4-channel gain stage with DC-servo for MiniDSP - Page 2 - diyAudio
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Old 11th July 2013, 11:00 PM   #11
Davey is offline Davey  United States
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Originally Posted by DrDyna View Post
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.
They already do. The 2x4 balanced version supports 2 VRMS (unbalanced) output voltages vice the "standard" unit which is limited to 0.9 VRMS. It sounds like you purchased the wrong device and/or your gain structure is still not correct.

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Dave.
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Old 11th July 2013, 11:17 PM   #12
DrDyna is offline DrDyna  United States
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Originally Posted by Davey View Post
They already do. The 2x4 balanced version supports 2 VRMS (unbalanced) output voltages vice the "standard" unit which is limited to 0.9 VRMS. It sounds like you purchased the wrong device and/or your gain structure is still not correct.

Cheers,

Dave.
I guess my whole point is they shouldn't make a device at all that's incapable of driving 80% of amplifiers past a few watts of output while at the same time showing it being used in that configuration in the product literature and advertising :P I don't like riding gear to the point of non-linearity..so my choice is purchase LabGruppen FP14000 amplifiers so that I can get 100w out of them with half a volt of input, or only buy drivers that are greater than 95db efficient.

I don't mean to sound like a turd, but I've got computer sound cards that are ~$15 dollar el-cheapo and have three times as much output voltage. Gain structure isn't even an issue, unless it's considered normal to have to have a completely additional gain stage device like a preamplifier before AND after a MiniDSP board.

Perhaps I purchased the wrong units, but I'm still really asking...for whom would these units be the correct units in that case? I mean, these units start reaching audible levels of distortion at somewhere around 0.6 or 0.7v it seems, so I have to ask if anyone actually owns a power amplifier that can deliver it's rated power at that input level. Gain Structure Shmain Structure! LOL!

Sorry, I'm just really grumpy with these things. They could have put something other than one microscopic transistor in the output stage and provided a jumper that allowed a range of output voltages which could drive anything.
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Old 11th July 2013, 11:35 PM   #13
Davey is offline Davey  United States
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Many amplifiers nowadays are spec'd with a voltage gain of 29db (x28). With a 0.9 VRMS limitation your amplifier should produce 25.3 VRMS into the load. That's approximately 80 watts into an 8 ohm load. (That's a considerable amount of power for most applications.)

You're incorrect that the miniDSP units produce audible levels of distortion at 0.6-0.7 volts. Only when you get to the clipping point at 0.9 volts does that occur.

You must have some other variable at work in your scheme that is causing a problem.

Cheers,

Dave.

Last edited by Davey; 11th July 2013 at 11:38 PM.
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Old 11th July 2013, 11:45 PM   #14
DrDyna is offline DrDyna  United States
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You must have some other variable at work in your scheme that is causing a problem.

Cheers,

Dave.
Correct, out of both my HTRs, and 4 stereo amplifiers, I don't own any equipment that can reach full power with 0.9v.
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Old 12th July 2013, 02:17 AM   #15
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Originally Posted by Davey View Post
They already do. The 2x4 balanced version supports 2 VRMS (unbalanced) output voltages vice the "standard" unit which is limited to 0.9 VRMS. It sounds like you purchased the wrong device and/or your gain structure is still not correct.

Cheers,

Dave.
But don't you have to run balanced input to the board and at 2Vrms in order to get the 2Vrms output?
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Old 12th July 2013, 03:13 AM   #16
Davey is offline Davey  United States
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But don't you have to run balanced input to the board and at 2Vrms in order to get the 2Vrms output?
No. You can run unbalanced input at either the 0.9 or 2.0 volt jumper setting and still get 2.0 volts output unbalanced.

miniDSP Balanced Kit | MiniDSP

Dave.
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Old 12th July 2013, 05:12 AM   #17
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No. You can run unbalanced input at either the 0.9 or 2.0 volt jumper setting and still get 2.0 volts output unbalanced.

miniDSP Balanced Kit | MiniDSP

Dave.
Thanks Dave and looking at the user guide (which I should have done first) I see how it all works.

The problem with the balanced board though is that it has op-amp buffers on the inputs (makes sense as it's balanced) and the outputs.

If I'm going to put the signal though additional stages I'd rather limit their number and also have control over what op-amps I used and how they were powered etc.

As such a buffer stage with gain and only on the output of the unbalanced board starts to makes sense.
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Old 12th July 2013, 05:29 PM   #18
Davey is offline Davey  United States
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I believe miniDSP recognized the need for (possible) further voltage capability quite a while back and that prompted the balanced version. Yes, it does add extra circuity relative to the direct interface of the ADAU1701. For users who don't like the selection of particular op-amps or circuitry implementation it's certainly possible to add. However, you do lose the compact/integrated format of the OEM boards.

Different strokes for different folks I guess.

Cheers,

Dave.
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Old 12th July 2013, 05:46 PM   #19
DrDyna is offline DrDyna  United States
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However, you do lose the compact/integrated format of the OEM boards.
That's pretty much exactly what prompted my almost aggravated comments, it doesn't seem like it would have been too hard to offer, say, 3v of output capability within the same form factor.

*shrug*
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Old 12th July 2013, 07:21 PM   #20
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That's pretty much exactly what prompted my almost aggravated comments, it doesn't seem like it would have been too hard to offer, say, 3v of output capability within the same form factor.

*shrug*
That's probably not possible with the current minimum power supply specification of 5VDC. That corresponds to +/-2.5V max supply voltages and the output devices need some "extra" voltage to operate correctly. This is probably why there is only 0.9VRms available (about 2.6Vpk-pk).

They could up the minimum power supply voltage to 12VDC, use a DC voltage inverter IC like the ADM660 to generate the negative rail, and then use a low noise analog gain stage to boost the output voltage swing capability to, say, +/-5Vrms (a typical max output level for analog preamplifiers).

This would require only minimal design input. Even I can design an analog gain stage... it's a mystery why they don't get to it, when many of their customers are begging for it and lack of output voltage is one of the main complaints about their platforms??? It's a big fail on their part IMHO.

-Charlie
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