[kato]

I'm building 4 channels of Amek M-2500 preamps. Anyone want to join in?
Here's the redrawn schematic by Olaf Matt.

I hope to offer help and build documentation when I'm done. But for now, all I have are questions.

The schematic calls for a resistor (Ri) 1k that sets the input impedance. The mics I'm using (Shure SM11's) are all dynamics.

Here's my dumb question: Is the Ri load resistor supposed to literally match that of the mic?
(The SM-11's output impedance is 200 ohms.) In other words, should I use 200 ohm resistor in place of Ri instead of 1k? What are the issues here?  



Thanks for any help,  Kato

[ppa]

for first, this resistor is important that it's of low value for having a good CMMR input,
for second thing it's important of low value to give a low input impedance for having best S/N value, increasing the signal power respect to the noise power. This value should be circa 1.5 kohm , less values give a load too low for the mic.   

[kato]

Thanks Pier,
The calculator on this page shows a 200 ohm input impedance would give unacceptable load loss. Although I don't understand why. In fact, it shows no real world scenario where you'd benefit from less than 1k.

I'm reflecting on preamps with variable input impedance like the Focusrite ISA-828. It's switchable between four different settings: • 600 ohm, • 1400 ohm, • 2400 ohm, and • 6800 ohm.

The MP2NV also. Come to think of it, I use this amp all the time and have never bothered to switch this. From the FAQ:

Q:      Does the mic preamp "Impedance" switch make any difference in the sound?

A:      Yes and no – with condenser microphones, the input impedance switch (300 or 1200 ohms) will make almost no difference due to the buffer amplifiers in the mics. With dynamic mics and especially with passive ribbon mics, the preamp input impedance will affect the sound, often quite a bit. A good rule of thumb is the mic preamp input impedance should be approximately five to six times the microphone’s rated output impedance (a 50-ohm ribbon mic feeding a 300-ohm input impedance, for example).

    There is no danger to the mic if you load it down however, so if it sounds good, don’t be afraid to use it. One example would be a modern ribbon mic with a 300-ohm output impedance on a guitar cabinet. Maybe it’s a bit muddy or tubby sounding; by lowering the input impedance of the preamp, you’ll filter out some of the low end, maybe enough to make it sound right. Maybe not…

It's hard to see how the 600 or 300 ohm settings  would ever be a good thing, unless you're trying to mess with the sound to get some purposely bad pseudo mojo or something.

Reading this:
Impedance Matching: a primer hoping this will all sink in a little.

I'm thinking about building these with fixed gain.  
Can I ask another dumb question?
If I make some fixed gain versions, is the 2200 uF cap necessary or can I leave it out?

[pedroplanet]

I've been thinking about making theese too!
any PCB layout?

[ppa]

if you have other questions , let me know, I'm here for these.

regarding the 2200uF cap , it's necessary using t1,t2,t3,t4 and t5, t6, t7, t8.
Using hand mached transistors needs this cap, because without this cap the current of these two groups of transitors are unbalanced.

Can be avoided this cap changing these 8 transistors with a IC super mached trasistor pair.
Discrete mached transistors were used because at that time there were no IC super mached transistors in the market for this use.

Can be used the complementary of the same IC super mached pair used in SSL 9000 preamps , it's the MAT03

http://www.analog.com/static/imported-files/Data_Sheets/MAT03.pdf  

the amek 2500 with this IC will be the same very low noise of the SSL 9000 preamp..............not bad.....I think

moreover it's necessary adding a diode to D5 change r12 and r11 values.
soon the schematic modified.




  

[SSLtech]

A rule of thumb is 10:1

I suggest trying a 2.2kΩ resistor.

Keith

[jensenmann]

Increase the 2200uF to as as big as possible. The bypasscap should be 100nF, not pF.
This preamp topology has been used 1000+ times, not only in the 9k.
Olaf did his prototype with BC560C IIRC, Graham Langley uses paralleled pairs of these in his Langley Design preamp (which sounds great, btw, and costs close to nothing, check audiomaintenance.com)

[kato]

Thanks Keith. Rules of thumb are helpful to those of us who know just enough to be dangerous. i.e. schematic-reading and soldering.   

