Whistle Rock Audio - New ML918 Discrete Op-Amp

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mikefatom

Well-known member
Joined
Apr 5, 2009
Messages
640
Location
Toronto, Canada
Ladies and Gents,

I present you the ML918!  :)

ML9181.jpg


As you've probably guessed it, it is the Jensen 918 op-amp that I have repackaged in a '2520' footprint.

This came to be from the personal request of a fellow forum member who has been looking for a 918 op-amp to use in a preamp.  The information being openly available, I thought it'd be a great opportunity to mess around with a discrete op-amp design the same way that I've enjoyed doing with the ML2520.  Needless to say that this op-amp has a very different vibe than 2520 clones out there.  Being the 'old brother' of the 990 op-amp, it has a more laid back tone compared to the 2520 which is more forward sounding.  Overall, it sounds clean and smooth all at the same time!

I will be offering kits for these in the near future for those who want a different flavor for their pres!

Here are some more pics...

ML9182.jpg


ML918PCB1.jpg


ML918PCB2.jpg


Cheers,

Mike
 
haima said:
looks nice!

what supply range does it like?

Hello haima,

This one likes +/-16V.  I have had it plugged in for burn in for a while now and it gets just a bit warm.  The 918 was designed to run on +/-15V, this says that it should run fine on anywhere from +/-13V to +/-18V.  A few resistor changes should allow you to optimize it for +/-24V operation.  ;D
 
mikefatom said:
This one likes +/-16V.  I have had it plugged in for burn in for a while now and it gets just a bit warm.  The 918 was designed to run on +/-15V, this says that it should run fine on anywhere from +/-13V to +/-18V.  A few resistor changes should allow you to optimize it for +/-24V operation.  ;D

cool! i'd love to try some... can't get enough! :)
 
Cool Mike, so that's what they look like  ;).Thanks for taking this on and making my custom request and also making them available to the rest of the prodigy pro community.I'm ready to bet that they sound just fabulous! .Glad to see you get them done and I'm sure you'll see a lot of orders on these 8)
Congrats! Eric
 
Dana:  Thank you sir!

Gary: Thanks!  I'll be putting a kit aside for you for sure. :)

Eric: Thank you!  I will be sending you the first pair as promised.  ;)
 
Nice job Mike!  Are you using the original transistors or the bc560/550 equivalents... looks like the "c" grade ones as the hfe looks to be pretty high on the originals.  I'll definitely take a couple when the time comes

dave
 
Hey Dave,

I am using the closest thing to the original transistors: PN4250A and PN2484 just like the modern 990 I believe. It is VERY hard (impossible??) to find PN4250A @ > 450Hfe (as it is called for in the available documentation).  Notice that the documentation does not mention the collector current corresponding to the 450 number.  However, I have been experimenting with transistors matched with Hfe > 250 @ Ic=2.5mA and it's been working great!  I have not tried matching the Vbe, because it should not have any bearing on the sound quality anyway.

With just 1% Hfe matching, I get about 5mV dc offset which is totally fine in my book if driving an output TX directly.

 
Very cool!  You answered the other question I had as well... that of dc offset.  Seems these'll be an easy substitution into pretty
much any circuit.  I look forward to trying these out.

Thanks!
dave
 
UPDATE:

I've decided to rework the PCB a bit.  The main change in the .1 revision (version 1.1) is the use of a matched pair of BC560C for the differential input stage.  The greater Hfe range really helps with keeping the DC offset reasonable when using DI straight to op-amp or other situation where large impedance differences are present at the inputs.  I have not noticed any change in the sound but I have not conducted extensive listening tests as of yet!

If I'm satisfied with the sound, I will organize the kits. 8)
This weekend will be a busy one...  :D
 

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