Solid state phono preamp design philosophy

jam · 2008-06-01 8:41 pm

I have discovered that the LT1115 might be similar to the LT1028 in which case Sigurd is correct

The conceptual schematic of the LT1028..........

Jam

dpuopolo · 2008-06-01 8:44 pm

Their suggested phono preamp

Their suggested phono preamp looks pretty good-especially the FET current source that's pulling its output stage into class A operation.

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1026,P1293,D1612

jam · 2008-06-01 8:49 pm

I think that the buffer is also key to driving the RIAA network and not loadong down the LT1115. Someone has got to build this.

Jam

Oops!.....posted too soon see also link above.

AndrewT · 2008-06-01 8:53 pm

Sigurd Ruschkow said:
Jam -
I downloaded the SPICE model for the LT1115 from Linear's web page, and installed it in MC. You see the results below.
It is way off. Instead of 1nV/sqrt(Hz) we see 1uV/sqrt(Hz).

Looking at the model data, I see no noise generator.

Sigurd

that 1uV is noise voltage with zero ohm Rs.
The other is noise per root Hz.

mlloyd1 · 2008-06-01 8:58 pm

someone did ...

adcom, years ago, in the 565 preamp

mlloyd1

jam said:
... Someone has got to build this.

Jam
...

jam · 2008-06-01 9:06 pm

mlloyd,

Good to know............I somehow missed this. I used to work for an Adcom service center, but then they sometimes put their own part number on ic's.

Jam

Sigurd Ruschkow · 2008-06-01 9:09 pm

Andrew -
not sure I follow you here. Are you referring to the graph I showed?
Which "other"?

Here's the SPICE model data for the LT1115:

* Version 2.0 Copyright ? Linear Technology Corp. 10/19/04. All rights reserved.
*
.SUBCKT LT1115 3 2 7 4 6 ;(+IN -IN V+ V- OUT)
RC1 7 80 70.736
RC2 7 90 70.736
Q1 80 2 10 QM1
Q2 90 3 11 QM2
*
C1 80 91 750E-12
RXC1 91 90 50
CXC1 91 90 400E-12
C2 1 98 3e-11
RXC2 98 8 1K
CXC2 98 8 1e-11
*
CIN 2 3 15E-12
RIN 2 3 2E4
DDM1 2 104 DM2
DDM3 104 3 DM2
DDM2 3 105 DM2
DDM4 105 2 DM2
RE1 10 12 -44.157
RE2 11 12 -44.157
IEE 12 4 4.5006E-04
RE 12 0 444390
CE 12 0 1.5789E-12
GCM 0 8 12 0 7.0854e-9
GA 8 0 80 90 0.014137
R2 8 0 100000
GB 1 0 8 0 267.31
RO2 1 0 79
*
RS 1 6 1
ECL 18 0 1 6 27.91
GCL 0 8 20 0 1
RCL 20 0 1E3
D1 18 20 DM1
D2 20 18 DM1
*
D3A 131 70 DM3
D3B 13 131 DM3
GPL 0 8 70 7 1
VC 13 6 3.6394
RPLA 7 70 1E4
RPLB 7 131 1E5
D4A 60 141 DM3
D4B 141 14 DM3
GNL 0 8 60 4 1
VE 6 14 3.6394
RNLA 60 4 1E4
RNLB 141 4 1E5
*
IP 7 4 0.00745
DSUB 4 7 DM2
* MODELS
.MODEL QM1 NPN(IS=8e-16 BF=3461.5)
.MODEL QM2 NPN(IS=8.0155E-16 BF=6428.6)
.MODEL DM1 D(IS=1e-19)
.MODEL DM2 D(IS=8e-16)
.MODEL DM3 D(IS=1e-20)
.ENDS LT1115

Sigurd

AndrewT said:
that 1uV is noise voltage with zero ohm Rs.
The other is noise per root Hz.

AndrewT · 2008-06-01 11:25 pm

Sigurd Ruschkow said:
Andrew -
not sure I follow you here. Are you referring to the graph I showed?
Which "other"?

post 294 refers to 1uV and the other you referred was 1nV/sqrt(Hz)

PMA · 2008-06-02 5:38 am

Guys, you are re-inventing wheel with the LT1115. It is a BJT, very low voltage noise, very low distortion OA, especially suitable for MC cartridge. It is very similar to LT1028, regarding noise and distortion. The 1st google click leads to Linear Technology website and datasheet. It has been in production about 10 years, AFAIK. Your BJT candidates are AD797, LT1028 and LT1115. New OPA211 is a bit worse, and new NS series cannot compete.

