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Old 20th May 2005, 08:01 AM   #1
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Default Why does my phono preamp measure well but sound bad?

I have posted the schematic of the RIAA preamp (MM) I currently use. The design is based on the circuit and procedure described in National Semiconductor Application Note 346: "High Performance Audio Applications of the LM833" by one Kerry Lacanette.

The R values were calculated so that preferred value, obtainable, compact caps for C9, C14 and C6 could be used.
Although they are marked on the sch as OPA2134s, the op-amps I'm actually using are SSM2275's because I had lots of them.
The circuit was implemented as part of a subsystem on a 4-layer pcb with good care taken over layout and grounding. Power to the op-amps is regulated and cleaned up with LC filters in each supply rail.
Measured RIAA response within 0.25dB of the spec, S/N was also good. On the scope (up to 20kHz) this thing appears to check out just fine.

I then did a listening test, first using a Stanton 500 cartridge - a disgusting thing at best - and then a Bang & Olufsen MMC3, which I had used for years before and which I always felt auditioned well.

First listening test with the new RIAA circuit was very disappointing. Not to mince words it sounded c**p. 'Tight', and like the musicians were playingin a metal drum. At this stage I had an NP0 ceramic SMD cap for C2, Panasonic EHCU film SMD caps for C9, C14, and an X7R ceramic cap for C6.
I traced much of the poor sound to the ceramic cap for C6 which I then replaced with a WIMA MKS4 film cap. (Big difference in footprint, so I bodged it underneath the board where I couldn't see it and feel guilty). That's how it is at present.

But I'm still not 100% happy with the way this preamp sounds. It's nice and puncy in the bass, the midrange seems smooth enough, the highs appear to be present and correct, and the overall sound comes over as balanced across the spectrum. But it still sounds a bit 'tight', like an early digital recording done with a Nyquist rate ADC.
My intuition tells me that the problem is hf related, but I may be wrong. I have previously designed simpler, single stage RIAA preamps which sounded just fine, so it's not a general 'all op amps are bad for RIAA, use transistors instead' issue.

I'm all out of ideas. Any analog audio experts out there who can shed some light on this...?

Regards

John H
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Old 20th May 2005, 09:03 AM   #2
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If I remember correctly, SSM2275 is a BiFET op-amp? If so, you may be falling foul of common-mode distortion because you're operating in non-inverting mode. I think Walt Jung wrote an AD paper about this, but by using non-inverting mode, you force both inputs of the op-amp to track the input signal and this changing voltage between the sources of the input FETs and the substrate causes distortion. Further, the feedback network imposes quite a heavy load on the op-amp.

What to do? The OPA2134 might be better. Alternatively, perhaps lowering the gain of the first op-amp might help (although you might need to do your RIAA sums again). I'd suggest changing the second op-amp to be inverting (for lower distortion, but higher noise), but some people get hung up about absolute polarity.

Other possibilities are the actual implementation of the circuit. You've replaced the ceramic capacitors (hurrah) but what about the power supply decoupling capacitors and that capacitor below the feedback network? Are they ideal? Did your PCB layout take account of where the currents flow? Are the tracks nice and wide to prevent voltage drops along them from being amplified?
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Old 20th May 2005, 09:20 AM   #3
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Default Hi-end solid state RIAA pre

Hi-end solid state RIAA pre:

http://astral-for.narod.ru/index/stones.htm


Attention! Do not use ceramic capasitors!
Use silver\mica or teflon!
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Old 20th May 2005, 11:07 AM   #4
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Why does it sound bad? Don't know but it may be a couple of reasons.

1 Not too much feedback with used opamp? The SSM2275 seems to have enough...

2 Too heavy load due to the RIAA network

3 Something unknown.

I think using a buffer is good thing, to unload the opamp itself. I have done this and I think it worked pretty good. I used a some better opamp maybe, a LT1028.

I have made an inverting vinyl amp monster also but this is too much for most people.

Have you by any chance tested with another known good vinyl amp so that you can rule out your other equipment like TT and pickup?
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Old 20th May 2005, 11:46 AM   #5
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Does the cartridge have the correct load capacitance? 47p seems a bit low. This will effect the HF response.
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Old 20th May 2005, 02:18 PM   #6
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Default RIAA woes

Hi everyone, and many thanks for all the constructive responses you've given.

Reading these I think my first plan of action will be to replace the op amp with a genuine OPA2134 and to investigate cartridge loading effects by experimenting with the value of C2, starting upwards. I will do all this this weekend and report progress on Monday.

The SSM2275 claims to have a Butler Amplifier front end which theoretically should be OK, but it's noteworthy that AD have discontinued this device, when I would not imagine there to be valid commercial reasons for them to do so.

