Hi,
I came across this amp for the first time in a junkshop, selling for peanuts, so I bought it over. The amp looks complete but when I opened up this amp, I realise that there is no transformer inside. There are two rectifiers and individual filter caps. At the back there are 2 multicore cables thet connect to the switch and this 2 power supply boards. I do not know anything about this amp, this is the first time coming across it. Does this amp originally use an external transformer?. Could not get the schematic, so I dont know what are the secondary voltages. Does anyone came across this amp or has a schematic or any info about this amp? Please assist.
Thanks
Carlos
I came across this amp for the first time in a junkshop, selling for peanuts, so I bought it over. The amp looks complete but when I opened up this amp, I realise that there is no transformer inside. There are two rectifiers and individual filter caps. At the back there are 2 multicore cables thet connect to the switch and this 2 power supply boards. I do not know anything about this amp, this is the first time coming across it. Does this amp originally use an external transformer?. Could not get the schematic, so I dont know what are the secondary voltages. Does anyone came across this amp or has a schematic or any info about this amp? Please assist.
Thanks
Carlos
From the age (~1983) this is among the original MF products. It's actually a integrated version of the Dr Thomas (first) MF amplifier. I think it's most likely a conventional transformer supply amp if the rectifiers are still in place but the transformers have been removed for some other purpose.
Whilst its a nasty surprise to buy something with no power supply, there is always a good reason for them being cheap.
I guess this one was just too cheap.
Look at the electrolytic voltages for some idea of the rail voltages. MF had a bad habit of rating the caps right at the DC rail voltage. Try to correct that after sorting a suitable voltage from the power rating and and the caps.
Some reviewers state the amplifiers were rated at 120WPC with the first 25W in class A. That means they would have to be substantial transformer(s) as the standing current would then be a total 2.5-3 Amps. per channel and the transformer should be rated at twice that as a minimum. That's 10-12A total current rating - a big transformer or two indeed and not cheap for DC rails of 50-55V necessary to realize 120W /8 ohm (if that is the actual power rating).
MF did not publish schematics - the best you will find is rough, hand drawn circuits with no parts details.
The high voltage supply connects to two 6800uf/63V and the low voltage supply connects to two 1000uf/25V caps. What could be the possible secondary AC voltages. Please advice.
Thanks
Carlos
It's class AB amp with small heatsink at the rear of enclosure. It uses Hitachi lateral MOSFET output transistors. Take a look at this excellent report for ideas how to upgrade and service it:
Musical Fidelity Synthesis Amplifier upgrades | Rock Grotto
Musical Fidelity Synthesis Amplifier upgrades | Rock Grotto
warning ,, steer clear of this website as the person who runs it is a absolute nutter and a very rude person
be warned
Thank you Ivanlunic. I have 2 of them in my position. The one is back to 1984-5 and the second last month.
It uses 2SK226/2SJ82 and yes the heat sink is relatively small. Although the amp has NFB of 31 (in low frequencies) to 25 at 20kHz the gate is limited by the use of 4V zener.
I upgraded myself the fist device doing almost the same that this reference is reporting (thank you too Jamesfeline) . I didn’t remove the fuses but I replaced the line amplifier opamps and the device sounds great.
Although the construction is very poοr the final amp uses one degenerated LTP for input stage and another LTP for VAS. I intent to completely upgrade (as an experiment) the newer one and see what will happen.
dtoli,
If you decided to keep speaker protecting fuses, I suggest that you clean both fuse holder and fuse caps with contact cleaning fluid.
This MF amp probably uses standard Hitachi app note circuit. Later MF amp that used Hitachi lateral mosfets (B200) had different topology of output stage.
If you decided to keep speaker protecting fuses, I suggest that you clean both fuse holder and fuse caps with contact cleaning fluid.
This MF amp probably uses standard Hitachi app note circuit. Later MF amp that used Hitachi lateral mosfets (B200) had different topology of output stage.
No, at the end I won’t keep them, because I’ll install fuses, for safety reasons, on the rails (out of NFB circuit) and a speaker protection using a relay. It was not a good idea to keep the fuses in NFB circuit.
