Hardy M1 mic pre /mitsos PCBs

[grantlack]

any ballpark current draw numbers from early builders on these and/or the 312 build parameters?  this will obviously fluctuate a bit, but i'm trying to weigh my options for power supply needs.

 

[bobschwenkler]

Depends on the op amp perhaps, but you can do a max current calculation by figuring out how many volts you'll want to put into how many ohms (say 600 worst case).

What I've come across suggests that 25mA is a reasonable allotment.

 

[mitsos]

Most of the 2520s I've measured in use have always measured low. 10-15mA I think. Bob is right, depends on the current output, but even at 600 Ohms, it's not that much.

But, then, how accurate is my mA meter? Here is some discussion as well as some math by harpo (a much smarter man than me):

http://www.groupdiy.com/index.php?topic=36081.0

FWIW, I've got a 2 channel 312 box with a 10VA toroid (with no servo,bias,or 1646), another with an 18VA, but I have yet to complete a 990 one because I messed up the faceplate : and the rotary switch is sort of flopping around in the air right now. It's working just not in real use yet.  I should test the current on these to see what's going on, but I used the 312s quite a few times already and they are working 100%.  I'd say the 18VA is a bit safer, the 10VA is cutting it close and will probably crap out going into 600 Ohms, according to the above calculations.  It doesn't seem like the toroids were overworked though (no heat/noise so far for example), but I should test to be sure.

Also, I've also heard of someone running an API console (32 in 48 monitor) off two power one 1amp supplies (one for each rail). How many opamps in that thing?

EDIT: I got the 990 working with a custom dual pot, 10K and 1K rev log. I used the output pad switch to switch between the two (had to cut a couple of traces to isolate it from the polarity swich but it works. Not ideal cuz there are too many wires flying around. I might eventually do ann add-on PCB for a custom dual pot (smaller value on rear gang) and the switch for hi/lo gain.

 

[seavote]

i am going with the op97 (+/- 15V) as i had already cut some traces and altered some components. now i'm committed to the work around i descibed above. mitsos method seems simpler.

 

[grantlack]

any ballpark current draw numbers from early builders on these and/or the 312 build parameters?  this will obviously fluctuate a bit, but i'm trying to weigh my options for power supply needs.

i decided to power externally, tossed another 511 psu on the pile and i'll sort out the 12V later.  could've sworn i saw the load and zobel values for the altran c-3982's earlier, but of course now that i'm finalizing my order i can't seem to track them down  .  i'm assuming it's 1:2 like the jensen, but you know what they say about assumptions.  anybody have those handy?

 

[kato]

i'm assuming it's 1:2 like the jensen, but you know what they say about assumptions.  anybody have those handy?

I don't have my notes handy, but IIRC, the 3982 has 10k secondary impedance, compared to 600 ohms (or 750 ohms?) on the Jensen JT16 secondary. So probably a different load resistor.  But I can't find a datasheet for the altran, anyone?

The 3982, again IIRC, can be wired 1:2. It's 1+1:4.

 

[grantlack]

indeed it is.

In the case of the M1, I tested with an Altran 3982. It's a 1+1:4 input trafo (wired it as 1:2), smaller than the JT16 I think.

maybe mitsos can shed some light on his test values

 

[mitsos]

Hey guys,

sorry, been out of town for a few days.  I don't think I used a zobel on the Altran input, but my little jig for doing zobel tests fell apart and I haven't made a new one. I won't be home for a while to check what exactly I used, but I wanted 1K2-1K5 seen by the mic, so I am sure it was between 4K-6K.

FWIW, hardy uses 6K19 in his M1 schematic (his zobel values are marked FS(factory selected) so I take that to mean he tests each individual trafo to find the ideal RC network).

hope that helps!

 

[pablobolche]

I am building this M1 build and new in diy and electronics and have read some of the caps specs and descriptions but a pratical Q, on selecting the 91pF feedback cap for the 22k rev log pot. What kind of cap is good for this.... polyester, ceramic, tantalum,low ESR, high ripple current.

Is there a "this kinda cap is good for a feedback loop / audio path / power line rule you can follow? Other than reading all posts regarding caps of course.

thanks for any help

 

[kato]

Ceramic, make sure the dialectic is specified as C0G or NP0.

 

[mitsos]

I am building this M1 build and new in diy and electronics and have read some of the caps specs and descriptions but a pratical Q, on selecting the 91pF feedback cap for the 22k rev log pot. What kind of cap is good for this.... polyester, ceramic, tantalum,low ESR, high ripple current.

Is there a "this kinda cap is good for a feedback loop / audio path / power line rule you can follow? Other than reading all posts regarding caps of course.

thanks for any help

As Kato says, ceramics are generally fine, besides them you can use silver mica or any good film cap that will fit. I've heard some like polystyrene, I'm not sure how much any of this will affect the sound.  Don't overthink it, or overspend on hi-fi caps. Unless you want to of course.

