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Old 22nd April 2002, 11:17 PM   #11
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Thumbs up rljone phono amp

Sounds like you have a great design here. The INA103 is a very low noise part with great distortion specs. 47dB is a lot of gain for one stage, but should be ok here. The bipolar inputs may be prone to demodulating RF signals, but your 150pF cap should save the day.

You might want to post a schematic here, it may be of interest to many readers.

I've done a similar design (but not with an IA), see schematic below. I don't get quite the headroom you do because of lower supply voltages. And I agree with your 16dB findings.

www.hagtech.com
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Old 23rd April 2002, 01:32 AM   #12
rljones is offline rljones  United States
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To remove grudge, I placed in series with each input to the INA103 a 1.2 microhenry, non-magnetic inductor, in parallel with a 50 ohm resistor. I also use a series 3.5 microhenry inductor on the output in parallel with a 39 ohm resistor. On the output side, since I run a balanced stereo system, I uses another Jensen transformer (PC mount).

As for schematics, I draw things by hand on paper which I can barely read. However, I do make PCB on a laptop PC (I normally Macs) and I can upload the board datafiles for ExpressPCB (http://www.expresspcb.com/) if there is interest. The problem is that most parts are surface mount and are kinda tricky to solder unless you have a steady hand, are very myopic (or have a magnifier), and have a particularly fine-tipped iron. The only non-surface mount devices are the INA103, two '431 precision zeners, two series transistors for the +/- 13V for the AD8610 op amp, and 5 electrolytic caps. (People can order the boards from them; $60 for 3 boards.) The boards have few markings once made, but the file can be printed and shows where the parts go. There is also a 12V relay to allow switching between the MM and MC, and this has it's own 7812 regulator on board; I spec'd out a very small relay so the whole circuit board is about 2 x 3 inches per channel, including the pcb mounted output transformer.

On other issue I didn't mention was matter of DC offset. It was too much coming out of the INA103, so I had to block it, using a 8 microfard cap paralleled with a 1 microfarad cap, both in series between the INA103 and and the RIAA circuit, with a 100K resistor to ground (f3 < 0.5 Hz). I've also made one version using a servo op-amp (with no series caps) to remove DC from the INA103, but have not yet listened to it. The DC offset was too much without it.

Power supply has 6 x 22,000 microfarad caps in a C-L-C-R-C arrangement, generating the +/- 22VDC supply. L is 20 mH and R is 2 ohms. These are present in both rails; the inductors are dual units toroids having one half in the ground as well as the high voltage rail. This power supply on a scope looks as clean as a 12V battery. (I can also upload this pcb information too.)

The two phono boards including parts, plus the power supply board and its parts (transformer, caps) and the MC input and ouput transformers, and chassis cost me about $900. (Each MC transformer is $250, so this is half the cost, so figure around $400 for only MM.) I spent more time calculating and measuring this phono section than just about any other single project. But is sounds REALLY good.
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Old 23rd April 2002, 11:52 PM   #13
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Thanks everyone. Having received good advice, I am not going to look at specs any more. I shall try 1NA103, AD797 and LM6172 for a start in a line stage and see how they fare.

Mohan
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Old 23rd June 2002, 01:00 AM   #14
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Reporting on progress. I built two line stages using AD797 and LM6172 (standard configurations) and listened to them for over a month. My preference continues to be with tube line stage. This does not mean that I have given up on ICs. I shall return to these from time to time. Thanks everyone for all the help and advice.

Mohan
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Old 23rd June 2002, 06:01 PM   #15
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Mohan, the AD797 would be a good choice for the phono stage as it has very high gain with low distortion, but for line level stages the AD8610 or AD8620 (dual version) is far superior to the opamps you are using.

The LM6172 is a high current bipolar opamp, a really poor choice for your application. Some people use it for headphone amps because of the high output current, but it is a finicky, cranky chip, and does not sound as good as the high end Analog Devices chips.

The AD797 is not a great choice for line level signals either. It shines for extremely low level signals such as phono cartridges, tape heads, and microphones.

Sorry I missed this thread when you originally posted. If it is not too late you might want to search HeadWize and Head-Fi, those forums are more opamp oriented.
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Old 24th June 2002, 04:45 PM   #16
alfsch is offline alfsch  Germany
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Question :
i design a dac with ad1852 and iam not shure, which opamp for filter-out is best: i have opa2134 or 2227, but now : maybe ad8610 is even better?

Hope somebody can tell me...
alf
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Old 24th June 2002, 05:39 PM   #17
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Yes, the AD8610/20 is significantly better than those Burr-Brown opamps.
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Old 24th June 2002, 07:22 PM   #18
peufeu is offline peufeu  France
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Default AD825

I have built an active crossover with AD825 chips and it sounds extremely good. I recommend them heartily.

Not a trace of grudge.

They need good decoupling though.

OPA604/2604 sounds veiled/dark.
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Old 25th June 2002, 12:23 AM   #19
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Thanks Morsel. I am glad that you found this thread. I shall try AD8620 and report.

Seeing that you are into opamps, can you advise whether you have any clear preferences in the topology of these devices as far as audio is concerned. Are there any opamps with single ended input or output? Am I dreaming? You never never know if you never never ask!!

Mohan
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Old 25th June 2002, 10:30 AM   #20
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The headphone amp that Tangent and I just finished uses Jung multiloop topology. You can find schematics and info here:

http://tangentsoft.net/audio/meta42/

The AD825 is nice but the AD8610/20 is better still.

I do not understand what you mean by single ended input or output.
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