Calibrating the John Hardy MPC-3000 Card

Yeah, I know I'm slow, just now I finished builiding a chasis and wiring all of the stuff.
I turn on the pre and plug my bass guitar through a DI Box, at some level(it's really a small notch) the sound becomes crazily fuzzy, some electro space fuzz. It happens when I plug a microphone too. I think it might be the servo circuits, maybe they're not calibrated right? How should I calibrate them? what values should I get?

Specs

Thanks,
Eliya.

You want to know how to calibrate the John Hardy MPC-3000 card? Ask John Hardy.

The noise that you are hearing could be caused by a small amount of DC voltage that is reaching the gain pot. This can cause a scratchy sound when adjusting the pot. Or, it could be a pot that is dirty or has some other problem at one or more locations around the resistance element.

A schematic and layout are available in the pdf for the MPC-3000 card at:

http://www.johnhardyco.com/pdf/MPC3000.pdf

Note that the MPC-3000 card was revised about three years ago. Older cards will have a different layout and will not include trim pot RV3 whch is mentioned later.

You will need to adjust the trim pot RV2 ("I-BIAS") with power applied to the MPC-3000 card. This is a 25-turn trim pot. It is the Input Bias Current Compensation trim pot. This adjustment must be done after the card has warmed up for at least 15 minutes, as follows:

1. Move jumper JP1 to the "ADJ" position (ADJUST). This disconnects the DC servo circuit so you can make DC voltage measurements at the output of the 990 without the DC servo interfering.

2. Connect a DC voltmeter with the black lead connected to the gold pin that is between the two large electrolytic capacitors that are between the 990 and the JT-16-B input transformer. This is the GROUND connection. The red lead of the meter should go to the end of RL1 that is nearest the connector end of the card. RL1 is a large resistor with 40 turns of magnet wire wound on the body of the resistor, located along the side of the JT-16-B input transformer. This connection point is the output of the 990, with a small amount of resistance to isolate the 990 from any cable capacitance. The meter must be able to read voltages as low as a millivolt or better.

3. You will make two DC voltage measurements. One will be made with the gain of the card at maximum (GAIN button pushed in, gain control fully clockwise), the other will be made with the gain control set to a much lower gain, the simplest way to do so is to deactivate the GAIN switch.

4. Make note of the DC voltage reading that you take at the low gain setting. Then, with the gain set to MAXIMUM, adjust RV2 until you obtain the same reading (within a millivolt or two) as you did at the low gain setting. Repeat this step until you get the same reading (within a millivolt or two) at both gain settings.

5. Move JP1 to the RUN position. This reconnects the DC servo. The DC voltage measurement should slowly go down to less than 100 microvolts or so (yes, that is MICROvolts, not MILLIvolts). If your meter cannot reliably read voltages in the microvolt range, do not bother to try to adjust trim pot RV3 ("SERVO"). It should not normally need adjustment anyway. This trim pot fine-tunes the DC servo circuit so that the final DC offset of the 990 will be just a few microvolts. I recommend you do not adjust it unless your meter indicates a problem. You must also be familiar with proper nulling procedures for your meter when making measurements in the microvolt range.

Thank you.

John Hardy
The John Hardy Co.
www.johnhardyco.com

Sorry for jumping in here:

Repeat this step until you get the same reading (within a millivolt or two) at both gain settings.

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John, do you mind explaining why this procedure does ensure correct input bias cancelation? Thanks!

Samuel

John, do you mind explaining why this procedure does ensure correct input bias cancelation? Thanks!

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The 990 discrete op-amp has, according to the specs, 2.2 microamps of current flowing from each of its inputs. This is the input bias current. This causes a small voltage to appear across the available DC paths to ground at each input. There are two DC paths for the non-inverting input: the 6.19k resistor that is across the secondary winding of the input transformer, and the secondary winding of the input transformer itself. This is a fixed resistance.

Meanwhile, the inverting input has two DC paths: the gain control path to ground, and the feedback loop back to the output of the 990. Since the gain control path varies from about 20 ohms to 10,020 ohms, it is NOT a fixed resistance. Therefore, the DC voltage that is caused by the flow of input bias current through this path will vary.