The vinyl-pickup-turntable combo with the B&O cartridge has been tested on another amp (Harmon-Kardon AVR30 or AVR60, I forget which; a big fat thing with Dolby ProLogic) and it sounds quite a bit better than on my RIAA.
The Stanton cartridge sounds s*** on everything but that's to be expected; it's a disco workhorse with bargepole cantilever that tracks at 3 gram.

I've posted a pcb fragment which shows the layout of one channel's RIAA stage. Red is the top layer, blue is the bottom layer. Brown is a mid layer used for powers, and there's a solid groundplane on the other mid layer

Cheers all

JohnH
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Old 23rd May 2005, 08:12 AM   #7
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Here's what I tried:

1. I trawled through dozens of RIAA circuits and found that the loading cap used typically varies from 47pF to 220pF and many circuits have switchable loading caps. I removed the 47pF loading cap (which was still a ceramic type) and replaced it with first a 47pF polystyrene and then a 220pF polystyrene cap. (Big squash-in). None of this made any change that I could hear.

2. I replaced the op-amp with a OPA2134, which also made no change that I could hear.

3. I invoked the 'critical ears' of some other persons and none of them could figure what I was on about; everyone said it sounded OK and that I was crazy. In my previous post, I said it 'sounds OK' on the HK AV amp. This is not accurate. What I should have said is that 'I remember it sounding OK 4 years ago'.

4. I made a CD copy of the particular phono disc on my HHB 830 Burn-IT CD recorder about 4 years ago, using the same turntable, same arm, same cartridge, same RCA cables, but equalised through the RIAA preamp of the Harmon Kardon AV60 amp and fed to the HHB CD recorder's analog inputs. At the time I'd put my entire vinyl collection onto CD, but I'd kept this particular phono disc (Bad Influence - Robert Cray Band, 1983) for testing phono stuff because it was in excellent nick.
I synchronised playback of the previously recorded phono on CD with 'live' playback of the phono disc and did A-B listening tests to compare the two.
I was very surprised that I could not hear any significant difference; if anything the raw phono through my RIAA had very slightly more 'presence'! My 'critical listeners' shared this view.

The source record involved in this test is 22 years old and 99% of my listening today is done on 16-bit CD or 24-bit DVD source. I think perhaps one forgets what stuff sounded like and expects too much from vinyl phono discs!

So I feel most embarrassed : that I have fallen victim of my own subjectivity and wasted all of your technical thoughts.

What a 'nana I have been.


Paper bag on head. . .
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Old 23rd May 2005, 09:00 AM   #8
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Quote:
Originally posted by John Hope
So I feel most embarrassed : that I have fallen victim of my own subjectivity ....
The mind is pretty powerful. I think more people should do some A-B listening so they really know that the mind is playing tricks.

Some (or very many) vinyl recordnings (or copies) sound far from what the vinyl itself can deliver. That's the sad truth but even more sad CD's sound not either so good as the CD medium can deliver. (Why want we DVD and SACD when MP3 is enough?)

About important things like load of the cartridge.

I have a Denon preamp which has varialble input capacitance with help from a capacitance diode, 200-600 pF. The truth is when I turn the knob is that the difference is in most cases zero! Isn't that amazing considering that soem people claims that this is _very_ important. I'll admit that some high quality recordnings with a special type of music may be affected by the loading.
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Old 23rd May 2005, 10:10 AM   #9
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Whether or not input capacitance is critical is dependent on the cartridge. Shures and the original Ortofon VMS20E used to be very fussy about loading capacitance because they resonated it with the cartridge inductance to equalise losses in the mechanical system. The effect of insufficient capacitance was perfectly audible. More recent cartridges tend to be less fussy.

If you're really worried about vinyl replay, I find that the most critical thing is temperature and humidity. Cartridges seem to want temperature over 70F (perfectly reasonable) but also relative humidity over 40%, and the more, the better. Maplin do a cheap digital thermometer/hygrometer.
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Old 23rd May 2005, 10:55 AM   #10
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Although I am not technically minded,I did note that the best cartridge you used for testing is several years old and I feel that this is where your problems may lie.I am a vinyl user and would not consider it valid to judge a new (not broken-in) phono stage with a cartridge that would most likely be showing it's age.There is a good consensus among the vinyl community that old cartridges have a use by date.Worn needles obviously,but also inner suspension and electrics fail after a certain period.There are correct alignment issues as well .You do not mention what table you are using,the arm etc.Synergy is all important in capturing that magical vinyl sound.I would consider it obligatory to use a cartridge that you can verify is working properly.I would recommend something like a Grado Prestige series cartridge for moving magnet,they are not too expensive or one of the Denon(DL-110,DL-160,DL103) range for moving coil.Here is an informative link to info on cartridges. http://www.cartridgedb.com/ I hope this will be of some assistance.
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