Pedro, OlafMatt made a nicely-designed PCB. It's re-posted in the "supergreen diy preamp schematic" thread. I haven't seen finished builds from this layout so I can't confirm it's error-free. So I'll try one channel first. If it works, I'll continue with the other three channels.

The PCB layout is tight for my skill level and eyesight. I etched the boards yesterday and started drilling this morning but my smallest drill bit is size #70 which doesn't give much room for error against the small pads. So I ordered smaller bits this morning. Also I slightly modified the PCB artwork for smaller drill holes If you're interested I'll post that file.

Thanks Pier for the circuit analysis. I look forward to seeing your schematic.
And thanks Jensenmann for the cap increase suggestion. I recall on the green, this cap is more like 4700uF rather than 2200. Too bad I just placed my Mouser order. I'll build these stock first and see how they sound.

[ppa]

I'm doing the schematic with the mat-03 transistor super mached pair and without the 2200uF cap.

I think this cap gives a minimal distortion that's near to zero , but I council that this cap should be for audio use or for swicthing power supply. I'm doing a second version with BC327 that gives ultra low noise to the preamp, a bit lower than mat-03 or other solutions, this version uses the 2200uF cap bypassed with one 1uF cap.

      

[kato]

I'm doing the schematic with the mat-03 transistor super mached pair and without the 2200uF cap.

I think this cap gives minimal distortion near to zero , but I council that it should be for audio use or for swicthing power supply. I'm doing a second version with BC327 that gives ultra low noise to the preamp, lower than mat-03 or other solutions, this version uses the 2200uF cap bypassed with one 1uF cap.

So I take it the BC327 [data sheet] is not a direct replacement and requires some circuit modification? They look similar - same pinout, same collector emitter voltage, etc. but I can't decipher if it's an actual replacement part. Again, it will be interesting to see what you come up with!

[ppa]

I'm doing the schematic with the mat-03 transistor super mached pair and without the 2200uF cap.

I think this cap gives minimal distortion near to zero , but I council that it should be for audio use or for swicthing power supply. I'm doing a second version with BC327 that gives ultra low noise to the preamp, lower than mat-03 or other solutions, this version uses the 2200uF cap bypassed with one 1uF cap.

So I take it the BC327 [data sheet] is not a direct replacement and requires some circuit modification? They look similar - same pinout, same collector emitter voltage, etc. but I can't decipher if it's an actual replacement part. Again, it will be interesting to see what you come up with!

using bc327 require one circuit modification: changing the value of r12 and r11 values that became 470 ohm,
t1,t2,t3,t4 and t5, t6, t7, t8 are changed with 2 bc327 only , t1,t2,t3,t4 are changed with one BC327 and the other trasistors with the other bc327. I'm referring to 1uF cap because jensenmann has counciled to add a cap of 100NF to the 2200uF cap, and i think that is a good idea.

[ppa]

this is the schematic modified for using the BC327

[kato]

Nice work! Interesting variation.  3 different ICs, and half the number of transistors.
I'll see if I can modify the original PCB to include these changes, extra resistors, an extra cap for the 5534, and two 2200uF caps, shouldn't be too hard.

Thanks for the design!

[pedroplanet]

As you're adding this, You could add a pad and phase invert on this pcb, so we save money
instead of buying jlm gobetween or something like that  

[ppa]

of course

[kato]

this is the schematic modified for using the BC327

PPA, I'm back on this thread having successfully built a single channel from the OlaffMatt layout, which sounds great by the way. I'm doing a 2nd channel with BC327 using your diagram.

Question about replacing 2 10uFs with a single 4.7uF removed.
It now occurs to me, caps wired cathode-to-cathode aren't really in series as they appear at first glance.

UPDATE: Just to check my blood crack content, I measured 2 10uFs in both "standard series" and "reverse series" and indeed, they measured the same.  Learn something new everyday. (good thread on the subject.) My question is answered. Thank you for your time.

New PCB layout underway with pad, phase, phantom, and that giant 2200uF cap onboard rather than dangling from the front panel pot as in the previous layout...

[kato]

Ciao Pier, if you're still out there.

I just got to the output. You changed Olaf's balanced output to unbalanced? Why you do that?

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