New development on old project: Roger Mayer RM58

[soapfoot]

IT's a really cool unit! Next time I'm on the same coast as mine I may have a go at recalibrating it, taking into account what you've worked out here

 

[groselicain]

well I won’t lie, calibrating this thing is not easy and is a bit of trial an error.

I followed @richiyobs advice, as well as @soapfoot advice too. R27 is a 50k trim, and has a lot of effect on the comp.Tweak a symmetrical waveform with this. Much easier to do with a scope and square wave generator than a sine. Then, with the input/threshold knob at a fixed position, set the signal generator to get 0dB at input. Play with VR6 and VR5 to get symmetrical waveform and same level at input and output of the main board. We don’t care about the makeup amp during calibration. That’s it for the audio path / signal detection.
For the meter, it is much more mystical but I was able to get decent results by setting up VR1 and VR2 alternatively, going back and forth on one and then the other so that the meter rests at 0dB GR. channels must be unlinked and release set to slowest.
It helps to be able to (true)bypass the channel so the meters sits in an « idle » position and then compare this idle position with 0 GR.
VR1 was used to fine tune the position when everything is set up.
My meters are 1mA. They are not super accurate for low GR (strange ballistics) but they are spot on at -10 and -15 dB and that’s close enough for me.

I then realized the compression kicked in a little too early (too low thresh) and too hard for my taste so I relaxed VR5 a bit

Everything was done with a 600 ohm load at the output, first with a scope and then with Protools and some audio. Then readjusted and double checked with the scope again before closing up the rack enclosure. I am using 100k pots for the attack instead of 22k, so I’ll probably change that in the future.

Thanks a ton @soapfoot and @richiyobs , can’t wait to use it in a real context. Very happy with the sound.

Can't thank you enough for writing this down!

 

[richiyobs]

well I won’t lie, calibrating this thing is not easy and is a bit of trial an error.

I followed @richiyobs advice, as well as @soapfoot advice too. R27 is a 50k trim, and has a lot of effect on the comp.Tweak a symmetrical waveform with this. Much easier to do with a scope and square wave generator than a sine. Then, with the input/threshold knob at a fixed position, set the signal generator to get 0dB at input. Play with VR6 and VR5 to get symmetrical waveform and same level at input and output of the main board. We don’t care about the makeup amp during calibration. That’s it for the audio path / signal detection.
For the meter, it is much more mystical but I was able to get decent results by setting up VR1 and VR2 alternatively, going back and forth on one and then the other so that the meter rests at 0dB GR. channels must be unlinked and release set to slowest.
It helps to be able to (true)bypass the channel so the meters sits in an « idle » position and then compare this idle position with 0 GR.
VR1 was used to fine tune the position when everything is set up.
My meters are 1mA. They are not super accurate for low GR (strange ballistics) but they are spot on at -10 and -15 dB and that’s close enough for me.

I then realized the compression kicked in a little too early (too low thresh) and too hard for my taste so I relaxed VR5 a bit

Everything was done with a 600 ohm load at the output, first with a scope and then with Protools and some audio. Then readjusted and double checked with the scope again before closing up the rack enclosure. I am using 100k pots for the attack instead of 22k, so I’ll probably change that in the future.

Thanks a ton @soapfoot and @richiyobs , can’t wait to use it in a real context. Very happy with the sound.

How many volts have you got at detector out? Just at 470ohm resistor that sets the fastest Attack? It should be about 5 volts if i m not wrong.... Since i m using a different jfet i setted It about 4 volts... Or i get Extreme compression and so more headroom for Thr pot
Thanks for your attention
Best

 

[thomasdf]

The tech documents mention a 4.4 to 5.4v bias if I remember correctly. On my unit I am using very different FETs so my bias is quite low (around 2vdc, from memory) and it works fine. That’s why I didn’t mention any tensions. I went by ear at some point and I think that’s the best one can do, unless using the original FETs.
If there’s a need for it I can reopen my unit and make measurements but I think that with the correct mindset and procedure anyone able to complete this build will be able to calibrate it and tweak it to taste

 

[richiyobs]

I think than with the correct mindset and procedure anyone able to complete this build will be able to calibrate it and tweak it to taste
[/QUOTE]
You re right ...
Wich fet are you using?

 

[thomasdf]

I am using FETs I bought at an antique electronic shop owned by an 80+ y.o. man
He sold me NOS FETs aimed at professional market. They are white ceramics with a black dome top, golden legs and 7125 P1069 written on them. They're supposed to be made by a famous manufacturer but I can't remember which one... Hewlett Packard, something like this...? I found NOTHING on the internet. I'll ask him again next time.
I went through 32 of them to find 2 matched pairs.

 

[thomasdf]

My unit is finally finished. Unbalanced input, output is transformer balanced, Sowter 1:1. Lovely sounding, colored for sure, a little bit wooly / crunchy in a good way, can really smash things up.. a bit more difficult to use in gentle settings. Many thanks for the project, support, PCB designs and all. My only wish would be to have a little more gain in the output amp because it is hard to have unity gain on gentle settings. Here’s a picture, I designed the front panel and had it cut / print by modushop who also provided the case. Meters are regular cheap 1mA with a custom designed scale I printed and glued onto the orig scale.

 

[chrispbass]

https://reverb.com/uk/item/11866606...essor-from-the-record-plant-in-nyc-modded-bra

 

[f.aiseur]

Hello There!
First of all, thanks everyone involved in this project, and the previous rm57 too while i'm at it!

Im starting to get the parts for my build, and i'd like to ask some confirmation about the resistors.
Do we need higher power ratings - in some spots? - or 1/4W everywhere will do?

Thanks!

 

[thomasdf]

1/4 all around, iirc, except in the PSU, depending on what type you go for