Also I generally recommend a rotary switch for the M1 gain, or a custom dual pot/switch like JH uses.  Since this is much higher gain than a 2520 (54db is what I set my 990s to), the larger the pot value, the less control you have over the highest part of the gain, so for example, the last 30% of gain might be in the last 5% of pot travel, so you get lots of gain for little pot travel.  You can partially solve this by using smaller pots, I've gotten OK use with 10K or 5K IIRC and 1K might get you acceptable results, but maybe not, so YMMV.

I am ordering some 23T switches for me... Pablo, since we are on the same continent, I can probably get some to you if the pots don't work out.

HTH

 

[Soeren_DK]

I have the Mitsos pcb laying around and thought combine them with parts from my old API312 project.

So I have some question.
- What does DC servo and Bias do? and can I leave them out?
- What about all the voltages on the board? anybody designed a power supply?
- I'm using Altran from the groupbuy, what resistor should I use?
- Any corrections when using Cinamag CMOQ-2S as the output tranni?

Cheers
Soren

 

[mitsos]

I have the Mitsos pcb laying around and thought combine them with parts from my old API312 project.

So I have some question.
- What does DC servo and Bias do? and can I leave them out?

They work in conjunction to eliminate DC offset on the 990 output when not using a cap between opamp and the CMOQ.  You first adjust the bias trimmer so that you get very little difference in DC between min and max gains (measure between output of opamp and ground with servo out of circuit), once that is within the tightest tolerance your DMM allows, connect the servo and watch it go to zero. There is a detailed description of the procedure in John Hardy's 990 pdf.
http://www.johnhardyco.com/pdf/990-2007.pdf

- What about all the voltages on the board? anybody designed a power supply?

I have a single-sided one that you can etch if you want. I tried posting it before but the links always disappeared. Let me know if you want it, I can email the files to you. On the other hand, the 4th rail is for the LEDs,  you could jumper that to the +V and G and use the positive rail.

- I'm using Altran from the groupbuy, what resistor should I use?

Which Altran input did you get? The 990 one (1+1:4) or the 312 one (model 3402-2, 1+1:8). If the former, you can use it in 1:2 (primaries in series) with about 6K as a load. I haven't done any tests for zobels on this trafo because I moved recently and the place is still a mess.

- Any corrections when using Cinamag CMOQ-2S as the output tranni?

With a 990 opamp? You should be able to run it as in the 312 (1:2 with an Aux output, will give you +6dB of gain), but test it, as it may work better as 600:600, with a pair of windings in series on each side. Check the PDF for the CMOQ and wire it like "Test Circuit 4" except leave out the 604R on the output side.
http://cinemag.biz/output/CMOQ-2.pdf

Cleanest way to configure the windings this way:  Connect Red wire to pad G and Orange to pad Y.  Then Blue to pad GY and Violet to Pad V and you place a jumper between the 2 pads for the Aux output Molex.

THEN:
Brown goes to the pad marked "BR",
Yellow goes to the pad marked "R",
Green to the pad marked "O"
Gray to pad marked "BL"

Unless I mixed up the colors, that should work!

good luck!

 

[Soeren_DK]

Thanks for that detailed answer Mitsos

They work in conjunction to eliminate DC offset on the 990 output when not using a cap between opamp and the CMOQ.  You first adjust the bias trimmer so that you get very little difference in DC between min and max gains (measure between output of opamp and ground with servo out of circuit), once that is within the tightest tolerance your DMM allows, connect the servo and watch it go to zero. There is a detailed description of the procedure in John Hardy's 990 pdf.

I'm using 990 clones from some group buy long time ago and they run on 24V, What IC did you use for the DC-servo?

Which Altran input did you get? The 990 one (1+1:4) or the 312 one (model 3402-2, 1+1:Cool. If the former, you can use it in 1:2 (primaries in series) with about 6K as a load. I haven't done any tests for zobels on this trafo because I moved recently and the place is still a mess.

I'm using the Altran C-3982 from the group buy. I remember that the where recommended for the 990 opamp. What do you recommend for R10, R12 and R17

I have a single-sided one that you can etch if you want. I tried posting it before but the links always disappeared. Let me know if you want it, I can email the files to you. On the other hand, the 4th rail is for the LEDs,  you could jumper that to the +V and G and use the positive rail.

