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Old 26th March 2003, 09:21 PM   #41
Electrons are yellow and more is better!
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Default Re: OPA627/OPA637

Quote:
Originally posted by carlosfm


- For CD-players: OPA627
- For phono pre-amps: OPA637

That's what I use, after many op-amp tests.
AD8610 not so bad either...and much cheaper.
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Old 9th April 2003, 08:16 AM   #42
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Default MC section

Has anyone implemented the MC section of this circuit? Most have done the MM, but no-one mentions the MC section.

I have a 0.4mV MC cartridge and I need the gain and loading of the MC section.

Does anyone have any experience or thoughts on the sound of this section?
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Old 9th April 2003, 09:05 AM   #43
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0.4 mV is just 6-10 dB weaker than normal. I think you can try to increase the gain a bit. If it's too noisy then you may add an another stage.

Opamps with < 4 nV /Hz will do the jobb. AD797, LT1028, LT1115, etc. OPA637 is also probably OK.
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Old 11th April 2003, 07:45 AM   #44
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Default Power supply inductor

Is the 30 Ohm 10Henry inductor required for the MC section. Am I reading the circuit wrong? This inductor would be HUGE.

Can anybody clarify?
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Old 11th April 2003, 07:59 AM   #45
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10H seems big but you could ask Torsten himself, ID here http://www.diyaudio.com/forums/membe...fo&userid=3984

One thing for sure you will need very quite supply voltage for the first JFET stage. You could without any harm increase the 2200 µF to 10 mF or more and exclude the inductor. You could also increase the resistance but the amp will get a longer start time. I can imagine up to at least 220 ohms would work.
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Old 11th April 2003, 10:42 AM   #46
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Default Re: Power supply inductor

Koinichiwa,

Quote:
Originally posted by rookster
Is the 30 Ohm 10Henry inductor required for the MC section. Am I reading the circuit wrong? This inductor would be HUGE.
You are reading the Circuit right.

Yes, the Inductor is 10 H / 30 Ohm / 10mA.

There is only one thing that you are wrong about. The inductor is not HUGE, it is quite small.

It is an item from Steinmusic in Germany.

Steinmusic Chokes

Click the image to open in full size.

Does that help?

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Old 11th April 2003, 11:59 AM   #47
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Rookster

I have been using a similar front end for MC. Tried both 2sk147 and 2sk170 and settled for two parallel 2sk170GR as a compromise between resolution, noise and bass. For power i use the contents of 4 Nokia NiMeH rechargable batteries connected in series, i doubt if a much better sounding solution exists and of course it will sound better than sharing a common supply with the riaa section. Plus you won't need the inductor.
What really bothers me about El-cheapo are the humongous output coupling caps, surely they needn't be that big? If i had to build this exclusively for MC i'd check what's the output offset is like and if need be would periodically readjust it just to get rid of the elcos.


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Old 11th April 2003, 12:23 PM   #48
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Koinichiwa,

Quote:
Originally posted by analog_sa

What really bothers me about El-cheapo are the humongous output coupling caps, surely they needn't be that big? If i had to build this exclusively for MC i'd check what's the output offset is like and if need be would periodically readjust it just to get rid of the elcos.
There is good sense in all of this. First, I have removed the DC blocking capacitor inside the negative feedback loop. This causes a lot of offset even with low offset, J-Fet input Op-Amp's. That is due to the very high DC gain of the circuit. Up to 1V offset is not unusual even with OPA637's.

Of course, you could use some for of external Offset adjustment or a DC servo. I never got either of these options to sound as good as an output capacitor with a suitable bypass combo as shown.

You CAN leave off the big electrolytic capacitors completely if your load will not be any lower than around 40kOhm, as I wrote in the text, which you clearly not bothered to read. By using very oversized audio grade (Elna Starget or Silmic) the influence of these on the sound is minimal as they in effect "bypass" the 2u2 Film Coupling Capacitor at low frequencies. To have a low sonic foot print you need these capacitors back-2-back and biased from the negative rail, plus very large values.

I have tested this whole circuit extensively and tried pretty much all options. For reliability (no DC on output) and smallest sonic footprint I found exactly the solution you dislike so much to be sonically most transparent.

I will still say that this circuit is not the final word for Phonostages, but in the context of invested materiels and costs vs. performance it is very good, IF BUILD AS DIRECTED, including observing the general passive parts quality.

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Old 11th April 2003, 02:52 PM   #49
Electrons are yellow and more is better!
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Quote:
Originally posted by Kuei Yang Wang
Of course, you could use some for of external Offset adjustment or a DC servo. I never got either of these options to sound as good as an output capacitor with a suitable bypass combo as shown.
Thorsten, have you checked my DC servo in my QSXM3 phono amp?
The servo can be seen in the datasheet of LT1115. See page 10 in the datasheet. If this circuit is to be used you have to investigate how OPA637 is built inside. When you have got everything right this is a cool way to connect a DC-servo.

The servo in my amp works really nicely.
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Old 11th April 2003, 03:23 PM   #50
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Koinichiwa,

Quote:
Originally posted by peranders

Thorsten, have you checked my DC servo in my QSXM3 phono amp?
No, it is the first time I noticed that. But I tried a good quality cermet offset correction pot and even that made the sound worse by more than the output capacitors do.

In the end you are still injecting a signal into the circuit. You have chosen a different junction but it is still the same story. Also, it is I think not readily applicable to the OPA637. I'd have to see the LT1028 schematic.

All first order servos I have tried where clearly audible, even when the "pull" range was minimised and audibly worse than a simple output coupling capacitor combo. I had little luck with 2nd or 3rd order servos, they tend to be, shall we say, tempramental (predictably so, look at the bode plot).

Hence the output coupling combo as it stands. These days I use a MKP of a few uF, a KP of 0.47uF (usually largerst value avialable) plus a few nF each of Styrene and Silver Mica.

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