WolfX700 Measurements of Topping Pre90/Ext90 Preamplifier/ Extender

[gvl]

The manufacturer of the signal source is very unlikely to give the full spec on the output capability of his product - because he has never measured it!

This is key. In other words pairing with low impedance input is crapshoot.

 

[EdW]

Curious, when using DAC with weak op-amp, would the distortion cause excess highs and recess low? Thanks!

Nearly all opamps would do the job just fine and would be capable of sinking/sourcing several mA without significant increase in distortion above the lightly loaded case. If the O/P is particularly weak (say a low power battery DAC) just set the PRE90 to +10dB gain then you’ll need less drive capability.

 

[Harmonie]

Low input impedance means the source has to supply more current to keep the voltage linear. If the source component has robust op-amps on the output that can supply the required current, all is fine. If not, the source component will increase distortion into the preamp. Same idea with power amps increasing distortion into 4 Ohm, 2Ohm, etc.

Low input impedance on the preamp also requires the source output impedance to be very constant with frequency, otherwise the frequency response will be affected. This is what the earlier post was referring to with regards to low frequency droop.

Seems quite an issue:
I might be wrong:
Is this impedance/current relation measured each time we have a new review here on sources, preamps aso ?
Or:
The fact that we just measure distortion, is the answer to everything ?

 

[Michal]

So let’s take a practical example. The sort of power amp you might be using this with would be the Benchmark AHB2. This needs 2v rms drive for its nominal output. You choose to set the PRE90 to +10dB gain in order to minimise the drive requirements for the signal source which means your signal source has to be capable of driving 632mV rms into the 2k load of the PRE90. So if the output impedance of your source is much less than 2k (say 100 ohms) then your source must be capable of 632mV absolute minimum. If the impedance is higher, say 2k, then you’d need 1264mV of output. But be careful: a high output impedance from your signal source suggests that it was never intended to drive a low impedance and probably will distort if loaded that way. A modern signal source will probably incorporate an opamp output stage and will do the job OK (boutique phono preamps excepted). Older gear probably won’t make the mark. The manufacturer of the signal source is very unlikely to give the full spec on the output capability of his product - because he has never measured it!

Look at post #83 and than #84. Do you think that JohnYoung could lie to me?

 

[EdW]

Look at post #83 and than #84. Do you think that JohnYoung could lie to me?

You have misread what I have said:
In the case you discuss with John we can analyse as follows - assuming the drive is from an opamp in your DAC (DACs usually have opamp outputs unlike older signal sources) then the signal will be attenuated by 3.5dB because of the 1k feed resistor and the low frequency cut off would be about 11Hz with the 4.7uF coupling capacitor - so OK

 

[seashell]

Why 2k?
A 2k input impedance perhaps indicates the use of an opamp in the inverting mode, rather than voltage follower mode.

[sarcasm]Inverting? Should we start the reverse absolute polarity discussion now? Was it designed to be paired with the E30?[/sarcasm]

 

[gvl]

nd the low frequency cut off would be about 11Hz with the 4.7uF coupling capacitor

How is this calculated? 4.7uF will add about 1k impedance at 35Hz, no?

 

[EdW]

How is this calculated? 4.7uF will add about 1k impedance at 35Hz, no?

The 4.7k is feeding 1k in series with the 2k I/P impedance of the preamp giving a 3k effective load so a time constant set by 3k and 4.7uF

 

[MusicNBeer]

Curious, when using DAC with weak op-amp, would the distortion cause excess highs and recess low? Thanks!

It causes compression in the signal, meaning amplitude peaks get "soft clipped" for lack of a better term. This is harmonic distortion which would be easily measurable at the preamp output if present.

Many source components will say "minimum load impedance" of 10K or so to avoid this and the frequency response issue.

One area that low load impedance helps is with noise pickup in the cables. With balanced cables, this is pretty moot though.

The 2K Ohm input impedance value is interesting because it's exactly the input impedance of the Adcom GFP-750 in passive mode. I have one and measured it. I remember back when that preamp was popular, people were always saying it sounded like crap in passive mode. My theory is this exact issue.

