[BUILD] Hairball Audio/MNATs JE-990

[PRR]

>> What is the result if I change out the resistors to accomodate 24v but then use them in a 16V module?
> Really poor performance I would assume.  Would be power starved.

Looks to me like very-little difference. The 33K or 68K set 0.9mA to 0.7mA in diodes. If this current were "'2X wrong", 0.35mA or 1.8mA, the voltage on one diode would change less than 50mV. There is roughly 600mV in the 150r and 88r7 resistors. 50mV change is less than 10% change in Q4 Q7 currents.

Q4 current, *also* Q1 Q2 parameters, sets input bias current and OSI. Input bias must tolerate much larger changes due to Q1 Q2 parameters. OSI has a very broad optimum, 10% change is hardly measurable.

Q7 current limits output current. The design value, 6.7mA, times Q9 beta, is the maximum output current. Rating is 260mA (13.8Vrms in 75r). If Q9 beta exceeds 260/6.7= 39, spec is met. This is right at MJE171's minimum. But a 10% change is not going to make-or-break performance.

I'd be inclined to use R10 R11 as 47K for all supplies +/-15V to +/-24V.

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Manual Typos:

page 4

PCB Orverview s/b "overview"

These componets are mounted on the bottom s/b "components"

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And.... nice project. Hope it does well for you.

 

[Echo North]

Great points PRR!

I'm going to try the 47K at +/-15V to +/-24V in the next couple weeks.

Thank you for analysis and the manual corrections.  R12 is also listed as 100Ω on the "resistor map", should be 110Ω.  I should also add the schematic in the manual.

Mike

 

[gridcurrent]

What is the result if I change out the resistors to accomodate 24v but then use them in a 16V module?

I asked Deane about that back in 1979.  He expressed concern about stability and insisted on using the resistors for the specific voltages.  Though I respected his opinion, have used 990's set up for bipolar 24 on 15 V supplies.  No issues noted. 
About the 990 being compatible with the 2520, not in many instances.  The higher input current is a limitation.  A 990 with a 10k fader becomes a noise generator in short time.

 

[prh]

Great points PRR!

I'm going to try the 47K at +/-15V to +/-24V in the next couple weeks.

Reviving an old-ish thread to check if anyone has tried 47k, and what was the outcome?

 

[Echo North]

Great points PRR!

I'm going to try the 47K at +/-15V to +/-24V in the next couple weeks.

Reviving an old-ish thread to check if anyone has tried 47k, and what was the outcome?

I have not, but feel pretty confident it would be fine.  We've run the 16V ones on 24V rails with no issue.

 

[prh]

I have not, but feel pretty confident it would be fine.  We've run the 16V ones on 24V rails with no issue.

Mmm.  Might try 47K in my builds  8)

 

[Potato Cakes]

Mike, I just built a pair of of these JE-990's, and I tested them with a CAPI VP25, and first one had low output and would not change level with increase in mic preamp gain. I figured I just messed something up so I built the second one and tried to go extra slow and be absolutely certain everything was placed and soldered correctly and that one did the same thing except it didn't matter if the pad was in or not. The output level went down slightly with the gain turned half way up then back to the starting level with the gain all the way up. I've made errors before on a couple of other DOA's I've built and they just wouldn't work, but I was able to find my mistakes. Just to confirm the preamp was fine, I put in a gar1731 and 2520 that I built and everything was back to normal.

This one has me a bit puzzled. I hate to bother you with what is seemingly user error, but the preamp not responding to input gain is a new one for me. Any insight to the matter would be greatly appreciated!

 

[Potato Cakes]

So I just finished a third one and the output level was much hotter, however, I still ran into similiar funny business with the gain input. I have about 5dB total gain change. I'm going to order a complete 990 and compare it to the ones I built. Like I said earlier, this is a new one for me.

 

[samguaiana]

Gonna hijack quick.

I bought two Lolas used with GAR1731s and they have poor headroom. I'm chalking it up to the 2520 style as opposed to the 990s. Now I'm looking to order four DIY 990s from Hairball. Do I order the 15/16v or the 24v?

Thanks!!

 

[Winetree]

I have a 51x rack and have built 2 lolas and built the 990's as 24 volt D.O.A.s
The problem is: The lola P.S.B.s do not have the 24 volt tabs. (pins 17 & 18)
So how can a 24 volt 990 ever get the power????? Why is there even the option?????
I had to change them back to 16 volts.  Order the 16 Volt.

 

[Potato Cakes]

The option is for preamps that to utilize the 24V rail like the 51X version of the VP312 and some of the JLM modules.

 

[Echo North]

As PRR suggested you can use 24V 990s in 16V and the other way around.  No issue there, just tweak head stuff.

The Lola has two op-amps in parallel which will give you as much head room as the 24V stuff.

Mike

 

[Unit7]

I have these beautiful kits on my bench right now, to be tested in Seventh Circle J99. If I'll like the J99 and will spend the time and money to build 8 of them eventually, I'd like to use the voltage, 16V or 24V, that is optimal for this op amp, even if it's 'tweak head' sillyness So..any advice?

 

[Echo North]

I have these beautiful kits on my bench right now, to be tested in Seventh Circle J99. If I'll like the J99 and will spend the time and money to build 8 of them eventually, I'd like to use the voltage, 16V or 24V, that is optimal for this op amp, even if it's 'tweak head' sillyness So..any advice?

I would just stick with the standard 33K resistors.

 

[JW]

So, I want to run these at +/-24V

For R10  and R11, I have 56K and 68K resistors on hand. Will these do, or do I need to 62K? (That is the stated value in the build manual) Interesting 47K keeps getting mentioned. That is the value for the Gar 918 at +/-24V

Also, first time, but no matched transistors?

 

[PRR]

> no matched transistors?

The design started with the LM394 dual super matched transistor...

 

[JW]

I build 5 of these Hairball 990's. All the same, checked over visually and I can't see anything I've done differently. 2 of the 5 keep blowing the 10 ohm power resistors in my API bus.

Any ideas where to look? Little DOA opamps are hard to repair but I'm willing to try replacing transistors if that's what it is. Maybe a bad batch of something? The other 3 990's are working nicely.

 

[Echo North]

I'd triple check the output transistors and make sure that they are not swapped and that the diodes clipped to them are in the right direction and not damaged. That's the most common thing I see.

 

[JW]

Ah. Okay, thanks for the direction. On one opamp, I had inserted one of the output diodes wrong, on the other I had somehow crushed one of them with the metal coupler thing. So, fyi, that's possible. . . might just be me.

One more question,

Is 1/8 watt enough for the power rail resistors? Converting +/-16V to +/-24V, it's easier just to clip the upright existing resistor and add resistance in series. But they're 1/4 watt to begin with, so if I use another 1/4 watt in series it will be 1/8 watt

 

[Echo North]

I would not change the power rail resistors. There is a post somewhere by PRR, maybe in this thread, that breaks down how unnecessary it really is. We've been using them at 24V with the original resistors for years.

Mike