Yes, it is a variation of Hitachi circuit. For example in the input stage it has the current source on top of LTP where there is also a transistor that delays the feeding of the input LTP at starting (probably to avoid the cost of a relay). On the driver it has the mirror on top and the driver LTP is under cascode transistors (those TO126 seen on the heat sink).
This is not a pure MF design. Power amp is a Dr Tomas design build by using cheap material / construction, probably is the reason that is not mentioned on their archive. Still this is the first device that MF produced. I’ve heard that they were building these devices in a kitchen?
Yes, it is a variation of Hitachi circuit. For example in the input stage it has the current source on top of LTP where there is also a transistor that delays the feeding of the input LTP at starting (probably to avoid the cost of a relay). On the driver it has the mirror on top and the driver LTP is under cascode transistors (those TO126 seen on the heat sink).
This is not a pure MF design. Power amp is a Dr Tomas design build by using cheap material / construction, probably is the reason that is not mentioned on their archive. Still this is the first device that MF produced. I’ve heard that they were building these devices in a kitchen?
Hi dtoli and all
Yes I too still have my Synthesis and have been stunned by the poor build yet captured by its good looks. I did build in speaker protection (by relay delay) in a rather quick solution to a 'thump' at power on in the right channel.
That was years ago and it still runs beautifully although probably long overdue for the recap as detailed in an earlier link.
Oh and yes I have heard the same rumour about the kitchen table build. Personally I wouldn't mind doing away with the metal car and redoing nicely in timber...
Yes I too still have my Synthesis and have been stunned by the poor build yet captured by its good looks. I did build in speaker protection (by relay delay) in a rather quick solution to a 'thump' at power on in the right channel.
That was years ago and it still runs beautifully although probably long overdue for the recap as detailed in an earlier link.
Oh and yes I have heard the same rumour about the kitchen table build. Personally I wouldn't mind doing away with the metal car and redoing nicely in timber...
Hi audiofirst,
I ‘ve made, since last June, of both of the amplifiers I own extensive changes and it is amazing how good they sound.
Really the construction is very poor, and it is obvious that they tried to reduce the cost in the expense of sound quality, but the amplifier design is very good.
Some of these drawbacks are:
· The use of fuses at the output in order to avoid a protection relay.
· The very small values of PSU caps. If you try to measure the rail voltage during a program you will discover how brutal is the variation.
· Regarding the stability of the amplifier, they use very big values of decupling capacitors.
· Main heatsink is inadequate to handle high volumes. It is only about 1C/W and considering a dissipation of about 55Watts can easily lead to temperatures of about 80 deg Celsius.
Still:
· Input stage is degenerated
· VAS is a cascaded LTP
I measured one channel using a sinusoidal signal in input and a resistive load (8R) and I found that clipping occurred at 39Volts (measured on the load resistor). That means 95W Rms output power driving one channel only (and using the old Hitachi mosfets).
I calculated that if both channels were driven with a monophonic program this value would be something like 2x64W RMS
I ‘ve made, since last June, of both of the amplifiers I own extensive changes and it is amazing how good they sound.
Really the construction is very poor, and it is obvious that they tried to reduce the cost in the expense of sound quality, but the amplifier design is very good.
Some of these drawbacks are:
· The use of fuses at the output in order to avoid a protection relay.
· The very small values of PSU caps. If you try to measure the rail voltage during a program you will discover how brutal is the variation.
· Regarding the stability of the amplifier, they use very big values of decupling capacitors.
· Main heatsink is inadequate to handle high volumes. It is only about 1C/W and considering a dissipation of about 55Watts can easily lead to temperatures of about 80 deg Celsius.
Still:
· Input stage is degenerated
· VAS is a cascaded LTP
I measured one channel using a sinusoidal signal in input and a resistive load (8R) and I found that clipping occurred at 39Volts (measured on the load resistor). That means 95W Rms output power driving one channel only (and using the old Hitachi mosfets).
I calculated that if both channels were driven with a monophonic program this value would be something like 2x64W RMS
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