Why not disconnect the mute switch from the low level phono signals, then use the switch to operate a small mute relay in the preamp? In other words, why dirty up a sensitive low level signal in the first place?
I wish I could say that all datasheets followed a common convention, but it really varies from part to part. In theory, you are correct, bias cancellation should only leave a small residual current flowing in or out of the op amp inputs, denoted by the +/-. However, as these documents get moved between different systems (or systems engineers...) sometimes those symbols don't make it. Or even worse, sometimes they show up where they don't belong...
Mark,
IMO the short circuit switch is not the problem.
It’s triggering a huge bump shows that something is on the edge, so replacing the switch would be symptom fighting.
Hans
IMO the short circuit switch is not the problem.
It’s triggering a huge bump shows that something is on the edge, so replacing the switch would be symptom fighting.
Hans
When taking such draconic steps, it would be just as easy to place the Riaa amp in the TT’s plinth. 😛
Hans
Hans
Please do not take this as an off-handed or rude remark regarding a) Moving the Mute function into the RIAA box or b) Moving the whole RIAA circuit into the TT's plynth.
"Shouldn't have to do that". Period.
Btw, this is the final box. I'll be using it with or without the 1656 or 1612. I read the most praise and acknowledgement from folks that really do listen that the sonic King is the BB OPA627 and maybe seconded by another BB diFET the OPA2107. This is just my glean but even with my old ears I lean towards agreement.
And I was going to live with the limitation in choices for op amp rolling. But then I discovered this thread, saw the big players and figured this certainly was the time and place to throw a curiously disruptive input over the wall.
"Shouldn't have to do that". Period.
Btw, this is the final box. I'll be using it with or without the 1656 or 1612. I read the most praise and acknowledgement from folks that really do listen that the sonic King is the BB OPA627 and maybe seconded by another BB diFET the OPA2107. This is just my glean but even with my old ears I lean towards agreement.
And I was going to live with the limitation in choices for op amp rolling. But then I discovered this thread, saw the big players and figured this certainly was the time and place to throw a curiously disruptive input over the wall.
Yes course, you should leave the location of things exactly where they are now.
But I’m anxious to hear whether you tried shortening C4, that’s such an easy and simple thing to do.
Hans
P.S. is this a proto or is it your final version ?
But I’m anxious to hear whether you tried shortening C4, that’s such an easy and simple thing to do.
Hans
P.S. is this a proto or is it your final version ?
I could do that Hans and could deal with some Vos but since this is a 1-stage pre, all the DC gain would go to 60dB. There would be no warp or eccentricity or "rumble" immunity at all. Isn't that a worry? I also don't think it was the RIAA's intent to go down to 0Hz. Now yes, a hi-pass up front could help out. It's interesting that according to the TI circuit write-up the divergence from the RIAA spec in the very low-end could be lessened by increasing C4! BTW. What's your theory re shunting C4?
I'll try it but was interested in your theory. And yes, as above, I'm afraid this is THE Box.
Tell me where to put the R-C hi-pass. Do want to see the series R between the 47K and + input or up-front between the RCA and 47K?
I understand this is your final version and not a proto.
Shortening C4 is not meant as a permanent solution, just as a test because your bumps are at LF where C4 still has a high impedance.
Two possible outcomes:
1) it doesn’t change your problem, then the fast opamps that you don’t really need should be avoided and the opa627 is an excellent (but very expensive) amp.
2) it solves your problem and then it’s up to you how to find an alterative for a rumble filter if you want to proceed with the OPA1656.
Hans
Shortening C4 is not meant as a permanent solution, just as a test because your bumps are at LF where C4 still has a high impedance.
Two possible outcomes:
1) it doesn’t change your problem, then the fast opamps that you don’t really need should be avoided and the opa627 is an excellent (but very expensive) amp.
2) it solves your problem and then it’s up to you how to find an alterative for a rumble filter if you want to proceed with the OPA1656.
Hans
It's either a damper or a low-pass, see the attachment (where I left out the op-amp supply connections and input and output connectors out of laziness). Rdamp and Cdamp are the new components.
My preference would be the right circuit because of the RF filtering and because you could use larger resistors than 3 ns/Cdamp if needed (or so I expect), but either should help if my hypothesis is correct. The left circuit has the advantage that you don't need to open any connections, just hook up something in parallel to the input, and it has a very small noise advantage.
Attachments
Tonight I'll try shunting C4 and also try the damping as I still have the 15pF polystyrenes across the RCA inputs from the other test so will add a 220R in-series.
Btw, is there any prejudice against polystyrene caps? Are they considered inductive? My surplus PolyS caps are axials so are "jelly rolls" but I can see the crunched extended-foil edges through the transparent end caps.
Attached is the simplified bench test setup PDF dwg used yesterday which produced the results in post #632. Also attached are pgs 19-21 from the 1656 datasheet for convenient reference.🙂
Btw Marcel, what drawing package are you using?
Btw, is there any prejudice against polystyrene caps? Are they considered inductive? My surplus PolyS caps are axials so are "jelly rolls" but I can see the crunched extended-foil edges through the transparent end caps.
Attached is the simplified bench test setup PDF dwg used yesterday which produced the results in post #632. Also attached are pgs 19-21 from the 1656 datasheet for convenient reference.🙂
Btw Marcel, what drawing package are you using?
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I use KiCAD, and I think polystyrene capacitors with shorted windings such as yours should be low enough in inductance. They were used a lot in RF circuits before everything went SMD.
How is the mute switch implemented? Is it a mechanical switch or is it an electronic switch?
Tom
Tom
I am now listening to a 47yr-old pressing of "There goes Rhymin' Simon" - using an OPA1656-based RIAA EQ preamp - without sonic issue. Everything is hooked-up as per usual. Lifts and Lowers are soooo soft. The Mute switch is right there, right on cue, very well behaved. I could be hallucinating again (that was embarrassing) so I will report more later, tomorrow. For now, fingers crossed; all fixed with the attached circuit mod from MarcelvdG. Thank you!
PS
This mod passed the no cart, no TT 1Kohm loaded cable-only scope test b/c I could not get a trigger. Then it passed the Lift and Lower scope test with TT and MM Cart attached. Then it passed the system volume test where I Lift and Lower and just listen at increasing volume levels.
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The OPA1612 (LME49990's replacement I think) is on stage now behaving just fine. I now see some Ib across the 47K + Input and quite a few mV Vos but that's OK. I'm just kicking tires tonight.