Stereo phono input testing - low freq noise

Don’t think its a matching network, its emulation of the input hardware. The purpose is not to match source and load impedance. The inductance in particular can be a big contribution to noise if the phono preamp has significant current noise, any SNR without may be rather bogus.

Could not really find anything on measuring phono preamps- not even an AP app note on it. I did find some mention in one instance of parallelling a resistor- like 10ohms- across phono preamp input, but this mention was not really clear what there test equip was…

It comes down to the output impedance of the generator outputs on the QA vs an MM or an MC cartridge and the inductance of those particular typical cartridges.

A typical MM is typically 200-1000R with most hovering somewhere in the middle.

Phono stages were specified driven back in the day with mostly 600R or lower generators, so the QA is abolutely fine, as is for MM stages. Added inductance doesn’t help as the cartridge was a generator, not a series inductance to a generator. Depending on the input topology, it may make a difference.

The Denon above is a parallel FET direct coupled input on MM and MC, with MC being shunted to give a 100R input load to the MC cart when connected. It’s RIAA (MM and MC) deviation was specified as 20Hz-100kHz +/-0.2dB.

I haven’t bothered with MC testing as I haven’t fully characterised the QA generator DAC at very low output levels- has anyone done that?

I have tons of phono stages here, built into preamps and integrateds and up until the QA, all I ever tested was overload, noise, channel balance and a basic RIAA deviation using an inverse RIAA network/attenuator. THD was always a nightmare in the analogue domain of phono stages due to noise swamping THD, especially when most good preamps have none or close to none.

@restorer-john- thanks for the additional info. The Denon specs are impressive. I can only go by what I have measured, and I do see a lot of variation, but that is probably because of the age of the units since the majority are >30yrs old. Most of them do not sound as bad as they measure- at least when I listen to them which is not something that I regularly do.


@restorer-john: I have to say that your Denon looks exceptional to me, congrats! My attempts at measuring vintage phono stages fall very much in line with what @VAR shows/describes. I have a long way to go as far as building up a database of results. So far my phono stage testing is a couple of Sansui integrateds and then all vintage Marantz 22xx-series receivers. At least they’re pretty consistent, and they do sound good, even if they don’t measure all that well.

Also concur with Scott’s observation that best results are achieved from the Tape Mon Out (i.e. phono stage standalone), followed by Pre Out (phono stage plus preamp/tone controls), and the worst measurements are the overall Phono In to power amp output chain. Only makes sense since the noise and distortion from the ~40dB phono stage gain is further amplified in each subsequent stage. Scott is correct that it becomes pretty discouraging to measure that overall amp chain and maybe there’s not much point to it if you get a solid set of numbers from the phono stage standalone that hopefully match up to the original specs.

@GlasgowGrip- I am glad I am not the only one with phono stage results unlike @restorer-john. If I do a loopthru measurement of my system for THD it looks pretty reasonable. I measured a Sansui 7000 receiver a few days ago and even ran the QA403 off my laptop (battery only) instead of my normal desktop and there was no change. I even swapped out the test cables with some new ones. Not really any changes. For grins and database purposes, here are the responses I got on the 7000 (it is >50yrs old and pretty much all original):

Here’s a nice example of a Sony TA3650 from 1977. Absolutely untouched and original.

Not nearly as good as the Denon, but decent nonetheless. More in line with what I would expect from the topology.

Note, this one A-WTD as I was comparing to rated specifications. Which was a published S/N of 70dB WRT 2.5mV. At 5mV the S/N went considerably higher.

RIAA deviation:

We really have to be careful with input levels when comparing to rated specs on phono stages. S/N will improve massively of course if you input a higher input. 5mV is not a “standard”, but in the interests of comparing phono stages, we could agree to use that for MM stages.

Absolutely use the tape monitor output only, that is the way it has been done forever.

All these tests are done with the QA on my lab bench with no special treatment, hooked up to my PC that handles everything. No wireless keyboards or mouse, they mess up any high precision audio measurements. I will say however, the QA doesn’t play well with the Rigol DSO if I turn the b#stard on. Not that I care, my 35yo, old school CRO is my goto scope for audio. Everything is common ground earthed at my bench, except the QA being isolated USB. I have found a few times where grounding the QA helps with noise.

I recently bought some of these as I was sick of fighting with crimp BNCs and coax. So cheap and really flexible.

I got 600mm (2ft) as that is just right for gear to instruments without creating rat’s nests of cable.

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Here’s a freq response run on my 1969 vintage Sansui AU-555 47K phono input (it has 100K too but I haven’t run that yet). I used the RIAA hi-res playback weighting and the tape record jacks for the output. Probably not great but this is a pretty early solid state integrated amp and I’m just happy it’s working again. The QA-403 is a lot of fun so far!

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@restorer-john- I have seen several MM specify other voltages than the 5mv “standard”. In fact, the one that I posted was spec’d at 2 or 2.5mv, but I did not capture that data. I compared an RG58 cable with bnc’s on both ends to my normal RCA test cables which use bnc/rca adapters on the QA403x end, and there was no difference in performance. But I will order some of the cables you showed, though I would go with a 4’ length.

This looks like what I would get and would be happy with: +/
0.6 to 0.8dB

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You can never have enough cables I say. :slight_smile:

Do we want to see more phono stage test plots in this thread? Happy to test and post some more if anyone is interested.

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I think it may be helpful, but may make some of us feel bad that our plots are not as good as what you have shown. I may post one of my nicer ones a bit later today. I agree on you can never have enough cables… or hard drives or power amps :grin:

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Here’s the weighted reponse of a little phono preamp I made (little meaning small enough to fit inside a tonearm in principle…).

And expanded:

I have a lot of mains noise around my workbench alas so the spectrum is full of mains harmonics:

(All graphs taken at 5mV input (-46dBV))

Anyway the PCB (inputs on RHS, output on LHS, one RIAA stage, two filter stages making a high order Butterworth rumble-filter.

There are some more caps on the backside for the RIAA network, and decoupling caps there too.


Interested to hear the logic behind this project. Are you building an arm too?

I meant to get back to you guys, here is a Marantz 2230 phono stage that has been restored (caps, diodes, transistors, a few resistors). When I went to test it with the QA403, I realized that plugging into the Tape Monitor Out’s caused the damn thing to go into a low-frequency oscillation condition (a.k.a. “motorboating”). Based on a few suggestions I dug up over on AK, I did some trial and error substitutions of the transistors and finally landed on the need to add Miller capacitors to the final transistor stage, which fixes it right up.

So here we are:

Happy to hear any critiques of my tests - thanks!

Well done!

As the output from the internal phono stage is only 476mV, you can move the full scale input to 0dBV and get a useful drop in noise, but that’s looking really good for a 2230. A 50 year old receiver.

The phono stage is quite different between the 2230 and the 2230B…

Looks pretty darn good to me. Not sure about the “Miller Transistors”- beyond the scope of what I typically get into. Here are plots of a Marantz 2330 that I had measured a while back- all original (still):

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This 1980 preamp (AKAI UC-A5) has an amazing MM overload at a measured 290mV (1kHz), but even that is below its 340mV spec.

And, it clearly needs some serious attention in the regulated power supplies as the 50Hz and mains harmonics are way too high. Not hitting the MM overload could point to PSU rails being lower than they should be. THD is super low, but add in the noise and it’s a mess.

RIAA deviation is respectable within -0.2dB up to 20kHz (spec was 30Hz-15kHz, +/- 0.2dB), but its trajectory could be an issue out of band.

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The FR looks pretty darn good! I probably should start looking at the overload point more. I will post another response from a phono preamp later on that is “interesting”.