Roger Mayer RM57 build?

Great news ! Kudos !

I've done a little video but before i show you the first test, i do need to look at my PSU i think.

I am getting noise and haven't used screened leads for input and output!!

my PSU is really simple so i think i need to put a bit more filtering in!!

at the moment its just IEC into switch, pilot light into 18v TXF, into bridge diodes (1N4007) then 1000uf/50v electrolytic, then 7818 regulator then 470uf/50v electrolytic. I had diode from input to output for protection and that it!!
should probably up the 1000uf to 2200uf? and then have a 330nf cap before input to regulator then after output of regulator put 100nf cap.

whats your thoughts PSU wise for 18v dc nice and clean?

There shouldn't be any problem with using the PSU as is. An old boy radio engineer once said to me use 1000uF per amp, I normally have several times that per amp on the PSU's I make. You won't be drawing anything even close to an amp.

Ah ok, thanks Rob, i'll try and use the screen leads on input and output and see if the noise improves.

Meanwhile here is where i'm at!!!


https://www.youtube.com/watch?v=c4ZHW6nZwtA

That's sounding FAT !

My jfet choice for q10 and q14 is 2N5461 and I have BC560C in Q1 , q2 and q3 are 2N5088.
All others are 2N3391, I’m checking with Dan that my transistor choices are ok.

Thanks for your hard work!!! It looks like there is R19 22K in your schematic insted of 220K (value from original schematic). Values of R8 and R9 are reversed. R38 should be 3K3. R35 = 5K6 in original schematic. R27 = 47K. R24 = 1K2.

Pleas can you update your schematic with final choice of jfets and transistors?

wooooohohoh !! I like that !

Vac11, nice one for spotting those, I’m changing them now and will update!!
Thank you.

here is updated schematic,

will see what changes have improved and report back, thanks again for all your input.

mmmm. now it doesnt work, waveform all over the place and vu meter all the way to right with only adjust more to the right!!!

back to the drawing board!!

I think it will be difficult to obtain correct left/right tracking for stereo, since there is no adjustment to get the tracking right.
So: it only depends on the parameters of the FETs.
Ideally, you should have 4 exactly matched FETs, if you want the meters to indicate the effective gain reduction.
(My experience is that you need something like 25 FETs, to obtain a matched quad.)

I’m not that interested in stereo unit yet, just want to get this working correctly, but thanks for that info, well worth matching some FETS eventually once the unit is working correctly.

It was working but I had a few mistakes of different value resistors.
Now I have corrected them all it doesn’t work again!!!
The first part of the circuit which I’m thinking is the preamp I don’t get clear waveform at q13 emitter so I’m going to disconnect and go through each section again!!!

One quick question, these p channel jfets, should source go to gnd or drain?
Drain to gnd right?

Well, would be glad help to this thread

Spencerleehorton said:

One quick question, these p channel jfets, should source go to gnd or drain?
Drain to gnd right?

Click to expand...

Should be source to gnd.

Made a quick test in LTspice with RM input chain. It also shows P-channel fet principle.

ok, well i'm completely foxed with this at the moment, i'm getting signal through but the signal just seems to move and wobble, no compression, release and attack do not function but i can measure the 5v at B (with release at slowest) going into both the attack and release pots and the trim pot works to adjust this voltage.
maybe I have blown the 2N5461?, not sure but the VU meter just sits too much to the right? Q4 and Q5 have -0.4v and -2.8v on base? and 0v on collectors?
Im not measuring any voltages on jfets?

the weird thing is that before i changed the following it was kind of working!!??

R19 was 22k changed for 220k
R8 was 12k changed to 1.2k
R9 was 1.2k changed to 12k
R38 was 2.2k changed to 3.3k
R35 was 8.2k changed to 5.6k
R27 was 22k changed to 47k
R24 was 12k changed to 1.2k

And i think  i had FETs drain to gnd!!
Meanwhile i wait on some more jfets!!

Q11
E - 2.4v
B - 2.9v
C - 3.8v

Q12
E - 3.2v
B - 2.9v
C - 5.8v

Q13
E - 5.2v
B - 5.9v
C - 18.6v

Q9
E - 0.9v
B - 1.5v
C - 2.1v

Q8
E - 2.0v
B - 2.1v
C10.7v

Q7
E - 1.4v
B - 2v
C - 18.8v

Q6
E - 12v
B - 10.8v
C - 18.7v

Q5
E - 0v
B - –0.4v
C - 0v

Q4
E - 0v
B - –2.8v
C - 0v

Q1
E - 7.9v
B - 7.1v
C - 5.6v

Q2
E - 5.1v
B - 5.7v
C - 18.4v

Q3
E - 4.4v
B - 5.1v
C - 10.7v

Q15
E - 2.2v
B - 2.8v
C - 15.3v

Normally speaking you should be expecting to see around 0.6-0.7v between the base and emitter.  This could be + or - depending on whether it;s an NPN or PNP.

Yes I’ve used that rule before to fault find,  thanks for reminding me!!
Looks like I have problems with Q1, q4, q5, q6, q8 and q12.
Q4 and q5 seem worst off?
Along with q4?
Q8 not good either!!

well i've taken out the fet's to just look at signal and replaced Q4 and Q5 for 2N2222 this is the waveform i get at output.

The waveform seems chopped? on the scope at Q12 it looks this way so i'm going to poke around and see if i can see anything, but if anyones got any ideas?

Also would a J175 P-channel work? seems to have a slightly higher rating than the others?