Sigurd Ruschkow · 2008-06-02 9:17 am

OPA211 SPICE Model

Yes, Andrew, the model is 1000 times worse relative to the datasheet.

PMA - the OPA211 is a superb OPamp noisewise, IMO.

Attached is the noise plot for the OPA211 SPICE Model.
The model is very accurate. Sligtly too much at 10Hz: 2.7nV/sqrt(Hz) instead of 2nV/ sqrt(Hz) in the datasheet.

Siurd

AndrewT said:
post 294 refers to 1uV and the other you referred was 1nV/sqrt(Hz)

AndrewT · 2008-06-02 1:28 pm

But the nV/rtHz have to be multiplied by the bandwidth to get uV of noise.

20kHz takes 2uV/sqrtHz up to 0.3uV
If the bandwidth were 100kHz then 1uV drops to 3nV/sqrtHz.

Sigurd Ruschkow · 2008-06-02 3:05 pm

Now I see what you are trying to say.

ALL my presented graphs of OPamp noise are
the eqvivalent short-circuit input rms noise voltage spectral density. It is NOT the total rms noise over a bandwidth.

Microcap should present what the unit is when doing noise simulations but MC does not. I have mentioned this to them, and expect them to add this.

Sigurd

AndrewT said:
But the nV/rtHz have to be multiplied by the bandwidth to get uV of noise.

20kHz takes 2uV/sqrtHz up to 0.3uV
If the bandwidth were 100kHz then 1uV drops to 3nV/sqrtHz.

scott wurcer · 2008-06-02 3:50 pm

PMA said:
Guys, you are re-inventing wheel with the LT1115. It is a BJT, very low voltage noise, very low distortion OA, especially suitable for MC cartridge. It is very similar to LT1028, regarding noise and distortion. The 1st google click leads to Linear Technology website and datasheet. It has been in production about 10 years, AFAIK. Your BJT candidates are AD797, LT1028 and LT1115. New OPA211 is a bit worse, and new NS series cannot compete.

AFAIK LT1115 and LT1028 are the same part. I think Walt Jung talked them into relaxing DC specs and making a lower cost audio part.

PMA · 2008-06-02 4:02 pm

Re: OPA211 SPICE Model

Sigurd Ruschkow said:
PMA - the OPA211 is a superb OPamp noisewise, IMO.

I have measured LT1028, AD797 and OPA211 for voltage noise in the same setup, within 1 hour. No doubts OPA211 has higher voltage noise.

jam · 2008-06-02 4:24 pm

PMA,

How about you thoughts on a low noise power supply for a phono stage.

Jam

peranders · 2008-06-02 4:27 pm

scott wurcer said:
AFAIK LT1115 and LT1028 are the same part. I think Walt Jung talked them into relaxing DC specs and making a lower cost audio part.

Exactly, they look very much alike except for the DC performance.

PMA · 2008-06-02 4:30 pm

Just my 12 cents, when I tested them for stability, I had a feeling that LT1115 was a little bit more difficult to compensate at very low gains.

PMA · 2008-06-02 4:31 pm

jam said:
PMA,

How about you thoughts on a low noise power supply for a phono stage.

Jam

Should be a nice PSU, cascaded. I have not had time yet to solve this issue.

scott wurcer · 2008-06-02 4:41 pm

PMA said:
Just my 12 cents, when I tested them for stability, I had a feeling that LT1115 was a little bit more difficult to compensate at very low gains.

Jam's schematic got a little clipped, remember there's a mask option 1028/1128 so I'm not sure where they picked the comp cap to be for the 1115. In any case it's probably pulled back to the point where you're going to get some lot to lot phase margin issues at low CLG.

(what a compensation nightmare I forgot!) :bigeyes:

x-pro · 2008-06-02 4:44 pm

scott wurcer said:
AFAIK LT1115 and LT1028 are the same part. I think Walt Jung talked them into relaxing DC specs and making a lower cost audio part.

Here is a bit of evidence on that: noise traces for LT1115 and 1028 in their datasheets are identical (it is the SAME trace!

) .

Cheers

Alex

Solid state phono preamp design philosophy

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