Since the 990 is DC-coupled in this circuit, and it is a "differential" circuit, any difference in the currents/voltages that appear at the two inputs will be amplified by as much as a factor of about 500 (54.4dB of gain for the 990 at maximum gain), or as little as a factor of 2 (6dB of gain for the 990 at minimum gain).

If the input bias currents are properly compensated for, or nulled, it has the effect of causing little or no voltage to appear at either input of the 990, so there will be very little voltage showing up at the output of the 990 regardless of gain setting. There is no DC voltage at the inputs to be amplified. Therefore, the DC voltage readings at the output of the 990 will be about the same at high gain as they are at low gain.

Traditional audio designs get around this by putting a fairly large value electrolytic capacitor in the gain control path to keep the bias current of the inverting input from reaching the gain control where it might cause noise during adjustment. The capacitor also causes the gain at DC to be unity, so there will be no amplification of any DC differential signal at the inputs. But capacitors have various problems, which I won't go into at this point (unless someone really wants to know). Thanks.

John Hardy
The John Hardy Co.
www.johnhardyco.com

Thanks for your writing.

There is no DC voltage at the inputs to be amplified.

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So you are assuming that the 990 has zero offset voltage?

Therefore, the DC voltage readings at the output of the 990 will be about the same at high gain as they are at low gain.

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I see--it is a matter of degree. If we assume that the trim is set to perfectly cancel input current (i.e. not offset current either) and that the 990 under test has 10 uV offset, the output offset varies from 20 uV to 10 mV (gain 6 dB/60 dB). I wouldn't say that "the output DC voltage does not vary", but I understand your point.

So this method does only work with opamps having very low offset.

Samuel

Thanks for your quick and detailed reply john!

I already played with the trimpots(not the way you said) so, it might make problems now or It doesn't matter?

I love this place.

I already played with the trimpots(not the way you said) so, it might make problems now or It doesn't matter?

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Well, it's hard to tell what you may have done to the card, since you did not know what you were doing! Have you stuck your finger in a spinning fan lately?

There won't be any damage to the card, but there may be more DC voltage than there should be at various points in the circuit.

John Hardy
The John Hardy Co.
www.johnhardyco.com

So you are assuming that the 990 has zero offset voltage?

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No. The trimming procedure reduces the apparent input bias currents to an acceptable level. I trim each 990 during the manufacturing process so the input bias currents are fairly well balanced, but not all 990s will work in this trimming circuit. It depends on how well the trimming procedure was done.


Regarding DC offset voltage (at the output), the DC servo takes care of that. If properly trimmed with proper equipment and proper technique, the DC offset can be trimmed to a few microvolts. This is far better than required, but the cost is only one trim pot for the DC servo op-amp (currently the OP97FP).

So this method does only work with opamps having very low offset.

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The input bias current compensation circuit has a wide adjustment range. The DC servo circuit can handle a large amount of DC offset at the output of the 990. But I have not tried other op-amps, so I cannot comment. The Jensen Twin Servo mic preamp that I manufacture has two trim pots for the input bias current compensation of each 990, one for each input of each 990.

John Hardy
The John Hardy Co.
www.johnhardyco.com

If I could be so bold as to offer a summary worded a little more simply... Please correct me if I am wrong, John.

The trimming procedure reduces the amount of offset created by the opamp and reduces the amount of "work" needed from the servo circuit to get the final offset to the absolute minimum.

Not having the opamp trimmed in circuit can cause some distortions due to imbalances.

Hope that helps clarify a bit more. Its nice to have expalnation on this stuff from the guy who makes it. Yet another thing that makes this forum valuable and awesome.

Peace!
Charlie

If I could be so bold as to offer a summary worded a little more simply...

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Sounds pretty good. The input bias current compensation circuit reduces input-related DC errors, the DC servo circuit reduces output-related DC errors. How's that?

John Hardy
The John Hardy Co.
www.johnhardyco.com

[quote author="John Hardy"]You want to know how to calibrate the John Hardy MPC-3000 card? Ask John Hardy.[/quote]
:grin:
life is good

thanks John

life is good

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Life can be very simple (and very good), if you let it.

John Hardy

Life is good. But in the case of my own life, I'm still wondering about the "simple" part. :wink: