to keep it running not so hot 😢. Would 16-0-16 be ok? Since 18-0-18 is just on the limit of that capacitors
16v RMS is just about right i think.
@@myalbatech thanks I will order then one with double 16v maybe 120w that will be enough since the space is limited
why did you only change the 1 capacitor to 33mF not both of them? and wouldn't 47mF been better.
and what about changing the volume to attenuator?
yeah very nice
Because there is no room! Higher capacitor values would be better still yes.
Does putting the bigger caps on the power rail help with bass?
Here are some comments from the owner: 'Even from a cold start the bass is definitely fuller, deeper, tighter since you worked on it, and the soundstage has more detail with a wider sweetspot than before and just improves as it heats up. I noticed I played around less and just listened to the music.'
@@myalbatech Sounds like a worthwhile upgrade.
My A1 sounds magical and very easy to listen to but the bass seems a little out of control compared to my other integrated an Ion Obelisk 3X/ XPac1.. but I prefer the sound of the A1, other than bass.
Neither have been serviced.
I got a b1.