 

[Max]

So, based on that measurement from Wolf, this has granular volume adjustments down to -24db, but lower than that are jumps of -5db and -10db. Am I interpreting that right?

No. You can see the whole volume adjustement in the first page and the granularity is "continuous" from the beginning to the end. The second measurement has been made by Wolf to answer quickly my question about THD vs volume adjustment. This measurement requires to adjust manually the volume and to measure at each point ... so, Wolf has well analyzed the first part where the gain switching occurs then has just used big steps down to finish the second part.

 

[Neale]

I was hoping Topping was going to release a pre amp that includes a DAC. Why can't someone, anyone make a single box with a DAC and a couple analog inputs. This is all anyone who hangs out at ASR needs .. we know good sound come from a digital source, but we all have a couple of analog devices we still use ...

 

[Vini darko]

I was hoping Topping was going to release a pre amp that includes a DAC. Why can't someone, anyone make a single box with a DAC and a couple analog inputs. This is all anyone who hangs out at ASR needs .. we know good sound come from a digital source, but we all have a couple of analog devices we still use ...

Aylic s50 pro streamer @£170 does that. Only single ended and not sota but the price reflects that.

 

[MRC01]

Relay of course.

SSR or EMR? Is the volume silent as you move it, or can you hear the relays clicking? What is the expected relay lifespan?

 

[Music1969]

I've seen comments that high output impedance with DACs is a way to cheat for a higher SINAD.

Does similar apply to this Pre, with low input impedance? Does that aid SINAD measurement ?

 

[Vini darko]

I've seen comments that high output impedance with DACs is a way to cheat for a higher SINAD.

Does similar apply to this Pre, with low input impedance? Does that aid SINAD measurement ?

It can mean being able to run less gain and have a lower noise floor as a result.

 

[VeerK]

The 2k impedance is interesting, assuming Topping intends for the Pre90 to be used with their eventual power amps. Definitely sitting on the sidelines for now

 

[EdW]

I've seen comments that high output impedance with DACs is a way to cheat for a higher SINAD.

Does similar apply to this Pre, with low input impedance? Does that aid SINAD measurement ?

Generally low value resistors generate less noise
Noise voltage = SQRT(4KTRB)
where SQRT stands for square root, K is Boltzmann’s constant, T is temperature in degrees Kelvin, R is the resistor value Ohms, and B is the bandwidth of interest (Hz)
so a low source resistance from the DAC is good here

The noise trade offs in the preamplifier would depend on the architecture of preamp. The lowest distortion virtual earth feedback design will give lowest noise when the input resistance is low. A voltage follower architecture would have slightly higher distortion (could still be around -120dB linearity) but without the need to load the signal source.

 

[gvl]

The L30's pot is wired directly to input, perhaps this pre's design is similar. Is this the "virtual earth feedback design"?

 

[sq225917]

Re the battery shipping. They can leave the battery in the plastic cardboard strip it was supplied in or put it in the remote with an electrically insulating pull tab fitted to stop it making contact on both electrodes. I assume its a sliding door type remote where the electrodes touch the battery on the front and side, so option one is the only choice for them.

One battery per box, perfectly legal for intl Air shipping.

 

[restorer-john]

There's no industry standard first of all.

Sorry on this one, you're mistaken. 2K is ridiculous and you know it, John. You've gone after one metric at the expense of source compatibility and performance. It's a mistake to pass the buck to the source components. A preamplifier is designed to take varying source impedances and levels and buffer them without messing up the performance of the attached components.

47-50K is the defacto input impedance for line sources into a typical preamplifier/integrated line input. It's also the defacto input impedance of an MM RIAA stage and has been this way for many, many decades. Go peruse a few hundred service manuals for amplifiers and preamplifiers and tell me what the "standard" is. I can tell you, it's closer to 50K than 2K.

20-25K was favoured by pro power amps input impedances in the 1980s.