Schematic for Pro-ject phono box

[mzperx]

They are fully pin to pin compatible.

You have to care only about a supply voltage range, because it is decreased at AD8620. (max +/-13V)

[raffells]

After a longish absence I have aquired an early version 1 ,ie wired input but slightly later circuitry.(220ks)
I have 3 questions.
I need to increase the gain for moving coil cartridge by a smidgeon.Can Anyone suggest a value and which resistor.

I have a whole range of opamps available.
Can anyone suggest which is best position for best pairing.
Currently intending to use OPA627/opa2228.
others opa134/2134/5532/5534/jrc4580

The 100R item (R3) using a pasive preamp with leads total maximum length less than a metre .
What is miniumum safe value/. 68R+ ? or can I use higher 121R as I have spare Vishays.
Thanks in anticipation. Dave

[bobolix]

OPA2228 is not a dual OPA627, it is something like dual OP37. Therefore it has lower voltage noise, very low offset and lack of unity gain stability. Its input noise resistance is 750 ohm approx., which means for MC the voltage noise will be prevailing, whereas for MM the current noise is dominant. It is a good compromise, but the noise figure for the MC is not very good (it strongly depends of the actual pickup impedance).

From the noise point of view the OPA134 aren't suitable for the input stage at all. The 5534/5532 and the 4580 are quite good and very similar considering the noise, but the 5534/5532 has significantly lower THD. Though, some people say the 4580 "sounds better" than the 5532.

I repeat, the best choice for the MC is the OPA211 as single and OPA2211 as dual, but the dual version is not available recently neither as samples.

[PMA]

If everything goes well, there might be a brand new reference ProJect MC preamp .... . At least prototyping has been finished.

[bobolix]

Originally posted by PMA
If everything goes well, there might be a brand new reference ProJect MC preamp .... . At least prototyping has been finished.

You have luck I'm ill, otherwise I'll go you kill ...

[raffells]

Many thanks Bobolix.
Sorry to mislead you but I have many spare opamps from other home projects over the years and this is why I was asking.
The pair I have are in fact OPA228 not OPa2228s as I stated.
Unfortunately I havent got.opa211 or 2211.s.
.
Firstly the single expensive 627 in the first position .At the moment seems very quiet.I am using 2200mf Elna caps/schottky diodes as well as a number of Vishays. Silver mica caps replaced ceramics.

I have now had a play around for a few hours with a couple of different pairings and the 5532/4 do not sound at all to my liking.
Ive used them for umpteen years and can identify their dull and hard sound easily.
The pairing of 627/4580 worked quite well and seems better than 627/2134.The latter are brand new and need a few hours to settle in. Tonally both are much better and im liking the 2134s in at the moment.Actually sounds better tto be than the SE.
Note I do NOT now need any more gain or need to change the 100r resister value.Found a couple of Vishays.

My Only real questions are about 270R*/100mf* in the early version against 300R/47mf in the mark 2 versions.Should I replace * *with the later version values.... and what will the effect be.? I pressume Its something to do with bass end.(Im still learning).Their was one odd diode that was smaller than the other three.Just curious why?

I do hope you gett better soon.
Cartridges are Goldring outputs .45mv MC in a Townshend Rock.Believe imp is approx 12ohm.

Dave

[bobolix]

Well, the OPA627 is really very quiet, but the OPA228 is at least 3 dB quieter being fed from MC, although it need not mean it sounds better, as the noisiness is only a part of the auditioning result.

Concerning the resistor value you cited, it influences only the overall gain virtually. The capacitor value is much more important, because if it is too low, then it causes the bass roloff. The higher value is better and I recommend to keep it or even raise it to 220 uF.

Concerning the diodes size ... hard to say. It can be a difference between the cases only - frequently the same parts are fabricated in several types of cases. I don't believe it could have any influence to the sound.

[raffells]

Once again many thanks.
I have now run in the OPA2134s and swapped the oxide resisters for vishays.Sadly the extra noise and more revealing neutral nature shows this opamp up as coloured and more distortions as per the specs.Two burr browns in series seems a bad choice.Unless you have loads of splashy noisey components in there.
As I cannot use the OPA228s in the first stage and the recomended ones are SOIC Im now back to the better sounding JRCs for the moment.
I hope to find a top quality Analogue devices to balance things.
I may even go browndog/soic as the better new ones seem to be those.Even the 4562 seems a good choice if I can buy them in the uk.
Do I need to do anything else to get them to work?

The 100 Mf cap situation is interesting.
Having tried 3 different types ,Blackgates,Silmic,standard each one shows its sonic characteristic quite clearly.
Looking at the specs of chips they all seem to state the need for bypassing caps within a few mm of the chip.
I may have found a way to cool that wall transformer which also gives off large amount of emf. Dave

[bobolix]

Concerning the Analog Devices products, they are very good experiences - although a kind of compromise - with AD8671/72 - single/dual, both in SOIC.

[PMA]

Originally posted by bobolix


You have luck I'm ill, otherwise I'll go you kill ...

Hopefully the new prototype is released for production, the production should start in 2009.