4-channel gain stage with DC-servo for MiniDSP - diyAudio
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Old 12th June 2013, 04:21 AM   #1
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Default 4-channel gain stage with DC-servo for MiniDSP

I'm painfully aware of the MiniDSP's output voltage limitations. I have some consumer amps that specify 1V RMS input voltage to reach full output power. More input voltage is needed to enter the "dynamic headroom" territory, too. Since we all know that digital clipping should be avoided, it seems that a prudent solution is an analog gain stage after the MiniDSP, so I designed one.

This uses the LME49740 (a quad version of the LME49720/LM4562) for the gain stage and the TL074 for an (optional) DC servo. The gain is set to 10dB, which should provide additional headroom and gain, and even allow some pro amps to be used. This could easily be increased to 20dB (10x gain) if desired.

I haven't properly set up the servo yet, but here is a preliminary schematic (attached), omitting things like bypass caps. This is one of four identical channels - use one board for each 2x4, two boards for each 2x8. This seems like something that many, many people are asking for. I can use some myself, so I plan to do a run of PCBs if there is enough interest.

Your comments and suggestions are of course welcome.

-Charlie
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Old 12th June 2013, 05:22 AM   #2
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With so low gain it's hardly necessary with a DC servo. The servo is also too slow.
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Old 12th June 2013, 11:41 AM   #3
Davey is offline Davey  United States
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Charlie,

Have you actually found the 0.9 VRMS output of the miniDSP units to be a problem/limitation?

I know, on paper, this would prevent some amplifiers from achieving their rated outputs, but most of us seldom (if ever) need to use that rated capability.

Cheers,

Dave.
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Old 12th June 2013, 03:18 PM   #4
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Quote:
Originally Posted by Davey View Post
Charlie,

Have you actually found the 0.9 VRMS output of the miniDSP units to be a problem/limitation?

I know, on paper, this would prevent some amplifiers from achieving their rated outputs, but most of us seldom (if ever) need to use that rated capability.

Cheers,

Dave.
YES, definitely. Let me give you a specific example:

I recently built a 3-way open baffle system and used an Adcom GFA-2535 on each side. Because boost was being applied to the woofer at the low end of its operating range, I needed all of the amp's output capability. The woofer was an 8 ohm unit of only 85dB/W sensitivity, and I bridged two of the channels to provide 200W to the driver. Checking the manual, I found that in bridge mode the input sensitivity for the amp was 1.75V RMS (!) to reach the rated 200W power. With the MiniDSP driving the amp I could not really reach full power using the 4x10 (unbalanced). There was definitely no "extra" output voltage available for driving the amp a little harder, for transients, etc. Just to keep up with the other drivers, I had to use all of the dynamic range of the MiniDSP (operate all the way up to 0dB internal levels) on this channel, which didn't make me feel at ease either. I basically had to turn everything up to "max" but because I was using source material with some dynamic range to it, which caused the average SPL to be rather weak. I needed additional gain after the MiniDSP and before the amp(s).

So, yes - I think that there is definitely a need for a gain stage like this.

-Charlie
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Old 12th June 2013, 04:36 PM   #5
Davey is offline Davey  United States
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Charlie,

You might consider adding an optional capacitor location across R2 which would allow to create a 6db/octave shelving filter....for possible dipole EQ (or maybe something else.) That allows to "unload" a possibly large EQ section out of the miniDSP and should simplify (possible) gain structure issues.

Cheers,

Dave.
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Old 12th June 2013, 05:20 PM   #6
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Quote:
Originally Posted by Davey View Post
Charlie,

You might consider adding an optional capacitor location across R2 which would allow to create a 6db/octave shelving filter....for possible dipole EQ (or maybe something else.) That allows to "unload" a possibly large EQ section out of the miniDSP and should simplify (possible) gain structure issues.

Cheers,

Dave.
Good point. I'll add that. If nothing else the cap can be used attenuate the gain at HF.

To do a shelving filter you need to add one more resistor in series with the cap. You can do a shelving filter very easily in the MiniDSP as well, and can change the shelving corners as needed, but I guess it would make just as much sense here where you have available gain. My Active Crossover Designer tools can calculate the biquad coefficients for a first order shelving filter if you want to implement one in the MiniDSP.

-Charlie
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Old 12th June 2013, 06:01 PM   #7
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Here's a question - according to my sims there is a pretty good DC startup transient until the input DC blocking capacitor charges up. With the values shown, this seems to take a couple of hundred milliseconds before settling down to low levels. The peak magnitude is the product of the level of DC offset present in the input signal and the gain, but it is not something that you would want to reach your drivers.