That would be nice. mail: niehans at gmail dotski com

With a 990 opamp? You should be able to run it as in the 312 (1:2 with an Aux output, will give you +6dB of gain), but test it, as it may work better as 600:600, with a pair of windings in series on each side. Check the PDF for the CMOQ and wire it like "Test Circuit 4" except leave out the 604R on the output side.
http://cinemag.biz/output/CMOQ-2.pdf

Cleanest way to configure the windings this way:  Connect Red wire to pad G and Orange to pad Y.  Then Blue to pad GY and Violet to Pad V and you place a jumper between the 2 pads for the Aux output Molex.

THEN:
Brown goes to the pad marked "BR",
Yellow goes to the pad marked "R",
Green to the pad marked "O"
Gray to pad marked "BL"

Unless I mixed up the colors, that should work!

good luck!

I'm using the 990 opamp and I don't thing I need the AUX output. What is it for? a guitar amp?
Is it still the wirring you have described above if I don't need the AUX output?

Sorry for all the questions...  :-[

Cheers
Soren

 

[Soeren_DK]

bump

 

[grantlack]

Soren-
A lot of this is spread throughout the thread(s), but here's the consolidated version:

Servo IC: OPA604 has been used with apparent success, that's what I bought but have yet to test for lack of opamps.
Resistors: can't recall off-hand what I used, I'll update if I can find where I scribbled it.
Aux: you can use this to drive a meter directly or as a handy split if you're doing live sound.
Hope some of that helps.

 

[mitsos]

oops, this must have slipped to the second page before I saw it..

With my 990s I am using the OPA604. I believe (they are on loan to a friend) I am running them around 20-22V because of a power trafo issue (not enough VAC) but I plan on remedying this as soon as I get a new one.

I did not use a zobel with it, just a 6K or so load R as my trafo is wired 1:2.  6K / (2^2) = 1K5 Ohms impedance on your input.

The instructions for the output transformer wiring are how I would wire a quadfilar trafo like the cinemag CMOQ series (whicha are spec'ed 150:150:150:150) to work at 600:600.  Sorry if it's confusing, maybe a diagram would be better.  Take a look at the original 312 schematic (http://www.waltzingbear.com/Schematics/API/API_312.htm). The output trafo is very similar to the CMOQ, the primary side is 150Ohms, and the output is 600. The VU winding is 150 Ohms.  What I suggested you do is, instead of leaving the fourth winding (vio/gry in the 312 schem) for a VU or other use (backup recording, monitor, etc) is to put it in series with the brown/red, so you get 600:600.  That is what I tried to describe above. Does that make any more sense? 

The Aux output connector in this case would not be an output but a jumper to make the series connection between one pair of windings. You can do the same by just connecting the wires directly together.

In the end, I would try it both ways, 1:2 (like the 312)  will give you 6dB more gain and might be a bit more current hungry but I don't think that it would make much difference.  Try both ways and see if you like one sound over the other. 


I'll shoot you the PSU files later today.

cheers!

 

[zayance]

I'm thinking of making some M1 type Preamp using Mitsos PCB, and after reading some here and there.
i had a noob question on this one... I wanted to use OP97 as Seavote, and before reading this thread i was actually going to cut traces and bring
the needed Supply voltage to the DC Servo...
Now i wanted to cut trace 1, as mitsos suggested, and cut trace 2 isolating the Pin 4 of the IC, means both ways of the Pad, and Wire jumper the track to still
be able to feed the -24V to the DOA and the IC output also if needed....
But that's for the DC Servo section, for the Bias, i understand that some DOA's Would need minus voltage for that, but i'll be using JH990C for them so positive is
what i need...
So for that i would need to cut the trace that goes from the C2 Bypass Cap to the Jumper Pin, and Wire Jumper again the +15V to it.

Sounds good?

But is there much of a difference to use the +/-24V rails for the Bias while the DC Servo is at +/-15V (that's the noob question)
Better to be isolated?


Thanks for the clarifications.

 

[mitsos]

Did I recommend cutting traces?  ;D

Actually, I could have laid that part out a bit better, in a way that would allow for easier cutting of traces, but it didn't occur to me until after I made the PCBs, so it would involve a few jumper IIRC. But that's all a year or so behind, I'll be building up a pair of 990s soon, so I'll post pics when they're done, but FWIW I'll be using OPA604s as servos with no trace cutting. They are not super ultra high precision, but in this design (and many others) it works just fine.  From what I understand, it's in the twin servo that the servo opamp starts to make audible differences, but YMMV.

Good luck with this, I'm happy to see people still building these, makes the hours spent with Eagle (and not with my family) worth it!!

cheers!

 

[dmp]

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.
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.
Also, I ordered transformers from Jensen and they arrived with the zobel components, same exact values listed in the Hardy doc.
My plan is to build a second pair with Altran transformers and see how they compare to the Jensens.
Mitsos, great job with these and I'm really glad you took the time to make these! I built API's as well and the Jensen 990s blow them out of the water for acoustic guitar and vocals.