Favorite op-amps for RIAA preamp? - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th August 2007, 11:27 AM   #1
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
Default Favorite op-amps for RIAA preamp?

I'm updating a phono section I built rather a long time ago. It's the two op-amp design, with a non-inverting 1st stage having a gain of 3.15X, followed by the inverting RIAA stage. I think it's from a Jung book. Both amps were originally 5534s, and the design used 100uF electrolytics after each stage because of the large offsets. I've seen it published with LT1028 for the front end, and that's how I'm using it now, with no coupling caps needed. I'm also using the AD845 for the RIAA section. Just curious, what would everybody recommend for op-amps in this thing, or ditch it altogether? I do have two OPA627s available. Only caveat is that I drive a fairly low impedance stepped attenuator- 7kohms.
  Reply With Quote
Old 8th August 2007, 11:53 AM   #2
diyAudio Member
 
Join Date: Jun 2004
Location: far East
depends on whether you make a single step amplification or a double step.

I achieved very good results with a AD797 in a one step amplification(and RIAA correction).

In a 2 step amplifier one can use the LT1028, LT1115 or AD797 as the first amplification either with or without active RIAA correction. If one uses passive correction between the two amplification step I prefer the AD797.
The second step amplification can with a good "normal" opamp which has to be able to drive de 7K attenuator. A Opa637 will suffice.
the offset is very diffcult to get rid of ecause of the high amplificationfactor. I always use the Blackgate NX Hi-Q, 47 uF, 6,3 Volts as cap. Its small, not too expensive and works like a charm.
A DC servo will give a sound degradation.

Rob
  Reply With Quote
Old 8th August 2007, 01:31 PM   #3
diyAudio Member
 
Join Date: Jun 2004
Location: far East
depends on whether you make a single step amplification or a double step.

I achieved very good results with a AD797 in a one step amplification(and RIAA correction).

In a 2 step amplifier one can use the LT1028, LT1115 or AD797 as the first amplification either with or without active RIAA correction. If one uses passive correction between the two amplification step I prefer the AD797.
The second step amplification can with a good "normal" opamp which has to be able to drive de 7K attenuator. A Opa637 will suffice.
the offset is very diffcult to get rid of ecause of the high amplificationfactor. I always use the Blackgate NX Hi-Q, 47 uF, 6,3 Volts as cap. Its small, not too expensive and works like a charm.
A DC servo will give a sound degradation.

Rob
  Reply With Quote
Old 8th August 2007, 01:41 PM   #4
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
The LM4562 will easily drive the stepped attenuator.

I have used the LT1115, LT1028 and NE5534 and as well, the lowly LM833. I haven't used the AD797 for a phono preamp -- just for noise measurements and in the super-reg.

By the way, we had a great time driving upstate -- took the Ontario Parkway across to Lockport, then over to PA and OH.
  Reply With Quote
Old 8th August 2007, 04:16 PM   #5
jcx is online now jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
fundamental question: mc or mm?

until you decide the input op amp can't be selected appropriately with 30 Ohm to 5KOhm source impedance range the optimum for one sucks for the other
  Reply With Quote
Old 8th August 2007, 07:27 PM   #6
diyAudio Member
 
Join Date: Mar 2004
Location: Arkansas
MM or MC?

IIRC.... the AD797 has back to back diodes which limits the input of .6v max... which might play havok if used in the second stage.

Also, for lowest johnson noise, it's best suited with low impedance input... so it's probably not well suited for the first stage if using it with MM cartridges.

I would love to see something really beautiful using the LT chips mentioned earlier.
__________________
Writing is good exercize for the texticles!
  Reply With Quote
Old 8th August 2007, 07:50 PM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
the AD797 has back to back diodes which limits the input of .6v max
That's the differential input limit; the opamp would have slammed into the rails long before that. Remember, in an opamp, the differential input voltage is always near zero. There's no issue at all in using a 797 at higher inputs.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 8th August 2007, 09:57 PM   #8
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
Thanks all! This is a mm preamp, and right now it sounds very good with the LT1028 front end (flat) and AD845 in the active RIAA stage. My guess is it could be better with the National parts in the RIAA stage (I'm ordering some tonight), and I might as well try the OPA627 since I have 'em. I used to use that part in atomic force microscope designs, and it has some really good noise and offset properties that were equaled by nothing else with the same bandwidth, but I'm not sure that translates to the ultimate audio opamp- those new National parts look really impressive, especially for the money. BTW, the reason I use a low value stepped volume control is I don't use a buffer afterwards, and usually only go through about ten inches of cable to a digital interface.

Using the Linkwitz inverse RIAA network, the preamp is within a few tenths of a dB of the right curve. I found the Hagerman network with the additional HF time constant, and it's also within a few tenths of that, though I could make it better by fiddling the values a bit. No idea if it's worth it, since the differences are up at 20kHz, and I can't hear much above 15! edit- cart is an Ortofon OM30.
  Reply With Quote
Old 9th August 2007, 03:29 AM   #9
diyAudio Member
 
Juergen Knoop's Avatar
 
Join Date: Jan 2007
Blog Entries: 3
Hi Conrad,
you mentioned the LT1028 in other threads too.
Perhaps you have some advice for me.
I have experienced random oscillations at 1MHz and can't get rid of them. Maybe they are no oscillations at all, but some sort of noise pickup.
Due to the random character I can't trace it down and any circuit changes which seems effective, don't work some times later anymore.
Is there a general issue with LT1028? I have heard collegues reporting problems and the "mystery" oscillations showed up in different LT1028 circuits and even in the high gain, wide bandwith instrumentation amplifier from the datasheet I tried to build.
regards
  Reply With Quote
Old 9th August 2007, 11:31 AM   #10
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
IMO, the LT1028 isn't any more susceptible to oscillations than anything else, but it is a bit unusual. One of the ways it achieves its low noise is via high current through the front end. It's also sort of pointless to use high value (high noise) resistors with it. Thus, I bypass heavily, with 47uF Oscon caps right up close to the part. I'm also using 475 ohms feedback and 221 ohms to ground. Some opamps give oscillation trouble if you put an RF or loading cap right from the non-inverting input, to ground. I've never understood this, but a few hundred ohms series resistance on the ni input to isolate the cap will help. Naturally you've fueled my general paranoia, so now I have to go back and check the circuit more carefully!

Put in my Digikey order yesterday, so I should have half a tube of LME49710 parts, plus a mess of polypropylene caps, arriving tomorrow.

Last night I compared the AD825 and OPA627 in the RIAA stage. It was hot, so I had a lot of background noise from fans, but have to agree with other posts about the AD825 sounding aggressive, and the OPA627 having a smoother wider soundstage. I don't understand why they should sound much different at all, and offer the caveat that I need to check out the 627 on the bench. It's always risky to pop a new opamp in a socket and reach conclusions without checking the performance. There may also be some issues with offset affecting stages further down the line. I also have a vestigial comp cap from the 5534s, and I need to be sure it isn't affecting the offset pins of any other amp.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Favorite low power chip amps johnr66 Chip Amps 12 17th December 2010 10:29 AM
Your favorite diy and commercials amps and cases ? gaetan8888 Everything Else 0 1st July 2009 12:15 AM
My favorite preamp 7n7is Tubes / Valves 13 30th March 2008 02:51 PM
MC Riaa preamp help/opinions please duderduderini Tubes / Valves 15 6th March 2007 02:42 PM
RIAA Preamp Justcallmedad Solid State 4 2nd September 2005 01:52 AM


New To Site? Need Help?

All times are GMT. The time now is 02:28 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2