Musical Fidelity The Preamp Schematics - UK Vintage Radio Repair and Restoration Discussion Forum

pedro_couvreur · 21st Jun 2022, 12:15 pm

I have received a Musical Fidelity early Preamp, THE PREAMP first model, and have a problem on the output signal on one chanel.
Does anyone have the schematics for this preamp ?

Thanks ins advance.
Pedro Olievira

RojDW48 · 21st Jun 2022, 3:45 pm

I also have one of those - but not the circuit diagram. Here's a link to the Mk 11 iteration, which might help. Take care - it's a nice pre-amp, but very cheaply made - the pcb trace is very thin (!) as I discovered when I repaired mine some years ago. https://gtkc.net/musical-fidelity-the-preamp-ii

pedro_couvreur · 21st Jun 2022, 4:24 pm

Thanks for the reply and link for info.
My preamp is older that the the one on the link, it has the power trnsformer inside.
I attached a picture of my model. Serial number 470 !

Pedro Oliveira

wd40addict · 21st Jun 2022, 7:12 pm

At the time MF made a fuss about the low flicker noise transistors they used. Reality seems to be it's based on opamps!

RojDW48 · 21st Jun 2022, 8:42 pm

Quote:

Originally Posted by pedro_couvreur

Thanks for the reply and link for info.
My preamp is older that the the one on the link, it has the power trnsformer inside.
I attached a picture of my model. Serial number 470 !

Pedro Oliveira

Yes - same as mine. The fault I had was in the phono stage - MF were not at all helpful. Happily it was a cracked resistor, which I discovered by gently nudging the resistors in that bit of the circuit. That was probably 15 years ago - still going now.

mhennessy · 22nd Jun 2022, 7:49 pm

The earlier version used NE5534s, but used the trick of turning off the input transistors and connecting their own to the compensation pins. They did the same thing with the LM318 used in later versions of the B200 and similar amps, which, having measured both versions, did make a useful contribution to the signal to noise ratio.

In the case of The Preamp, they used the SSM2210, which is a pair of matched transistors. It's worth saying that this only applies to the main phono pre-amp, with an additional SSM2210 used in the MC pre. The main pre-amp is nothing more than an NE5532 set to give about 15dB of gain. Plus the tape output is buffered by the other half of that '5532.

The schematic I have was reverse-engineered by someone else, and looks like it might have a mistake around the volume control - which I think is a conventional pot ahead of the line gain stage, unlike the active gain stage used in the later version (which was extremely problematic in the A1 and related designs). As it's not my schematic, I'm not sure I should share it. There is a URL included, but the site has gone, and there's no sign of the original schematic that I can find on the Wayback Machine.

Having said all that, it's a simple bit of gear, and reverse-engineering the relevant bits won't take long. It's the only way to deal with most MF stuff as schematics are rarely available. Assuming the fault is in the line amp, it's just an op-amp! Check for dirty switch contacts and clean the volume pot, and check the muting relay isn't sticking. These were hand-assembled, so poor soldering or similar might be a possibility. Also check the electrolytic caps in the signal path. And check the tape record output. Of course, if the fault is in the phono pre, that's more complicated, but you have a working channel to compare against.

pedro_couvreur · 23rd Jun 2022, 7:59 pm

My device uses LF353 output gain/tape buffer opamp instead of the NE5532.
The volume control is a classic voltage divider aproach, ccw to ground, cw from input and wipper to the input of the output gain stage.

Thank you everyone for the support. I plan to document the circuit of my device, by being very paitient . Buy the way, this 'THE PREAMP? is serial number 420 and not 470 as stated before.

As soon as I have the schematics, MC and MM circuits, I will share it with the forum.

Pedro Oliveira

pedro_couvreur · 8th Nov 2022, 10:50 am

After some time, I finally reversed-engineered my preamp.
Schematics and layout are attached for everyone to use.

If someone finds some incorrections, please give feedback.

Interesting fact: the circuit for the RIAA MM pre-amp and the circuit of the MC pre-preamp are based on a Texas Instruments application note - AN-222 Super Matched Bipolar Transistor Pair Sets New Standards for Drift and Noise.

