Hardy M1 mic pre /mitsos PCBs

[mitsos]

I did not cut traces and the servo is fine with 24v. I have to check what opamp I used but I think it was OPA604.
As long as the opamps don't fail, I don't think they will affect the sound??? They connect to the audio circuit through heavy low pass filters - just a slow DC correction.

I have no idea why this is, it's just what I've heard, albeit from people who's opinion I trust, but I have not yet tested the twin servo with different servos.  I would think there'd be diminishing returns after a certain point of precision and drift with these opamps, but I'm curious to try it and see. Need more 990s I guess.

My plan is to build a second pair with Altran transformers and see how they compare to the Jensens.

I have the same test planned, I hope one of us gets it done this century!!

I built API's as well and the Jensen 990s blow them out of the water for acoustic guitar and vocals.

There are a million ways to build these PCBs, but by far, the 990 kicks ass! Amazing little opamp!! Thank you Deanne Jensen wherever you are, and of course John Hardy for still making them!

 

[Cardinen]

Hi,

I am building 2 M1 pre with red Mitsos boards, Altran-3982 In , bias and servo dc (using OPA604).
i have read the entire thread but i have a few questions:

i didn't use the zobel for the altran as mitsos said, just the R load (5.1K). is fine ??

At the moment i don't have the out trafos, but i have two spares Lundahl 1539 (2:1+1);
Is possible to use these as output transformer ? maybe wired in reverse ?

Thanks 

 

[kato]

Wire your output transformer 1:1.

 

[dmp]

i didn't use the zobel for the altran as mitsos said, just the R load (5.1K). is fine ??

Try it and see. Let us know how it sounds.
Basically, I don't think anyone has tried to figure out if the altran needs a zobel. I still haven't built channels with the altran, just Jensen

You can also do an impedance balanced output as shown in the John Hardy paper.

 

[CJ]

zobel networks operate well out of your hearing range,

so you will need golden ears to hear any difference sound wise,

upper harmonics of hi notes will be attenuated,

but they do work well at preventing high frequency oscillation,

you will definitely hear a difference if your amp was oscillating,

 

[Cardinen]

Ok thanks for the explanation CJ. i'll go with my trafos wired 1:1

 

[plinker]

I've found a potential option for the servo; the OPA454 specs much better than the OPA604 (I think).  It would need the Browndog SO to DIP adapter.  Does anyone see a problem here?

Linear has a couple of options, but they have low slew rate.  Does slew matter in this servo application?

Thanks,
Jim

 

[dmp]

The servo can be very slow - look at the components around it.  It is just correcting the DC offset of the DOA - not in the audio band.
The important specs I believe are a low DC offset. And if you are using 24v, you need a chip that can handle it. I've used OPA604 in 10 channels @24v and they work great.

 

[pH]

I made M1 type 990 preamps with these boards and they work and sound great. I did a 12 position grayhill switch for gain up to 70 dB in 5 dB steps.

Hey dmp, any chance you could post your resistor values?

 

[dmp]

I think this is what I used. Goes from 15dB to 70 dB in 5 dB steps.
Step 1 (70dB) has the 1.5 ohm on the switch, and the 4.7 on the pcb and they are in series. As you go up in the steps, the resistors keep adding.
The gain formula is G = 20*log(2*10000/R), where the 2 is the transformer gain.

4.7
1 1.5
2 4.7
3 7.5
4 20
5 30
6 47
7 91
8 150
9 270
10 470
11 910
12 1500

 

[dmp]

Here is the resistor values for 5dB to 60dB:
1 20
2 15
3 27
4 47
5 91
6 150
7 270
8 470
9 910
10 1500
11 2700
12 4700

 

[pH]

Wow, that was fast. Thanks a bunch.

Paul

 

[pH]

Okay, these are nearly built up.

Question here about power ...I want to fit these into my Seventh Circle rack which is being underused and puts out +/-24v and phantom from a switching power supply. Would I need additional filtering and/or regulation before supplying the M1 cards?

Thanks
Paul

 

[mitsos]

SCA says the CH2 rack puts out +/-30V and their modules appear to use local regulation to get the voltage down to the necessary range for each.  You may be able to run a JLM99V, I think they have a version that runs on higher voltage, but most others would go up in smoke. 

If you have room though, it shouldn't be hard to put a couple of regulators on a perforated board.

 

[pH]

You're right, it is running hotter than +/-24v from the psu. I guess I will local regulation for these.
Thanks,
Paul

 

[pH]

Finally getting these buttoned up, ready to test.
I just want to verify, do I only adjust the 990 input bias current? Is there no further dc offset calibration? (as per the JH doc)

Thanks,
Paul

 

[mitsos]

Follow the procedure in the hardy document and you'll be fine.

good luck!

 

[pH]

Got one of these fired up today, sounds excellent!

Still need to get resistors for my gain switch. I used Mnats bipolar power supply board for extra filtering/regulation off the Seventh Circle supply, Hairball 990 kits and Jensen transformers. A great addition to my SCA rack.

Thanks Mitsos (and dmp) for your help.

-Paul