Measuring a phono preamp with QuantAsylum 402

[radix]

I'm new to the QA402, so I wanted to check out a few things with the experts. I have an MC phono preamp that's setup for 62dB gain or switchable to 68 dB.

To measure its performance, I believe I want to do this:

Get a baseline for self-noise and THD+N
- Shunt the L- and R- with 50R caps
- Hookup my BNC-RCA cables input-to-output (I'd use a barrel connector)
- Measure the self-noise (no signal) and THD+N (1kHz) to get a baseline (these are -114 dBV and -81 dBV respectively)

To measure FR
- Select the User weighting and RIAA Record Response
- Use EchoChirp @ -68 dBV (for 0.4 mV) output

To measure gain:
- Select the User weighting and RIAA Record Response
- Use multitone
- view Gain dB
- Should be around 62 dB

To measure THD+N
- Use GEN1 @ 1 kHz and -68 dBV output
- View THD+N

I imagine I'm missing something. Also, are there any measurement threads I missed that go over this already?

Thanks,
Marc

 

[amirm]

If you want to be compatible with my measurements, I use 5 mv for MM and 0.5 mv for MC simulation. With most phono stages, these tests become basically SNR as the distortion is far lower than the noise at lower frequencies.

I run THD+N vs output voltage to graph the point where it clips to show headroom.

 

[LTig]

I'm new to the QA402, so I wanted to check out a few things with the experts. I have an MC phono preamp that's setup for 62dB gain or switchable to 68 dB.

To measure its performance, I believe I want to do this:

Get a baseline for self-noise and THD+N
- Shunt the L- and R- with 50R caps

50 Ohm is too high for MC, go for 10 Ohm. For MM I've heard that 1 kOhm would be a realistic value.

 

[radix]

50 Ohm is too high for MC, go for 10 Ohm. For MM I've heard that 1 kOhm would be a realistic value.

Ok. How do you wire that up? I guess a BNC to short piece of RG6 then solder on a 10 ohm?

 

[LTig]

Ok. How do you wire that up? I guess a BNC to short piece of RG6 then solder on a 10 ohm?

Just get an RCA connector and solder the resistor directly into it.

 

[Blumlein 88]

50 Ohm is too high for MC, go for 10 Ohm. For MM I've heard that 1 kOhm would be a realistic value.

Most MM phone stages are 47kohm are they not?

MC is more variable with some cartridges looking for as much as 300 ohms. Though 10 to 30 ohms is more common.

 

[radix]

Most MM phone stages are 47kohm are they not?

MC is more variable with some cartridges looking for as much as 300 ohms. Though 10 to 30 ohms is more common.

So you are talking about putting a load resistor parallel to the preamp input when driving it from the QA402? There's no cartridge involved here. The preamp specs says the low gain (62 dB for 0.4mV) has an input impedance of 470 ohm and the high gain (68 dB for 0.2mV) has an input impedance of 117 ohm.

I was originally talking about putting a 50 ohm terminator on the L- and R- inputs to the QA402 to make the balanced input single-ended.

 

[Rja4000]

Most MM phone stages are 47kohm are they not?

Isn't that the preamp 's input impedance ?

That was quite standard in old Hi-fi era, but I don't know where this value comes from.

 

[LTig]

Most MM phone stages are 47kohm are they not?

MC is more variable with some cartridges looking for as much as 300 ohms. Though 10 to 30 ohms is more common.

For noise measurements you have to use a resistance which is similar to the output impedance of the source, not the input impedance of the preamp, to get real world values for SNR. My MC pickup has around 10 Ohm and the recommended load is 200 Ohm so this is what my DIY preamp uses.

 

[Blumlein 88]

For noise measurements you have to use a resistance which is similar to the output impedance of the source, not the input impedance of the preamp, to get real world values for SNR. My MC pickup has around 10 Ohm and the recommended load is 200 Ohm so this is what my DIY preamp uses.

My apologies I misread the post thinking it was about what input impedance the QA unit should mimic. Everyone is correct for what you have in mind to use a low value for source impedance.

 

[radix]

I've made a few measurements. I seem to have an interference problem, especially at 16 kHz, which I am not sure what it would be. Any suggestions would be appreciated.

When I measured the self-noise of the QA402 with short BNC connectors, I get this

If i switch to RCA cables to BNC/RCA plugs and use two barrel connectors, I get this

There's a little spike at 60 Hz, a smaller one at 180 Hz, and then a large spike around 16 kHz. The 60 and 180 I can figure out, but I don't know what's causing that 16 kHz spike. It must be noise from something. Maybe I need to move the testing to someplace further away from my regular computer workbench.

So far the DUT results for a 1 kHz tone and a FR plot look like this. Obviously those noise artifacts are being a big deal for it. This is without any source or sink impedance added at the QA402, just straight to/from the preamp. I think I need to solve my interference problems before hacking some cables.

Marc