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Old 14th February 2013, 02:34 AM   #301
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Hi,

The xo is okay but will restrict the performance; the lme49710 are fine in the buffer; but unfortunately, they don't sound good in the I/V so just about anything will be better. What can you get for reasonable money that is fet input ?

Opa134 are good, opa132 are the upgrade from 134, opa1641 are smooth and warm, opa827 are good and opa627 and ada4627 sound best imo. lt1028 are too bassy, ne5534 are a little dull. Opa602 and ad8597 are okay. Probably ad8610 will sound good (limited to +/-12V and now replaced by ada4627). Any others you can get ?

cheers

Last edited by KlipschKid; 14th February 2013 at 02:42 AM.
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Old 14th February 2013, 02:46 AM   #302
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KK have you tried AD8065 for I/V? - they're very cheap on Taobao (1.9rmb I paid). I found them a little coloured at the top end as input buffer on my amp, not particularly unpleasant. Haven't tried in I/V as prefer something more neutral but as you're a fan of FETs I wondered if you'd listened? Dynamic specs do look good for I/V.

On eBay, its about $5 a pop for AD8065AR.
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Old 14th February 2013, 02:57 AM   #303
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Hi,

I haven't tried them - they're very fast but they didn't make my wishlist 'cos of noise. I used the parametric searches and set noise limit to 5nV/Hz, already quite noisy. The only ones left on my ad wishlist are ada4898; not fet but worth a try.

BTW, I'm not fan of any particular design - it's just the strong output from this DAC seems to want something that can behave very linearly and fets seem to have stolen the show for this.

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Old 14th February 2013, 03:04 AM   #304
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Thanks.

I don't really understand why noise is so important with DACs - if I'm building a phono preamp then sure, noise is potentially a problem because the signal is at most 5mV say with a MM cartridge. But with a DAC the signal's around 2V, so that's about 50dB higher. By my handwaving reckoning if 0.3nV/rtHz is the bees knees for MC phono then adding 50dB to this gets us close to 100nV/rtHz. Sure, we'd like to do a bit better than phono with digital but do we really need that much less noise?

Yes I agree with you 100% about linearity being king and FETs being better at HF linearity. However in my experience there's a negative correlation between linearity and noise - by sacrificing some noise we can gain linearity. This seems to be because when an LTP is involved (as it is with most opamps) degeneration buys us more linearity at the expense of noise. For DAC duty, I want maximum linearity hence higher degeneration - this comes with higher noise.
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Old 14th February 2013, 03:19 AM   #305
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I agree to some extent. I'm using class-T amps, bi-amping, and relative to classA, and this DAC, they're noisy as hell. But I seem to get a better soundstage with lower noise - more detail and better placed. Christopher Cross, Sailing, has some nice chimes on it that flow across the room better. Stuff like that....

I'm think about replacing the I/V op amps with a very simple CFP - two transistors that make a unity gain stage. What do you think ? I'm sure someone must already have done this and a quick search gives me this :

FET compound pair?
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Old 14th February 2013, 03:28 AM   #306
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You mean doing I/V passively then buffering it? My money's still on LC filtering before any active stages - once you've tried that, you won't go back. The soundstage improvement was the biggest shock when I first heard it Once there's a passive filter there, the active stage becomes much less critical.
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Old 14th February 2013, 07:10 AM   #307
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Hi,

I wasn't thinking passive - I would want to avoid coils unless they are tiny air-core jobs. But just for arguments' sake, I could use the 4 air-core output coils (10uH) I have from an old Charlize TA2020 amp that burned, and add some caps to make a filter ?

I was thinking two fets connected as a complementary feedback pair, or Szilaki, with a constant current. Here's a description :

http://www.epanorama.net/sff/Misc/Am...dback_Pair.pdf

Anyway, I'm not going to do it for this DAC. I'm very pleased with the OPA627 and ADA4627 so I'm going to leave this DAC alone and "work" on the two others - AK4399 and dual AD1955.

cheers
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Old 14th February 2013, 07:17 AM   #308
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10uH is a bit on the low side as regards the (too high) corner frequency it will create, but its better than nothing yep.

What's the idea behind using two FETs in a CFP? Because you don't think a single FET is linear enough? I don't have any experience to speak of with JFETs so couldn't comment. As regards bipolars in this configuration I'm not sure that the linearity is gained in the area where we need it - high frequencies and low level signals. AP distortion plots tend to measure lower freq linearity and high levels.
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Last edited by abraxalito; 14th February 2013 at 07:23 AM.
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Old 14th February 2013, 07:43 AM   #309
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As I understand it, CFP with fet should have lower output impedance than single fet so it'll drive the bjt buffer better.

I have some Mundorf silver/oil caps also sitting around: 2x 6.8uF and 2x 8.2uF - big expensive caps that were doing duty as tweeter protection caps when I first built this hifi. I could give that a try... maybe cut too much... reduce the coil size..

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Old 14th February 2013, 04:08 PM   #310
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Salas is quite Jfet expert here....
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