LT1028 mm phono preamp? - diyAudio
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Old 12th July 2004, 12:50 PM   #1
miziq is offline miziq  Slovenia
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Default LT1028 mm phono preamp?

http://www.linear.com/prod/datasheet.html?datasheet=165

any comments on this. Its the chip that is used in Mark Levinson amps. Noise should be really low by specs and its ment for high quality audio.

Click the image to open in full size.
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Old 13th July 2004, 09:00 PM   #2
x-pro is offline x-pro  United Kingdom
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Unfortunately, this circuit suffers from the same problem as most of one-OPAMP circuits of this type - the output is essentially loaded on a quite large capacitance with only 10 Ohm resistor in series. Good for low noise but not much else...

x-pro
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Old 14th July 2004, 05:31 AM   #3
Werner is offline Werner  Europe
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The LT1028 is one of the three low-noise IC amplifier champions. Up to you to find the other two

Was used in the Michell ISO HR and in the first Trichord Delphini.
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Old 15th July 2004, 05:53 PM   #4
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Quote:
Originally posted by Werner
The LT1028 is one of the three low-noise IC amplifier champions. Up to you to find the other two

Was used in the Michell ISO HR and in the first Trichord Delphini.

LT1115? AD797?

Maarten (Who will certainly try the LT1028 in a next project)
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Old 25th April 2007, 01:52 AM   #5
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Default LT1028 vs. OPA227 or OPA627?

My Forte model two pre-amp has these op-amps in it also, and I was wondering if the 227s or 627s sounded better even thought the Linear Circuits LT1028 has lower noise.
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Old 25th April 2007, 07:46 AM   #6
Calvin is offline Calvin  Germany
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Hi,

apart from noise there´s imo not much to worry about the ´sound´ of the used OP. Sound of a Phono stage is in first case made by the precision and dimensioning of the equalizing network and a good power suply and grounding!
Even the topology of the circuit has a greater influence. I´d generally opt for a 2 or three-stage design, where You can optimize on gain, values and noise-figures.

jauu
Calvin
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Old 25th April 2007, 08:12 AM   #7
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I have used LT1115 and LT1028 and am quite pleased with them. To avoid probelms I have used a buffer at the output. It gives me also less problems driving a lowohmish RIAA network.

Schematic here

Picture here
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Old 26th April 2007, 01:47 PM   #8
jcx is offline jcx  United States
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none of the previosly mentioned op amps are low noise - when used with a typical mm catridge input Z

the low voltage noise in LT1028/1128/1115, AD797, LM4562 is acheived by heavy bias of the input Qs which gives large base current noise and input current noise is multiplied by the source Z to give output noise

for mm source Z low noise FET inputs are best: AD743/745, OPA627/37 with input noise current order(s) of magnitude smaller than those "low noise" bjt input op amps

for bjt input op amps those with lower bias/base current noise such as LT1007, OPA227 are better than the high bias "low (Volatge) noise" LT1028, AD797 op omps in this application

http://www.diyaudio.com/forums/showt...ht=#post673663

http://www.diyaudio.com/forums/showt...ht=#post674460

http://www.diyaudio.com/forums/showt...609#post328609

some Vnoise plots:

http://www.diyaudio.com/forums/showt...ht=#post843381
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Old 26th April 2007, 07:04 PM   #9
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jcx, you are right in theory but in real life the LT1115 has substantially lower noise than the vinyl itself. In other words: No noise is added and when the cartiridge not is in the grooves you won't hear any irritating noise.

BTW:Is it grooves or tracks when you talk about vinyl?
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Old 27th April 2007, 05:10 AM   #10
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Quote:
Originally posted by jcx
none of the previosly mentioned op amps are low noise - when used with a typical mm catridge input Z

the low voltage noise in LT1028/1128/1115, AD797, LM4562 is acheived by heavy bias of the input Qs which gives large base current noise and input current noise is multiplied by the source Z to give output noise

for mm source Z low noise FET inputs are best: AD743/745, OPA627/37 with input noise current order(s) of magnitude smaller than those "low noise" bjt input op amps

for bjt input op amps those with lower bias/base current noise such as LT1007, OPA227 are better than the high bias "low (Volatge) noise" LT1028, AD797 op omps in this application

A useful exercise is to calculate e(n)/i(n). This gives you the impedance where the influence of current noise and voltage noise will be equivalent.

A FET input low noise amplifier is usually a good mm choice as mentioned since current noise will dominant. Make sure the 1/f noise is reasonable as well.

In my designs, I direct coupled the cartridge. You don't want large bias currents in this case. You might want to include about a 1K00 series R (assuming non inverting stage). This would be placed between the 47k and the + input. It's purpose is to keep from destroying the input FET from static etc. Sometimes the FETs will degrade but not die. You may start to get increased 1/f noise.

This may not be an issue these days, but it was with early FET input op amps.

I think the LT1028 and similar parts would be good for MC types since these are very low impedance. All the noise will be dominated by e(n) in this case.

Al Clark
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