Pedro Oliveira

21st Jun 2022, 12:15 pm	#1
pedro_couvreur Diode Join Date: May 2020 Location: Oeiras, Portugal Posts: 5	Musical Fidelity The Preamp Schematics I have received a Musical Fidelity early Preamp, THE PREAMP first model, and have a problem on the output signal on one chanel. Does anyone have the schematics for this preamp ? Thanks ins advance. Pedro Olievira

21st Jun 2022, 3:45 pm	#2
RojDW48 Nonode Join Date: Jun 2014 Location: Bristol, UK. Posts: 2,074	Re: Musical Fidelity The Preamp Schematics I also have one of those - but not the circuit diagram. Here's a link to the Mk 11 iteration, which might help. Take care - it's a nice pre-amp, but very cheaply made - the pcb trace is very thin (!) as I discovered when I repaired mine some years ago. https://gtkc.net/musical-fidelity-the-preamp-ii __________________ '....don't go mistaking Paradise for that home across the road!' (Bob Dylan)

21st Jun 2022, 4:24 pm	#3
pedro_couvreur Diode Join Date: May 2020 Location: Oeiras, Portugal Posts: 5	Re: Musical Fidelity The Preamp Schematics Thanks for the reply and link for info. My preamp is older that the the one on the link, it has the power trnsformer inside. I attached a picture of my model. Serial number 470 ! Pedro Oliveira Attached Thumbnails

21st Jun 2022, 7:12 pm	#4
wd40addict Octode Join Date: May 2009 Location: Stevenage, Herts. UK. Posts: 1,518	Re: Musical Fidelity The Preamp Schematics At the time MF made a fuss about the low flicker noise transistors they used. Reality seems to be it's based on opamps!

22nd Jun 2022, 7:49 pm	#6
mhennessy Dekatron Join Date: Jul 2007 Location: Evesham, Worcestershire, UK. Posts: 4,244	Re: Musical Fidelity The Preamp Schematics The earlier version used NE5534s, but used the trick of turning off the input transistors and connecting their own to the compensation pins. They did the same thing with the LM318 used in later versions of the B200 and similar amps, which, having measured both versions, did make a useful contribution to the signal to noise ratio. In the case of The Preamp, they used the SSM2210, which is a pair of matched transistors. It's worth saying that this only applies to the main phono pre-amp, with an additional SSM2210 used in the MC pre. The main pre-amp is nothing more than an NE5532 set to give about 15dB of gain. Plus the tape output is buffered by the other half of that '5532. The schematic I have was reverse-engineered by someone else, and looks like it might have a mistake around the volume control - which I think is a conventional pot ahead of the line gain stage, unlike the active gain stage used in the later version (which was extremely problematic in the A1 and related designs). As it's not my schematic, I'm not sure I should share it. There is a URL included, but the site has gone, and there's no sign of the original schematic that I can find on the Wayback Machine. Having said all that, it's a simple bit of gear, and reverse-engineering the relevant bits won't take long. It's the only way to deal with most MF stuff as schematics are rarely available. Assuming the fault is in the line amp, it's just an op-amp! Check for dirty switch contacts and clean the volume pot, and check the muting relay isn't sticking. These were hand-assembled, so poor soldering or similar might be a possibility. Also check the electrolytic caps in the signal path. And check the tape record output. Of course, if the fault is in the phono pre, that's more complicated, but you have a working channel to compare against.

23rd Jun 2022, 7:59 pm	#7
pedro_couvreur Diode Join Date: May 2020 Location: Oeiras, Portugal Posts: 5	Re: Musical Fidelity The Preamp Schematics My device uses LF353 output gain/tape buffer opamp instead of the NE5532. The volume control is a classic voltage divider aproach, ccw to ground, cw from input and wipper to the input of the output gain stage. Thank you everyone for the support. I plan to document the circuit of my device, by being very paitient . Buy the way, this 'THE PREAMP? is serial number 420 and not 470 as stated before. As soon as I have the schematics, MC and MM circuits, I will share it with the forum. Pedro Oliveira