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Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 22nd July 2021, 12:58 PM   #5741
kevinkr is offline kevinkr  United States
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Salas DCG3 preamp (line & headphone)
ADA4625-2 is the dual version, but it's somewhat expensive. Might also consider the OPA1642?
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Old 22nd July 2021, 02:07 PM   #5742
hpasternack is offline hpasternack
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I see. I was looking at the first seven pages of the data sheet (up to Absolute Maximum Ratings) and there's no indication that I can see that the -2 is a dual version until later in the document. The specs for the -1 are better so I assumed the suffix just referred to the chip grade. I thought possibly you were working on a one-channel-per-board variation of the DCG3.

The AD823 is in plentiful supply, so I guess you are looking to experiment with different servo amps.
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Old 22nd July 2021, 02:49 PM   #5743
kevinkr is offline kevinkr  United States
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Salas DCG3 preamp (line & headphone)
I think I paid about $12 each for ADA4625-2, its DC performance is very good but there are likely better devices for this use, and it may not be stable in this application either, need to check carefully for oscillation. .
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Old 22nd July 2021, 03:41 PM   #5744
hpasternack is offline hpasternack
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Quote:
Originally Posted by kevinkr View Post
I think I paid about $12 each for ADA4625-2, its DC performance is very good but there are likely better devices for this use, and it may not be stable in this application either, need to check carefully for oscillation.
The ADA4625-2 is supposed to be unity-gain stable so oscillation shouldn't be a problem, I would think.

For my latest project, I selected ADA4627-1. It is also unity-gain stable. It has much better offset and input bias current specs than ADA4625-1, but worse noise specs. It's quieter than the AD823 across the audio band, but comparable below 100Hz. It should be no worse than the AD823 in the DCG3, which is already dead quiet.

I believe the resistors in the servo circuit will be larger sources of noise than the op-amps themselves, when using low-noise op-amps.

Unfortunately, there is no dual version of the ADA4627. There are a lot of op-amps out there that you can use, but if you're intent on chasing the secret sauce, I guess getting just the right part is important.
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Old 22nd July 2021, 03:58 PM   #5745
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
Choosing between unity gain stable very low input bias current op-amps like JFET and CMOS because of the high source impedance low pass input filter in the servo circuit.

U1 servo subjectively affects to an extent. ADA4625-2 promoting even more resolution, AD823 a warmer tonal balance still with very good resolution.

OPA1642 OPA2156 OPA1656 have been tested too. Those tend to ADA4625-2 effect.
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