In praise of simple op amp circuit phono stages. - diyAudio
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Old 29th July 2014, 09:05 AM   #1
fap is offline fap  Australia
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Default In praise of simple op amp circuit phono stages.

G'day all, I've had a bit of an epiphany of late, as I've been 'rediscovering' that the simple one op amp 'full feedback' phono stage circuit can be quite ok, if a little refinement and TLC is applied in construction.

I have a number of simple kits of this type in my collection and over the last couple of days little refinements like good dual op amp choice, matching channel components, using better capacitors in some sections, improved supply rail bypassing, and even 'optimising' the impedance of the RIAA network can really improve things sonically.

Whilst I'm still not a huge fan of the simple full feedback op amp phono stage circuit, I now have to admit that if done right, they can be pretty good and very pleasant to listen to. Regards, Felix.

Last edited by fap; 29th July 2014 at 09:32 AM. Reason: Spelling.
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Old 29th July 2014, 10:03 AM   #2
vulejov is offline vulejov  Serbia
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What was the reason to start a same topic..

Thoughts on the sound quality of one op amp full feedback phono stages.
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Old 29th July 2014, 10:35 AM   #3
fap is offline fap  Australia
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G'day mate, oops I forgot about that one! In all honesty though, some phono stages have unbelievable circuit complexity, but is it really necessary for good audio? Regards, Felix.
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Old 29th July 2014, 10:49 AM   #4
ahaja is offline ahaja  Poland
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Felix
Step forward - go to Calvin's homepage and search there for Platina ... a litle more complex but you get much better sound.
Of course don't forget about PSU ... poor = poor sound.
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Old 30th July 2014, 10:09 AM   #5
DF96 is offline DF96  England
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The right level of circuit complexity is needed for good audio. A single opamp phono stage involves too many compromises but is acceptable for uncritical applications or where cost must be kept down.
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Old 30th July 2014, 11:15 AM   #6
andyr is offline andyr  Australia
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Quote:
Originally Posted by fap View Post
G'day mate, oops I forgot about that one! In all honesty though, some phono stages have unbelievable circuit complexity, but is it really necessary for good audio? Regards, Felix.
I agree with you, Felix, in that some phono stages are amazingly complex.

However, I would also say that some "complexity" is required to get good sound.

Peruse the Salas "Simplistic" thread. Here is a simple concept - 2 gain stages with a passive composite RIAA network in between. There are several variations on this circuit - the original was shown in a Silicon Chip article in the late '80s - but the Salas incarnation seems to be a real winner!


Regards,

Andy
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Old 30th July 2014, 11:58 AM   #7
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The right level of complexity for me uses two opamps. The second is your basic RIAA stage. The first is a buffer/gain stage that relaxes requirements on the second. There are many ways to skin this cat.
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Old 30th July 2014, 05:31 PM   #8
rayma is offline rayma  United States
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Originally Posted by Conrad Hoffman View Post
The right level of complexity for me uses two opamps. The second is your basic RIAA stage. The first is a buffer/gain stage that relaxes requirements on the second.
Yes, even the discrete Vendetta is similar in overall topology: first a head amp, then the passive 75us, and finally the 3180us/318us RIAA gain stage.
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Old Yesterday, 09:57 PM   #9
fap is offline fap  Australia
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G'day all, something that I've seen used on simple phono stage as a possible circuit 'enhancement' is using a large value capacitor between the cartridge and the op amp input.

At least one well regarded commercial phono stage manufacturer (Graham Slee) does this and claims reduced low frequency input noise. I have seen reference to this phenomenon in one of my kits as well. Yet most op amp designs couple the cartridge directly into the op amp with a 47 k resistor hanging off this input to earth.

At least one audio designer that I have spoken to is adamant that there is no advantage to using a large value input capacitor. I have actually tried using a large value input capacitor and apart from a long RC time constant induced turn on delay, I noted no other 'benefits'.

Does anybody have any other thoughts or comments of this? Regards, Felix.
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Old Yesterday, 11:26 PM   #10
andyr is offline andyr  Australia
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Quote:
Originally Posted by fap View Post
G'day all, something that I've seen used on simple phono stage as a possible circuit 'enhancement' is using a large value capacitor between the cartridge and the op amp input.

At least one well regarded commercial phono stage manufacturer (Graham Slee) does this and claims reduced low frequency input noise. I have seen reference to this phenomenon in one of my kits as well. Yet most op amp designs couple the cartridge directly into the op amp with a 47 k resistor hanging off this input to earth.

At least one audio designer that I have spoken to is adamant that there is no advantage to using a large value input capacitor. I have actually tried using a large value input capacitor and apart from a long RC time constant induced turn on delay, I noted no other 'benefits'.

Does anybody have any other thoughts or comments of this? Regards, Felix.
Hi Felix,

All the phono stage circuit schematics which I've seen, couple the cartridge directly to the 1st gain stage - not just opamp-based ones.

Wouldn't a series input cap mean you have a roll-off given (approximately) by the equation: 160,000 / (47K * C-in-uF)? So you'd need to use, say, 2.2uF as the series cap to get a low enough roll-off frequency - is that the 'large value' that G Slee uses?


Regards,

Andy
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