One solution is a NC relay connected between the output of the gain stage and ground. After a few seconds delay, power is applied and the relay opens, removing itself from the circuit. This would require some additional circuitry and several relays, which seems expensive. Another option would be to use an LED-LDR (e.g. Vactrol) in series with the 1k resistor (R4) just after the DC blocking cap. Initially the LDR resistance is well over 1MOhm, and it's a simple matter to design a circuit that ramps up the current to the LED and slowly drops the resistance down to a couple of hundred ohms. Since the voltage drop across the LDR resistance element (and thus the current) is low in "normal operation" when the LDR resistance is low, its distortion will also be very low (see postscript), so under normal operation is should "disappear".

Does anyone happen to know the nominal level of DC offset output by the MiniDSP?

-Charlie

PS: here is a link to some distortion measurements for various LED-LDRs done by member jackinnj:
Comparison of Light Dependent Resistors

Last edited by CharlieLaub; 12th June 2013 at 06:10 PM.
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Old 12th June 2013, 07:57 PM   #8
Davey is offline Davey  United States
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The miniDSP units already have DC blocking capacitor on their outputs so no (steady-state) DC will appear at your circuit input. You may have some DC on your circuit output, but it probably won't be much and could be reduced by using FET-input op-amps.

If the op-amp were in an inverting configuration you'd need another resistor in series with the capacitor. But you're showing a non-inverting configuration. Any first-order shelf can be achieved by using/adjusting just three components....R1, R2, and capacitor.

Cheers,

Dave.
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Old 12th June 2013, 08:44 PM   #9
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Originally Posted by Davey View Post
If the op-amp were in an inverting configuration you'd need another resistor in series with the capacitor. But you're showing a non-inverting configuration. Any first-order shelf can be achieved by using/adjusting just three components....R1, R2, and capacitor.

Cheers,

Dave.
You are correct, but only if you want the shelf to transition between "full gain" and 0dB gain. If you want to transition between "full gain" and some other smaller gain, you need another resistor in series with the cap. The additional resistor makes the shelf implementation more flexible.

Quote:
Originally Posted by Davey View Post
The miniDSP units already have DC blocking capacitor on their outputs so no (steady-state) DC will appear at your circuit input. You may have some DC on your circuit output, but it probably won't be much and could be reduced by using FET-input op-amps.
Hmmm... without the DC servo, my circuit sim shows the gain stage having only about 1.5mV of DC offset for 10dB gain. Most amps are AC coupled anyway, so this level of DC offset should not be a problem.

If the MiniDSP is AC coupled, maybe I should just pair this down to the bare minimum - no servo, no input coupling cap, and try to balance the input impedances to reduce the offset. I would definitely keep the input LP filter, however.

Actually, in that case, I already developed a board for the LME49270 that can implement all sorts of HP, LP filters, LT, etc. I should just use that! Some details can be found here, in a thread from 2011:
active crossover board capabilities - I want your input
What's ironic, is that I made a run of these boards and then put them aside after I discovered the MiniDSP, in which you can implement many circuits without having to actually "build" them. But now it seems my analog circuits will be useful afterall! I can implement a tailored "boost" circuit like LT or LF-boost shelf using the analog boards and do the other filtering in the MiniDSP for an analog-digital hybrid crossover. During development I can model the whole thing in the MiniDSP and then move only those circuit elements that require lots of boost into the analog domain.

-Charlie

Last edited by CharlieLaub; 12th June 2013 at 09:03 PM.
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Old 11th July 2013, 09:21 PM   #10
DrDyna is offline DrDyna  United States
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Quote:
Originally Posted by Davey View Post
Charlie,

Have you actually found the 0.9 VRMS output of the miniDSP units to be a problem/limitation?

I know, on paper, this would prevent some amplifiers from achieving their rated outputs, but most of us seldom (if ever) need to use that rated capability.

Cheers,

Dave.
Yeah, I just came around looking for a thread about this as I've just been working on a naked driver dipole and I keep having issues with gain.

The problem is audible distortion before amplifiers even get close to rated input. With 2 MiniDSP units driving the multichannel input on a receiver, and the inputs on an EP2000 for subs, I can't even get close to clipping before the MiniDSP starts making unpleasant noises.

They should just go ahead and make a version of these with some actual voltage capabilities so we don't have to build gain stages and ride them way past the point of linearity to drive an amplifier to full output.

MiniDSP's with actual discrete output stages would be great.

Quote:
Originally Posted by Charlie
.....MiniDSP and then move only those circuit elements that require lots of boost into the analog domain.
Nah, I'd rather just have a DSP that wasn't anemic, I don't really want to have to put 3 preamplifiers after the MiniDSP just to have the capability of flicking the clip indicators should I choose to...a feat any $300 receiver can do with it's subwoofer output, and so should these DSP boards.
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